Eight things I learned by reading Section C8

I know, I know: a listicle. How 2011! But this is a site about heritage, after all… Here’s the pitch. I’ve been reading Section C8 Unreinforced Masonry Buildings from The Seismic Assessment of Existing Buildings, written by a team of researchers, academics and professionals and published at eq-assess.org.nz. (I mentioned it in the post on bed joint sliding shear.)

You might think it sounds tedious, but it’s not. It’s well-written, well-illustrated, and provides some really useful and clear categorisation of structures and the ways they can fail. I needed to read it: this is the work I want to do. Some of you might be in the same position. But even if you’re not interested in doing seismic retrofit, if you’re involved in any way with building works on heritage buildings made from URM, you should probably leaf through the first few sections.

It would be presumptuous and preposterous for me to write a “review” of Section C8. I don’t understand it well enough to do so. But I thought I’d share my own highlights, as a way of enticing others to have a look. We’ll see how it goes.


Christchurch

On a more serious note, this article includes images of damaged and collapsed buildings from Christchurch. The following is presented with sincere respect to the 185 people who died in the 2011 earthquake and to their families. Let’s work to try to stop something like that happening again. Kua hinga te tōtara i Te Waonui-a-Tāne. HT


 

Section through a cavity wall. Note also the change in thickness at the first floor, creating a ledge for the joist to sit on. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings, at eq-assess.org.nz. Attributed to Holmes Consulting Group.

1.  Spot cavities with algebra

It was common practice to have a vertical cavity in brick walls. The cavity provided a barrier to moisture, and the outer wall could be made from higher-quality bricks that gave a finer appearance to the building. (The cavity is the black line running down the wall in the picture above.)

There are various complexities pertaining to the ties that were used to hold the layers of wall together across the cavity, how those ties have lasted, and how to assess the relative motion of the outer wythe (the veneer of good bricks) and the inner wythe(s). I won’t go into those here beyond noting that they exist. The question is, how do you know whether there’s a cavity, if you haven’t made a hole in the wall?

Algebra to the rescue! Well, multiplication, anyway. Brick are standard sizes. They’re usually 110mm thick (taking “thickness” as the dimension going into the wall). With some mortar in between wythes, that means that two wythes (layers) ≈ 230mm, three wythes ≈ 350mm, and four wythes ≈ 450mm. So if your wall thickness isn’t pretty close to one of those numbers—check for a cavity.


Insufficient connections between (floor and ceiling) diaphragms and walls leading to out-of-plane collapse. Section C8, as above, eq-assess-org.nz

2. Loose diaphragms fall out (sorry)

We’ve heard on site about floor joists sitting on ledges and maybe falling off when the ground shakes. We’ve also heard, on almost every site we’ve visited, about how tying the diaphragm into the walls at floor height, ceiling or both, can improve the structural performance of the building. What Section C8 makes clear is that the diaphragm can act to redistribute loads from out-of-plane to in-plane walls. If the diaphragm deforms too much, though, it won’t be able to support the walls effectively. In fact, too much diaphragm deformation can actually shove a face-loaded wall right off the edge of the building.


In-plane sliding on a damp-proof course. Section C8, as above, eq-assess.org.nz

3. Slip-‘n’-slide on a damp-proof course

For walls loaded in-plane (along their long axis), one of the possible failure modes is in-plane sliding. I wrote about one version of this in my post on bed joint shear testing. Section C8 points out that damp-proof courses can also be vulnerable to sliding.

To stop moisture wicking upwards through porous masonry, a layer of damp-proof non-porous material was commonly included in brick walls. Bitumen, slate, lead, or similar waterproofing was laid down in a continuous layer, usually not far above the foundations. (At a building I saw this summer, the DPC is granite, but that’s exceptional.)

Turns out, those damp-proof materials are softer than the masonry, or perhaps bond less well to the surrounding materials. Or maybe the change of material just provides a stress concentration. I don’t know! Still, I’d never seen or heard about DPCs as a site for sliding until I read C8.


Parapets tied back to a roof with raking braces — but are they sufficiently restrained in the vertical direction? Section C8, as above, eq-assess.org.nz. Attributed to Dmytro Dizhur.

4. Vertical tie-down for parapets

It’s a common sight around the traps to look up and see a steel brace holding back a parapet. Job done, right? Maybe not. Section C8 points out that such braces may not have enough capacity to deal with vertical displacement, especially when shaking of the roof plane is amplified by the brace and transmitted to the parapet. Quoth C8: “The danger of non-robust strengthening is that the parapet still fails, but collapses in larger, more dangerous pieces.” Not good. The parapet may need to be drilled and post-tensioned onto the top of the wall below.


As axial load increases, masonry walls gain strength from confinement. From Section C8, as above, eq-assess.org.nz. Attributed to Dunning Thornton.

5. Look out above

I suspect if you’d asked me whether masonry buildings suffered more damage at the top or the bottom in earthquakes, I’d’ve said the bottom. Makes sense, right? The walls crush and they turn over? Wrong. Generally speaking, the more axial load that is on the masonry walls, the stronger they are and the better they resist disintegration. Of course, this depends on the building form, the shaking, and other things, but the relationship between axial load and strength is useful to know.


Failure modes. Left, out-of-plane failure: instability of wall insufficiently tied in. Right, in-plane failure: spandrel failure, diagonal tension cracking, toe-crushing of piers. [Right, Section C8, as above, eq-assess.org. Attributed to Sharpe. Left, CCC Heritage, licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 New Zealand License]

6. We don’t talk about failure here

Section C8 enumerates the failure modes of URM buildings. I won’t write a list or a summary, but I will say that I found it really clarified my thinking about how buildings work to consider a finite list of failure modes. When I look at a structure now, I feel much better informed about how to break it into chunks and think about how each chunk might move and how it might fail. From that perspective, reading C8 was like taking your American mate to the cricket and telling him where to look.


Hierarchy of vulnerability, Section C8, as above, from eq-assess.org.nz

6b. And what to look for

An afterthought to the above. In assessing the structure, the C8 guidelines suggest working from left to right on this hierarchy chain, with the idea that the most vulnerable components of the building, those which are likely to fail first, are at the left. No good wasting time and money diagnosing a complex in-plane failure mode if the parapet’s not secured.


Ten steps. The assessment procedure, Section C8, as above, from eq-assess.org.nz

7. Ten simple steps…

I suppose I haven’t much to add to the image above. These are the guidelines which C8 provides to engineers as advice on how they should approach assessing a building. As with #6 above, for me this helped to understand how engineers divide the building into a set of observations and parameters which allow a model to be created—and don’t bother creating one that exceeds the complexity of the structure! I will be trying to hold these ideas in my head on our next site visit (St Paul’s Church on the 13th of March) and to think about how I’d approach the task of assessing the building. Thankfully we’ll have Peter Liu from EQ STRUC with us to show us how it’s really done!


A video from the Uminho Research Group on Historical and Masonry Structures. This is apparently a strengthened model. Still, watch the upper sections of the wall crack at the floor line and rock.

8. Walls rock

Walls under face load can be modelled as rocking. This means if the load is perpendicular to the wall, the wall can be “assumed to form hinge lines at the points where effective horizontal restraint is assumed to be applied… At mid height between these pivots… a third pivot point is assumed to form.”

When I read those words I recalled John O’Hagan talking about this at Hopetoun Alpha, but I think I understand it a little better now. To me, it feels different to think about a masonry wall in an earthquake as two rigid panels teetering one atop the other, as opposed to thinking about n bricks shuffling about independently, or as one rigid surface.

And this is the note on which I’ll leave this post. C8 offers quite a lot of guidance about how to make simplifying assumptions that allow analyses to be made, the rocking walls being one. It also offers suggestions for how to calculate important parameters if you can’t or haven’t tested them—things like tensile strength of the masonry. The impression I had, on reading these guidelines, was that the task of doing this kind of work myself someday seemed not impossible. Surely that’s the most sincere praise I can offer.

Testing mortar on-site with EQ STRUC, February 2018

What’s this then?

I’ve mentioned here before that I’m working for Salmond Reed Architects this summer. Recently, I had the opportunity to observe an on-site test of the material properties of some mortar at a heritage building in Auckland, and I thought I’d share some of the details here. The site is a late-nineteenth century two-storey brick building. (To clarify some jargon used henceforth:  bricks = unreinforced masonry = URM). I’ve been given permission to share the test procedure, but I haven’t sought permission to disclose exactly where the testing was being carried out, so that is a deliberate omission from the post. Still, I thought the process was novel and interesting enough that some of you might enjoy hearing more about it.

Borderline punching shear failure, wall anchor. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Dymtro Dizhur, et. al.

So you think you’re pretty tough

If you’re going to assess an existing building, you have to decide how you think the building might fail. For URM buildings, there is a hierarchy of failure modes from most likely to least likely, and an engineer needs to work down through  the hierarchy, examining all of the possible modes. At a certain point, the assessment will find a mode that causes something unacceptable to happen under the predicted load. That doesn’t mean that the assessment stops there: but certainly, something needs to be done about the potential failures and their resulting risk to life.

So far, so tidy. But there’s a problem. Heritage materials are far from homogenous. The 1920s concrete at the St James Theatre  is soft and drummy, whereas on another site I recently saw 1920s concrete that was described to me as “rock-hard”. Bricks can be low-fired and soft, or fired at high temperature and hardened. Stone’s anisotropic. And mortar’s really idiosyncratic. How much lime went in? Were there shells in the sand? How wet or dry has it been throughout its life? It’s hard to predict the strength of hand-mixed materials from a time before the standardisation of products. The true in-situ strength of the material, the actual number, will make a difference to which failure modes come out of the analysis as critical weaknesses, and how much work you have to do to the building. Hence also the dollars involved. So how do you find out how strong the materials at your specific building truly are, and hence, how they will fail? You test them.

Sliding shear along a defined plane. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Dunning Thornton.

In due course

Mortar’s not brick glue. Its primary purpose is not to stick bricks together. Instead, it provides a slightly compressible joint between the bricks as they sit in their stacks, allowing them to expand and contract without cracking each other to shards. Generally, mortar is softer than the bricks, especially lime mortar, and this is a good thing.

Notwithstanding the above, the mortar is the thing that stops the bricks sliding across each other if the wall gets shoved along its length, for example by an earthquake. The test that I observed, a bed joint shear test, examines how well the mortar prevents the bricks sliding across each other. It seeks to establish a cohesion value for the mortar. Key takeaway: it’s a mortar test, not a brick test.

Diagonal tension cracking, piers. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Dmytro Dizhur.

The cohesion value influences several important failure modes: embedded anchor pullout; punching shear for plate anchors; (diagonal tensile strength leading to) diagonal tensile cracking and spandrel shear due to flexure; and bed-joint sliding. In this post, you’re seeing  pictures of several of these modes. For the building that was being tested, the  EQ STRUC engineers told me that the cohesion value was going to be used for calculating bed joint sliding shear.

Bed joint sliding, stair-step crack sliding in low axial load walls. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Bothara.

The bed joint shear test is carried out by taking a brick out of the wall and using a hydraulic jack to push on the bricks either side of the hole. When the brick that’s being pushed starts to move, that means that the mortar has failed. Simple. In a moment, I’ll show some images of the steps. But before that… this.

Bed joint sliding shear

Bed joint sliding shear is calculated thusly:

Bed joint sliding shear, equation C8.33, from Section C8 (Unreinforced Masonry Buildings) of The Seismic Assessment of Existing Buildings, from eq-assess.org.nz

I hope it won’t totally destroy my credibility if I tell you that my first reaction to seeing the above was yuck. But on closer inspection, the equation is pretty straightforward, and considering its terms really helps to understand what is being measured.

Firstly, the µf(P + Pw) bit. That’s just saying that the load pressing on the bricks makes them rub on each other, which makes it harder for them to slide across each other. More load on top: harder to slide the bricks.

Secondly, the tnomLwc bit. tnom and Lare the thickness and the length, so really that’s an area: the area of the bed joint. And the cohesion value is the stickness. How sticky is the mortar, per area? That’s what this part is describing.

In the real-world bed joint shear test, the engineers try to make the equation even simpler, because the friction bit mightn’t be completely obvious: exactly how much load IS on the wall at the time you test? You can measure the height of the wall, but what about superimposed loads? So, to avoid worrying about this, the EQ STRUC team carried out the testing on bricks just underneath windows, where there wouldn’t be much imposed load. That means the cohesion can be found by knowing how much area of mortar you were testing (the bed joint size) and how hard you shoved the brick.

Opening up and finding a stretcher course

The test, in detail

First, catch your brick. I’ve noted above that the best spot to test is under a window. Another limitation is that the test should only be carried out with stretcher bricks in a stretcher course. If you’re not sure what that means: stretchers are laid along the wall, headers are laid across the wall. Like so:

A brick wall, anatomised.

The tests I saw were all carried out on the internal wythe of the brick wall. (A wythe is a layer of thickness–ie, a wall that’s three bricks thick is laid in three wythes, usually interlocked. There’s more complexity to this but that’ll do for now.) So, to carry out the test, you need to open up the linings and identify the right course to test.

Using a mortar saw to remove a brick; and a closeup of the mortar saw

Secondly, a single brick is removed. This means using a mortar saw to take out all the mortar above, below, and to the side of the bricks. On the picture two up you can see that these are the two bed joints and the two head joints. The brick gets extracted whole and can be replaced later.

A brick is removed. The mark reads “W Hunt, Auckland”

Mortar is removed from the extracted brick, and set aside for future testing—think of it as something like a concrete cylinder test. This testing isn’t part of determining cohesion.

The head joint is removed from the far side of a brick adjacent to the hole

Thirdly, the head joint is removed from the far side of one of the bricks adjacent to the hole. This is to give the brick that will be shoved some room to move. If the mortar wasn’t removed, the test results would be affected by the compressibility of the mortar in the head joint.

A jack exerts force on the brick, shearing the mortar in the bed joints above and below

Finally, the jack is inserted. It’s pumped by hand, exerting a force on the brick. The mortar in the bed joints above and below the brick resists the force. Since we’re testing under a window, there’s basically not much friction from the weight above. So the thing that’s stopping the brick moving is the two bed joints.

When the shove gets strong enough to move the brick—that’s deemed to be the point of failure, and the value is recorded. This is the peak value, meaning how much force would have to be exerted on the bricks by an earthquake to get them to start moving. Once they’re moving, they still need some force to keep them moving. So the engineers reset the jack, and pump it up once more until the brick begins sliding again—at which point, the residual value has been found.

The brick on the other side of the hole can now be tested. The engineers wedge something into the vacant head joint on the first brick—to prevent further sliding—and then cut out the head joint on the second brick, before testing again as described above. With a set of values from paired tests carried out around the building, an average value can be determined. Remember, we know the area of the bed joints, we know how much force we used, we’re ignoring friction, so there’s only one unknown: the cohesion value.

Engineers read the gauge on the jack during bed joint shear testing

Mouthfeel

There’s a somewhat subjective element to the tests. As for most things, it takes experience to determine exactly when sliding has begun. The tester gets a certain amount of physical feedback from the unloading of the jack as the brick slides, but even then, the exact moment of failure and the associated peak value are not precisely defined.

And this is only appropriate. After all, the mortar isn’t going to be homogenous throughout the whole building. Nor will other conditions be exactly similar. What’s required is something more like a geotechnical value—which is to say, a good rigorous estimate of the cohesion, into which some safety margins can be built. It’d be possible, with more elaborate equipment, to measure the load and the deflection more precisely; but a more precise number wouldn’t really be more meaningful.

Thanks!

Thanks are very much due to Romain Knowles and Antti Wallenius from EQ STRUC. Thanks also to Phillip Hartley from Salmond Reed Architects for getting me in the door at the site.

Britomart CPO and the City Rail Link with Jenny Chu, October 2017

Regular readers of this irregular journal may remember that sometimes your correspondent organises tours but can’t attend ’em. Unfortunately, I missed the Britomart visit in favour of a crook two-year-old—although luckily she was well enough to come down to town with me and hand over the hard hats to the site visitors.

Thanks to the great kindness of Matt Goodall, I have a few pictures from the tour to share with you all. My apologies for the delay in getting these on the web.

This brief post will (hopefully) serve as something of a placeholder. It has been suggested that we might be in with a shot of going back in 2018—which will no doubt be welcome news to the sixty-odd people who signed up but didn’t get a spot.

Those of you who are interested in following the project may also enjoy seeing an animation of the proposed work and the well-illustrated work-in-progress blog.

Many thanks to Jenny Chu and John Fellows of the City Rail Link for their kind hosting and for giving generously of their time.

CPO, general view of the interior. Sandrine le Drilling Machine lurking in the background. Note wrapped columns and protected ceiling. Image courtesy of Matt Goodall, all rights reserved.
Detail of column base. Image courtesy of Matt Goodall, all rights reserved.
Drilling operations. Image courtesy of Matt Goodall, all rights reserved.
Backfilling trench. Image courtesy of Matt Goodall, all rights reserved.

Hopetoun Alpha with John O’Hagan of Compusoft Engineering, October 2017

Concrete classicism

A lightning-fast summary: Hopetoun Alpha was built in 1875. It’s mass concrete, meaning concrete with no internal reinforcing steel. Built for the Congregationalist church, this is a Neoclassical temple in the Doric order, with a charming and luminous timber interior. It’s in Beresford Square, close to the intersection of Pitt St and K Road.

Hopetoun Alpha, exterior

If you’re reading this, you likely know that concrete is pretty strong in compression, but not great in tension. It’s hard to crush, but it doesn’t like to bend. You’ll also have a pretty good idea that 140-year-old buildings can move around a bit: slowly, as the ground settles over the course of the years; and quickly, in extreme cases—for instance, when the ground shakes. So the task that John O’Hagan of Compusoft Engineering has accepted in assessing a building like this is twofold. Firstly, to evaluate how well the building has coped with all the slow movement and the vicissitudes of time, and secondly, to consider how well it might handle an earthquake or a severe weather event. John shared some of the process of making this assessment with us, as we walked around the building.

To understand how the building will perform, you need to know exactly what’s made from, and how the pieces are put together. Over the last few weeks, John and the owners of Hopetoun Alpha have carried out a number of investigations to establish this. They’ve been underneath the building, into the roof void, and everywhere in between. John began today’s tour by explaining the structural system of the building.

Alpha’s anatomy

The building is a long rectangle, shaped somewhat like a shoebox. Its two long walls, the side walls, get narrower in two steps as they get higher. At ground level, they’re about 640mm thick. The site slopes, but the walls come up to establish a level for the floor. At floor height, they step in to 420mm, creating a ledge both inside and out. On the outside, the ledge demarcates the plinth on which the building sits, and the walls change colour to emphasise this. On the inside, the step creates a handy support for the floor joists.

Hopetoun Alpha, basement. John indicates the ledge on which the floor joists rest

It’s a very common detail for “masonry” buildings of all kinds. As we’ve learnt on our tours, brick buildings often contain a similar step (or sometimes just a socket in the course) into which floor joists can be inserted. The problem is, of course, what happens when the walls move so much that the joists fall off their ledge or out of their sockets!

The walls step in at ceiling height, too, and there the step supports the ceiling trusses, which are likely made from jarrah. John shared a drawing of the trusses.

Hopetoun Alpha, drawing of ceiling trusses. Image courtesy John O’Hagan / Compusoft Engineering

He noted that although the trusses are statically indeterminate, they were well analysed by the designer for resisting gravity loads. The steel ties that you see connecting the vertical members to the bottom chord are wrapped around and pinned, forcing the verticals into tension. The diagonals carry compression. The trusses are sound and strong, and seem to have performed well. John mentioned that nails seem to have been a scarce resource at the time of construction, as there were very few to be seen in the timberwork! Fasteners of all kinds were clearly at a premium: a single bolt connects the front truss to the gable end. (As you can imagine, this is not ideal.)

The trusses span the main hall laterally, but there is far less roof structure in the longitudinal direction. What’s there is more or less entirely cosmetic—boxwork, panels, trim, ventilators. This meant that the engineers needed to wear abseiling harnesses to get up in the ceiling for a look—there’s not much to stand on and a fair way to fall.

Hopetoun Alpha, interior from mezzanine. Note bowed ceiling panels, and mezzanine cutting across windows.

As you can perhaps divine from the photo above, the mezzanine or gallery wasn’t part of the original build. The windows on the long walls would probably look different if this were the case (c.f. the smaller auditorium at the Auckland Town Hall.) The addition of the mezzanine meant some additional supports were needed underfloor. Cast iron columns carry the load down to neat brick piers—far neater than the original footings, which seem to have been pragmatically cast inside a few handy barrels!

Hopetoun Alpha, concrete footing (and extraneous foot, far right). Note the remains of barrel hoops and staves—easy formwork!

Mass (of) concrete

The entrance to the building presents the greatest challenge for modelling and analysis. There are large volumes of concrete in the pediment and the stairwells on either side, and, as noted, this section is poorly connected to the roof truss.

Hopetoun Alpha, portico. John O’Hagan explains the structural system of the building

There are decent-sized spans between the columns—approaching two metres—and of course the concrete is being asked to cross that gap without tensile reinforcement. This part of the building may yet require more analysis.

She’ll be right

When the mezzanine was put in, Hopetoun Alpha was also extended at the rear. This allowed a stage to be built and an organ installed, and to accommodate this a large opening was made in the back wall of the existing hall. And the building didn’t fall down.

Hopetoun Alpha, model, showing the opening in the rear wall (the blue rectangle)

It didn’t fall down, but over the long years, a large crack has developed in the wall. This is probably the result of the foundations moving slightly outward, and of tension forces in the reduced thickness of wall above the stage. Whatever the cause, the crack extends from below a round window above the stage (not visible from inside) to the top of the proscenium. Then the crack starts again at the left-hand edge of the bottom of the stage aperture, and continues down to ground level.

Hopetoun Alpha, basement. John points out a deep crack in the transverse wall below the stage.

How significant the cracks are, structurally speaking, is yet to be determined. But it seems to me that some form of strengthening will be required to make up for the diminished capacity of the wall.

Solution, general form

I want to preface what follows by saying that John made it clear that he didn’t want to talk about specific solutions for Hopetoun Alpha. It’s too early for that—too early, even, to say for sure whether work is needed at all, until the NBS rating is determined. Instead, we talked about some hypothetical solutions for a building of this type. Please read the rest of this post in that spirit. Your mileage may vary.

Hopetoun Alpha, view from the mezzanine

The major concern in a building of this size and style would be the out-of-plane response of the long walls. This means, if an earthquake shook the building from side to side (as opposed to back and forth), how well would the long walls cope with being flexed?

It might be sufficient to support them by connecting the floor joists to the walls in the basement. (At the moment the joists are just resting on the ledge you saw in the picture above.) As well as that, you’d probably put a plywood diaphragm across above the ceiling panels to tie the walls together at the top. By doing this, you’d end up with the long walls far better supported by the in-plane elements of the building.

Hopetoun Alpha, longitudinal section showing position of mezzanine. Image courtesy John O’Hagan / Compusoft Engineering

But if floor and ceiling diaphragms weren’t enough, the mezzanine might present an opportunity. Site visitors may remember hearing about the truss inside the gallery at the Auckland Town Hall. There’s obviously no room, and no need, for a truss inside the mezzanine at Hopetoun Alpha. But the mezzanine itself has some inherent strength, aided by its tongue-and-groove flooring. This could be enhanced with some inserted material. If the mezzanine were then connected more securely to the wall, it would serve as a brace at about halfway up the wall height. The walls could then be modelled as rocking about the pivot of the mezzanine, improving their performance. Tying the structure together better would be completed by more securely fastening the pediment and the rest of the portico to the main building.

Thanks!

Grateful thanks to John O’Hagan for his time and enthusiasm. Thanks also to the Ashton Wylie Charitable Trust which owns the building, and to Paula King, who made it possible for us to go and see it. We’ll stay in touch with this project as it progresses.

Pitt St Buildings with Tracey Hartley of Salmond Reed

I’m going to work for Salmond Reed over the summer. Knowing that I love to have a dekko at an old building with the covers off, Tracey Hartley  from SR very kindly invited me along to a site visit she was making to the Pitt St Buildings. A brief post, then, to share with you a couple of details from the project.

On the (new) roof, Pitt St Buildings, facing south towards K Road

Braced back

The main aim of the work at Pitt St is to replace the roofs and restrain the parapets. The parapets are large, ornate and slender. They needed thorough bracing, but that bracing had to be as unobtrusive as possible. The original design was a conventional diagonal bracing approach, but that was too obvious from the street and created future roof maintenance problems. Following close collaboration between the architects and the engineer, a design was developed that not only met the seismic strengthening requirements but also was acceptable aesthetically for this important 5th elevation of the building.  Most of the braces and their triangulated members have been tucked under the roofs and exposed steelwork minimised to the gables. At the bottom left edge of the picture above you can see a hatch—I ducked though it to have a look at the rest of the brace.

Pitt St Buildings, detail of parapet bracing inside roof space
Pitt St Buildings, detail of the brickwork of the parapet, inside the roof space. Note the rounded courses, corresponding to decorative roundels on the street side. See also the line of bolts on the timber at the gables. I was told by the contractor that they assist restraint of the brick panels.

Draining the pool

Designing efficient water run off is part of Tracey’s expertise. She’s developed an instinct for understanding how water flows through and over a structure. My photograph below doesn’t show quite the right angle, but Tracey explained that she’d advised on the design of the connections between the bracing and the parapet, to avoid potential water traps. For example, the C-section channels that run horizontally are packed out slightly from the wall, allowing water to run behind them without getting trapped.

Pitt St Buildings, detail of connection between brace and parapet

On our way to look at another part of the roof, we passed by a beautiful piece of crafting—a welded-lead cap connecting the new stainless-steel gutter with the existing downpipes. This nifty improvisation is the work of Chris the artisan plumber.

Pitt St Buildings, lead gutter cap

Moving to the Pitt St/K Road corner of the building, we inspected the timber and steel components of the new roof. The roof has been altered from its original profile: it now has a central gutter, instead of sloping right down to the back of the parapet. Keeping the water away from the brickwork protects better against intrusion.

Pitt St Buildings, K Road corner. Framing for new roof, with steel parapet bracing visible. Note also the original tie-rods crossing over at the lower left. They appeared to be slack.

The repositioning of the gutter at this corner section also serves a structural purpose: the short rafter members that go from the gutter to the parapet are also tying the parapet back to the steel beam that runs around the corner section. The timber running along just below the top of the parapet is fastened into the brick. As you know, attentive reader, this kind of heritage-structure win-win design is catnip for me.

Detailing

All the water on that roof has to go somewhere. In this case, the gutter leads to a smallish aperture in the rear gable wall called a corner sump outlet. Tracey wanted to make sure that the water would pass through the outlet, visible in the picture below, without backing up, overspilling, getting behind the lead flashings (the grey step-shapes on the right) or exerting too much pressure on the outer wall.

Pitt St Buildings, detail of water flow path for new gutter. The water passes through the gap in the bricks.

In the end, the solution she devised with Adam the project architect and René the contractor was to increase the fall somewhat, and form a stainless-steel sump box before the hole. The increased fall makes the choke-point a bit larger, and the tank protects the surrounding fabric if water backs up at this point. This is the kind of on-the-fly rethinking that takes decades of experience to spot and to remedy. The author (somewhat uncharacteristically) kept his trap firmly shut.

Pitt St Buildings, preparation for re-roofing, Pitt St side

Passing around the corner to the Pitt St side of the building, we saw more re-roofing and parapet work. There was no angle to photograph it from, but it was possible to peek under the bottom of the timber roof sarkings that you see in the photo above. Underneath were more steel parapet braces—these ones appeared to be long straight members, concealed entirely under the roofs. They have to be long because they can only rise at a shallow angle under the small roof spaces. As at the corner section, these braces connect to a longitudinal beam.

Tracey, Adam and Chris worked on a plan to connect the gutters of the new roofs to the existing Colorsteel™ gutters on the parts of the building that sit behind this section—to the right, in the picture. They settled on a plan that involved joining the new stainless steel and the old Colorsteel™ on the vertical section of a step (the riser, as it were), as opposed to joining it on the flat, where water could pool at the joint. The plan also involved overlapping new and old materials for some distance below the joint. Pernicious stuff, rainwater.

Thanks!

Do have a look next time you’re going along K Road. The project is also going to involve repairing the awning—Heritage Society regulars will remember both Peter Boardman and Jason Ingham talking about how strong awnings protect passers-by from falling parapets. Hopefully, the newly-strengthened parapet won’t tumble, but if it did, you’d be grateful that the ties were in better nick than this.

Pitt St Buildings, canopy tie pulled out of pilaster.

Thanks very much to Tracey, René, Chris, and Adam for letting me have a look. More soon—two site visits coming up in the next two weeks. Registration links at the top of the page if you’re keen.

The City Rail Link, Auckland Baptist Tabernacle, Mercury Theatre, Hopetoun Alpha, and the Pitt St Methodist Church with Edward Bennett and John Fellows, August 2017

Terry Gilliam’s 1985 film Brazil, designed by Norman Garwood, tells its story partly by creating contrasting spaces. There are cramped domestic interiors; imposing civic buildings; sparse and frightening chambers of horrors. The look of the film, Gilliam said, came from “looking at beautiful Regency houses, Nash terrace houses, where, smashing through the cornices, is the wastepipe from the loo… …all these times exist right now and people don’t notice them. They’re all there.*”

On Friday, as site visitors toured around four significant buildings in the Karangahape Road precinct, Brazil was on my mind. Mostly, this was because I knew we were going to go past the ghost of what used to be my favourite cafe in Auckland, named and themed after the film. But as we toured, it seemed to me that the film’s aesthetic echoed something about the sites we were looking at. All of them had hidden beauty; odd spaces; unexpected textures and histories to reveal. The face they show the street doesn’t always match what’s inside. And all of them exist in the anachronistic mish-mash that is K Road, a space that’s being opened up and re-invented by the imminent arrival of the City Rail Link tunnel.

In company with Edward Bennett, K Road historian, and joined along the way by John Fellows of the City Rail Link, we learned a little more about the tunneling, discovered a couple of the loveliest interiors in Auckland, and even climbed through a trapdoor on a folding ladder—seemingly a recurring theme of these site visits. Follow me and I’ll show you some of what we saw.

Auckland Baptist Tabernacle

Auckland Baptist Tabernacle, Queen St.

The Auckland Baptist Tabernacle is a study in hierarchies. Front on, its Classical rigour is imposing—its design was based on the Pantheon in Rome. But from any other angle than dead centre, the building reveals its more prosaic brickwork—to me, generous and well crafted, but to Victorian tastes, horribly patchy and common. The walls were intended to be stucco’d to a shiny white, but this never happened.

Well-proportioned windows at the Auckland Baptist Tabernacle. Note the brick lintel and the irregular colour of the bricks.
Auckland Baptist Tabernacle, looking down into the main hall from the gallery.

Inside, the Tabernacle shifts gears again. The spaces are large—indeed, this was the largest room in Auckland when built in 1885—but not imposing. It’s perhaps not the authoritarian space that the portico might suggest. The authority, Edward explained, came from the moral rigour that the congregation practiced, and was intended to set clergy and flock on a more level footing.

Structurally, the room is noteworthy for the curved rear wall, intended to bounce sound back into the room. There are slender cast iron pillars supporting the gallery. But, most of all, this is a large span. And the span had to be crossed without the aid of structural steel. Luckily for the church’s builders, then, that they lived in a country where 2000-year old kauri grew strong and straight! Thirteen good-sized ‘uns were ordered up from the North, and were duly sawed to size. We climbed through a hatch in the ceiling to have a look. Here’s where it got a little Brazil.

In the ceiling, Auckland Baptist Tabernacle

As you can see above and at the top of this post, there’s a large-ish ceiling space above the main hall. Truthfully, I was a little preoccupied with my fear that a site visitor would put a foot wrong and crash sixty feet to their doom (“how can Santa Claus get in if we don’t have a chimney?”), but nevertheless I managed to cast an eye over the structure. There are large kauri rafters, long straight members which make up the top and bottom chords of the truss. The hall doesn’t run the length of the building—there are sizeable rooms behind for other kinds of functions, and so the building is divided about half way by a brick shear wall, which goes up through the whole building almost to the underside of the roof.

As the tour continued, I butted in to a conversation that Professor Jason Ingham was having about the Tabernacle. For those of you who don’t know him, Jason is responsible—among a number of other things!—for developing methods to analyse the strength of unreinforced masonry buildings. Jason explained that this kind of building is a classic example of a structure that isn’t explained well by conventional structural dynamics. Instead, said Jason, the ceiling has to be thought of as a flexible diaphragm (not a rigid one), and assessment and strengthening should be designed on that basis. That doesn’t, of course, solve the problem that (like most churches) you are dealing with a big empty box with long not-so-strong sides. Still, there may be more strength in the building than conventional analysis would suggest.

Mercury Theatre

Mercury Theatre, the stalls and the corner of the gallery. Showing the “restored” but perhaps over-garish colour scheme

Next stop was the Mercury Theatre, opened in 1910 and Auckland’s oldest surviving theatre. It has been through a number of reinventions in its time: as a picture palace; a 1970s black-box theatre; a church; a language school; and so on. Like the Tabernacle, it’s a brick building, but in the intervening 25 years between the Tab’ and the Mercury, structural steel was introduced: so the Mercury’s large roof is held up by I-beams, not kauri. [Edit (25 Aug 17): Thanks to Mike Skinner on the K Road Heritage Facebook page who pointed out that the Mercury’s roof beams are timber and provided a picture.]

The theatre is ornate, having kept most of its plasterwork intact even through the austerities of a 1970s all-black paintjob. When it was last restored, the paint was scraped back revealing the bright blues and reds you see in the photo. These colours were duly reinstated—but Edward’s opinion is that the bright colours would’ve been more muted in the original, overlaid with paint effects: in fact, he thinks the bright blue was probably an undercoat.

Pressed-metal ceiling, Mercury Theatre foyer

There’s a large expanse of lovely pressed-metal ceiling still to be seen in the entrance foyer, and Edward explained that at the time of construction, this was believed to be a fireproof material. Sadly, fires in other buildings with pressed-metal ceilings disproved this notion, and these ceilings were mostly torn out, becoming quite rare.

Complex forms beneath the gallery, Mercury Theatre

For my part, I enjoyed the profusion and contradiction of the forms and decorations of the theatre. It’s hard, on first sight, to get a sense of the exact extent of the space and its orientation, and this slightly warren-like quality is exacerbated by the theatre’s position, tucked down the lane, its façade declaiming bravely and boldly at an audience who are not there to watch.

John Fellows now took the stage at the Mercury. This was the perfect place for him to speak, as, come 2019, ground will be broken next door for the new Karangahape Station, part of the City Rail Link. The project involves digging a large pit at the south edge of the theatre, a pit which descends some ten stories. The station’s platforms will extend underneath the Mercury, underneath K Road, and underneath some of Pitt St on the other side.

It’s an audacious project, but of course one with plenty of precedents in all the major cities of the world. John explained that careful consideration has been given to minimising the impact that the CRL will have on the surrounding  buildings, both during construction and in operation. For example, the tunnels that will take passengers down from the Mercury Lane entrance to the station will veer out under the roadway, rather than passing under the theatre. This is to avoid noise and vibration passing up into the structures above.

John also explained that the results of subsurface core sampling have been encouraging. The soil, at the depth where the work has to be done, is East Coast Bays sandstone—common throughout Auckland. This soil can vary widely in its strength, but the good news is that the stuff underneath the Mercury is stronger than expected. This will make shoring up the pit next to the Mercury easier, and makes settlement less likely.

Decorative profusion, Mercury Theatre

As a sidenote, John described the system that is protecting the heritage buildings of Albert St, where the cut-and-cover tunnel work for the lower end of the line is currently proceeding. A network of over 1300 laser sensors is trained on the buildings’ exteriors, measuring in real time any deflections that might occur. If the movements of the buildings were to exceed the design parameters—hold the phone! The work stops immediately until the problem is resolved.

John had  plenty more to say about the plans for the station, about its design programme, mana whenua, use of local materials, bicycle integration, green design, and other topics. He said, just as Jeremy Salmond said at the Melanesian Mission, that he doesn’t see the purpose of trying to make a new building look like an old one just to “blend in” with its surroundings. Instead, John says, why not try to design a building that in 50-100 years will become a historic building in its own right? There was more to say and more to ask about all this, and the good news is that there will be an opportunity to hear more from John when he speaks at an ACE event in September. Keep an eye on their Facebook page for details.

We site visitors moved on to one of the loveliest hidden treasures in the city: the palm court in the disused K Road entranceway to the Mercury. To increase foot traffic to the Mercury, shortly after it was opened the owners purchased a narrow sliver of land and built a barrel-vaulted entranceway that took punters down into the theatre. As I mentioned, some will remember it as Brazil cafe. Now it’s a fast food joint. With brick-pattern wallpaper.

The Palm Court, Mercury Theatre

Tucked away, though, in between the Mercury and the now-disconnected entranceway, is the palm court. This was intended as a scene of Hollywood glamour to pass through on the way to the movies. Designed by Daniel Patterson, topped with a stunning leadlight dome, the room has retained its glamour and charm through decades of disuse. Fashionistas, artists, clairvoyants: what a studio space! Get in there, you muggs! (The author confesses to having once practiced one of the three professions listed above.)

Hopetoun Alpha

Hopetoun Alpha

Hopetoun Alpha is a delight. I felt the same sense of joy and astonishment as when I first entered St-Matthew-in-the-City, last year. It’s a light, delicate, finely-proportioned space—a Leipzig shoebox, just like Auckland Town Hall. Before you even get inside, the portico is unusual enough to warrant a better look. It’s painted a bold red with a pale blue soffit, creating a sense of interiority in comparison to the pale sides.

Red portico, Hopetoun Alpha
Blue ceiling, portico, Hopetoun Alpha. Note the marked curvature of the wall.
The “oak” door, Hopetoun Alpha, in fact made of kauri. Note the “ashlar” lines on the wall, which is in fact made of concrete.

From the pictures above, you can see that the front wall is curved, once again to produce sound reflection and natural amplification inside. The wall looks a bit like ashlar, doesn’t it? But in fact it is mass concrete, unreinforced. Timber trusses span the walls, just like at the Tabernacle. Speaking of timber and things that look like other things, the main door to Hopetoun Alpha appears to be oak—but scratches on its surface show that the oak is a paint effect, and the door is kauri. Fashions have changed, and now real fake oak is rarely seen.

It’s inside that Hopetoun Alpha truly shines. We were all delighted with its lightness and grace.

Interior, Hopetoun Alpha
Interior, Hopetoun Alpha. Detail of decorative elements. Slender cast iron columns.

Like many other buildings of its age and general type, Hopetoun Alpha and its owners are now having to give consideration to earthquake strengthening. There’s some hope that the gallery or mezzanine could act as a diaphragm, strengthening the outer walls.  [Edit: Edward Bennett kindly corrected me: the gallery was inserted into the 1875 building in 1885, “which is why it rather awkwardly passes in front of the windows”. The point I was trying (and failing) to make is that perhaps a retrofit can strengthen the gallery or be concealed inside it, to brace the long walls. HT]

Visitors to the Auckland Town Hall will remember that its gallery conceals a large truss designed to brace the long walls. Subsequent to our visit, I spoke with John O’Hagan of Compusoft Engineering, a firm known to site visitors from the St James Theatre visit last year. John’s supervising some investigations into the materials, foundations, and structural members of the building. We may yet have the chance to return and learn more.

Pitt St Methodist Church

Pitt St Methodist Church, with the 1962 porch

Last but not least we arrived at the Pitt St Methodist Church, nipping in through the Wesley Bicentennial Hall, for which there’s sadly no more space in this post. The Pitt St Methodist is determinedly Neo-Gothic, echoing the style of an English parish church, and deliberately eschewing the Classical. It’s a brick building, spanned with timber arches, and incorporating wrought-iron tie rods to muscularly and pointedly restrain the springings of the arches. Edward explained that this style reflected the Neo-Gothic designers’ conception of the power of the Gothic—Gothic church-builders would have done this too if they’d had wrought iron.

Pitt St Methodist Church, interior

Earlier, I wrote about John Fellows’ contention that to design for a great historic building, you make a great contemporary design. Here at Pitt St, there are two shades of this theory in evidence. The first is the organ, which was rebuilt and rehoused in the 1960s into a large “tabernacle”, looking something like an enormous jukebox. Secondly, there is the porch, added on at the same time. The porch is concertina-folded, with windows and doors shaped as stylised versions of the Gothic ogive. It’s very likely inspired by the Coventry Cathedral of a similar date, says Edward. Both the organ and the porch inspired mixed feelings from visitors, some feeling that they added a new dimension, others that they detracted from the original form of the building.

Pitt St Methodist Church, the organ in its 1962 “Tabernacle” rehousing.

Heritage buildings are living things, truth to tell, and there’s no one point at which you can freeze them and say, that’s it. For me, the comment that resonated was Paula King’s—she works for the Trust that owns Hopetoun Alpha. Paula said that using Hopetoun Alpha for good things “keeps its battery charged”; and keeping it charged gives it the power to last longer and speak louder, perhaps loud enough that future generations will still be able to hear it.

Thanks!

Our thanks to Edward Bennett and to John Fellows. You can read more about K Road’s buildings and their history at the kroad.com site, written by Edward. You can also read about the plans for Karangahape Station on the City Rail Link’s site.

* The quotation at the start of this post is from Bob McCabe’s book Dark Knights and Holy Fools: The Art and Films of Terry Gilliam: From Before Python to Beyond Fear and Loathing 1999 p.141.

Disclaimer: all ideas, information, insight are Edward’s and John’s. Errors of fact or interpretation are all my own work. HT

Ōtāhuhu Station with Sara Zwart, Tessa Harris, Joshua Hyland, Tony Berben, August 2017

Site visitors at Otahuhu Station

Sadly, your correspondent was crook and did not attend the site visit. Many thanks to the professionals for their time and effort.

I can highly recommend having a look at the Ōtāhuhu Station case study published by the Auckland Design Manual.

Update: I’m delighted to be able to share with you some pictures and thoughts by Yi (Sophie) Huang, a civil engineering student who attended the visit. Thanks Sophie!

The Maunga Moana facade by Graham Tipene. Image courtesy Sophie Huang, all rights reserved.
The most interesting part for me was the stormwater collection system and the water treatment pond.

At the station, there are two small rain gardens. The one we saw is approximately 5-7 square metres. There are plants, grass and flowers above the ground surface. Beneath the soil, the bed is deeply filled with sand and other materials which have a very good permeability, to absorb the stormwater. The storm water is collected underneath the rain garden then discharges through a stormwater channel to the water treatment pond. After several processes, the water is good enough to release into the wetland. We did not have a chance to see the water treatment pond, because the traffic was busy and it was too dangerous to walk there.

Site plan. Wetland on the right. Image courtesy Sophie Huang, all rights reserved.

The stormwater system was built to reduce the possibility of rainwater being contaminated by oil underneath Ōtāhuhu Station. Furthermore, one of the iwi consultants was very concerned about water quality: she strongly suggested that it would be a good idea to include an on-site water treatment pond. Sarah Zwart, of Jasmax, said that if the iwi had not persisted in the idea, the on-site water treatment pond might not be there.

Site visitors watch an informational video about the artworks. Image courtesy Sophie Huang, all rights reserved.

There were several difficulties involved in the construction process of this project. Firstly, constructability: several things did not go as planned. There was a problem with the piles, and this had to be solved. The schedule and money at that time were very tight. Tony Berben, of Aurecon, said “most of the time we think on our feet. There are always problems we did not expect; we had to find solutions for each of them.”

Tessa Harris artwork on window and other features. Image courtesy Sophie Huang, all rights reserved.

Secondly, time and money were constrained. Joshua Hyland, of AT, said it was very important to make sure the right materials were delivered on time.  Because of the tight schedule and budget, they didn’t have time and money to reorder and wait for the right material to be delivered.

Purapura whetū mahau by Tessa Harris under awning. Image courtesy Sophie Huang, all rights reserved.

We saw all the art works from the case study. They look simple, amazing with deep meanings and were not expensive. For example, at the station entrance, there is an x-shaped pattern, the purapura whetū mahau [by Tessa Harris of Ngā Tai Ki Tāmaki]. Joshua Hyland said it represents the past and future, and ancestors.

Professionals offer advice on the train. Image courtesy Sophie Huang, all rights reserved.

I learnt so much and was amazed by the two hour site visit. On the train back to the CBD, the professionals gave us tips on finding jobs, networking with people and how to be better engineers. They are super-friendly people and professional in their field. Moreover, I made amazing friends through the trip as well.

Auckland Town Hall with George Farrant, May 2017

The clock mechanism, Auckland Town Hall.

To potential clock room taggers and graffiti artists: All tags, names and graffiti will be promptly removed and you will be forever haunted by the ghost of the clock tower.

 Thus read the notice meeting the eyes of Auckland Town Hall site visitors on Monday and Tuesday, as they put their heads through the trapdoor at the top of the ladder to the clock room. As Ray Parker, Jr. said, I ain’t afraid of no ghost: but it seemed to me what we saw on the tour, the fruits of the restoration work carried out from 1994-97, was certainly the resurrected spirit of the original design of the Town Hall. No effort was spared to return the building to something approaching its original state, and, at the same time, to make it safe and strong for the future. True to form for ghosts, the structural upgrades are in many cases invisible—or at least, hidden from the eye where members of the public can ordinarily go. On this visit, we went behind the scenes.

Bluestone on the lower floor, Oamaru stone above, but right at the top the true building material of the Town Hall: bricks.
Town Hall exterior, northeastern corner. Bluestone on the lower floor, Oamaru stone above, but right at the top, the true building material of the Town Hall: bricks.

BIG EMPTY BOXES

The Auckland Town Hall is essentially a brick building. It is faced with Oamaru stone and with Melbourne bluestone, the latter brought over by the Australian
architects of the Town Hall—a prime example of coals to Newcastle in this basalt-bottomed town. There’s reinforced concrete in the foundations, in the form of piers and floor beams. Structurally, however, the Town Hall suffered from some of the usual flaws of unreinforced masonry buildings: vulnerability to face loading of external walls, and insufficient shear strength.

 

In out-of-the-way areas, remedies for these problems could be visible. On lateral cross walls, shear strength was improved by adding a 100mm-thick concrete skin to the bricks. That doesn’t stop the building rocking itself off its foundations, so the basement-level concrete was tied into new piles, hand-dug for lack of headroom. Longitudinal walls at the upper level had fibreglass glued to them, and this was covered with plaster. For various reasons, some of the internal brick walls had new openings cut into them. To retain the shear strength of the wall and to leave a record of the intervention, these openings were finished with a visible internal frame of structural concrete. This frame-within-a-frame motif, used to signify a modern alteration, was lifted from parts of the original design.

The original (non-structural) frame-within-a-frame design.
The original (non-structural) frame-within-a-frame design.
In the light well, new openings cut into the brick shear wall were denoted by the frame-in-frame treatment.
In the light well, new openings cut into the brick shear wall were denoted by the frame-in-frame treatment.

Strengthening the main performance spaces was trickier. Of necessity, these rooms are high-ceilinged and large, and, designed for natural light, their walls have many openings, separated by slender columns. Out-of-plane loading would wreck them. But the walls are beautiful inside and out, and the spaces are well-beloved and well-known in their current form. Structural enhancements had to be invisible.

The wall of the Great Hall. The curtains cover large windows--note the slender piers between openings.
The wall of the Great Hall. The curtains cover large windows–note the slender piers between openings.
George explains the design solution underneath the gallery which conceals the truss.
George explains the design solution underneath the gallery which conceals the truss.

In the Great Hall, the solution was obvious—once someone had thought of it. A gallery runs around three sides of the room, providing extra seating. It was easily large enough to conceal a gigantic U-shaped horizontal truss, which provides stiffness to resist lateral movement of the weak outer walls. A plywood diaphragm hidden in the ceiling cavity tied the tops of the walls together. In the somewhat smaller Concert Chamber, the gallery is small too, and it doesn’t extend around three sides of the room. With no opportunity to conceal a truss, the strengthening in the Concert Chamber took the form of reinforced concrete columns inserted into 400×400 slots cut into the wall—or, in one case, cut right through the wall and out into the weather (Oamaru stone is pretty soft!). As part of the refit, air conditioning was inserted into the walls, and the vents are partially hidden by the decorative plasterwork.

Air conditioning vents between plaster corbels, Concert Chamber. The steelwork is inserted into the columns between the windows.
Air conditioning vents between plaster corbels, Concert Chamber. The steelwork is inserted into the columns between the windows.

 OUT OF THE FRYING PAN, INTO THE FOYER

 For a number of years, prior to the restoration project, the floor tiles in the foyer often exploded. This alarming phenomenon was at first put down to excessive compaction caused by floor buffing machines, but the installation of a sprinkler system into the concrete slab on which the tiles sat revealed the true problem. The reinforcing bars in the slab were in the wrong place—the lower bar sitting far too close to the top surface. What was causing the tiles to explode was the floor slabs deflecting under the weight of concertgoers: alarming indeed! Thankfully, none of the floors failed, but many of the tiles, under strong compression, did.

The problem for the design team was how to support the floors without changing the proportions of the spaces, since there is nowhere to hide any supplementary structure. The deflection was reduced by adding carbon fibre strips to the underside of the floors—likely the first time that this material had been used for structural repair in NZ. With the floors strengthened, a repair job had to be done on the tiles.

The tiled floors of the foyer. Floors extend over two levels. In the centre, the round tile is an encaustic tile, stained orange by the acid bath. The square brown tiles are original--I think!
The tiled floors of the foyer. Floors extend over two levels. In the centre, the round tile is an encaustic tile, stained orange by the acid bath. The square brown tiles in this photo are original–I think!

The fanciest tiles, made with a light-coloured slip poured into a relief-moulded dark-coloured base (encaustic tiles), came through OK, barring some orange stains caused by an overzealous acid bleaching. But the plain, square, brown tiles which cover the greatest part of the floor were seemingly impossible to source: they couldn’t be bought, and, scour the world though they might, the team could not find a manufacturer capable of exactly matching the original colour, given an understandable reluctance on the part of modern potters to use lead oxide in their glazing. All seemed lost—until one day, a project manager from the Town Hall team had lunch at a well-known franchise restaurant specialising in Scottish food. To his utter astonishment, the kitchen tiles at McD’s appeared to be an exact match, and he nearly earned himself a cell next to the Hamburglar by bursting unannounced into the restaurant kitchen with his tape measure get the exact size of them. To cut a long story short—the tiles matched matched perfectly, and McD’s eventually agreed to give the Town Hall enough of their custom-made tiles to repair the floors.

In a similar spirit of desire for perfection, George mentioned several other examples of the lengths to which the project team went to get as close to the original design as possible, including scraping the walls painstakingly to find the original wall colour (not to be mistaken for the colour of the primer or the basecoat). They trawled through archival pictures to find the patterns of the original leadlight windows. Of course, the pictures are in black-and-white, but the glass colours were revealed by the discovery of one large window, which had literally been rolled up and stashed away. Picture the restorers hunting in a dark basement for scraps of coloured glass. That’s dedication.

 A TICKING TIME BOMB?

The clock tower rises above the administrative offices of the Town Hall.
The clock tower rises above the administrative offices of the Town Hall.

On Monday’s tour, George willingly expressed his “diffidence” over the threat that earthquakes pose to Auckland’s buildings. He qualified his position to the extent of saying that the risk is non-zero—and with a non-zero risk in mind, the clock tower on the southern end of the Town Hall presented a serious engineering challenge. It’s extremely heavy, and being taller than the adjoining structure, it would have a different period under earthquake acceleration.

The exposed steel frame in an upper storey of the clock tower. Note the steel rods running across the window instead of solid beams.
The exposed steel frame in an upper storey of the clock tower. Note the steel rods running across the window instead of solid beams.
In the storey below, the exposed steel frame (white) joins up with steel inserted into the walls (grey). In lower (public) floors, the steel strips are hidden in the walls.
In the storey below, the exposed steel frame (white) joins up with steel inserted into the walls (grey). In lower (public) floors, the steel strips are hidden in the walls.

The initial design solution, a steel framework inside the tower designed to hold the tower up, was rejected—by George. It would have dramatically altered the staircase below it, which winds up to the council offices. George’s name was mud among the engineering team for some weeks, until an alternative solution occurred: why not hold the tower down, instead of up? This developed into a solid steel frame, in the upper tower; connected to cross bracing cut into the walls, in the storey below the steel frame; connected to thin steel strips inserted into the walls of the stairwell, and anchored into the foundations. These steel strips are 200×19 galvanised steel flats, sitting in 270×120 slots packed with a Denso felt, and tensioned. This holds the tower together, using the tension in the steel against the crush strength of the masonry, but doesn’t eliminate the possibility of swaying. In addition to the post-tensioning, then, a transfer truss connects the tower to the top of the longitudinal walls of the Town Hall, holding it fast. The truss is hidden under a sloping roof. One final touch—in the clock tower, where the steel frame sits, instead of steel cross-members going over the windows, the bracing consists of four 40mm steel bars, painted in a dark colour. You’ll see it (at night) now that you know it’s there, but it’s far less noticeable than steel beams would be—seen out of the corner of your eye, you might pass it off as a mere apparition.

 FURTHER READING

There’s a really lovely post on the Timespanner blog with some great archival images of the construction of the Town Hall.

I also sent site visitors a link to Downer Senior Engineer Mark Hedley’s 2014 paper on the strengthening of five major civic buildings in Auckland.

 THANKS

 With sincere thanks to George Farrant, redoubled since he generously agreed to host a second tour in the face of extremely high demand. For he’s a jolly good fellow, and so say all of us.

Roselle House with Peter Reed, and the Melanesian Mission with Jeremy Salmond, Andrew Clarke and Dave Olsen, April 2017

Visitors on the terrace at Roselle House...
Visitors on the terrace at Roselle House...
Visitors… on the terrace at Roselle House…
...and in the attic at the Melanesian Mission.
…and in the attic at the Melanesian Mission.

WHAT I DID ON MY HOLIDAYS

Over the Easter break, heritage enthusiasts from the U of Auckland visited building works at two 19th-century masonry buildings. The first was the 1870s mansion Roselle House, now part of St Kentigern Boys’ School. Next was the 1850s ecclesiastical training school, the Melanesian Mission, which gives its name to Mission Bay.

Melanesian Mission. Dressed-stone sill and jambs (?) around small attic window, photographed from the scaffold.
Melanesian Mission. Dressed-stone sill and jambs around small attic window, photographed from the scaffold.
Roselle House. Brick “relieving arch” built to ease strain on large internal lintel. (Note, this was hidden in the original by plasterwork, and will be covered up again.)
Roselle House. Brick “relieving arch” built to ease strain on large internal lintel. (Note, this was hidden in the original by plasterwork, and will be covered up again.)

RE-USE

One of the major topics of discussion at both sites was re-use, and how making the buildings useful for their current occupants supports their preservation. Renovating a building usually means making changes to its fabric and there are consequent losses of heritage material. To make such changes, consent is required from heritage authorities, and this has to be negotiated. Part of the negotiation comes down to demonstrating the overall benefits to the building that can be expected from the project, even if those benefits come at some cost to what is currently there.

The Roselle House tour
The Roselle House tour

At Roselle House, the school is on- trend, transforming library space into learning commons. There was a discussion of the decision-making and consenting processes that were required to allow a large opening to be cut in a wall for a new entry. Cutting the hole meant losing some heritage fabric, but the future use of the building required it. Peter Reed and his colleagues discussed how the building’s elements were classified through its conservation plan, and how the Heritage Impact Assessment (for the new aperture) was devised. (In other parts of Roselle House heritage material that has been removed has been stored for re-use when the building is made good.)

At the Melanesian Mission, a new restaurant housed in an adjacent contemporary-style building provides the financial oomph required to care for the heritage site. At the Mission, there are fewer obvious changes to the building itself than at Roselle House, but its aspect will be significantly altered by its newly-built neighbour. Jeremy Salmond termed the Mission (and other built heritage) “vertical archaeology”: a record of the past, its people, their hopes and their achievements. That’s what makes it worth the care we lavish upon these buildings, he said. Jeremy’s belief is that you best complement a good old building with a good new one, rather than attempting to replicate an older building style and thus fudging history. Yes, the Mission’s visual surroundings change, but that’s the price of maintaining the history it embodies.
Roselle House. Preparations for pouring the shear wall. Above, note existing timbers, which have been included in the design calculations. See also, for interest, the plaster oozing through the laths—this is how the plasterwork adheres to the walls.
Roselle House. Preparations for pouring the shear wall. Above, note existing timbers, which have been included in the design calculations. See also, for interest, the plaster oozing through the laths—this is how the plasterwork adheres to the walls.
Roselle House. Looking down into the cavity for the shear wall. It will sit on a slab broad enough to avoid overloading soil bearing capacity, which could lead to overturning.
Roselle House. Looking down into the cavity for the shear wall. It will sit on a slab broad enough to avoid overloading soil bearing capacity, which could lead to overturning.

HOW TOUGH IS OLD STUFF?

Both Roselle House and the Mission are being strengthened against earthquakes. There’s a good deal of new material going into each site, but, interestingly, the pre-existing fabric of the site is also having its strength recognized and used. U of A research on heritage fabric was mentioned in dispatches, and no doubt a number of you site visitors (and your professors) are working on how to assess the strength of old materials.

At Roselle, new concrete bearer beams span under the floors, and the walls, floors, and ceilings are being strapped together and connected to these beams. Plywood diaphragms at floor and ceiling are the order of the day. But the main earthquake-resisting structure will be an internal shear wall. This will be poured anew, but it incorporates pre-existing timbers, and their strength was calculated and incorporated into the shear wall’s design.

At the Mission, a good deal of new steel has gone in, to secure the gable ends and the long walls against out-of-plane loading. Jeremy Salmond and Andrew Clarke described sending their design drawings for the steelwork back and forth to each other, and they both stressed the importance of designing every detail sympathetically to the building’s original programme. For example, the 200mm beam that spans the top of the walls in the Mission Hall has been custom-welded with an angled rear flange: instead of looking like this |____| in section, it looks like this |____\ . Why? So that it fits under the slope of the roof: thus the beam will not protrude over the edge of the wall. The beam has the same dimensions on its exposed face as a now-removed timber strip that used to run around the top of the walls. When the walls are refinished, the steel beam will have the same visual effect as what has been lost.

Melanesian Mission. An internal wall is drilled at regular intervals. The Mapei grout is pumped into the holes, starting at the bottom, until it begins to flow out of adjacent holes. The process is repeated three times.
Melanesian Mission. An internal wall is drilled at regular intervals. The Mapei grout is pumped into the holes, starting at the bottom, until it begins to flow out of adjacent holes. The process is repeated three times.
Melanesian Mission, detail of another internal wall, showing the insertion tube. The hole will be re-grouted with lime mortar, so it won’t be noticeable.
Melanesian Mission, detail of another internal wall, showing the insertion tube. The hole will be re-grouted with lime mortar, so it won’t be noticeable.

But to return to the strength of the existing materials at the Mission: the engineers made an assessment of the capacity of the masonry walls, using for their calculations some results from Jason Ingham’s research. An initial plan to tie the wall together with threaded rods was abandoned in favour of a Mapei- brand lime-based grout or slurry. Regularly spaced holes were drilled in the mortar, and the sludge was pumped into the wall. (Pumped by hand, so that the pressure didn’t get high enough to pop off the other side of the wall!) The result: the void spaces between the rubble are filled, and the inner and outer skins of the wall are bonded together. And it’s invisible. So the original material, supported by some chemical wizardry, gets retained, and can now resist greater loads.

Efflorescence on the bricks, internal walls at Roselle House
Efflorescence on the bricks, internal walls at Roselle House
The highly porous volcanic stone of the Mission. The lime mortar has been renewed as part of the project, but the Mapei-grout holes are yet to be filled.
The highly porous volcanic stone of the Mission. The lime mortar has been renewed as part of the project, but the Mapei-grout holes are yet to be filled.

WATER WATER EVERYWHERE; or, THE CONSEQUENCES OF DESIGN DECISIONS

Water in the walls was a recurring theme. At Roselle House, a chain of unfortunate decisions caused considerable harm to the fabric. First, wooden verandahs were replaced with terrazzo in the 1930s, sealing off the underfloor without ventilation, and causing the timber bearers to rot. Next, sagging timber floors were replaced with concrete. Uh-oh! Now the ground water, under pressure, wicked up the rendered plaster internal walls, moving between the brick and plaster, or between the plaster and its hastily re-applied paintwork. Wherever the water went, efflorescence remained, in the form of salty stains and crystalline growths. One of the major tasks of the project is to remove the old concrete floors and to draw the moisture out of the bricks with a special clay, in a process known as poulticing. The terrazzo stays, but it will be ducted to allow proper underfloor airflow. Peter made the point that the consequences of the 1930s renovation decisions took decades to become obvious, but have also created problems for occupants for many more decades. Earlier attempts to fix the problem only made it worse. Think twice about messing with an original design!

Water has a more subtle place in the walls at the Mission. The walls are made from chunks of basalt, taken from Rangitoto, and piled up in random courses, held in place with a lime mortar. Dressed blocks of scoria form the quoins. Both scoria and basalt are highly porous, and so, in wetter months, the walls have always been permeated with damp. This, says Jeremy, is not really a problem: the walls were made to be wet—notwithstanding that water entry did ruin the original plasterwork and create efflorescence. Problems have been caused by later attempts to “solve” the dampness, in particular by repointing with Portland cement, by plastering the inner face of the walls, and by treating with an “invisible chemical raincoat”, the latter occurring in 1977. These treatments tending to combine to retain moisture within the walls—the opposite of what was intended—and deteriorate the lime mortar, so much so that Jeremy described the walls as being “two dry stone walls with sand between them.” That doesn’t sound like a structure that would resist earthquake shaking very well! In combination with the Mapei re-grouting and the steelwork, the walls have been re-limed, and will surely be much the better for it.

The Main Hall chimney at the Mission. Steel rods run down the stack to the fireplace.
The Main Hall chimney at the Mission. Steel rods run down the stack to the fireplace.
Looking up from the ground floor at Roselle House to the stub of chimney. At some stage in the building’s life, the chimney was removed from the ground floor, but the rest was left to hang on in there... somehow!
Looking up from the ground floor at Roselle House to the stub of chimney. At some stage in the building’s life, the chimney was removed from the ground floor, but the rest was left to hang on in there… somehow!

A NOOK ABOUT CHIMNEYS

Two contrasting treatments for chimneys deserve mention. At the Mission, an elegant brick chimney stands above the rock wall on the western side of the hall. The chimney has been post-tensioned with steel rods, which connect an upper plate to the floor slab, holding the stack firmly together against shaking. The solution allows space for a flue to be inserted, so that a gas fire can simulate the cozy effect of a real one.

At Roselle House, the project team discovered that in some long-forgotten she’ll- be-right renovation, a chimney which poked out of the roof had had most of its lower extent removed. This is a trick somewhat akin to climbing out on a tree branch and then sawing it off behind you—expect things to start going downhill fast! As Peter explained, there were six tons of bricks sitting up in the roof and upper storey with very little holding them in place. In this case, the majority of the remaining chimney material has been removed, and a lightweight replica will be installed to keep the roofline looking the same. A steel brace for the chimney-stump was discarded, as it would have needed to be excessively large.

Roselle House. A ceiling rose clings on to its lath, awaiting the re-finishing of the room.
Roselle House. A ceiling rose clings on to its lath, awaiting the re-finishing of the room.
Melanesian Mission. The roof sarkings, seen here from above, have been exposed by the removal of the shingles. The sarkings are being nailed off as a diaphragm, stiffening the structure of the Mission. Note the bolted connections between sarkings and purlins.
Melanesian Mission. The roof sarkings, seen here from above, have been exposed by the removal of the shingles. The sarkings are being nailed off as a diaphragm, stiffening the structure of the Mission. Note the bolted connections between sarkings and purlins.

I very much enjoyed the visits, and I’m sure that other site visitors felt the same. We’re extremely grateful to Jeremy Salmond and Peter Reed of Salmond Reed, and to Andrew Clarke and Dave Olsen of Mitchell Vranjes, as well as to the contractors and project managers who allowed us to come on site.

Continue reading “Roselle House with Peter Reed, and the Melanesian Mission with Jeremy Salmond, Andrew Clarke and Dave Olsen, April 2017”

University of Auckland heritage buildings with Neil Buller and Peter Boardman, March 2017

Yesterday a large group defied the weather forecast and enjoyed a tour of several of the University’s heritage buildings.

Neil Buller, architect and project manager for the UoA, battles a sore throat and traffic noise, talking to the crowd about 10 Grafton Road.
Neil Buller, architect and project manager for the UoA, battles a sore throat and traffic noise, talking to the crowd about 10 Grafton Road.

For me, the recurring theme of the afternoon was the University’s special nature as a client. It’s unusual to have a client whose heritage properties span such a range of ages, styles, and typologies. The University’s buildings mostly have high levels of occupancy and usage, making it more critical that they be demonstrably safe and sound. And the University also has a kaitiakitanga role and a concern for preserving its history. For all these reasons and more, the University seems to take a more scrupulous position about how much retrofit it is prepared to do. The projects we saw were targeting 100% of the New Building Standard (NBS), which is a higher target than many clients choose to set.

It’s also a target that necessitates more physical intervention. Some of the conversation on the tour turned around questions of the practice of making these interventions visible, of leaving good records of work done, of reversibility, and of the suitability or otherwise of certain building technologies for heritage sites. Sometimes repairs can bring problems with them, if, for example, they bring moisture where it’s not wanted. We also heard a familiar story about working on heritage buildings–starting to fix one problem leads to finding several more!
Neil describes the proposed post-tensioning system for the Clock Tower Annexe, where work is due to start in October 2017.
Neil describes the proposed post-tensioning system for the Clock Tower Annexe, where work is due to start in October 2017.
I enjoyed the opportunity to hear and see some details about the specific problems heritage buildings experience, and how these are addressed. At Bayreuth (the 1903 Italianate building at 10 Grafton Road), tie rods have been inserted both parallel and perpendicular to the floor joists, binding the structure together. The brick and concrete Merchant Houses Belgrave, Okareta and Mona (12-16 Symonds St), dating from the 1880s, were suffering from water intrusion, and required structural improvement. The basement slab was relaid, with a ventilation system designed to improve airflow and remove moisture. Floors were taken up, the joists re-fixed to the walls, tongue-and-groove floorboards re-laid, plywood diaphragms used to stiffen the structure. Roof timbers were refreshed. All in all, a major refit, and one that necessitates tradeoffs between the future of the building as a whole and the integrity of its heritage fabric.
Crossing the road, we saw an elegant intervention at the General Library, where the services and stairwell were too stiff and inflexible in comparison to the abutting structure. In a decent shake, they’d likely knock the rest of the building to pieces. The solution proved to be “strengthening through de-strengthening”, in that vertical saw-cuts were used to weaken the walls, while the structural members of stairwell and library were tied together. There was an interesting conversation about the value of taking the time to allow designs to be re-thought. In this project, the initial design proposed masses of steel, strapping the disparate parts of the structure together. The final solution proved to be far more minimal, essentially a steel rod lashing elements together across the stairwell, forming a lattice. A conference panel on Alistair Cattanach of Dunning Thornton’s design for the library, with some photos, drawings, and some more information, can be found here.
After the tour, we moved inside to examine drawings and photographs. Above my not-so-hot picture of the design for the Library. Look up, next time you're on Alfred St!
After the tour, we moved inside to examine drawings and photographs. Above my not-so-hot picture of the design for the Library. Look up, next time you’re on Alfred St!
Lastly, we looked at the Annexe to the Clock Tower (aka the Old Arts Building), built in the 1920s. Peter Boardman, of Structure Design, who accompanied the tour, was responsible for the highly effective post-tensioning system on the Christchurch Arts Centre, which saved it from major damage. If you’re not familiar with the Christchurch project, imagine a netting of steel cables wound around the outside of a stone building and tightened! The Clock Tower Annexe is getting a more high-tech version of this treatment, where steel rods are being inserted through the major structural members of the building. The Annexe is made from concrete faced with stone, and the steel rods, once tightened, will add some very necessary tensile strength to the building.
I’ll finish by mentioning that in response to a question from the audience about the most important lessons from the Christchurch quakes, the presenters agreed that the biggest lesson was this: doing something is better than doing nothing. Buildings which had been strengthened, even those with ad hoc solutions, survived better than those which had not. Peter suggested, as an example, that shoring up verandahs at a cost of a few thousand is often a way to reduce stresses throughout the structure, and provide protection from falling decorative elements. The two presenters also reinforced the importance of communication and compromise between members of allied professions (like architecture and engineering), and I’m sure you will all concur with this sentiment. We’re very grateful to Neil Buller and Peter Boardman for their generosity, frankness, and expertise.