ClockTower East Wing, University of Auckland, with Neil Buller and Peter Boardman

UPDATE 15 October 2018: Tiago Almeida of Structure Design got in touch to let me know about a paper that the participants in the job had written and presented at the concrete conference. I highly recommend a read of it: it contains a good overview of the full scope of the works and has some great illustrations.

We’ve been doing these site visits for a while now. In March last year, a large group of site visitors heard Neil Buller of the U of A’s Property Services talk about planned works on the University ClockTower’s East Wing. Peter Boardman of Structure Design was along that day, too—he was there primarily to talk about the work he’d done on the Symonds St Houses. This week, we got the band back together, and went to see the progress at the ClockTower.

ClockTower, view from the site gate.

The ClockTower, the East Wing, the Annex(e), B119, B105, the Cloisters…

All the above are legitimate names for some part of the building you can see above. The East Wing was built as part of the original construction of the ClockTower, in 1923-26. The great Roy Lippincott was the architect—I’ve written about another Lippincott building, Nelson House, on this blog. The ClockTower, aka the Old Arts Building (there’s another name!) will likely need no introduction to the audience of these posts, but its East Wing is less iconic.

Originally built as student accommodation, the East Wing has served as offices, meeting rooms, and administrative space for much of the last few decades. It’s undergoing a major seismic upgrade, targeted at 67% NBS. The target is based on a 500-year return period earthquake, and the building is designated Importance Level 2. The interior has been modified considerably since the building was first constructed. At the moment it is fully stripped out, and it will be getting a contemporary refit. It’s also going back to being a teaching space.

Plans, proposed refit of ClockTower East Wing. Note the symmetrical plan of the East Wing itself. Extending at the top right are the cloisters which link the East Wing to the main ClockTower building. Image courtesy Neil Buller, drawn by Architectus, all rights reserved.
The drilling rig atop the stair tower, ClockTower East Wing. Image courtesy Neil Buller, all rights reserved.

Stronger, tougher, independent

The most arduous part of the work at the East Wing is to strengthen the walls. The building has a reinforced concrete inner shell, which is clad in an outer shell of masonry. The strengthening regime requires inserting long steel rods down the walls from top to bottom. The rods will be tensioned, squeezing the stones more tightly together.  In the horizontal direction, the masonry is being tied more firmly to the concrete. The ClockTower connects to its East Wing by a covered walkway, known as a cloister. In the cloister, rods have been inserted horizontally as well as vertically, binding the open-walled space together. A seismic joint has been cut mid-cloister, separating and de-coupling the ClockTower and the East Wing and giving each of them room to wobble about at their own rate if an earthquake strikes.

Capstones removed, top of the exterior wall, ClockTower East Wing. Picture taken in December ’17.
Capstones marked with Roman numerals to allow them to be correctly replaced. Roman numerals are used, says Neil, because they’re much easier to carve with a grinder–all straight lines!

Drilling the walls

At the roof level, the capstones have been carefully removed. At regular intervals, the drill has been worked down through the masonry parts of the wall to the foundations. As the drill is lowered, the workers add on extra length to the drill bit, carving holes down to the foundations as far as eleven metres below. If the drill jams—and sometimes it does!—in some cases a pilot hole needs to be drilled through from the inside to release the bit.

Once the hole is drilled, steel rods are inserted, then grouted into place. Grouting a wall can be tricky—unseen cavities and naturally porous materials can leave you pumping oceans of grout into a small hole. To prevent this, the hole is lined with a fabric “sock”, which deforms to fit snugly into the drilled void, but prevents the grout from branching out into the wide blue yonder.

Holes drilled down through masonry wall. Holes ~120mm diameter?
Photo taken in December ’17.

With the rods installed, a stainless steel plate connects the rod-tips together. They’re then mechanically tightened, binding the whole system into a whole. Post-tensioning works by putting the entire wall into compression. When the wall gets shoved by a quake, it wants to rock or overturn. The side that’s being shoved up gets put into tension. (To understand this, put your hands on your hips and bend sideways: you’ll feel your muscles getting stretched on the side you’re bending away from.) Stone, brick, concrete—these materials don’t like tension. They’re good at squashing, bad at stretching. By adding extra compression through the post-tensioning system, the walls get to stay in an overall compressive state, even when tensile stresses are created by rocking. The tensile stresses aren’t big enough (hopefully!) to overcome the pre-existing compression created by the post-tensioning.

Once the rods have been tightened, the capstones are drilled out to conceal the protruding rod tips and nuts. Then they’re mortared and dowelled back into place. It’s important to fix the capstones back tightly so that a shake doesn’t dislodge them. They are not something you’d want landing on your head.

Rod inserted into hole and grouted. Yellow cap is for worker safety. Note stainless steel plate connecting rods and generating compression in wall. Note shaped edge of capstone, to avoid water running into wall cavity (?) Picture taken Feb ’17. Roof of cloisters.
Stainless steel plates overlap. Tensioning rod through centre. Capstone will be hollowed out to cover bolt heads, etc. Feb ’17.

A bigger, sturdier foundation

So much for the walls, but what are the vertical rods going down into? They’re not going to help much unless they’re sturdily connected to the ground! Significant work is going into upgrading the foundations and increasing their capacity. The ground has been dug out on the outside of the building, and a new foundation strip poured against the existing one. In the interior, digging is in progress to create a second new foundation inside the existing wall. The new foundations, inner and outer, are interconnected at intervals. Soon, the base of the original wall will be sandwiched between two new foundations, with the vertical post-tensioning wall rods tied into this newer, larger foundation unit.

ClockTower, East Wing. New external foundations being prepared. Photo courtesy Neil Buller, all rights reserved.
Base of exterior wall, ClockTower East Wing. The timber is formwork for poured concrete foundation. This new concrete foundation abuts existing foundation. The dark layer of stone at the base of the wall is granite, creating a damp proof course through which water cannot travel up the walls. This was very hard to drill through! Picture Feb ’17.
Interior, ClockTower East Wing. Preparation for new internal foundation to be poured, abutting existing foundation. Note at corner in foreground, reinforcement coming through hole. This is where the new inner and new outer foundations connect.
Horizontal drilling, ClockTower cloisters. Photo courtesy Neil Buller, all rights reserved.

 Tie me up, tie me… across

In the cloisters, the drilling work has been carried out horizontally as well as vertically. Workers have drilled through the concrete vaulting of the arches, installing horizontal ties to bind the open-air structure together. The tie rods have been hidden with round pattress plates, designed to imitate the tie rod end plates that are pretty ubiquitous on older buildings. At the moment, they’re a bit shiny, but they’ll soon dull down and become essentially invisible.

Cloisters. New steel pattress plate spreads bearing load. At centre of plate, rod extends through cloister arch into wall of ClockTower. Photo courtesy Neil Buller, all rights reserved.
Cloisters. Steel bracket, used in location where drilling is not possible. Bracket styled after decorative newel post in main ClockTower building.

In one spot, drilling proved impractical, owing to the geometry of what was above. To increase the capacity of that area, a steel bracket was designed and inserted, taking up the work that the internal tie rods would have performed. In keeping with heritage principles, the bracket has been designed to be sympathetic to the character of the building, but not to pretend to be an original feature.

ClockTower, ground floor interior. Black dots on walls are the location of ResiTies, inserted to bond masonry outer wall to existing concrete inner wall.

(Not) losing face

To prevent the masonry and the concrete shell delaminating, they are being bonded together with a close-spaced grid of special ties. They’re called ResiTies, and they’re a stainless steel twist, which looks not dissimilar to a decent-sized drill bit. The system uses a resin to bond both ends of the tie, locking the masonry layer and the concrete layer together. Apparently they go in pretty easily, but it certainly seemed like a big job to install these throughout the building. The manufacturers reckon they’re good for holding together brick cavity walls, too. You can read about them here: the link goes to a commercial site but, just to be clear, I have no relationship of any kind with Helifix.

ResiTie inserted. Note epoxy blob holding stainless steel tie.
ClockTower, East Wing. First floor. Concrete floor slab, patches of drummy concrete removed. Photo courtesy Neil Buller, all rights reserved.

Augmenting the concrete

The internal floor of the building is concrete. As you will know, reader, internal floors can be pretty important when buildings are strengthened. They transfer forces between walls, and allow the structure to act as a box. Diaphragm improvement is one of the most common things we’ve seen on our tours—it’s often in the category of low-hanging fruit when it comes to improving a building’s NBS score. The East Wing is no exception.

Over the years, a certain amount of moisture has found its way into the building. This, combined with the fact that the concrete was made with unwashed beach sand, has led to some deterioration of the internal steel reinforcement. (You can tell that you’ve got unwashed sand when you find shells in your concrete, as they did at the East Wing—it’s a dead giveaway.)

On the ground floor, the undersides of some of the concrete beams have been carved away, the surface rust removed from the internal steel, and then they’ve been re-sealed. On the first floor, the team went over the floor slab with a hammer, inch by inch, whacking the concrete, listening for the ringing sound that means the concrete is drummy. That’s happened where steel has rusted and expanded, cracking the concrete, or where salts in the sand have caused adverse reactions, or both.

The drummy bits of the concrete floor slab have been raked out, leaving the floor surface more than a little Lunar. Neil pulled out a bit of the reinforcing mesh and snapped it. Not much capacity left there!The engineers have prudently decided to discount the existing reinforcement in the floor slab entirely. So, to reinforce the floor and help it do its lateral-load-transferring work, the plan is to use strips of fibre-reinforced polymer (FRP). The FRP strips will create a lattice which will resist both tension and compression. A thoughtful site visitor double-checked: FRPs? Compression? Yes, says Peter Boardman. The lattice pattern allows the FRP strips to act like a truss.

 

ClockTower, first floor. Drummy concrete removed from floor slab. Blue lines indicate proposed location of fibre reinforced polymer strips which. Lattice of strips creates truss which can resist tension and compression.

Speaking of trusses

ClockTower, East Wing. Timber ceiling battens. Timber trusses above.
Trusses, slightly better image. The trusses are mostly sound, with minor water damage in the area shown. Steel brackets will mitigate lost connection strength.

A brief note at the end, then, to say that the timber trusses that form the roof are in pretty good nick, bar a few rotten ends which are getting bypassed with steel brackets. The building’s going to be sealed and air-conditioned, and some of the plant is going up into the roof void, with the rest perching discreetly beside the cloisters. On the day we visited, the roof-level scaffold was going up, and soon the building will be wrapped to allow the concrete roof tiles to be replaced with more authentic clay ones. There’ll be the usual plywood ceiling diaphragm enhancement, too.

It’s good to be back, and thanks!

Having seen the building last year, it was great to get a chance to come back and see how the work is being done. As our ad-hoc society continues to mature, expect more “return to-” tours further down the line.

We’re sincerely and warmly grateful to Neil Buller for organising the site visit, to Peter Boardman for sharing his time and his knowledge, and to Todd and the Argon team for letting us come and get in the way of a tight timeframe. As University of Auckland students, it’s great to have the chance to use our own campus as a learning tool. We really appreciate your co-operation. Thanks also to Phillip Hartley of Salmond Reed Architects for taking me on-site at the East Wing over the summer.

 

 

St Paul’s Church with Salmond Reed Architects and EQ STRUC

Today’s visit to St Paul’s Church marked the start of the third year of activities for our ad-hoc society. Simeon Hawkins from the church’s congregation led the way. Sean Kisby of Salmond Reed Architects and Peter Liu and Dr John Jing of EQ STRUC came along to tell us about their work on the nascent project of strengthening and refreshing St Paul’s Church.

St Paul’s Church, panorama from the Symonds St entrance looking East

The church, explained

Simeon, whose Master’s thesis focusses on the church, gave us a brief overview of its history. The current St Paul’s is the third building to bear the name. It’s a Gothic Revival building, begun in 1894. The church is cross-shaped, as convention dictates. The main body of the church is stone, with brick veneers inside from shoulder height to roof and brick structure below the floor. The roof members are timber. The transepts (the arms of the cross) are also timber, and the chancel, the head of the cross, where the altar is, was made from reinforced concrete and wasn’t completed until 1936.

The building is not quite as its architect William Skinner conceived it. He’d intended for a gallery to sit above the door, housing the choir. The rough masonry you can see around the base of the church was to be covered with timber panelling. The chancel was to be faced with stone. And, most noticeably, no spire was ever built atop the northwestern stairs. Still, the building is Category I in Heritage New Zealand’s list, all the more poignantly since the HNZ listing says “St Paul’s invites comparison with Sir Gilbert Scott’s only New Zealand work, Christchurch Cathedral”.

A door to nowhere five metres in the air denotes the spot where the choir gallery was to be built.

For my part—although I don’t have favourite children!—I have a soft spot in my heart for St Paul’s. Something about the cheerful pick-and-mix of its irregularly-sized stone arches, its glorious melange of materials, and its air of patient worshipfulness makes my heathen heart glad.

Fine stonecarvings, southern entrance, St Paul’s Church

First, do no harm

Sean Kisby picked up the thread. Salmond Reed, he explained, have a dual role at the church. Firstly, there’s a fair hatful of material repairs that need to be done: there’s stone to be replaced; roof coverings to be refreshed; lead and copper flashings to replace, too. It takes expertise in heritage materials to understand what should be done, and how best to do it. Tracey Hartley (and others) from SRA are supplying this knowledge.

In parallel, there’s a real gem of a design project to be done. The building needs seismic strengthening, with the aim of reaching 67% of the New Building Standard. The internal circulation needs to be improved—at the moment, masses of people must file up and down a tiny rear staircase. The main stair, a lovely wooden spiral, has literally rotted away, the victim of a “temporary” roof over the void that was to contain the spire.

St Paul’s Church, spiral staircase seen from the crypt

With the access reconfigured, and the rooms beneath the chancel refreshed, the long-term plan is to build out into the carpark, and (best-saved-for-last here), at long last to design and build a spire for the church. Surely that’s a commission to gladden the heart of any designer.

Strengthening the church

“Bring the building up to 67% NBS” I wrote above, glibly. So, how might that be done? Peter and John from EQ STRUC have been considering that question. Over the last few months, they’ve carried out an analysis of the building, including a LiDAR  scan of the building which created a detailed 3D image of the church. The LiDAR generates a point cloud, essentially a myriad of measured dimensions, allowing a viewer to see details of colour and ornamentation as well as capturing idiosyncrasies in the plan. The engineers also need to know how the mass of the building is distributed, and the point cloud helps to identify this, making later finite element modelling in Etabs or SAP2000 more accurate.

Dr John Jing explains the proposed strengthening solution to site visitors, St Paul’s Church

The EQ STRUC blokes were too modest to say this for themselves, so I’m going to say it for them: they’re really good at assessing and then using the strength of existing materials. This is important, very important, for heritage buildings. The conservative approach would be to discount the strength of unreinforced masonry down to almost nothing, necessitating a larger and more intrusive engineering intervention. Think whacking great steel frames marching down the aisles.

For the nave and the aisles, the solution that EQ STRUC have devised uses the existing materials to their own advantage. I need to caveat this by saying that, as yet, this is just a proposed solution, but here it is: the plan is to use the mass of the walls to dissipate earthquake energy by allowing the piers to rock.

How does this work? Let’s work our way down from the roof.

Looking up at the roof truss, St Paul’s Church

Firstly, a diaphragm will be inserted above the sarking and below the roof, allowing forces to be transferred. (The nave floor’s getting a new diaphragm, too.) The existing timber trusses will be enhanced with steel, stiffening them considerably. At the junction between the trusses and the wall, a moment joint will be created, meaning that the roof and wall can’t rotate towards or away from each other.

The walls and piers of the nave. Spandrels above openings. St Paul’s Church.

Along the walls, between the openings, the spandrels will be strengthened with fibre-reinforced polymer wraps (FRPs). This will greatly increase their ability to resist tension, helping them to stay intact in a quake. [see footnote for a follow-up on this][update April ’18]

A pier, St Paul’s Church.

Lastly, as noted above, the piers will then be permitted to rock. In a design-level earthquake, the pier will form two hinges: one above the plinth (at the second moulding) and the other below the capital (the leafy bit). The pier will then wobble back and forth on the hinges, dissipating energy as it does so.

How do you stop the pier from toppling over, and bringing the roof down with it? By tuning the stiffness of the moment joints up above, the quantity of deflection at the piers can be controlled. It’s elegant—if not easy—and the great virtue of the solution is that you don’t need to ruin the spatial qualities of the interior with ugly external bracing.

The rose window and the western gable, St Paul’s Church

The real doozy of a problem might be the western gable end. How best to brace the wall, given the aesthetic and historic constraints at play? The solution may come from the planned completion of the choir gallery, absent from the church for so long. The gallery would run along the wall just below the level of the two long windows in the picture above. As at the Town Hall and Hopetoun Alpha, a gallery at the midheight of a tall wall can be a structural godsend, concealing crucial structural bracing, and reducing the risk of a potentially deadly of out-of-plane collapse

Peter Liu explains strengthening schemes to site visitors, the crypt, St Paul’s Church. Composite image.

To the Crypt!

Beneath the nave lies the crypt. Although you enter the church at ground level from Symonds St, the ground slopes away steeply to the east, and there’s a generous space beneath the floor of the nave. The space is used for a range of church activities. Here, in this unfussy rough-brick room,  it might be forgivable to insert some supplementary steel structure. EQ STRUC are proposing to do so, thus completing the rocking system described above by giving the piers a solid base to wobble on. (The slender piers of the nave are supported on the chunkier brick piers of the crypt, which you can see in the photograph above.)

Tension cracking in the transverse brick arches of the crypt, St Paul’s Church

Down in the crypt, we could see old signs of damage caused to the church  by the city growing around it. The church abuts the motorway trench of busy Wellesley Street, dug in the 1970s (? don’t quote me on this date!) As the ground settled post-trench, the foundations rotated outwards, creating tension cracking like that shown above. Don’t panic, though: a geotechnical report showed that the settlement has long subsided, and there’s no fear of further rotation and damage.

St Paul’s, south side, coming up the slope from the carpark. Timber transept meets masonry nave. Note irregular finishing of wall, perhaps intended to join to masonry veneers?
St Pauls, concrete chancel/transept and masonry nave.

 What are you made of?

As noted above, and as you can see in these images, the church is made from a variety of materials. There’s stone, brick, reinforced concrete (RC), and timber. How, I asked, would you assess the interaction between the RC and masonry elements of the building?

Here, it seems, the timber transept is invaluable. The timber’s elasticity should take up any differential movement between the RC and masonry sections. This means that the RC and the masonry were able to be assessed as two separate structures, removing a layer of complexity from the analysis.

Given the building’s Category I status, it’s not easy to make a case for destructive testing to establish the strength of materials. EQ STRUC have a database of material tests from similar structures to fall back on, for establishing likely material properties.

Signatures of the builders? Stairwell, St Paul’s Church.

Talk to me

In the crypt, I asked Peter, John, and Sean about collaboration between architects and engineers, hoping for dark and ghastly tales to befit at least the name of the surroundings. Instead, they talked about the pleasure of collaboration, of working together and modifying each others’ solutions to arrive at the best result. In all seriousness, such collaboration is a theme of our Society, since we’re composed of would-be members of each profession. It’s great to hear that the working world embraces this philosophy of co-operation.

Dr Jing made a comment that intrigued me. “You need to draw,” he said. “I always take a pen and paper to meetings and to site.” Calculations can be confusing, explanations are often ambiguous, but sketching makes for clear communication. Architects will be unsurprised by this, I think, but it’s food for thought for student engineers.

Thanks!

Sincere thanks are due to Sean Kisby from Salmond Reed Architects, and to Peter Liu and Dr John Jing of EQ STRUC, for their generous sharing of time and expertise. We’re also exceedingly grateful to Simeon Hawkins and Esther Grant from St Paul’s, and to the marvellous people who came and offered coffee and hospitality. Thank you for sharing your beautiful church with us. We look forward to seeing the project develop.

Footnote

back to article

Bricks in the spandrel area, closeup. Photo courtesy Tracey Hartley, all rights reserved.

Tracey Hartley touched base, following up on the proposal to use FRPs to reinforce the spandrel areas. Regarding the bricks in the spandrels, she notes “I believe they are a rough pale brick with large joints, originally coloured over with a red ochre finish and lined out (tuck pointed) to make the brickwork smarter looking.” The final design may involve restoring this finish, so covering with FRPs may be more difficult. Also, the breathability of FRPs needs to be determined so that moisture isn’t trapped in the brickwork.

I thought it was interesting to include this as a footnote, to illustrate how the process of design is a compromise or collaboration between the expertise and priorities of the professionals working on a project. Working together, the architects and engineers will devise an acceptable solution.

Update April ’18

back to article

I ran into Dr Jing today and we talked about the FRPs for the spandrels and piers in the nave. Dr Jing explained that the FRPs in the spandrels were never going to be added onto the surface of the bricks, as I had thought. Instead, they would be mounted into little slots cut into the bricks. The slots would be ~3-4mm wide and span across several brick units, being anchored at top and bottom into bolts. This near-surface mounting would make the FRP intervention much less visually intrusive than if it were covering the surface (as per Tracey Hartley’s note above). To reiterate, it was never the plan to mount the FRPs on the surface: the EQ STRUC team were always intending them to be near-surface mounted.

One final point. I hadn’t picked up that the piers were also to be FRP wrapped, not just the spandrels. The FRP wrapping would naturally add great confinement strength to the pier. However, by varying the length and position of the wrap, the EQ STRUC engineers can force the plastic hinge to form in the places they want it to: at the top and bottom, as per the pictures above.

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.

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.

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”

St Patrick’s Cathedral and St Matthew-in-the-City with Peter Reed of Salmond Reed, July 2016

Peter points out the chapel and kitchen his firm installed in the aisle of the church.
It was great to see both familiar and new faces at Friday’s site visit to St Patrick’s Cathedral and St Matthew-in-the-City.
The tour, given by Peter Reed, centred around the philosophy of strengthening iconic buildings. Peter described two contrasting methods for strengthening. The first, which he referred to as the “honest” method, involves adding visible bracing (usually steel) to the interior and/or the exterior of the structure. The honesty of this approach is that the strengthening doesn’t pretend to be part of the original fabric. It’s also more reversible if and when new technology emerges. In contrast to this approach, what Peter called the “concealed” method involves inserting bracing into the fabric of a structure, and then making the insertions as invisible as possible.
Peter points out the position of the hidden steel with a laser pointer. A large drawing is propped open below.
Peter points out the position of the hidden steel with a laser pointer. A large drawing is propped open below.
St Patrick’s contains both methods–in the main body of the church, steel has been inserted into the walls and hidden. In the tower, which some site visitors scaled, the steel bracing is not concealed. This is partly because it wouldn’t be possible to create a straight path from the tip of the spire, and partly because this area is not accessible to most visitors. Some of our crew made it into the belfry, though!
Peter describes stonework patterns, salt crystallisation, and wind vortex degradation.
Peter describes stonework patterns, salt crystallisation, and wind vortex degradation.
Next, the tour moved to St Matthew-in-the-City. (If you didn’t make it onto the tour, and you’ve never been there, do yourself a favour and drop in there one day. It’s simply stunning. I don’t believe there’s anything like it in this country. When you go, GO INSIDE.) Structurally, says Peter, the building is identical to a Gothic cathedral of the 12th through 15th century — apart from the Portland cement mortar which holds the blocks together. And herein lay the crux of the talk–how on Earth can we strengthen something as unique as this? “Honest” bracing would have to be pretty exceptional to escape severely defacing the building, and “concealed” bracing requires extensive drilling, which would be ground-breaking, very tricky, potentially in contravention of heritage principles, and, last-but-not-least, outrageously expensive. In fact, best practice might be to do nothing and wait for technology to catch up with the problem, hoping nothing too seismic happens in the meantime.
Peter points out the chapel and kitchen his firm installed in the aisle of the church.
Peter points out the chapel and kitchen his firm installed in the aisle of the church.