Christchurch Arts Centre, with Owain Scullion of Holmes Consulting

In memoriam

As with earlier posts about Christchurch, the post following is written with  respect and sympathy for the victims of the quake sequence and for their families. As students who want to work with heritage buildings, we can learn from Christchurch and be part of doing better. Ka mate te kāinga tahi, ka ora te kāinga rua. HT

Christchurch Arts Centre, tower restored by stonemasons.

What I did on my holidays

With exams out of the way, I was delighted to be offered the opportunity to go down to Christchurch and have a look at the reconstruction and strengthening work taking place at the Christchurch Arts Centre. The Arts Centre is a cluster of historic buildings, the majority of which made up the original campus of the University of Canterbury. It’s a closely-spaced collection of good-sized masonry structures, dating from the second half of the 19th century and the early years of the 20th: more or less the exact kind of buildings that copped a fair hiding in the Canterbury earthquake sequence. A ~$300 million, multi-year programme is restoring and strengthening the buildings in the complex.

Christchurch Arts Centre, Girls’ High entrance. A Corinthian column is cased for protection during the building works.

I had the great pleasure of going through the site with Owain Scullion of Holmes Consulting. Owain patiently explained the strengthening programme to me, and showed me the current state of the works. What I thought I’d do, for the audience of this blog (who are mostly clustered up here on Pig Island) is talk about a few of the technical details that I was able to see, and try to give an impression of how the work is proceeding. I can hardly claim to be comprehensive, but I’d like to give a snapshot of what I saw. There’s a site map here to see where the buildings are as I refer to them; scroll down a bit on the linked page to find it.

Christchurch Arts Centre, Chemistry building. Horizontal and vertical stainless steel cables are connected to the structure through junction boxes, enhancing the compression in the wall caused by gravity loads to ensure that the wall stays in overall compression during shaking.

A stitch in time

Buildings at the Arts Centre had been strengthened before the quakes began. (Holmes’ involvement with the Arts Centre stretches back well before the 2010 quake.[update]) Regular attendees of Heritage Now events may remember Peter Boardman talking about his involvement with applying post-tensioning to the Chemistry building at the Arts Centre, which happened in the 1980s. As Peter told us, there was controversy over the appearance of the post-tensioning, which takes the form of a visible network of cables wrapped around the building. But what is undeniable is that the system largely preserved the Chemistry block during the quakes, despite the collapse of the building’s (unstrengthened) tower. As a measure of the post-tensioning system’s success, it has been renewed and reinstated, this time using stainless steel cables. (For more about how the post-tensioning cable system performed, you might like to read Dmytro Dizhur (et al)’s paper from 2013, which also talks about the post-tensioning of College Hall at the Arts Centre. TL;DR: it worked bloody well, and had some fringe benefits like increased out-of-plane capacity that weren’t predicted by the retrofit designers.)

Christchurch Arts Centre, Chemistry building. The post-tensioning terminates at steel plates. Note the cable entering the structure on the right to travel along the inside face of the wall. The stone buttresses to which the plate is attached were the original design mechanism to resist out-of-place forces in the gable end walls, but unfortunately their capacity to do so is insufficient.
Christchurch Arts Centre, Girls’ High, pre-quake tension brace retrofit, attaching to gable end. Note valley beam at springing of arches.

Not all of the pre-quake work was suitable for unmodified reinstatement. However, I did see examples of earlier retrofits being integrated into the new strengthening systems. For example, in the Girls’ High building, a system of tension braces had been installed to restrain the gable end and to transfer loads towards the core of the building. In the 2010-11 quakes, the loads were transferred pretty successfully, but in doing so, the valley beam at the base of the tension braces pushed on an internal wall, moving it by 50mm. This internal wall is now being beefed up and stiffened to drag loads out of several surrounding rooms.

Christchurch Arts Centre, Girls’ High. A brick internal wall is being strengthened to collate loads from adjoining rooms.

As an aside, Owain mentioned that a fringe benefit of working on these unique structures is that you get to see examples of fine nineteenth-century crafting. In the Girls’ High building, tusk tenons connect the floor joists to the beams, a detail that points to the relative cost of fasteners vs. workmen’s time in the early days of the NZ construction industry!

Christchurch Arts Centre, Girls’ High. A profusion of materials require strengths to be assumed or determined: timber sarking; bricks and mortar; timber beams; steel tension braces (upper left), plus masonry walls (not shown).

The right ANSR?

Owain explained that it has been important for Holmes to test the properties of the materials at the site—particularly for critical structural elements. For example, the internal wall mentioned above that’s aggregating loads has been subjected to pull-out testing to determine the cohesion value of its mortar, in a process analogous to the bed joint shear tests I’ve written about on this blog. The testing allows the engineers to close the loop on their modelling.  Having made assumptions about the material properties of the elements, the engineers model the structures in a software package called ANSR. Using data recorded in the last few decades of earthquakes around the globe, the engineers subject the model to a succession of simulated earthquakes. This modelling identifies structural hot-spots that need to be addressed. Materials testing allows the team to to be satisfied that their underlying assumptions are valid and that they can therefore trust the results of the modelling.

Christchurch Arts Centre, Girls’ High. First floor interior, showing lower edge of timber ceiling trusses with hammerbeam-style half-arches. Hidden structural steelwork supports the gable end.

Variations on a theme

For an all-too-brief moment, I got to put my steelies on and go up the scaffolding, even if I did have to wear the hard-hat-of-shame marked Visitor. At the top of the walls of block DA, aka Girls’ High, below a steeply-pitched roof, some supplementary structural steel has been inserted. The frames support the gable end, and diagonal braces lead down to a PFC that runs along the top of the wall. (Jargon alert! PFC = “parallel flange channel”, eg a C-shaped section.)

Christchurch Arts Centre, Girls’ High. Taken from the scaffold outside the room in previous picture. Note the PFC running along the top of the wall, and at left the diagonal brace heading towards the gable end. The thicker timber beams at the left are the upper chord of the trusses shown in the previous picture.

At the time I visited, the workers were setting up to drill some three-metre holes horizontally down through the wall. My first thought was that what I was looking at was similar to what’s being done at the University of Auckland’s ClockTower Annexe, where a series of drilled holes accommodate rods that are putting the entire wall into compression. I was wrong. In fact, here the drilling was to allow for Macalloy bars to be inserted, which will hold down the PFC securely and hence restrain the gable end against excessive rocking. The three metre length of the drill holes won’t get them to ground level—it’s just to allow the Macalloy bars enough space to develop good friction. Plenty of length is needed, because the drillholes tend to be smooth, meaning that the grout or epoxy that holds the bars in place doesn’t get a very strong bond to the walls of the hole.

Christchurch Arts Centre, Girls’ High. In a ground-floor room, a concrete core has been created and attached to the existing masonry. This necessitates building up and moulding window reveals. Dowels secure the concrete core to the masonry.

In another part of the building, however, there was a remediation technique being used that’s closer to what’s happening at the ClockTower. (Incidentally, although the ClockTower is about 50 years younger than the Girls’ High building, the coursing of the masonry is similar.) A ground floor space is being converted into a movie theatre, and shotcrete has been used to create a concrete core inside the masonry. This concrete core is connected to the exterior material by a closely-space system of dowels, similar to the Helifix screws that are being used at the ClockTower. (At the ClockTower the concrete core was part of the original design.)

Christchurch Arts Centre, Great Hall

The great Great Hall

One of the best things about the site visits we’ve done is getting to see some of the unique and remarkable interiors of heritage buildings. (St Matthew-in-the-City is a standout example for me.) While I’m sure that College Hall aka the Great Hall at the Arts Centre is well known to many people, it was a complete (and stunning) surprise to me. It’s an incredible interior: ornate, stately, and imposing. My first reaction was that I’d never seen anything like it in New Zealand; but then, as I looked more closely, I realised that it reminded me a little of St Paul’s Church on Symonds St which we visited earlier this year.

Christchurch Arts Centre, Great Hall. Successive layers of materials. The piers contain concrete cantilevers and post-tensioning systems.

There’s a roundabout connection to be made between the two buildings, in that Benjamin Mountfort, who designed the Great Hall, also worked on  ChristChurch Cathedral, to which St Paul’s Church has been compared by Heritage NZ. That part I’ll confess I looked up: what spoke to me immediately, though, was the palette of materials and the hierarchy of how they were deployed. At the Great Hall, moving horizontally upwards, timber panelling gives way to fine red bricks, then to Oamaru stone, then timber hammer beams and a a patterned timber ceiling. St Paul’s follows a similar pattern of material proportions and transitions. Gothic Revival, innit? One day St Paul’s will look as stunning as the Great Hall does.

At the Great Hall, the strengthening has involved inserting concrete cantilever columns inside the piers of the long side walls, enhancing the in-plane and out-of-plane capacity of the walls. Post-tensioning was also re-installed and enhanced. I suspect that the much larger ground accelerations expected in Christchurch compared to Auckland meant that a rocking-pier model that EQ STRUC proposed for St Paul’s wouldn’t have worked for the Great Hall—and, of course, the site topography’s different.

(Here’s an interesting article on Newsroom about the work at the Great Hall and the UNESCO prize it won, including a few images.)

Christchurch Arts Centre, Engineering building. Note missing gable end, steel wire strapping, vertical cracks through masonry.

Next up

Once the Girls’ High work is complete, the works will move on to the eastern end of the Engineering building, where extensive damage is still visible. A gable end is missing; there are deep vertical cracks in the masonry; and, in the building’s tower, the Oamaru stone quoins and the basalt wall have parted company. Steel ropes lash the structure together for temporary support. When works begin, tall steel buttresses will be attached to the walls, allowing the connections between walls and diaphragms to be cut and reinstated without triggering a collapse. Stonemasons are working to recreate the lost finials and many other decorative elements of the building, using archival photographs as visual references.

Christchurch Arts Centre, Engineering building. Steel buttress frames await re-use in supporting the structure. Cut off at bottom centre is the poster for the Teece exhibition.

A colossal amount of work has been done to restore the Arts Centre complex. While planning for the Engineering Building restoration is doubtless far advanced, I’m sure that lessons learned from working on the other buildings will be carried into the final part of the project, and will go on to inform strengthening and repair of similar buildings.

ChristChurch Cathedral

Christchurch

Walking around the city, it was sad and sobering to see the two hulks of the great churches standing unrestored, the Cathedral and the Basilica. The first sadness is for the human tragedy, certainly, but then you feel the loss of the buildings and the stories they contained.

Inside the Chemistry building at the Arts Centre, I spent time at a small exhibition in the Teece Museum which contained artefacts from Greek and Roman burial sites. One way of reading the objects in this collection was suggested by the inclusion of a Piranesi engraving, emphasising the splendour of decay and Romanticism of ruin.

The Cathedral of the Blessed Sacrament, aka the Christchurch Basilica. Shipping containers as temporary supports are a not uncommon sight in the city.

Without a doubt the Cathedral and Basilica would fit the bill for the Piranesi treatment, and I do love Piranesi. But for me, the most affecting object in the Teece show was the tiny child’s sarcophagus, and the memorial plaques beside it. Death and loss seem overwhelming, all-consuming, and hope is lost. But what can survivors do in response except make beautiful objects, and offer dedications to the memory of those who (to quote the plaques) “well deserved” to be remembered? As hopeful humans, we keep piling up stones, making new, making good, even though we know that nothing we make can truly last forever.

I’ve just been reading Joseph Conrad’s Nostromo, and my thought above is expressed far better by him. “…for life to be large and full, it must contain the care of the past and of the future in every passing moment of the present. Our daily work must be done to the glory of the dead, and for the good of those who come after.” From afar, I’ve been unconvinced that it’s worth reconstructing the Cathedrals. But, faced with the sight of them, and seeing them on the same day as the Arts Centre, I’ve changed my mind. It’s worth trying to restore those churches, just as the Arts Centre has been restored. What else are poor hopeful humans to do? We’re fortunate, then, to have engineers, architects, historians, stonemasons—and many others—to shoulder the task.

Thanks!

With sincere thanks to John Hare, Owain Scullion, and Alistair Boyes of Holmes Consulting, for inviting me to visit the Arts Centre and for their generosity with their valuable time.

Update

back to article

John Hare checked in to provide some more background on Holmes’ involvement with the Arts Centre project.

“The strengthening being done now is now part of a much more comprehensive programme that was always intended, but is now being amalgamated into the repairs. That commences in 1978 or thereabouts, when the Arts Centre came into existence. The general approach was:

  • Stage 1 – secure the buildings to make them a good as they could be, ie tie in the floors and roofs and add a few elements to take out the worst of the structural weaknesses, and address the most critical life safety falling hazards – chimneys for example.
  • Stage 2 – protect as much as possible of the fragile heritage secondary structural and non-structural elements
  • Stage 3 – full seismic upgrading.

“In practice, money was always short so Stage 1 was done bit-by-bit with parts of stage 2 added in where practical. A big push in the late 90s saw most of the Stage 1 worked completed with outside help, but most of the work to that time was done by Jim Loper, the on-site Master of everything! And later in the piece, with Chris Whitty, now the Site and Restoration Manager.

“We did a few big studies, starting as far back as 1996, as I remember it, for the Stage 3, but other matters were judged more important, such as fire protection (which I supported at the time – fire would be hugely destructive there).

“As a consequence, only the Old Registry (HA) and the Old Girls High (DA) were strengthened to anything like what we now regard as a reasonable level (I think both were to 67% at the time, therefore around 50% against current code).

“The original post-tensioning (College Hall and Chemistry) was quite adventurous at the time! Done to a relatively low level and as such, prevented the worst of the damage, but needed significant upgrade for the current work.”

NZ International Convention Centre/Albion Hotel/Berlei House (Nelson House)

Thanks to Engineering New Zealand, I recently had the opportunity to attend a site visit to the NZ International Convention Centre. The NZICC is being built in central Auckland across the road from SKYCITY. The site takes up most of the block, and on two of its corners there are heritage buildings.

Albion Hotel, and, far left, Berlei House (Nelson House). Retrieved from Wikimedia Commons, public domain licence, taken by Ingolfson (2008)

The two buildings have required different treatment. The Albion Hotel, a five-storey URM building completed in 1873, is scheduled in the Auckland Unitary Plan as a Category B site. It has remained intact. Berlei House, (aka Nelson House), was designated a Category 2 Historic Place by Heritage NZ. To allow for sufficient land area for the conference centre, the developers were granted permission to demolish it, retaining the façade. You can read the Heritage Impact Statement prepared by Dave Pearson Architects on Auckland Council’s website.

Although the visit didn’t focus on the heritage sites, I was able to ask a few questions about Berlei House and the Albion. I thought that the treatment of the façade and of the retained building was interesting enough to merit a brief post.

Berlei House, steel gantry supporting facade during excavation. Courtesy nzicc.co.nz, all rights reserved.

Berlei House

Berlei House was completed in 1931. It was designed by Roy Lippincott, best known in these parts for the University of Auckland Clock Tower and for Smith & Caughey’s on Queen St. The Berlei House façade is brick at the base, with precast concrete panels forming the upper half. Large elaborate windows create slender piers over most of the wall height.

Berlei House, original (?) construction blueprints. Courtesy nzicc.co.nz, all rights reserved.

On both sides of the site, deep excavations have been made to allow for a capacious carpark. For the Berlei House façade, this meant that temporary works in the form of steel gantries were required to support it while the pit was dug.

Berlei House, excavation begins. Courtesy nzicc.co.nz, all rights reserved.
Berlei House, excavation, showing retention. Courtesy nzicc.co.nz, all rights reserved.

And what a pit! As you can see above, the excavation went down and down, with the façade nimbly supported on the very edge. But the gantry wasn’t staying, and the façade wasn’t going to have to hover on the brink like this forever.

Richard Built, BECA Technical Fellow in Structural Engineering, was one of the tour presenters. As we passed through the Berlei house façade on our way into the site, Richard explained that a concrete structure had been constructed to support the façade and to direct perimeter loads into the foundations. The façade’s not taking any loads beyond its own self-weight.  We were not permitted to photograph inside the site, but from the street I snapped a couple of shots, showing the new frame hiding behind the façade.

Berlei House facade, author’s photograph Feb ’18. Look for the concrete frame through the window openings.

With the job still in progress, it’s possible to see the holes where steel rods have been inserted to connect the façade to the concrete structure. They are held in place with an epoxy grout. Lateral load resistance is now provided by the concrete frame.

The Albion Hotel and the facade of Nelson House during site excavation, seen in a drone photograph. Courtesy skycityentertainmentgroup.com, all rights reserved.
Rendered drawing of finished project, view from above the corner of Hobson and Wellesley Sts. Courtesy nzicc.co.nz, all rights reserved.

Albion Hotel

I saw less of the Albion. In the later part of our tour, we went through a corner of the exhibition hall, which will be at street level on Hobson St, next to the Albion. We were able to see the plain, windowless North wall of the building. Barnabas Ilko of Fletcher Construction explained that there is 400mm of space between the Albion and the NZICC—which seems tight. Then he explained that the NZICC wall has to fit into that space, and it’s 225mm thick! Oh, and the precast panels are over 10m high. Piece of cake, right?

NZICC, piling around the base of the Albion. From nzicc.co.nz, all rights reserved.

The Albion Hotel, like the Berlei House façade, is also sitting on the edge of an excavation. This one is deeper: the land at the site slopes down from Hobson St to Nelson St, so on this side where the slope is higher, the hole must go down further to make a level basement. Incidentally, Richard Built mentioned in his presentation that the lateral pressures on the structure from the slope are greater than the vertical forces from the weight of the building!

We were told that the Albion Hotel was permitted to settle a maximum of 20mm. Somehow, the work was completed without exceeding that limit—when you look at the depth of the wall below the building, the challenge of achieving this target seems enormous.

Thanks!

There was a lot more to know and to see at the NZICC. The structural design is on a fairly heroic scale (40m spans, 4m deep trusses, columns supporting 13MN loads, etc) but as interesting as it is, it’s not germane to the theme here. Engineers might like to keep an eye on the Engineering NZ newsletter, as Fletchers have promised opportunities for future visits.

Thanks to Mervin Tibay for organising the visit, and to Richard Built, Richard Archbold (the project architect, from Warren & Mahoney) and Barnabas Ilko of Fletcher Construction.

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.