St Paul’s Methodist Church, Remuera, with Josiah Campbell and Richard Newman

From St Paul’s Church in Symonds St to St Paul’s Methodist Church in Remuera—22 minutes by number 75 bus. Like the Anglican St Paul’s (which we visited in March), the Methodist St Paul’s is undergoing seismic strengthening designed by EQ STRUC. The construction phase is well underway, and we were able to see some of the structural enhancements being installed. Josiah Campbell of EQ STRUC led the tour, and we were fortunate to be accompanied by Richard Newman from the contractors Aspec.

St Paul’s Methodist Church, Remuera. Point cloud model. Image courtesy EQ STRUC, all rights reserved.

St Paul’s Methodist Church is a Gothic revival brick building, designed by E. A. Pearce and completed in 1922. That puts it close to another site we’ve visited this year, the University’s ClockTower, which also dates from the ‘twenties and finds its stylistic roots in the Gothic. St Paul’s is a handsome building, with a reserved, dignified aspect from the street. It doesn’t contain the kind of flights of fancy that you’ll see at the ClockTower, but we were able to appreciate the craft and the sense of proportion that went into its design and construction.

And we were also able to see the kind of weaknesses that afflict buildings of this age and type. Reflecting on the visit, it occurs to me that the building contained examples of many of the kinds of problems—and solutions—that we’ve seen in our travels. In this post, I’ll try to give a sense of the structural solution (as I understood it) and also illustrate a few of the interesting details along the way.

St Paul’s Methodist Church. Site plan with structural additions at roof level highlighted. Note at the right the concrete-core shear wall, a later addition to the building. Image courtesy EQ STRUC, all rights reserved.

Not plane sailing

As with the ClockTower, St Paul’s Church, and other large masonry structures, a key issue at play in assessing and strengthening St Paul’s Methodist Church is the in-plane and out-of-plane capacity of the walls. (In-plane means pushing along the length of the wall, out-of-plane is pushing across the wall.) St Paul’s walls are relatively thick and sturdy—up to 410mm in places and thicker at the buttresses—and neither dizzyingly tall nor riddled with fenestration, so they are pretty good in-plane. Where the walls are less robust is out-of-plane, and a good deal of this weakness comes down to the way that connections were originally made between the walls and the roof.

Structures resist in-plane loads by transferring them to walls that lie parallel to the incoming forces. That’s a fancy way of saying that a box is stronger than four playing cards leaning against each other. At St Paul’s, the structural intervention is designed to connect the parts of the church together and to transfer loads to the parts of the building that are best placed to resist them. The new structure provides some additional strength, but its most important job is to allow the efficient  use of the existing strength of the building.

St Paul’s Methodist Church. A view of the top of the rear wall, brick veneer with concrete core. The diagonal white line running through the picture marks the position of the ceiling linings, which will be reinstated. This allows the steel PFC at the top of the picture to be installed, as it will be concealed by the ceiling. In other, visible parts of the building, a less obtrusive (and more expensive!) approach has been taken.

As it happens, there are some pieces of the church that are going to come in handy for just that reason. Most buildings that stick around for a few decades will have been modified in that time. In the case of St Paul’s Methodist Church, an extension project in the 1960s saw a concrete wall installed at the rear of the church, replacing the original wall. This newer concrete wall is hidden by brick, but there are construction drawings which give details of its dimensions and specified reinforcement. Josiah has supplemented the drawings with a GPR scan of the wall: the radar gives good indication of the reinforcement bar spacings, and from there it’s possible to drill to determine the bar diametersor in this case, the figures are on the drawings.

St Paul’s Methodist Church. A view of the original timber ceiling truss (which runs across the building) and the newly-inserted eaves truss, running longitudinally. The large metal attachment on the Reid bar is a “banana” clip, designed to allow the bar to be tensioned. The timber member running parallel to the white RHS is one of the underpurlins mentioned in the plan above.

The helpful part of all this is that the concrete wall’s capacity can be calculated, and it serves as a strong point from which to support other elements of the building. From this wall at the rear of the church, two eaves trusses are being installed along the long axis of the building. The trusses, made up of criss-crossed Reid bars and shortish lengths of RHS, connect the front and back walls together. This new metalwork also connects to the original timber trusses which run across the church, binding them into position. The front or street wall of the church is thus being given support by the other walls. The strength the eaves trusses provide to the gable end is being augmented with some steel wall braces that run across the face of the wall. In addition, the design uses the strength of some of the existing timber underpurlins.

St Paul’s Methodist Church. Connection between top of wall, timber ceiling truss, and eaves truss. The flat section of grey beneath the ceiling truss is the padstone (see below). This has been augmented by the addition of a bond beam (the concrete step at the right edge of the picture). The bond beam sits on top of the existing brick wall and is connected into the joint by a steel leg.

Looking across the building, quite a bit is being done to support the two long walls. As I mentioned above, the 1960s additions to the building have good drawings, and the engineers also found drawings made for the original construction. Anecdotally, I’m aware that the existence of original drawings is far from being a given when you’re working with older buildings. But even the designer’s drawings can only tell you so much. For example, the 1920s drawing set included a section across the building at the point where the timber roof trusses meet the top of wall. The drawing showed a solid unit—not a brick—underneath the bottom of the timber truss. Great, thought the engineers. There’s a bond beam. (A bond beam is a member that runs along the top of a masonry wall, and helps to transfer loads.)

However, when the work began in earnest, the ‘bond beam’ proved only to be a padstone, a short section of stone or concrete used to spread out the point load from the bottom of the truss onto a wider section of the brick wall. As with so many heritage projects, improvisation and redesign-on-the-fly was required, in this case taking the form of a series of bond beams cast in situ and connected to the truss system with embedded steel legs.

St Paul’s Methodist Church. Having come down from the internal scaffolding, site visitors inspect the placement of Helifix ties. The ties are being used in both horizontal directions: through the wall, to connect the brick wythes of the cavity wall together more securely, and also along the wall, to stitch cracks, especially around openings.

When describing this problem and its remedy, Josiah made an observation that I hadn’t heard before, which struck me as valuable wisdom. He wanted to avoid taking any of the old material away, he said. This was not for reasons of heritage best-practicealthough he’s certainly keenly aware of that, and earlier he had talked about how decisions have to be made about every scrap of original fabric that’s removed, from leaky roof vents to crumbling lintels. But the point that Josiah was making was that there are internal forces in play inside the existing pieces of the structure. Removing them comes at a risk. If you take things away, the internal forces will realign themselves, seeking equilibrium. If it’s not done carefully, unexpected movements or even failures could occur.

St Paul’s Methodist Church. Elevation looking from inside the church towards the road. Note the cross-bracing for the steelwork in the tower does not extend to the ground (see below). The wall braces are connected to the new internal trusses. Image courtesy EQ STRUC, all rights reserved.

You can see that it’s important to have the whole system of the building in your mind. Another example of this kind of thinking is to be found in the single new truss that spans transversally across the building. It’s being installed at the front end of the church. Notionally, says Josiah, you’d like to have it in the middle, to bridge the centres of the two long walls. But the choice to shift it to the front is part of a more holistic structural plan. Located at the front, the truss can provide connections and support for the parapet brace that’s needed in the porch, and connect to the steelwork that’s going into the tower.

St Paul’s Methodist Church. The roof is being retiled.
St Paul’s Methodist Church. A close-up view of the windows on the street side of the church, taken on the climb to the top of the tower.

Going up

At the time that we visited, we couldn’t get into the tower, but we did have the pleasure of climbing right up to the top of it from the outside. Because of the nature of towers—tall and waggly—I think it’d be fair to say that the inserted steel structure is being called on to do a bigger share of the work in the tower than it is in the main body of the church, where the design is more about using the strength of existing materials. Inside the tower, a braced frame will climb up the interior—Richard tells us it just fits. At the bottom, though, it wouldn’t be acceptable for the steelwork to block out the corner of the church, so the lowest storey of the frame—about three metres in height—has no diagonal braces. Instead, the four legs at the corners of the braced frame plunge down into a pretty hefty block of concrete, which will hold them steady.

St Paul’s Methodist Church. A concrete lintel above one of the windows was badly damaged and needed to be removed. The use of beach sand in the original concrete led to corrosion in the reinforcing steel, and the expansion caused the concrete to crack. Care needed to be taken in the removal, as it was in a delicate state. A good view here of the cavity construction of the wall. The two wythes of brick (three wythes lower down) are separated by an air gap, to keep moisture from getting inside. This can weaken the wall, but St Paul’s is not a severe case of this. Helifix ties are being used in places to connect the walls across the cavity. Image courtesy EQ STRUC, all rights reserved.
St Paul’s Methodist Church. A new lintel replaces the one above.

Climbing the tower, we got a better chance to appreciate the beauty of the church. I’d been clued in to the charm of St Paul’s Methodist by the intricacy of the timber ceiling trusses, but from the outside, we could also see the lovely slenderness of the windows with their stylish blue stripe. We saw a repaired lintel, sign of a common problem with 1920s concrete— the use of beach sand in the original mix, causing the reinforcing steel to corrode and expand.

St Paul’s Methodist Church. A site visitor inspects the top of the tower, where original lead has been removed and will be replaced. The round knobs cover fixings, as can be seen at the bottom right of the picture.

One last surprise awaited us at the top of the tower—or awaited me, I should say. I’d never seen a lead roof, at least not up close like this. It’s being re-leaded to prevent leaking, but the original round fixing-covers are going back on. They added a little hint of rococo to the Gothic, I thought. With knobs on, isn’t that the phrase?

Thanks!

It’s always a privilege to get to visit an active worksite, and we know that we are stopping real work getting done and taking up the valuable time of our guides. It makes a big difference to us—thank you for helping us to build up our experience and knowledge. Many thanks to Josiah Campbell for finding the time for this visit and for slides and for taking all our questions in his stride. Many thanks also to Richard Newman for his generous use of time for our visit. Both of these guys’ passion for the building and for the work they do was evident. Thanks also to David from the church for the background information and for permission to visit.

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.”

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

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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

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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.

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.