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.”

NZSEE Conference 2018

This weekend I attended the New Zealand Society for Earthquake Engineering conference, entitled From Inangahua to Kaikoura and Beyond. The NZSEE publishes conference papers (with a small delay) at its website, so it’d be unnecessary as well as presumptuous to attempt to summarise individual presentations. What I do want to do, briefly, is set down my impression of the two conflicting schools of thought at the conference about the …and Beyond part of the theme. Why? Because the way in which this apparent dichotomy gets resolved might make a big difference for heritage conservation.

Blinded by Science

One camp says that engineers have started to believe their own hype. Dazzled by the potential of Big Data, and the promise of sophisticated software models, engineers have started to believe that they can not just model events and natural phenomena but predict them. They’ve been quoting coefficients to two decimal places as though that were meaningful, and producing special studies to permit themselves to avoid meeting minimum performance standards.

Instead of getting more and more specialised, this argument says, and instead of running ahead of the capabilities of the profession and the reality of commercial practice by dreaming up increasingly complex methods, let’s adopt a basic risk model that covers the whole country more or less equally. And let’s not focus on a minimum standard of safety, but on designing structures that might still be useful for something once the dust settles.

In the Weeds

It’s a technical conference. Many of the presenters were there to talk about their research into specific avenues of knowledge: how earthquake waves propagate; how concrete floors deform; the probable intensity of shaking at a given time and place. Instead of generalising across the country and working from first principles, their goal is to discriminate between individual cases and to know how to treat ’em. Research, modelling, and better, more ubiquitous instrumentation holds the key to tailoring solutions to specific problems.

Heritage in the hinterlands

I’m not qualified to judge; but I do have an opinion. For what it’s worth (you decide), I’m leaning towards the idea that—for the moment—we need to accept a different level of risk in different places. I got interested in working with heritage and engineering in part because I was thinking about the fate of small-town New Zealand in a post-Christchurch era. I don’t want to see the embodied history in those places obliterated. To my way of thinking, it’s clear that if we adopt the school of thought that everywhere in the country should be treated more or less the same, we’re going to have to demolish a lot of old buildings. They just don’t make enough money to pay for their own repair.

At Pompeii, the state-of-the-art archaeological practice is to leave everything in the ground. There’s masses of unexcavated material. But too much has already been excavated—we can’t conserve it all— and it’s getting ruined by exposure. Besides, the science is advancing so much that it’s smart to leave plenty untouched. Analysis methods that haven’t been developed yet might be able to tell us marvellous things about Roman life—if we don’t get too eager and stuff up all the raw data.

What’s the analogy? The pace of innovation in earthquake engineering is immense. In my opinion, it might be smarter to use the science we have to tell us where shaking is likely to be less intense, and, for now, do little or nothing to historic buildings in those places. Yes, it would be riskier living in an old building in Paeroa than in Ponsonby. And it’s all very well for me to say, since I don’t live in an old building and I don’t live in a small town. But the alternative is that we bulldoze our history too hastily. Let’s be smart about fixing the worst stuff, and be equally smart about leaving the less risky stuff for a later, wiser time.

Thanks

To the kind and generous real engineers who spent their time talking to a wannabe engineer! And also to the speakers and organisers of the conference. It was excellent.

Eight things I learned by reading Section C8

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

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

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


Christchurch

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


 

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

1.  Spot cavities with algebra

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

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

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


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

2. Loose diaphragms fall out (sorry)

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


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

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

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

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

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


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

4. Vertical tie-down for parapets

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


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

5. Look out above

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


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

6. We don’t talk about failure here

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


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

6b. And what to look for

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


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

7. Ten simple steps…

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


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

8. Walls rock

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

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

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