A brief note: I run this site for recording the visits of the Heritage Society. However, the material presented on this page isn’t explicitly about heritage matters. I’m an engineering student, and the sight of the old engineering building being neatly demolished over the fence led me to enquire about the possibility of educational site visits for students. We were welcomed enthusiastically by the project manager and the contractors. The idea of site visits seemed popular among students, and as a result, quite a few people who wanted to come on the tour weren’t able to get a place. I thought I’d use the site to share images and stories, even though it’s somewhat at a tangent to the material I normally post. So, if you’re here for the heritage stuff, head to the home page. If you’re here for the Engineering Building, read on, but feel free to stick around and explore the other things on the site. HT.
Nearing structural completion, March 26, 2019
A massive amount of work has taken place in the seven months since our last visit to B405. With the building nearing structural completion, we had the opportunity to make one last visit. This time, we were most fortunate to have the company of Brendan Donnell from Structure Design, who has been the principal engineer on the project from concept to construction. With Brendan on site, assisted by lecturers Charlotte Toma and Hugh Morris and, as always, by the indefatigable Mike Renwick, we were able to learn more about the specifics of the design of B405.
A major theme of the conversation today was ductility. As readers of this post will likely know, any ordinary material can be stretched a small amount, little enough that when you let go it will snap back to its original shape. (Snap! That’s elastic deformation.) However, if you apply sufficient force to the material, at a certain point you stretch it so much that it can’t go back to its old shape! The stress above which the material starts to change shape permanently (aka plastically) is called the yield stress, often denoted f_y.
When you design a ductile component of a building, you are saying that if the structure experiences a big load, you’re going to allow the material in that component to be stressed past its yield point. You’re going to permit some plastic deformation. If you’re going to allow it to get loaded to twice the yield stress, that’s a ductility of two. In some parts of the country, and with some structural systems, buildings can be designed for a ductility of five or more, meaning that certain components of the building will be expected to deal with loads that take them to five times their yield stress without failing.
The trick is, of course, to specify which components are going to do the yielding. In B405, that’s the buckling-restrained braces (hereafter BRBs). (Those who want to know more about what BRBs are will find help in earlier sections of this page: go thataway ↓ ) Yielding the BRBs in an earthquake is actually a good thing: that’s how the energy from the shaking is dissipated. B405 is designed for a ductility of two, meaning that the BRBs have to be able to deal with loading that takes them to twice their yield stress. The rest of the building shouldn’t be stressed that far, but it does have to deal with the deformation that comes from the yielding of the BRBs.
A ductility of two is reasonably conservative, and, as a result, the rest of the building is fairly stiff. After all, it has to be less ductile than the BRBs, to ensure that the BRBs are the elements that undergo plastic deformation. So a decision to peg the ductility at a given level requires careful consideration. On the one hand, a ductility of two means that the overall cost of materials is going to increase a bit, since keeping the majority of the components elastic means making them a certain amount beefier. On the other hand, if there is a major earthquake, the relatively low ductility design should allow the building to be put back into service more easily. High ductility buildings are designed to stay standing in big quakes, but to take more damage in doing so, even to the point of needing to be demolished afterwards. The University decided that continuity of operation in the face of disaster was critical, and worth paying a little extra to obtain.
Another benefit of low ductility was that it made the building easier to analyse. Brendan explained that the design used the Modal Response Spectrum method, which is, as he said, “just linear.” Given the comparative stiffness of the building, it was felt that a linear model of building performance would be close enough to the actual response to allow for good design assumptions to be made. The tradeoff between the effort and cost of a time-history model (which could potentially have shown lower deflections) and the benefit of slightly reduced demand on facades and members was considered, and settled in favour of the simpler analysis model.
One aspect of the design for which deformations must be carefully considered is the cladding. As noted above, the building is fairly stiff, but as a consequence of the analysis method that was chosen, the predicted upper-storey deflections are set at a higher, more conservative level.
I always wondered how cladding is hung off a building. Brendan explained that at B405 the cladding does indeed hang, its weight supported by the beam above. (Supporting the cladding weight is another reason why the edge beams are not perforated.) At the bottom of the cladding panel, there’s a bolted connection, and the bolt goes through a slot. This slot allows the panel to shuffle and swing if the building deflects: far better that than shearing off a bolt or cracking the panel.
Returning to the BRBs for a moment, we talked about the decision to use them as the lateral load-resisting system for B405. Brendan explained that the University has been keen to employ BRBs, having also put them into service in the new Science building. (I’ve seen them at the NZICC, too.) Part of their appeal is that they are excellent energy dissipaters, and once they’re done dissipatin’, they can be swapped out for another one.
(Not entirely without effort, of course: for one thing, you’d have to get the new BRB into the building somehow, and the old one out; and for another, the installation of the BRBs required the steel fabricators Grayson Engineering to use a computer-controlled boring machine to get the holes in the gusset plates just so to fit the 1mm tolerances for the true-pin connections! Some more detail about this is below in earlier posts.)
I was curious about the load path that involves the braces, since we’ve been considering these things in Civil 713. Looking at the elevation in the image two above, if the building were loaded left-to-right, the diaphragms would apply their loads to the BRBs through compression struts, carried back into the braces by the collector beams at the edges of the floor plates. (The need for axial load capacity is another reason why the edge beams aren’t perforated the way the inner beams are.) Then, looking at the top of the building, the line of BRBs that heads off to the right would be in compression, and the other line in tension. At the mid-height, where the BRBs reverse direction, the actions reverse polarity. Brendan explained that the complexity of that mid-height connection node required a decent effort in computer modelling to resolve the detailing.
Detailing for sustainability and performance
Brendan made some really interesting remarks about sustainability in design. Pointing out that a new building like B405 represents a considerable carbon cost, he described a number of measures that had been put in place to try to ensure that the building would have a long and useful life—making the carbon worth expending. In particular, there was a big effort made to ensure that internal gravity structure was widely spaced: a few columns, working bloody hard, rather than a lot of lighter ones. This allows for the more flexibility in the uses that the building might be put to in a future incarnation.
To achieve this open-plan design required good detailing and a few clever tricks. The secondary beams span a not inconsiderable 13 metres. We’d noted on previous visits that the beams are highly penetrated to allow services to flow through. The part that I hadn’t fully comprehended, though, was that the beams started life at about 530 deep, and once cut and reassembled, made it up to about 800 deep—so there was a structural dividend from all the effort that went into making perforations for services. The material savings start to get considerable when you realise that there are 8 kilometres of beams in the building!
One tasty little detail, a consequence of the spans and the open plan layout, is the inclusion of viscoelastic strips between the bottom of the composite floor and the top of the beams. These act as miniature dampers, helping to stop footfalls from causing too much bouncing in the floor plate. As Brendan explained, it’s not helpful to have a lively floor when you’re in a lab, looking through a microscope at a sample.
Another small but noteworthy detail was the use of reinforcing bars in the floor slabs, in preference to mesh. Part of the thinking around this had to do with worker safety. When the slabs are poured, if the work is not done carefully, a buildup of concrete in one place can lead to a collapse of the supporting decking. A fall could lead to a serious injury for workers—but, with a network of reinforcing rods at around 300 centres, there’s no gap large enough to fall through.
One of the (many!) aspects of the site that have gotten much further advanced since we last visited is the atrium, which will serve as the main entrance to the building from Grafton Road. Above the entrance, a gallery projects, cantilevering from the main body of the structure. Looking at it from across the road, you will notice that at present it is connected to a number of straps, which are pulling it tightly downwards.
The reason for this slightly unusual sight is to avoid excessive deflection when the cladding is placed on the outside of the gallery. The saw-tooth configuration of the glazing makes it susceptible to too much deformation. Since the gallery is cantilevered out decently far, the cladding’s weight makes the structure deflect a reasonable amount. The beams are precambered upwards to deal with this deflection, but the total range of movement is too much for the cladding to handle.
Instead of loading the cladding onto the panel piece by piece, letting the gallery deflect gradual downward, and maybe cracking the panels, the whole element has been “pre-deflected”, using the strapping. As the cladding panels are installed, the tension will be gradually slackened off in the strapping, meaning that there shouldn’t be much net movement in the gallery.
Brendan also answered a question that’d been bugging me since I leaned out the window of the Leech last year and saw a most unusual triangulated set of structural members. The unusual brace you can see in the image above takes the lateral load from the gallery and transfers it to the beam-column junctions above and below. This accommodates the greater ceiling height in the atrium, without loading the structural column in mid-span—a definite no-no in Brendan’s load transfer scheme. It was great to have that piece of the puzzle fitted into its rightful place.
Sincere and warm thanks to Brendan Donnell for taking the time to lead us around the building; to Charlotte Toma for organising and liaising with Brendan; to Charlotte and Hugh Morris for coming on site and answering questions and helping out; to Mike Renwick for cheerfully and efficiently organising the visits and liaising with Hawkins; and to Hawkins for letting us onto your site. We enjoyed being there, and we’re looking forward to enjoying our new building!
Inside the building, August 28, 2018
From March to August is only five months, and yet it’s almost beyond belief how much the building has changed since our last visit. For one thing, the steel is up as far as level 12, so the building now covers almost as much space in the sky as it will when it’s finished. For another, with the cladding going on, B405 has lost its skeletal appearance and started to bulk up. Fairly soon, it’ll feel like the building has always been there.
Back again to check in
And yet, inside the building, certain elements retain a familiar air. Regular site visitors or frequent readers of this record will remember the last trip we made, at which time the steel tray of the composite deck was roofing over the third floor level, but no concrete had yet been poured inside it. That day in March, we started off in the easternmost corner of the first floor, where we once again found ourselves today. One of the themes of the visit, for me, was seeing how construction methods and structural ideas which had been described to us on previous visits (and in lectures) are now being put into practice. This first familiar room was no exception.
An important concept that we’ve heard about before at B405 is building up to build down. This seemingly counter-intuitive practice is part of making sure that the building gets finished on time. With the third floor slab poured, steel could go up to level six, then nine, then twelve and there’d be time later to complete the work on intermediate floors, without holding up progress on higher levels. This method also takes advantage of the fact that the composite floors don’t need to be propped during construction. Because of the up-then-down method, and perhaps also because machinery is still moving about at ground level, there is no concrete slab on level one as yet, even though the walls are in place.
If you skim down a bit on this page to some earlier images, you’ll see a picture of a column in this easternmost room, bedecked with temporary props. With the composite floor now in place at level two, these props are no longer needed to give restraint to the ends of the beams, and they’ve been removed. As we travelled up the building, we could see temporary props in place on higher levels, serving the same purpose. Many of the visitors on site today have done or are doing Charles’ paper CIVIL 313, and those of us grappling with NZS 3404 for the first time have come to realize how much of a beam’s capacity is determined by its restraint against twist or lateral movement. A prop in the right spot can really help!
Isaac Holden of Hawkins had a number of interesting things to say about the floor slabs which are taking over from the props and providing the necessary restraint. Since the Canterbury Earthquake Sequence, the amount of steel required in slabs has “nearly tripled”. (Damage limitation is the aim here—it’s not that slabs fell in Christchurch, but rather that the quake-shaken floors needed too much repair and some came close to failure. Although the buildings were still structurally sound, the cost of repairs or the danger of even attempting them meant that demolition became the preferred option.) The increase in prescribed reinforcement makes the sequencing of placing the steel into the decks critical. Isaac described going back through the drawings and making sure that all the steel would fit within the 160mm of concrete. It did fit; and the slab pours have gone well, he says.
Incidentally, although the site and the building is highly engineered and very high-tech, here’s how they make certain to install exactly 160mm of concrete depth on the slabs: they use a stick with a mark on it. (Sometimes old ways are best.) The concrete depth is important, and not just for cover of the reo. The beams beneath are pre-cambered, meaning that they curve gently upwards in their original, un-loaded state. The weight of the concrete squashes them back down to level. Where the composite deck runs across some of the primary beams, there’s only a shallow space between the top of the beam and the top of the slab—too shallow for concrete to cover it reliably. The gaps will be filled with an epoxy grout, covering over the rebar and forming a level slab. This occurs with some of the shallow box section primary beams around the lifts and services core.
We spent some time looking once again at the form and composition of the beams. It’s hard to look past the degree to which the beam webs have been sliced and trimmed to provide openings. Of course, in the middle of the spans, they’re mostly carrying bending, which they’ll do in their flanges. It’s at either end, where the shear forces are higher, that the webs need to be continuous.
To make the beams, standard sections have been cut in half along the web. The openings are cut, and then the beams are put back together—on site, you can see the weld-line clearly. The specific pattern of openings gives a clue about how much force each beam is resisting: in some of the larger interior primary beams, the openings are spaced wider apart, indicating more demand.
Above and below the holes, and once again depending on the load in the beam, the web posts between the openings have been augmented with stiffeners. Charles explained that the web posts are subjected to several concurrent design actions: axial load, bending, and shear. When the combined design actions pass the threshold of what an unstiffened web post can provide, the stiffeners are required.
So, why cut holes in the first place? On lower floors, it’s easy to see the reason. Services are going in, threading their way through the beam openings. Given the multipurpose nature of the building (teaching space, offices, labs), there are myriad requirements for services, and the fitout process for these is complex. Aiding in this task is a detailed 3D model of the building and its systems. Using a tablet, the workers can see how the existing building and what is yet-to-be-built fits together, right there, on site, on the screen. This level of modelling detail aims to make the fitout go faster and run more smoothly.
Another “callback” to a feature we’d seen in an earlier visit happened on a higher floor. We were inspecting the buckling-restrained braces (BRBs) which form the lateral load-resisting system (in civvy-speak, that means the things that stop it falling over sideways.) Scroll down to the last visit for more about the BRBs and how they work, but the exec. sum. is that a diagonal steel member resists tension nicely when the building shakes one way, and a concrete-filled jacket on the steel member stops it buckling in compression when the shaking cycles B405 back the other way. For this to work reliably, it’s really important that the BRB doesn’t deflect out-of-plane. On an earlier visit, we’d seen an interior BRB braced against this out-of-plane possibility by an outrigger beam. Today, we saw heavy stiffeners welded to the gusset plate, serving the same purpose of holding this critical connection steady and stiff.
As a half-trained engineering student, an interesting part of the puzzle of visiting a site is decoding the unfamiliar—or perhaps, translating the unknown in terms of the known. As I waited for site visitors to arrive, I pondered what on earth the weird zig-zags of steel sitting on the back of a truck might be. It was only once inside the building that a site visitor pointed out to me what they were. Flipped over by about 120 degrees, they made a lot more sense!
On the other hand, as an endemic lecture-rat, veteran of a thousand theatres, it didn’t take me long to recognise the shape of a lecture hall, spanning between levels three and four. As an aside, Charles mentioned that the diagonal bracing (Reid bars) have recently been tested by UoA researchers. (I’ve also seen them in action lately at a church in Remuera.)
On our way back down and out of the building, we stopped in on a lower level to examine some of the instrumentation that is being built into the structure. Arrayed around a column, one of several that is getting such treatment, was a ring of strain gauges. The gauges, and the other instrumentation, will give real-time data on how the building is responding to stresses applied to it: wind loading, live loads, seismic activity, and even the loadings imposed by the construction work. It’s part of a whole suite of design features which aim to make the building itself a pedagogical tool. The features range from glass walls into the plant room to the decision to pin-joint all the connections, an articulation of engineering principles which will certainly chime well with the showier elements of Building 401 and the adjoining atrium.
Today’s visit was led by Charles Clifton and by Isaac Holden of Hawkins, and accompanied and organised by the indefatigable Mike Renwick. Wayne Wihongi came along once again to keep us safe. We’re sincerely grateful to all of them for their time and their wisdom. If not for their willing help, we’d never have gotten within a bull’s roar of getting on site. Many thanks.
Structural steel, March 7, 2018
The new semester has begun, and a group of visitors went to check out progress in the work at B405. Hugh Morris and Charles Clifton were back at the helm, and we were accompanied by Prof. Robert Tremblay from Polytechnique Montréal, who was in town to give a talk to SESOC.
As any engineering student who has looked out of an east-facing window knows, the building is beginning to rise out of the ground. In recent weeks, structural steel has arrived at the site and has been erected. Steelwork began at the southeast corner, where a column was lowered onto its hold-down bolts and supported with a temporary brace. (Have a look at the previous story to see the hold-down bolts on the pile cap. ) The workers then constructed a corner: two perpendicular bays of steel frame, which could stand up by themselves. It wasn’t a coincidence that these first two bays contained diagonal braces—this meant that the corner was able to resist lateral loads as well as horizontal ones. The corner became a box, and from there, the rest of the structure was built out northwards and westwards, bay by bay.
The structural system
To a first approximation, what’s been constructed is as follows. Deep piles sit in the ground. Atop the piles are pile caps, connected by ground beams. Steel columns, two stories high (plus a bit more) are bolted to the foundations. Once situated, the columns are filled with concrete. Primary beams span between the columns, with secondary beams spanning the primaries and carrying the floors.
I mentioned diagonal braces and the lateral load. To resist wind and earthquakes, the building uses buckling-restrained braces (hereafter BRBs). The BRBs have a steel core, which is connected to the frame and resists the lateral loading through tension and compression. In a severe earthquake, the steel core is designed and detailed to yield in both tension and in compression, so that it has nearly the same strength and stiffness under tension loading as it does under compression loading. To stop it buckling in compression, it is contained within a concrete and steel jacket, which is not connected to the framing system and therefore does not carry the axial forces from the lateral loading. There is a slip plane between the core and the mortar and concrete jacket to allow the core to yield freely in both directions.
The workers are constructing the building in an up-down fashion, meaning that the next step will be to put the steel up to level 6, then build the floors back downwards to level 4. This helps to keep the worksite covered, and also allows for faster progress because there’s more flexibility in the timing of work stages.
[Update! It’s started…]
You’ll have noticed the pin connections everywhere throughout the structure. “Why so many pin joints is anyone’s guess,” said Charles. It would be possible to connect the members with welds or bolts, and the BRBs could be connected that way too, and perhaps more cheaply. But the two guides did suggest some reasons for this choice. Charles’ take was that the architect had preferred the look of the pin joints. Hugh made the point that, considering the building as a teaching tool, the pin joints helped to express the structural system more clearly to future generations of curious students.
To divert for a second, if you’ve never looked at B401 (or the old atrium) from the standpoint of expressed structural concepts, you might enjoy doing so sometime. Mike Renwick also noted that B405 will contain building instrumentation allowing future students to see the strain gauges wobbling as the building flexes on a windy day.
The choice of pinned connections has had some structural implications, too. Charles assisted in the peer review of the design, and he identified the need to support the BRB gussets against out-of-plane rotation. Short “outrigger” beams can be seen, connecting the gussets to the neighbouring secondary beam.
We were joined on the tour by Paul Wikiriwhi and Isaac Holden from Hawkins. Paul kindly explained a number of aspects of the construction, but a recurring theme of what he said was the importance of considering constructability in engineering design. Take the pile and pile cap shown above. The original design called for the piles to be 1200mm in diameter. Hawkins’ pre-construction review identified that with all the reinforcement, and with the need for the hold-down bolts to extend down through the pile, there simply wasn’t going to be enough room in the pile to get good concrete poured. As a result, the piles were redesigned with 1500mm diameters; bigger, but more practical.
There was another good reason to be careful about the hold-down bolts. Originally, they had been designed to go down four metres into the pile, although this was later reduced to 2.5m. You’re only seeing the last 200mm or so of the bolt sticking up in the image above. However, that 200mm was critical. If the pile cap or the bolts were not positioned just right, the steel column simply wouldn’t fit on top.
It was worse than that. Steel design is usually done to a tolerance of ±3mm. But the BRBs needed to be located very exactly, and very firmly. The tolerance was only 1mm. That meant that the BRB had to be exactly the right length, and the frame had to be in the perfect place to accept it. The tolerances were so critical that although the braces were manufactured in the USA, they had to have the holes in each end drilled in New Zealand.
Why? Because the ambient temperature at any given time is different in NZ and the USA. Summer here, winter there. A difference of 30°C in the temperature between the two places would mean that the BRBs arriving in NZ would experience thermal expansion of almost 3mm, far exceeding the design tolerances. [Thanks for the explanation, Charles!]
(Incidentally, at the NZ International Convention Centre I recently saw BRBs that were pin-jointed but had an eccentric-cam pin, allowing ±4mm tolerances. File that one away, engineers: might come in handy.)
Tolerances also came into play on the concrete floor. The floors are steel-concrete composite decks, and over the last week or so I’ve watched the workers put in the reo. The slab was poured last weekend, so there wasn’t much to see up top when we visited. Paul talked about the importance of designing cover depth to allow enough room for the reinforcement. Charles pointed out, too, that post-Christchurch we are designing diaphragms with a lot more reinforcement, for improved load transfer.
A bit more on the columns at B405. We were fortunate to find one lying down and able to be inspected closely. Charles noted that the columns are 50mm thick at the lower levels, reducing to 20mm at the top. As mentioned above, the columns are filled with concrete once they’ve been positioned, presumably with some kind of concrete boom pump. The concrete adds a little bit of stiffness to the columns, takes some axial load, but it’s most important for its fire performance. If a fire causes the steel to weaken, the concrete will take up enough load to support the dead weight of the structure.
An interesting footnote: strain your eyes to see the blue dot on the column above, just below the floor plate. That’s a steam vent. What steam, you ask? The steam from boiling concrete, of course! That’s right: Charles recounted seeing a concrete-filled steel column in Japan that had been through a severe fire. The concrete had boiled. The column was square in section at top and bottom—and perfectly round in the middle, inflated by the boiling concrete. Best avoided with a vent.
We peered into the column, observing that the beam connection plates pass right through it. To achieve this, the column has to be cut just below the bottom of the plates—you can see the seam in the picture above, inside the column. The seam is then welded to 100% strength. Column-to-column connection happens on the chamfered lower end of the column: that’s a 100% weld, too. These are expensive and technical welds, but the capacity in the columns and their resultant spacing must make up for this expense. And after all, it’s a twelve-storey building.
Last nugget. A cute trick. The little fins you can see sticking out at the base of the column are cleats. A short plate will temporarily connect those cleats to the ones on the column below, supporting the steel until the surrounding structure arrives. Normally, the cleats would have to be taken off for the column-to-column weld to be done—else how do you get the welder in there to do it? But instead, the Hawkins team are scalloping out the cleats, leaving a space to do the whole weld in one hit. Nice one.
We can’t thank Hawkins often enough or well enough for letting us come on. We know it interferes with real work. We really appreciate it. Thanks also to Mike Renwick for nimble scheduling, and to Charles Clifton, Hugh Morris and Robert Tremblay for satisfying our curiosity. Let’s do this again some time.
Visit to the foundations, November 22, 2017
With exams completed, a group of students were invited on site to view the foundation works and the nascent traces of the new building. Richard Zhang from Tonkin & Taylor led the tour party, and we were accompanied by Alvin and Wayne from Hawkins, the contractors.
There’s a world going on underground
At the time of our visit, piling has essentially been completed. The deepest piles have been sunk approximately 20 metres into the soil, which is East Coast Bays Formation of the Waitemata Group. The largest piles are 1500mm in diameter, and they run around the perimeter of the new building and through its centre. The remainder of the piles are 600mm in diameter. Richard told us that the piles develop 500kPa of skin friction in the rock.
I make no bones about being a bunny, so, while the numbers above sounded impressive, I wasn’t sure exactly what they signified. Richard explained that the piles weren’t atypical for a building of this size (twelve stories), but that they were somewhat deeper than might be expected because the upper layers of the soil aren’t especially strong. Pleasingly, though, for Richard and his team, the soil samples which have been removed during piling have matched the specimens taken from the preliminary boreholes. This is good because it means the piles will develop the load-bearing capacity they were designed for. In some cases, the soil has even proven to be slightly better than early samples predicted.
Interestingly, we learned that a row of the building’s new piles are offset a small distance from where they would have been in a strictly symmetrical arrangement. This is to avoid the location of some of the larger piles from the old building and any consequent weakening of the surrounding earth which they may have caused.
The new building is going upwards, and it’s also going down—or at least, stretching itself out to its full extent. To find an extra half-a-floor’s-worth of space inside the building envelope, the design cuts into the existing ground level at the southwestern corner of the site—in the direction of the School of Architecture. The cost of doing this, explained Mike Renwick, project manager from the Faculty of Engineering, is quite modest compared to the value that the extra space will deliver over the 50-year design life of the building.
One of the risks of the excavation is settlement of the surrounding buildings, especially the architecture building. To guard against this, the building’s position is being carefully monitored. Pore pressure in the soil on the proximate corner of the building is also being checked. I asked what mitigation measures would be used if the architecture building were found to be settling excessively. Richard suggested that sealing the face of the cut surface and/or propping the retaining wall would likely be the most effective solutions.
Whole lotta shakin’
One unusual feature of the foundation design is the provision of a vibration-isolated slab. Several of the high-tech testing machines that the faculty and students will operate need to be kept still and level. While most of the advanced labs will be on higher floors, clearly, the top of a twelve-storey pendulum is a bad place for this equipment! To accommodate these requirements, a section of the foundation is being kept separate from the surrounding structure. The vibration-isolated lab will be strictly monitored for performance, and the machines can be further isolated at their individual bases if this is required.
Talk the talk
There’s no better way to feel like a new chum on site than not to know the names of things. “What’s that—pointy—bendy—metal thing?” you ask. “Next to the—concrete thingy?” And everyone rolls their eyes. Your correspondent, always keen to wedge his foot further into his mouth, asked for a glossary of what we could see.
Most of what’s above is self-explanatory, but what you might not’ve figured out is that the gaps in the starter bars are to allow the heavy vehicles (like the digger you see above) to move between “rooms”. Alvin explained that, later, the workers will drill into the ground beam and socket home the reo (heh, more jargon: reo = reinforcing = [in this case] starter bars). An adhesive will be used to hold the steel in place.
Sincere gratitude to Mike Renwick for organising the visits. Thanks also to Liam Wotherspoon and Rolando Orense from academic faculty, to Richard and Hamish Maclean of Tonkin & Taylor, and to Wayne and Alvin from Hawkins for keeping us safe and answering questions.
We’ll be making more visits throughout 2018. Keep an eye on your inbox for the chance to register. Happy new year! HT.
Visit to the demolition site, August 9, 2017
What’s behind those doors?
A curious group of students from postgrad and undergrad Civil Engineering assembled beside these innocuous-looking doors, this Wednesday afternoon. Most of us used to eat our lunch back there—but what’s left of the building behind the doors? We went through them to have a look, guided by Hugh Morris and Charles Clifton.
Yes, the atrium is largely unchanged. You can see that the demolition works begin at its East wall. We would spend most of our time beyond that wall, but we paused briefly to note how helpful it was that the full weight of the atrium roof is supported by the central columns, with lateral support coming from building 401. This makes it a lot easier to demolish the wall without needing to support or brace the beam ends on the demolished side. We also admired the way that the timber roof beams follow the shape of the bending moment, getting more slender towards their tips.
Into the demo zone
Looking across the fence at the demolition works, it’d occurred to me that the building in its current half-up, half-gone state is a lot like a real-life cross-section. We’ve all seen cross-sectional drawings—but how often do you get the chance to step into that gooey cake-slice and walk around? What was really wonderful, then, was how bare the interior of the building was. As you can see above, it’s been stripped right back to the bare bones. That was what we were there to see.
The structural system
Hugh and Charles quickly analysed the system and described it to us. The floors are made from long precast units, shaped like this in section: Π . Charles described how they’d’ve been poured into troughs containing pre-tensioned bars and cut free once cured. The sections butt neatly up against each other: ΠΠΠ. Those upside-down U-shapes are deep enough to carry loads over a relatively long span, which Charles estimated at 14 metres. Once the precast units were in place, a topping layer would’ve been laid over them. The whole floor acts as a rigid diaphragm, stiffening the structure and tying the framing elements together at each floor level.
The floor loads are carried into the primary beams, which run longitudinally along either edge of the plan, with another set down the middle. From the beams the loads go to the columns, just as you’d expect. On closer inspection of the central members, the lecturers noticed that the columns were noticeably thicker in the longitudinal direction than in the transverse.
So why were the columns thicker longways? This is because, in addition to carrying their share of the floor vertical loads, they are supporting the structure laterally in the long direction. They are one of three rigid frames in that direction. The beams are rigidly built into the columns so when the building is subjected to lateral loading in the long direction these frames resist this loading. In the short direction, the lateral support is from the big cast-in-place concrete shear walls that were now clearly visible at each end of the big open space, a space that used to house several lecture theatres. These moment frames are one-way acting so it is actually a disadvantage for them to be very stiff in the transverse direction. Designing the system this way would have made the internal reinforcing at the beam-column intersections simpler, too.
Details and detailing
Looking closely at the beam-column intersection, we noticed that the depth of the secondary beams / floor units means that there is a significant distance between the top of the beam and the bottom of the floor it supports. There’s a stubby length of column poking up above the beam, and the floors are attached to the column. If an earthquake happened, there might be some movement in that short section of the column.
One interesting aspect of this design that Charles noted and praised was the way that the downstand supports of the precast units are only bearing-supported onto the top of the beam. Because the columns are connected laterally into the floor at the topping level, when the columns sway in an earthquake the bottom of these precast units might have to slip a few millimetres on the top of the beams. This detail allows that to happen without causing structural damage.
At either end of what remains of the building, large shear walls provide transverse stiffness. The shear walls would’ve been cast inside large timber formworks, bound together with strong steel wales to prevent the formwork bulging outwards under the bursting pressure from the wet concrete inside. The planks of these formworks have left long lines on the shear wall, giving it the classic striated look of in-situ concrete.
A keen-eyed site visitor pointed out that the water you can see in the picture comes from leaks. The leaks likely occur at the junction where one concrete pour stopped, at the end of a workday, and another one began the next day. These massive concrete walls wouldn’t’ve been poured in one go. Bear in mind, too, that this was an internal shear wall until someone smashed half the building down, so it wasn’t expected to get wet!
“Wouldn’t having a junction where the two concrete layers touch each other make the shear wall weak in shear?” I asked, foolishly. The site visitor kindly pointed out to me that we only expect the concrete to carry compression loads, not tension. Too true. (Later, Charles would describe a rocking shear wall designed for a Christchurch building, an idea that makes my knees shake a little!)
Spandrel dos and dont’s
You might’ve noticed that demolition has been halted for a few days—just long enough for us to get in for a site visit, in fact. The reason for that is the discovery of asbestos in the building, in pipe lagging and in a number of other places, which has meant a halt in the process while cleanup takes place. We were in a cleaned floor, of course, but the advantage of the asbestos removal for our purposes was that it had created yet another cutaway view for us to inspect.
The image above shows a closeup of one of the columns on the exterior (East) side of the building. In the original construction, once the building’s structure was up, precast window units were inserted into the gaps between columns and floors. Running between columns as they do, they are properly classed as spandrels. You can see the attachment points to the columns and the floor slab in the photo—it’s a supporting bracket. There are upper and lower brackets on each precast panel. The bottom supports the weight onto the perimeter floor slab/beam and the upper one ties them laterally into the columns. We were able to look at this detail because material had been gouged out for the asbestos cleanup—I think the radiators used to run along here, perhaps?
Hugh and Charles took the opportunity to issue a cautionary lesson: Don’t connect your spandrels too tightly. The reason is that this can increase the bending moment in parts of the column that weren’t intended to carry it. I crudely sketched what I thought they’d described and showed it to them, and I’ve made a version here for you to see.
This can be a particular problem with older buildings. In fact, said Charles, some earthquake strengthening solutions work by weakening the connections between columns and spandrels, allowing for greater independent movement of the elements of the building. For another example where weaker = stronger, see the notes on this site about the General Library retrofit.
Thanks a million, and what next?
We really enjoyed the visit. For a novice like me, it was highly informative, and I hope others picked up a few of the finer points that I missed.
We’re extremely grateful to Hugh Morris and Charles Clifton for their time; to Mike Renwick for his generous enthusiasm; to Martin from the UoA and Wayne from Hawkins for their patience.
The possibility of future site visits to the construction of the new building has been floated, and the signs look positive for that. The new building structure is long span composite floors on steel beams with the lateral load resisting system being a buckling restrained braced frame. It is very different to the old structure, which will be a memory only within the next month. Watch your inboxes for future opportunities to visit.
(Edited 11-7-17 with amendments by Charles Clifton.)