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High tension

30 Sep 20 Post-tensioned concrete has been used to finesse the structural design of Warwick University’s new arts faculty building. David Taylor reports

Boasting what is reputed to be one of the world’s top 50 arts faculties, it’s only natural that the University of Warwick wants a building that matches the faculty’s exalted status.

At present, the university’s seven arts departments are housed in two separate buildings but, as part of a five-year development plan for the campus, these seven departments are soon to be brought together under one roof.

This article was first published in the Jul/Aug 2020 issue of The Construction Index magazine. Sign up online.

Designed by renowned architect Feilden Clegg Bradley Studios and engineered by consultant Buro Happold, the new building is nothing if not ambitious. Comprising four wings of varying heights and intersecting at various angles, the building’s main entrance opens onto a huge open-plan area featuring a grand central staircase rising through multiple cantilevered and mezzanine levels.

The building is being built by main contractor Bowmer & Kirkland and is due for completion in time for the 2021-22 academic year. 

The feature staircase rises from a large entrance lobby to second storey level and is just one of a complex series of intersecting bridges and staircase connections linking different levels and wings. 

The building is set out on a 7m grid and the original structural design features 300mm-thick reinforced concrete (RC) slab floors with an insitu reinforced concrete frame and a number of large steel plate girders at the lower levels to support the structural loads and allow the long spans required by the design. 

Steelwork being fixed for a large transfer beam at Level 1
Steelwork being fixed for a large transfer beam at Level 1
 But while the proposed structure was fine from an engineering point of view, it fell short of the architect’s vision of a light, open space. With Fielden Clegg and Buro Happold retained by the client, main contractor Bowmer & Kirkland employed Cambridge-based architect MCW Architects to take the project through to realisation on a design-build basis. 

The concrete frame subcontract was awarded to Whelan & Grant which proposed the substitution of post-tensioned (PT) concrete floor slabs and transfer beams instead of the heavy reinforced concrete slabs and fabricated steel plate girders.

This was a job for a specialist, and so Whelan & Grant brought in Leeds-based CCL, one of the UK’s leading experts in the field, to design the post-tensioned structure. “There was absolutely no post-tensioned concrete in the original design,” says CCL’s technical director Matthew Gilliver. “It was all traditional RC frame with some special steel beams. I guess the change to post-tensioned beams was mainly to reduce height and improve sightlines in lecture theatres, although some of it was value engineering. But mostly it was necessary to satisfy the actual architectural needs,” he says.

“The main features of the building are the transfer structures and the central atrium where there were going to be some steel beams crossing over left, right and centre,” explains Gilliver. Here, post-tensioned concrete had the advantage. “Basically, you can span further with a thinner PT slab than an RC slab because the tendons give you better deflection control. You get more strength for your money and you compress the slab, which exploits concrete’s compressive strength,” he explains. 

CCL’s re-engineered design also offered the benefits of reducing costs, complexity, material consumption and the number of trades involved, bringing significant value to the project. 

“If this had been traditional RC frame, Whelan & Grant would have supplied all the formwork, the concrete, the reinforcement and the labour to fix the reinforcement and they’d do all the concrete pours. They don’t have any design responsibility,” says Gilliver; structural design remains the consulting engineer’s responsibility. 

“When it goes to PT, they [Whelan & Grant] still do all of those things and all we are is like a sandwich filling: we supply the PT design, the reinforcement schedules and the design under them as a subcontractor. And then we actually, on site, install the PT tendons and do the stressing of them,” Gilliver explains.

Besides resulting in a more slender, lightweight structure, the use of PT concrete slabs and beams allowed nearly all structural steel to be eliminated from the design.

The large fabricated steel plate girders specified in the original design were concentrated in the lower levels of the building where they acted as large transfer beams to distribute loadings across long spans. 

In one of the two wings located at the front of the building, the first level up from the ground-floor slab is a mezzanine originally designed as a conventional reinforced concrete slab with steel members supporting the lecture theatre areas. CCL’s brief at this level was to eliminate the steel elements. 

“It was mostly about removing the extra trade. With the existing design, you would have finished the concreting and then have to wait for the steelwork to be connected,” explains Gilliver.

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The CCL team switched the conventional 400mm RC slab to a 300mm PT slab which was not only thinner but stiffer, enabling the steelwork and two concrete columns to be removed from the specification as well as meeting the acoustic requirements for the auditorium. 

“Steel beams originally went across the top of the lecture theatre. They were plate girders which would have had to be designed and fabricated separately. We also realised that if you sat at the back of the lecture theatre and looked down, the beams would then be in the line of sight. So when we went for the PT alternatives they asked us to make them shallower but wider,” explains Gilliver.

Underside of a large transfer beam after casting, showing exposed soffit
Underside of a large transfer beam after casting, showing exposed soffit
 “In terms of mass the PT beams definitely weigh slightly more [than steel] because concrete’s heavy and chunky,” he concedes. “But it would have weighed more overall. We’ve thinned the PT floor slabs quite a bit so we’re reducing the loads coming down.”

A prominent feature of the building’s front elevation is the 3.5m cantilever overhanging the ground floor façade. The columns supporting the upper storeys are located above this cantilevered space and so, to enable the load from the upper storeys to be transferred through to the ground floor slab, a series of post-tensioned transfer beams was required at 7m intervals for the front elevations. 

Level 1 was perhaps the most complex slab layout because of the requirement for 16 transfer beams to carry the load from the upper storeys through the structure to the ground floor slab and the piled foundations. 

Post-tensioning allowed most floor slabs to be reduced in thickness from 300mm to 220mm. “But at Level 1 we kept the slab at 300mm because below that was the auditorium and the client wanted to make sure the sound didn’t spill out,” Gilliver says.

The complex architecture meant that each storey was designed according to unique criteria, considering the structural loads and forces present during construction and post-completion. Each transfer beam therefore had to be designed individually to address the structural load requirements of its location. 

With the layout of the building changing from floor to floor and each of the four wings changing independently of the others, there was a need to consider further changes in the building structure at different levels.

For example, where one wing ceases to have a direct connection to the core of a neighbouring wing, it was necessary to cast a reinforced concrete beam along the outside edge of the floor slab to create a portalised moment frame for lateral stability. 

Throughout the building, CCL was responsible for incorporating ‘pour-strips’ to prevent a build-up of restraint from the cores as the PT slab for each level was poured and stressed. 

“With post-tensioning you apply a compressive force to the slab – you tension the cables and they basically squash the slab ever so slightly,” explains Gilliver.

Transfer beams cantilever out 3.5m over the main facade and carry the columns for the structure above
Transfer beams cantilever out 3.5m over the main facade and carry the columns for the structure above
 “But in order to do that, you have to let the slab move. If it were connected to two cores, one at either end, the slab wouldn’t squash because the cores would take all that horizontal force. 

“So we put a pour-strip in by leaving out a metre strip of concrete within the slab between pours. Then when we tension each one separately – the compression gets into the slab and then we fill in that strip later.”

Lockable dowels were considered as a quicker and easier alternative to pour-strips: “This is a proprietary product; you cast a bar into one side of the pour and a socket into the other side of the next pour and the bar is free to move inside that socket until such time as you fill it with high-strength resin,” explains Gilliver. “But they can only be used in certain locations and they have a limited load capacity. They also leave a line on the soffit that wouldn’t suit the architecture on this project,” he explains.

Construction of the concrete frame is progressing to schedule and CCL is due to complete its work by the end of September. 

Nothing these days escapes the dead hand of Covid-19, but according to Gilliver the impact of the virus on this project has been relatively mild. “It’s a university and during the lock-down there were no students around, which helped,” he says. 

“The site stopped for about two weeks while they did an assessment and got advice from the government and Build UK. Then they returned with a reduced labour force. I assume the overall programme must have slipped at some point but I do know that the programme to completion – certainly for Whelan & Grant – has stayed the same. They’re just doing a lot more smart-working with the people they have.” 

This article was first published in the Jul/Aug 2020 issue of The Construction Index magazine. Sign up online.

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