Feature

Grafton Architects' Kingston University

Grafton Architects’ hybrid concrete structure sets the stage for some monumental spaces at Kingston University, writes Tony Whitehead

It is a slightly odd but undeniable fact that a university’s status is profoundly tied up with the type of property it occupies - red-brick, plateglass or even ivory tower. So it’s understandable that when universities commission a new building, they are particularly keen to get it right. 

The new Town House at London’s Kingston University, for example, is the sort of high-profile mixed-use building that has in recent years become increasingly popular with academic institutions. Its six storeys house lecture theatres, study rooms, dance studios, an auditorium, a library and a cafe.

But it is more than simply a space for students to meet, study and recharge laptops. It is a signature project – intended to become the building that people primarily associate with the university. It is now Kingston’s “front door”, its distinctive presence in the town, and the medium through which it seeks to engage with the local community. 

All of this is reflected in the distinctive design from Pritzker Prize-winning Grafton Architects. “The client really wanted no barriers between itself and the town,” explains Grafton director and project architect Gerard Carty. “So the colonnades of the outer layer give the building a monumental style and a civic presence, but they are also permeable. Passers-by walk underneath and through them, and they can see in. There’s no ‘them and us’.” 

This dramatic concrete colonnade is certainly striking, and its brutalist echoes firmly associate Kingston with the democratic new accessibility offered by the universities and polytechnics founded in the 1960s and 70s. The choice of concrete as the main construction material is absolutely central to this effect, evoking the iconic campuses of, for example, Lancaster and East Anglia.

Interestingly, the building was originally conceived by Grafton as an in-situ concrete frame with precast colonnades, but this was changed to a predominantly precast hybrid structure after discussions with the client, contractor Willmot Dixon and structural engineer AKT II. “Originally the interior frame had in-situ slabs articulated with beams to reduce the depth needed for the larger spans,” says Carty. “The overall shape and design didn’t change much with the switch to precast, but it did involve a seismic shift in our thinking about how to deliver it.”

As Carty says, one of the key concerns for the architect and AKT II was the long spans required to create some of the interior spaces: “For example, there’s quite a large double-height lecture theatre and a courtyard-cum-auditorium which is triple height and requires spans of more than 15m. We had to find a cost-effective way of spanning these spaces which stayed true to the design intent.” 

The solution for the interior frame involves 550mm2 precast columns arranged on a 6.4m grid running along the front of the building. A similar arrangement supports the rear of the structure and these shorter spans are bridged mainly by pre-stressed, hollowcore slabs 225mm deep. But in between, and where longer spans are required, these are achieved using 229 precast, pre-stressed “double T” beams cast in C60 concrete and weighing up to 18 tonnes each.

As Carty admits, the double T system is more usually to be found in multistorey car parks: “But in fact they are very efficient and several advantages result from using them,” he explains. “Firstly we can vary the depth of the downstand part of the T depending on the span and loading involved – so we have three variations: 610mm, 810mm and 1,010mm-deep including the ‘tabletop’ formed by the TT.” 

The depth of the tabletop is 60mm and above this is a 115mm-deep reinforced structural screed. This effectively creates a hybrid precast/in-situ slab of 175mm above integral downstands which vary between 550mm and 950mm depending on the space they are spanning. 

“Being pre-stressed and variable in depth means you end up with what we call a bony design – very efficient in terms of material usage,” says Carty. “Another environmental positive is that they are made with standard steel moulds that can be used repeatedly, so there is none of the waste that would result from throwing away used timber forms.”

Kingston's hybrid structure: A kit of concrete parts

Above the in-situ ground-floor slab, all the concrete construction for Town House was carried out by PCE, a company that, while it does not manufacture precast elements itself, has developed a specialisation in designing, procuring and installing them.

So at Kingston the majority of the internal frame, including the double-T beams, was manufactured  by Banagher Precast Concrete, which appropriately enough, considering some of the long spans involved, has a particular expertise in pre-stressed bridge elements. Meanwhile some of the core sections were made by Shay Murtagh, and the architectural elements for the external colonnades were supplied by Techrete.

“We take a kit-of-parts approach,” explains PCE’s business development manager, Simon Harold. “We are focused on buildability and are happy to use hybrid systems – so although most of the concrete at Kingston University is precast, the slabs are actually composite – precast beams with an insitu structural screed.”

As Harold explains, the frame assembly technique is focused on speed and accuracy: “The internal columns have cast-in shoes and these feed onto bolts in the column or beam below. Once in place, the crane is released for the next job, and the column is fine-adjusted for verticality using the bolts. The mechanical connection is then encased in structural grout and the whole shoe is hidden within the raised floor above the slab.”

Similarly, sleeves within the beams are fed onto dowels or bolts on columns and then grout-stitched together. 

“At Kingston the structural screed reinforcement was tied into reinforcement projecting out of the top of the double-T slabs to enhance the composite action,” says Harold. “We also needed to tie the screed into the stability elements of the building - the three cores – so these had Kwikastrip cast within them to make it easy to get the reinforcement out and connected up to the mesh in the screed.” 

PCE used ground granulated blast-furnace slag (GGBS) cement substitute in the mix to reduce the embodied carbon of the concrete. “For most of the elements we were able to use a relatively high proportion of GGBS, 40%,” says Harold. “That would normally slow curing times and present problems for our suppliers. However, they were able to compensate for this by making adjustments to the super-plasticisers used, and by keeping the moulds warm.”

In all, the interior frame of Town House comprises 229 double-T units, a further 318 beams, 275 columns, 50 solid wall units and 52 core sections. The exterior frame required a further 300 beams and columns, together with 175 mullion and spandrel panels. Despite these daunting numbers, an impressive time-lapse video on PCE’s website shows the building’s structure being constructed in a little over 12 months.

The Ts also perform a vital acoustic function: “Since we have in the same building a dance studio with loud music and also a library, it was essential to have sufficient sound absorption. The T shape helps break up sound waves as they hit the ceiling and, importantly, we have placed sound-absorbing woodwool slabs between the verticals of the Ts.” 

At first glance, these grey woodwool downstands blend into the grooved soffit created by the double-T structure. Closer examination reveals that the aluminium-framed woodwool slabs also carry features such as lights and smoke detectors.

The grooved aesthetic of the ceilings is repeated elsewhere in the building. In the ground-floor auditorium, for example, walls are formed from vertical exposed concrete mullions with more woodwool sound absorption placed between them. 

The auditorium itself features a kind of amphitheatre, the giant steps of which can be used as meeting space or, when the auditorium is closed off by sliding doors, as seating for performances. In keeping with the utilitarian design of the ceiling, these steps are formed from football terrace technology – simply L-shaped precast elements placed one on top of the other. “We decided that bare concrete might be a little uncomfortable,” says Carty, “so we’ve provided timber tops to the steps in maple, which is kinder to sit on.”

Outside, the building envelope is created by walls of brick interspersed with architectural concrete mullions that frame the windows and stiffen the walls. “For the exterior concrete we developed a special mix with the supplier, Techrete,” says Carty. “The planners wanted the building to be in sympathy with Surrey County Hall opposite, which features Portland stone – so the mix for the exterior mullions and the colonnades has a pigment to create the Portland colour, and a white dolomite aggregate with a grit-blasted finish to give it an attractive stony texture.”

Town House’s colonnade comprises some substantial elements – many of the columns are 12m high. However, it remains structurally independent from the interior frame, being simply tied back to it. The resulting absence of cantilevered slabs meant there were no issues with inserting insulation at the slab edges to prevent cold bridging. 

The precast terrace slabs supported by the colonnade provide useable outdoor space for students and contain planters from which climbing plants are already growing. The plan is that vines will eventually entwine around most of the exterior columns, softening the architecture with greenery, and adding, perhaps, a touch of Ivy League style to Kingston University’s new high-street presence.

Queens of cool: Grafton, concrete and thermal mass

Concrete has featured strongly in many of Grafton Architect’s award-winning designs – including the UTEC University Campus in Lima for which it won the inaugural RIBA International Prize in 2016 (CQ 259).

“The root of our liking for concrete probably stems from our early days when we were doing a lot of secondary schools in Ireland,” explains director Gerard Carty, who has been with the practice – led by Pritzker Prize winners Yvonne Farrell and Shelley McNamara – since 1992. “Inexpensive, basic, concrete framed structures with exposed blockwork walls were the order of the day. We learnt to value its resilience and robustness as well as the way that exposing the concrete cuts down on finishing and allows its thermal mass to be put to work.”

This ability of concrete to absorb the excess heat that can be generated in highly populated buildings is particularly useful at Kingston University, where the large numbers of students and computers could easily overheat Town House in all but the coldest months of the year.

“The building skin is well insulated to reduce heating costs in winter, so it’s important to have a way of regulating and releasing heat during the rest of the year,” says Carty. “For this reason we’ve included a Thermally Active Building System (TABS) within the structural screed.” 

This involves a system of pipes that are laid directly on top of the double-T “tabletop” (see main feature) before reinforcing mesh and screed is added. Cold water can then be passed through the pipes when necessary to supercharge the cooling effect of the concrete’s thermal mass.

“We did look at making it a heating system too, by also allowing warm water through the pipes,” he adds, “but the heating requirement was so low we decided it would not be worth it.” 

The deployment of the TABS within the thermal mass of the concrete helped the building to achieve its BREEAM rating of excellent.

PROJECT TEAM
Architect Grafton Architects
Structural engineer AKT II
Main contractor Willmott Dixon
Concrete contractor PCE
Precast frame suppliers Banagher, Shay Murtagh
External frame supplier Techrete

Photos: Ed Reeve; Dennis Gilbert