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'Toblerone' beams help ISG meet Yorkshire office demands


Contractor is employing a clever stitched long-span solution that sees steel stuffed with concrete on an innovative new headquarters in Leeds.

Project: TPP headquarters, Horsforth, Leeds
Client: TPP
Region: Yorkshire & the Humber
Main contractor: ISG
Architect: Bowman Riley
Structural engineer: Curtins
M&E subcontractor: Silcock Leedham
Steelwork subcontractor: Harold Newsome
Concrete subcontractor: Treanor Pujol
Completion date: July 2015

It has not been long since Frank Hester decided to set up his own company to deal with IT issues in the NHS, having seen his GP wife struggling daily with IT connectivity.

Some 16 years later and TPP, the business he founded to deliver increased healthcare IT performance, has flourished – to the extent that its headquarters now need much larger premises.

Main contractor ISG is overseeing the construction of its new 8,500 sq m head office in the village of Horsforth, on the north-east outskirts of Leeds.

The facility will be large enough to house up to 650 staff and has been designed with a traditional mill-style stone façade on its front elevation, in a move to help the building blend sympathetically with the surrounding architectural vernacular.

“The Deltabeam sections are a bit like Toblerone chocolate”

Anthony Mitchell, ISG

Its rear façade, however, features a more contemporary look, with a glazed pavilion-style structure.

“The building is in two parts, really,” ISG project manager Anthony Mitchell says. “It will have the look of a very traditional mill building from the front, but TPP wanted a modern, contemporary office feel at the back of the building.”

Although the two-storey ‘pavilion’ will look very different to the ‘mill’ building, the structural frame itself is similar throughout.

A steel-framed structure launches off a two-storey cast-in-situ reinforced concrete basement that doubles as a car park.

On the pavilion side of the building, the steel frame rises two storeys above the basement, with the mill reaching up five levels above the reinforced concrete slab.

Long-span solution

With the client specifying big, open floor plates throughout the building, the design immediately pointed towards a steel frame to accommodate the larger spans.

But some of the spans at Horsforth reach 17.5 m, which is long even for standard structural steel sections.

This meant the project team opted for an unusual composite steel/concrete beam solution that would allow the slim slabs that were needed, while meeting the open floor plate requirement of the client.

Deltabeam, manufactured in Slovakia by specialist company Peikko, is a truncated triangular hollow steel section, which when filled with concrete acts as a composite beam that is ideal for bridging large spans, while maintaining slim decks to help maximise storey height.

“They are a bit like Toblerone chocolate,” Mr Mitchell explains. “The beams are hollow triangular sections, which get filled with wet concrete and tie into the slabs.

“They give the opportunity to have the entirely open-plan office space the client is looking for.”

The Deltabeams are coupled with hollow-core precast concrete panels 1.2 m wide, 500 mm deep and as long as 15 m.

The largest panels weigh as much as 12.5 tonnes and are supported on a flange that protrudes from either side of the bottom of the beam.

Stitched planks and beams

Stitching bars of 16 mm-diameter steel reinforcement are fed through the hollow-core planks and into the Deltabeams.

“It’s a very clever system. The stitching is key to bringing the whole process together”

Anthony Mitchell, ISG

The beams are then filled with concrete; here, a standard C40 mix has been used, which ties everything together and ensures the whole slab and beam acts as a diaphragm.

“It’s a very clever system,” Mr Mitchell says. ““The stitching is key to bringing the whole process together though. The planks sit around 70 mm onto the flange, which gives just enough room to concrete around them and get the Deltabeam cast.

“That does mean the Deltabeam and slabs have to be supported until the required concrete strength gain has been reached.

“Generally, the beams can then be loaded after seven days, which can have an effect on planning the rest of the site activity around the installation of the slabs.

“Until the stitching has gone in and the concrete has reached its seven-day strength, the self-weight can be too high, so it has to be supported.

“It means we have to sequence work around it and that has an effect on what we can do and when. It is essential to get the whole floor plate in as quickly as possible.”

Contemporary-traditional mix

The pavilion section of the building will feature floor-to-ceiling bi-fold doors opening out onto a 1.2 m-wide balcony that runs around the rear and side elevations.

The façade for the mill building is altogether more traditional and features locally quarried stone blocks. These large blocks measure between 225 mm and 290 mm deep and will take time to bring up to the full height of the building.

That has seen the team install a watertight board on the inner leaf of the mill building’s front and side elevations in a bid to move the brick and blockwork from the critical path.

“It’s a steel-framed system skin on the edge of the precast frame,” Mr Mitchell says. “We have lined it with an all-weather defence board to create a weatherproof line.”

Across the rest of the mill building the apex roof will be a huge expanse of slate, once again intended to help the structure blend into its surroundings.

The completed building is set to hit its BREEAM Very Good target and features natural ventilation throughout, with particular focus on the thermal mass of the concrete slab’s role in helping to keep the finished building cool.

“There’s a fair-faced finish on the precast concrete planks that is exposed,” Mr Mitchell says. “It means the building benefits from the heat sink properties of the concrete.”

Traditional Yorkshire downpours aside, next summer should be the test of that theory, with the team due to hand over the completed scheme by July 2015.

Critical co-ordination

With precast concrete and steel sections being used across the development, the team employed a tower crane to feed material across the site, with some of the outermost Deltabeams lifted into position using a mobile crane.

The Deltabeams themselves are temporarily stored at the edge of the site in a section of the basement, before being lifted into position, fixed and cast.

Even with the beams travelling all the way from the Peikko manufacturing base in Slovakia, the site team has managed to co-ordinate all the deliveries so that the beams have arrived at the right time and in the right order, thanks largely to a foreman dedicated specifically to co-ordinating the call-off of the beams.

“It takes around two weeks for the beams to be delivered,” Mr Mitchell says. “We have a foreman who looks after all the delivery details.

“When you have deliveries coming from other countries you have to give plenty of notice for everyone involved and stick to what you say.”

The mill building, too, was constructed using the central tower crane, with the team pushing its build in an effort to gain time on the project’s timetable.

“We worked on the basis that we wanted to get the mill building up first, with the pavilion following,” Mr Mitchell says.

“It’s all about pre-planning. We were buying time at the front of the project so that we knew there would be time if we got behind later on in the scheme.”


Mass-slab basement

The site of the TPP headquarters falls significantly from the mill building at the top to the pavilion at the rear, which required a significant cut to locate the double-storey basement car park.

An additional area for car parking was requested by the client, situated beneath the front of the mill building and outside its footprint, thus increasing the amount of cut and reducing the lay-down space.

Despite its scale, the building is supported on mass slab foundations cast directly onto the underlying rock. This slab measures up to 2.5 m thick beneath the tower crane, but it thins to 1 m across the rest of the site.

“The basement features reinforced concrete walls sitting on a mass concrete slab,” Mr Mitchell says.

“In places we had to go down to 4 m to reach the ground-bearing strata. That was further than we initially thought. In the end we found that across around half the site we had to excavate further.”


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