A Perfect Partnership: Precast Concrete and Steel Work Together for B&Q
When B&Q planned a major project to build its two-storey superstore in Stevenage, Hertfordshire, one of its requirements was that the main sales space had to be at first floor level and needed to be capable of supporting heavy racking systems or a uniformly distributed load of 20kN/m2 for the storage and display of its goods. In order to deliver this high structural strength, precast hollowcore flooring units from Tarmac Topfloor were used in combination with a steel frame superstructure from Barrett Steel Buildings.
To help deliver this retail build, Barrett Steel Buildings, the steelwork design and build specialists for Simons Construction, (the project’s main contractor), turned to Tarmac Topfloor to supply prestressed concrete hollowcore flooring units to create the main sales floor and flooring for ancillary areas of the B&Q store. Around 6,000m2 of different precast flooring units were manufactured at Topfloor’s Weston Underwood site.
Grand Designs
To ensure that B&Q’s requirements for the two-storey structure were met, the design of the steel frame and concrete flooring units was an important stage, which posed a number of design challenges. Jonathan Davis, Barrett Steel Buildings’ associate design director, explains: “What was unusual about this B&Q superstore was that the main sales floor had to be located at first floor level and needed to withstand a large uniformly distributed load and ‘point loads’ from heavy racking systems with 4.5 tonne leg loads together with wheel loads from 650kg fork lift trucks. Essentially, the first floor design needed to replicate the typical ground floor scenario.”
“B&Q also specified tight deflection criteria, which means that the amount of floor movement when loads are placed on it is lower in comparison to the usual criteria for a floor of this span. Ordinarily, the overall floor deflection would be around 50mm, but B&Q’s criteria restricted it to 10mm, so our design had to be adapted accordingly.”
“It was decided that the building would be a beam and column structure on the lower level with a traditional portal frame above. The superstore, measuring seventeen metres high, was to have a ground floor elevation of eight metres and the 400mm deep precast units would be installed above this.”
“We used a ‘composite beam’ design between the steel and precast units as this was considered to be the most efficient method of construction,” continues Davis. “To deliver this, the steel beams were designed with pre-welded shear studs and ‘ties’ so that they could be attached to the precast beams - a design feature which, importantly, will provide the building with resistance to progressive collapse.
“We looked to Tarmac Topfloor as their hollowcore flooring units were capable of withstanding a large uniformly distributed load and punching forces from the racking system. Furthermore, their strong technical support and design services were crucial to the success of this project, so for us this was a strong aspect of their offering.”
A good working relationship between Tarmac Topfloor and the design team at Barrett Steel Buildings was imperative to ensure that such a detailed design programme was met. Geoff Bailey, chief engineer at Tarmac Topfloor, explains: “The 400m-deep concrete hollowcore precast flooring units, which formed the main sales floor, were specially designed using an insitu concrete structural topping. The idea behind this was to allow the topping to be used to distribute the concentrated loads into the hollowcore flooring units as well as stiffening and strengthening both the units and the supporting beams. Importantly, we designed the prestressed units so that they were able to support various combinations of loads including construction materials, mobile elevated working platforms and materials handling equipment during the construction phase, all before the structural topping was laid.
“By using the topping in this way, there were other practical benefits. The weight of individual precast units would be kept to a level that meant they could be easily handled and erected on site. Once erected, the units provided the builder with a working platform for materials and equipment for the completion of the superstructure.“
Composite actio
Installation of the hollowcore beams in tandem with the steel frame was also crucial to the delivery of this project’s design.
Bailey continues: “During manufacture, the precast units were ‘roughened’ on top by brushing to achieve the best possible bond between the topping and the precast units, so allowing ‘composite action’ to develop between the two components of the floor. In doing so, the floor is given the required strength, stiffness and shear resistance to meet the loading specified by B&Q.
“By attaching the concrete units to the steel frame we were also able to increase its stiffness. Pre-welded shear studs on the steel beams were used to connect in situ concrete filling around the slab ends to the steelwork, creating one structure. Reinforcement around the studs and within the hollowcore units was then used to connect the in situ concrete filling to the precast hollowcore units. The benefit of this is that the floor does not sit independently on the steel frame but acts with it as one structure, achieving the required strength and stiffness within a reduced overall depth. All of this is done at the relatively small cost of the special work on the steel beams and units and the additional reinforcement.
Working to new heights
The onsite installation process undertaken on this retail build was also unusual.
Rob Morton, operations manager at Tarmac Topfloor, said: “Normally when installing units at first floor level, the installation process does not involve tying the steel and hollowcore flooring units together. However, in this project, to ensure that both Barrett Steel’s requirements for the deflections of the steel beams in the completed building were met and that the specified ‘robustness’ under the relevant Building Regulations for large public buildings were also achieved, this method was a good solution.”
By undertaking detailed discussions with Barrett Steel Buildings at an early stage, Tarmac Topfloor was also able to iron out a number of project complications to promote onsite work efficiencies.
Morton continues: “During early consultation with Barrett Steel Buildings, we were able to resolve a number of expected onsite difficulties. One such problem was how to lift the precast flooring units into the building. To overcome this, it was agreed that the purlins would be left off the roof and the hollowcore flooring units could then be lowered through the roof to the first floor level”.
“Significantly, this was one of a number of larger projects where we used a ‘positive lifting system,’ to install the precast beams. This meant that instead of the precast beams being suspended by chains, they were lifted with a device that clamped onto the edge profile of the beam.”
Complementary system
The success of the B&Q project demonstrates that precast concrete and steel can provide a complementary system to deliver retail buildings.
Jonathan Davis at Barrett Steel Buildings concludes: “ This project has demonstrated the success of using the composite beam design between the concrete units and steel beams, and will act as a benchmark for future developments. Tarmac not only had the right products to assist us, but their technical support was also highly beneficial. In fact, we are working with Tarmac Topfloor on future work with B&Q at the moment.”
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28 November 2007
