黑洞社区

鈥淚n the past five years, the post-tensioned flooring industry has revolutionised itself and can now build very quickly,鈥� says Bill Price, director of structural engineer WSP Cantor Seinuk, a firm that regularly specifies both types of flooring solution.

There are two principal reasons for post-tensioned flooring鈥檚 popularity. It is ideal for the booming high-rise residential sector and, because of escalating steel prices and supply problems, it is increasingly specified for non-residential schemes too. 鈥淲e were working on a hotel in London Docklands using a steel-framed solution,鈥� says Price. 鈥淎t Christmas they said scrap this and do it in reinforced concrete using post-tensioned concrete floors 鈥� apparently because of a 60% increase in steel costs and increased lead-in times.鈥�

Kevin Bennett, technical manager of post-tensioned flooring specialist Structural Systems agrees that demand has taken off. 鈥淎t the end of this year there will be six hospitals built with post-tensioned flooring; at the end of last year there were two and the year before that there were none,鈥� he says.

Post-tensioned concrete is perfect for high-rise residential schemes, according to Price, who has worked on several such projects. He says the thin floor slab means an extra storey can be gained on a tall building 鈥� in fact, two extra storeys were gained at a scheme at West India Quay, London Docklands. The concrete slab provides good acoustic separation between apartments and a flooring solution that creates very little dead space between floors is ideal as apartments have minimal servicing requirements.

Composite metal deck flooring is still popular for offices. The large void between the steel beams supporting the composite steel and concrete deck is usefully employed to run the complex services needed in offices. The increasingly large span between columns in offices is also best spanned by composite floors. 鈥淎 13.5 m span in concrete gets a bit frightening,鈥� he says. 鈥淵ou would need a slab between 300 and 350 mm thick, which makes the building heavier so you need bigger, more expensive foundations.鈥�

However, this doesn鈥檛 rule out post-tensioned flooring for offices. The recently completed Royal Bank of Scotland鈥檚 headquarters building in Edinburgh has post-tensioned floors. 鈥淲hen the value-engineering exercises were done, post-tensioned concrete was deemed to be the most cost-effective answer,鈥� says Price.

Composite metal deck flooring

Steel beams support steel profiled-metal sheeting that initially acts as shuttering for a layer of concrete and ultimately in combination with it to form a structural floor. The profiled-steel deck takes tensile loads and the concrete takes compressive loads. Steel mesh is usually incorporated within the concrete layer to prevent shrinkage cracking, and it also helps increase the fire rating of the slab to two hours, so fire protection doesn鈥檛 need to be added under the deck. Slabs are typically 130 mm to 150 mm thick, with thicker slabs used for very heavy loads. The steel beams add substantially to the overall floor depth and vary in size according to the design.

Although this flooring can be used for large spans, composite metal deck floor spans are typically less than 4.5 m, as the temporary propping that is needed for larger spans would slow down construction time. Large quantities of the very light decking can be delivered in one go. They are then craned up to each floor of the building where the sheets can be positioned by hand. These are secured in place using shot-fired nails. There is then the option of welding shear studs in place through the deck to the beam linking the concrete directly to the beam. This creates a composite beam and means smaller steel beams can be specified. Reinforcement mesh is then laid followed by the concrete.

Post-tensioned concrete

Post-tensioned concrete flooring works by putting the whole floor into compression laterally through the slab to counteract the tensile loads. This works in the same way that the compressive forces generated by squeezing a pile of books together stops the ones in the middle from falling out. This approach means only 20-40% of the amount of steel used in conventionally reinforced concrete floors is needed to resist the tensile loads.

The floor is built in tandem with a concrete frame. Formwork is placed over finished columns and ducts containing steel tendons are laid over the formwork in a grid-like pattern. The tendons are positioned where the tensile loads will occur in the finished slab 鈥� at the top of the floor slab over the columns and at the bottom of the slab between columns.

Conventional reinforcement is needed around the columns to prevent the columns from punching through the slab, and also to prevent cracking and improve fire resistance. Reinforcement is also needed at the perimeter of the slab to distribute the tensile point loads evenly along the edges. Special anchors allowing the tendons to move in one direction only are fixed at the slab edge. The day after the concrete is poured, jacks are used to apply 25% of the final tensile load to the slab and three days later the tendons are fully tensioned. Finally cement grout is poured into the ducts to bond the tendons in place and prevent corrosion.