Specialist cost update: Structures

The structural sector

Structural frames have an increasingly demanding role to play in modern commercial buildings, with the growth in use of larger clear-span floors and increasing numbers of mixed-use developments with conflicting structural grids. However, this has not had a knock-on effect on the selection of the type of structural frame.

The choice of whether to go for a concrete or steel frame is still mainly dependent on the building type and site-specific constraints. Steel, with its ease and speed of erection, still remains popular in the office market, despite price volatility in 2004 and early 2005. Concrete remains very popular in the residential sector thanks to its inherent sound and fire-proofing properties and low cost, although it is slower to erect than steel. One recent trend is the growth of post-tensioned concrete flooring in the residential sector to save space. Another is the emergence on drawing boards of tall diagrid steel tube structures first seen on London's City Hall and Swiss Re. These have the advantage of taking the loads down the perimeter of the building, creating a clear-span space inside the building apart from the core in the centre.

The piling sector remains steady – and because it is the first trade on site, this indicates that the same is true of the follow-on trades, including the frame. Piling is another sector that has seen recent innovations in the light of the drive towards more sustainable buildings. These include the use of piles for geothermal cooling and heating.

Piling Gary Bibby finds that sustainability is the watchword for the piling sector in 2006

Market overview

The market for piling during 2005 was quieter than had been hoped for by the major piling specialists at the beginning of the year. This was because of a number of projects either not coming to fruition or being delayed at some point in the planning process. However, despite the slower-than-anticipated activity, reports are that turnover predictions were broadly met and modest increases in turnover for 2006 are being predicted at between 2-5%.

Several of the major piling specialists are reporting an increase in early enquiries for pricing budgets. However, they maintain that the market still doesn't generally allow the piling specialist to commit to a project early on, thereby enabling them to advise on value engineering at these early stages, which they would prefer.

Hot topics

Reinforcement prices

In the early part of 2005 there were unusual increases in the cost of steel, which affected the reinforcement element in piles. Like other sectors, piling companies were either reluctant or unwilling to commit to fixed prices for this element. However after a downturn and the stabilising of steel supply costs, once again the piling companies are committing to fixed prices for reinforcement.

Piling and sustainability

With the requirement to cut energy consumption, clients and consultants are increasing their interest in the part piling can play in this area. The more forward-thinking piling companies are promoting geothermal piling, as it uses the piles already needed for the building to transfer geothermal energy from the surrounding ground to either heat or cool the building. Steel pipework is fixed within the reinforcement cage and encased in the concrete of the pile, which provides an ideal energy transfer medium.

Ground source heat pump systems are another area where some piling companies are investing by using some of their existing plant. The systems work by boring a deep well in excess of 100 m deep and about 250 mm in diameter that taps into natural groundwater. This is typically between 7°C and 14°C and can provide either cooling in the summer or supplement heating in the winter. The water is extracted from the well and circulated around the building before being returned to the ground or re-used to flush toilets. This can contribute towards significant reductions in energy use.

Lead times

Lead times for bored piling have generally returned to about six weeks. This is the same as at the beginning of 2005, following a slight dip in the middle of last year. The lead times for precast driven piles also remained constant at six weeks throughout the whole of 2005.

Insitu piles under construction. Once the auger piling rig has finished creating the hole, steel reinforcement is put in followed by concrete