Our structural special kicks off by examining the new thinking on tall buildings in the post-9/11 world, before offering tips on fine-tuning dealings with structural engineers and how to gauge costs of fire-protecting steel frames
Designers of Tall buildings have never had so much help. Recently an avalanche of guidance on designing tall buildings has been published. In June the US-based National Institute of Standards and Technology published its recommendations for improving the safety of tall buildings in the Final Report of the National Construction Safety Team on the Collapse of the World Trade Center Towers. And the British Council of Offices has just published Tall Buildings: A Strategic Design Guide. So what do these two publications have to say on designing tomorrow’s tall buildings?
The first thing anyone contemplating constructing a tall building has to consider is how much will it cost, because the taller you go, the more it expensive it gets. The BCO report contains cost models prepared for the publication by cost consultant Davis Langdon. These include the effects of height, plan size and complexity of building form:
- Tall buildings get more expensive the higher you go, because structural frames need to be heavier to take the considerable weight and resist wind loadings – and they are trickier to build than low-rise structures. The amount of lettable space reduces at the same time due to the amount of space that has to be given over to expensive high-speed lifts.
- Large floorplates are cheaper, too, so for a building of a fixed height, the bigger the floorplate, the cheaper it will be per square metre.
- Functional, plain buildings are cheaper to construct than more complex forms, such as the Swiss Re tower in the City of London.
This is a key issue because of post-9/11 worries about the safety of tall buildings. In its guide, NIST has made 30 recommendations, including:
- A call for building standards and codes to be improved to give tall buildings greater structural integrity.
- More reliable means for predicting failure in structures that are subject to multiple hazards.
- Escape routes such as stairs to be large enough to evacuate all the people in the building together and additionally have space to allow for emergency services personnel to go into the building at the same time as people are leaving.
- Designers to site stairs and exits as remotely as possible without making these hard to reach.
- Hardened lifts could be used during a fire or terrorist attack once these become available. Both NIST and the BCO reports say this.
Protecting the structure of a tall building from the effects of fire is crucial if people are to have time to get out, as was made all too clear by 9/11. Both reports make the following recommendations:
- Effective compartmentalisation of floors is essential to stop fire spread through lift shafts, and more fire doors and measures such as thermally resistant windows would also help minimise the spread of fire.
- Better active fire protection systems such as enhanced automatic smoke control and sprinkler systems would also help.
- Effective and robust communications in the event of a fire are essential.
The BCO report has a section on the increasingly important issue of how sustainable tall buildings are. The council supports high densities of increasingly mixed-use activity, which reduces the need for transport – and there is an additional advantage in that tall buildings are generally situated in city centre sites near public transport nodes.
With mixed-use tall buildings, there is also the potential to transfer waste heat from an overheated area to an area that is too cold.
However, tall buildings do need sophisticated facades to minimise energy consumption because of the high ratio of external envelope to floor area.
- British Council of Offices Tall Buildings: A Strategic Design Guide www.ribabookshops.com
- NIST http://wtc.nist.gov