This week NCE asks: Is it still acceptable to group lifts, services and escape stairs inside one central core on tall buildings?
The first official report into the World Trade Center disaster recommended redundancy in escape stair provision. In the recent incident in Milan, where a light aircraft crashed into the Pirelli building, occupants were able to escape down well separated emergency stairs outside the area of impact.
Faith Wainwright, Arup director
The World Trade Center disaster has caused us all to think hard about the best actions to take to make tall buildings safer.
However remote the chance of an event that brings extreme impact, blast or fire loads, we all need to feel that the buildings we occupy afford as much protection as possible.
We cannot expect to save those unfortunate enough to suffer the immediate impact of a disaster, but the thought of being away from that immediate impact zone yet then being trapped is an alarming scenario.
Would dispersed stairs and lifts in a tall building be preferable to a central core? I say no. A central core in a tall building is usually large, and can be designed with a structure that gives good protection to the shafts.
Separate cores would individually be smaller and on a like-forlike basis more vulnerable.
Splitting them would also tend to push cores towards the outside of the building - in fact, the most vulnerable place if you consider an airplane impact, as well as restrictive on the planning and use of the building.
There are things that can be done. Core walls can and should be of robust impact resistant construction, ie. reinforced concrete not dry wall sheet construction. Arranging the entrances to the stairs to be separated so they are accessed from different sides of the core is a good idea - often also good planning in order to minimise escape distances.
Other measures that aid resilience are pressurised shafts, redundancy in the mechanical and electrical support systems, blast resistant doors to lobbies, and voice alarm and information systems that can actually be understood.
Above all, it is important to remember that we are designing a building to be safe and to be used, not simply to be a bunker.
The decision whether to separate cores has to do with the constraints of the site, the building usage, plant distribution and so on.
Split cores can be used successfully in many buildings, but it is not a change that on its own will make a tall building inherently safer.
Lawrence Webster Forrest, principal fire engineer Kevin Thorp
Prior to 11 September, would such a question have been asked? Even now, an aircraft crashing in to a building has a low probability for most tall buildings, but a terrorist attack, in whatever form, has a higher probability. So why make it easier for the terrorists to achieve their aim?
A major problem with high rise buildings is the time taken to evacuate. Safety features are designed into the building to compensate for this, by giving early warning of fire, means to suppress the fire and protection of the escape routes so that these are kept clear of the effects of fire for long enough for the occupants to escape.
In the UK, our fire strategies have generally stood the test of time in that, while our occurrence of high rise fires is statistically similar to other countries, bearing in mind the number, size and type of high buildings, we have not experienced the major losses of life there have been elsewhere. Much of the reason for this is as a result of our fire and construction codes, our testing and approvals process and legislation and our robust approach to fire engineering.
In all but the smallest of buildings, it is a basic principle of means of escape to provide an alternative, fire separated route and, although not a requirement, where stairways are protected, to discount the one with the largest capacity, in calculating occupant loads for evacuation.
We should also consider the need for the emergency services to enter the building while everyone else is leaving.
Putting all the services into a central core means that a strike at the heart has the highest disaster potential. Before designing buildings to a central core standard we should apply risk analysis and ask the question what if?
Top of the list of questions would be what if the central core became unusable? 'What ifs' about the level of fire protection also need to be asked - what if the level of passive or active protection are made redundant?
Designs can then be produced which take account of the risks to the building
World Trade Center Building Performance Study:
Data Collection, Preliminary Observations and Recommendations was prepared by the American Society of Civil Engineers for the Federal Emergency Management Agency. In Chapter 8: Observations, Findings, and Recommendations Section 188.8.131.52. i, it lists design
features of the twin towers which might have made it impossible for victims on or above the impact floors to exit safely. These include 'grouping emergency egress stairs in the central building core, as opposed to dispersing them throughout the structure'.
For links to the report go to www. nceplus.co.uk/magazine
On 18 April, a single engined four seater Rockwell Commander 112 struck the 26th floor of Milan's landmark Pirelli tower, killing four. One wing lightly damaged the central lift shaft and a fire was started by fuel from the plane's ruptured tanks. Occupants not injured in the initial impact, including those on the floors above, were able to move away from the fire and exit via stairwells at each end of the building.
The average speed of an evacuee down one storey with normal headroom of 2.8m per floor is about 16 seconds. Evacuees will normally experience fatigue while travelling in a downward direction after about 300 seconds and 60 seconds in an upward direction.