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Users demand more safety provisions

Cover Story: After 9.11

Clients and tenants are driving calls for better evacuation and fire resistance provision in high rise, reports Alan Sparks.

Initial fears that clients would shy away from tall buildings appear unfounded. But as London gears up for its next wave of skyscrapers, more questions are being asked of designers.

'Now our clients want to know exactly what emergency evacuation has been considered, ' explains Richard Thiemann, director at tall building structural engineer Yolles.

'Once an explanation of the level of code requirements has been made, the client has generally felt assured. But this is definitely of greater concern to tenants than in the past. They are also seeking greater reassurance over robustness, ' he adds.

Arup Fire International director Peter Bressington adds: 'Trying to say that the lesson of September 11 is 'It's not a building problem it's a security problem' is just not relieving client anxieties.

'Tenants of tall buildings are demanding to know exactly how long it would take them to get out of the building if anything happens.'

Following the events of last year, Canary Wharf undertook a fire drill to see just how quickly it could complete a total evacuation. Timed at just 18 minutes, the effect on the tenants was no doubt reassuring, although lifts were used.

The use of lifts as emergency escapes has been suggested in the joint ICE/IStructE and ASCE/FEMA reports as a way of improving evacuation times.

But this is forbidden in fires according to current regulations. Firefighter access lift shafts can be used, but these have a higher fire protection, increased water tightness and have protected or back-up power supplies.

To retrofit a public lift shaft would be very costly and disruptive, according to Thiemann.

But there are simple and comparatively inexpensive options open to building owners that will reduce total evacuation times by 30%.

'By improving lighting, ventilation, handrails and most importantly training and information, buildings can very quickly become much safer, ' states US building use and safety consultant Jake Pauls.

Pauls calculated the evacuation time for the WTC at 100 minutes. But in 1993 when there was a bomb in the lower section of the building, it took eight hours for people to evacuate, mainly due to the lack of information available to them.

'Practising these drills allows tenants to know what to expect.

It is normal to have a period of queuing on the stairwells, but unless you expect that some people will panic.'

Bressington reports that many tall building owners are now requesting discreet computer simulations of post traumatic event evacuations. 'We recently studied a 50 storey office block in New York - and discovered a problem.

'Simulations showed that the final exit was fractionally too narrow and people would jam up there. Simply widening this door transformed the evacuation into a potentially smooth and rapid exercise.'

Significant increases in the ability of the building to absorb blast or impact are more difficult and expensive to achieve.

British codes demand that any structure over five storeys must remain standing after the removal of a single column. This is more than is demanded in the US but some designers and clients feel it is still inadequate.

'I know of one steel framed building under construction in London which has significantly increased its degree of redundancy following the events of September 11, ' adds Thiemann.

Popular in Asia is the inclusion of refuge areas in megatowers.

The compartments, often lobby areas, have added fire protection to allow tenants to gather in the event of a fire.

Client pressure can also yield changes later in the procurement process. A London office tower under construction will have more robust fire protection to its steel frame than originally specified. A higher performance Portland cement based coating will be used rather than gypsum-based material - at a higher cost.

But making buildings more robust and more fire resistant can never eliminate the risk of a major catastrophe caused by terrorists, says ICE structural board chairman Gordon Masterton.

'No matter how impregnable a building is designed to be, terrorists can always build a bigger bomb, ' he says.

'As engineers, we must instead ensure that clients understand what is realistic. We do not want to end up chasing our own tails.'

Eyewitness

Dan Cuoco, president of consultant Thornton Tomasetti Group, the first engineering consultant contacted by the City of New York after the disaster.

'We could see it from the roof of our offices so saw it live. I remember thinking it was remarkable the towers stood up after the impact but then I didn't feel there was a risk of collapse. I was just thinking that somehow they'd put the fires out. The building was sprinklered, the Fire Department was there.

'We mobilised around 35 engineers to site next morning, when already there were more than 1,000 working on the site with people climbing over the pile.

There was a lot of overhanging glass, buildings requiring temporary stabilisation. It was initially really a case of cordoning off anything which was ready to come down.

'It was engineering 'on the fly' - designs and drawings by hand.

We'd be told 'a crane must go here', so we used our most experienced people to do quick calculations and sketches which would be sent to steel fabricators, certainly no Autocad drawings.

'Our teams worked 14 hours a day, and the media began to recognise the role played by the engineering profession which went up a few notches in the public eye.'

Eyewitness

George Tamaro, principal, Mueser Rutledge geotechnical engineers, worked for the Port Authority on the design and construction of the WTC 'bathtub' foundations. His firm designed a system of anchors to support the foundation walls and prevent collapse which would have caused inundation of the site from the nearby Hudson river.

'I was called by Dick Tomasetti to meet with Commissioner Burton from the Department of Design & Construction. I had kept detailed records during the construction with photos, so I loaded up the car with safety gear, and headed down. I had projected in my mind what the actual site problems could be, but was horrified when I got there.

'Everyone was looking up, nobody was looking down. I know that site very well, I could probably go through it blindfold. I told them some of my concerns and of water coming into the site. Everyone was focusing on rescue, but I was afraid they'd move a crane causing some of the walls to collapse, or that a crane would topple over and kill more people. I saw them trying to move a 200t crane out over a piece of decking inside the slurry wall which would have gone straight through - and then decided to do some 'cartoon' drawings showing them the problems below grade.

'We couldn't do positive work to start supporting the walls until mid October when the wall started to move at Liberty Street. One wall moved 1.4m - walls had gone into the ultimate moment range of capacity. Our work was nothing to do with geotechnical engineering - it was structural, construction and demolition work - the only geotechnics was that the soil was pushing the wall in. The closeness to water was significant, the river was only 50, 60m away. There was always a risk of inundation - our job was to make sure it never happened.'

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