Examination of Witnesses (Questions 392
- 399)
TUESDAY 5 FEBRUARY 2002
MR ANDREW
ALLSOP, MR
PETER BRESSINGTON
AND DR
JOHN ROBERTS
Chairman
392. Can I welcome you this morning, and ask
you to introduce yourselves?
(Dr Roberts) I am a consulting engineer,
a director with a firm called Babtie Group. I live in Manchester
but I do a lot of work in London as well. I am here because I
am the chairman of a committee which has been set up by the Institution
of Structural Engineers; it includes members from most of the
professions in the built environment, and it is going to provide
advice to designers relating to tall building design, the safety
of people in tall buildings, following the 11 September events
in America.
(Mr Bressington) I am a director of Ove Arup &
Partners. I am responsible for its fire engineering group. I am
particularly interested in the events of 11 September. I chair
Arup's extreme events task force, and we were set up to look at
what happened and perhaps where the future takes us in terms of
design of tall buildings.
(Mr Allsop) I am from Arup and am head of the wind
engineering work that Arup is involved with.
393. Are you happy for us to go straight to
questions?
(Mr Bressington) In terms of safety, we have heard
talk and obviously most of the discussion has been around the
feasibility and the desirability of tall buildings. From a technical
point of view we are just talking about safety, and I do not think
there are any reasons from a safety point of view that tall buildings
should not be built; we just need to look at some of the issues.
Christine Russell
394. Could you tell the Committee whether you
believe the collapse of the World Trade Center was effected by
its structural design?
(Dr Roberts) The story on why it collapsed is not
yet fully understood by the people who have been appointed in
America to investigate it. The head of that team of investigators
is a Fellow of the Institution of Structural Engineers of the
UK, because it is a worldwide organisation, and he is a member
of our committee, so we are getting some direct feedback from
the investigations. All the signs are as you might expect that
it is quite complex and there is not one single factor that caused
the collapse of the buildings themselves. It is related to a combination
of physical damage by the plane impact which did remove a portion
of the structure; very severe fire—and most buildings, as
Peter will explain in more detail, certainly buildings in the
UK, are designed for fire from normal occupational hazards which
are essentially burning paper and wood and other cellulose materials,
and here a fire was delivered into the buildings from hydrocarbons
which burn a lot hotter, so there was very severe fire going on
in the building and buildings have fire resistance. This building
had a defined fire resistance but against a cellulose type of
fire, and the final collapse occurred by something that structural
engineers would call progressive collapse, which is where a part
of the building collapsing on to the part below it can trigger
what you would call a pack of cards, in loose terms, type of collapse.
We would call that a progressive collapse, where one or more floors
falling on to a floor below can trigger an almost immediate chain-reaction
collapse down to the ground, and that appears to be what happened
395. Is it true that our safety regulations,
certainly after Roman Point and the disaster there, in the UK
are already more stringent than in the States?
(Dr Roberts) I am very reluctant to say that. What
is true is that, post the collapse of Roman Point in 1968—which,
as I am sure people will remember, was triggered by a gas explosion
in a very particular type of building, and that was a system-built
block of flats which had on the evidence afterwards quite weak
connections between all the pieces and which progressively collapsed
down through about 22 storeys from an explosion quite near the
top—that triggered a change in the UK building regulations
which came in in the early 70s. There were a couple more collapses
in 1973 and 1974 of some school roofs in the London area, in Camden
and Stepney, and those regulations have been with us since that
time and they serve to limit the effect of what is called disproportionate
collapse from an accident. You have to guess what the accident
might be. It is not just to protect against a gas explosion or
a fire; it aims at producing robust buildings that will tolerate
some quite unusual accident happening in them. Those regulations
are in place for all buildings in Britain and have been since
that time but I think it is a huge step to jump from that and
say, "Oh, had this building been in Britain it would not
have collapsed". I do not think that is a step you can take
at all.
396. Do you have concerns about existing buildings
in the UK that may be vulnerable to what you described earlier
as progressive collapse?
(Dr Roberts) Yes, because buildings are not designed
for an event of that magnitude or those sorts of events. We hope
they will be robust to survive some of them or most of them, but
I do not think there is any doubt that there are serious accidents
that could affect some British buildings.
397. And is it premature to say yet whether
or not you believe it would be possible to strengthen further
the existing regulations in the light of what happened on 11 September?
(Mr Bressington) There are a few issues there. The
structure robustness is one issue and the other, obviously, is
fire and evacuation and management of those buildings. My feeling
is that certainly people will look at the codes. The codes are
really based on a fire which can happen in a building which you
would associate with that occupancy, so it may well be there are
more performance requirements in the codes. It therefore gives
people the opportunity to look at specific buildings in different
areas, whether structure or fire, and make decisions based on
a performance criteria. For instance, if you need 45 minutes to
clear a high rise building in terms of evacuation, you need to
try to do that based on the tenability, so I think it will move
that way. It may not be a prescriptive requirement but more of
a guidance.
(Dr Roberts) Looking back, which is very easy to do
with hindsight, is that in the UK buildings are required to have
defined periods of fire resistance, as you have no doubt heard
from some other evidence, (probably from manufacturers of fire
resistant materials), and typically in the UK large buildings
would be required to have two hours' fire resistance for the above-ground
structures. First of all, it does not mean that the building will
stand there for two hours in a very serious fire—it is a
sort of guide as to how long it will last but it is not absolutely
for sure that that will happen—and the other link that is
missing is that there is not any requirement in the UK to ensure
that you can get everybody out in two hours in a building with
a two-hour fire resistance, and that missing link is rather worrying
when you think about it.
Chris Grayling
398. If an aircraft hits the side of a tall
building, as happened in New York, you get a catastrophic impact
and fire but in the case of the World Trade Center buildings,
given the number of people who worked in those buildings, a very
large proportion of those in the building did get out. When the
planes hit the building, a lot of the impact burst out beyond
the buildings rather than being contained within it. If a plane
were to hit a medium-rise building with much greater density within
a geographical area, is there an argument for saying that an aircraft
landing in a 20-storey building with a larger number of workers
could have a greater impact on that building than a high-rise
building?
(Mr Bressington) Potentially, the World Trade Center
I would suggest was picked out because it was a symbol and they
decided they would attack that. They could have chosen to attack
a sports stadium where, in terms of trying to achieve their objective
which is to kill as many people as you can, they would have achieved
that end using that. So it is not just tall buildings with these
incidents because a lot of this is to do with security, politics,
and all sorts of other issues rather than design. Certain aspects
have come out of the World Trade Center and obviously more will
in terms of what happened there, and we just need to build on
some of those points and perhaps use them in the future. I do
not think it is to say that, because a plane flew into the World
Trade Center it is the sort of thing that is going to happen every
day of the week. Certainly a plane was also flown into the Pentagon
and that building was quite resilient in terms of attack but it
still did kill quite a few people.
399. So there is a danger that safety restrictions
around tall buildings become focused entirely on September 11?
(Mr Bressington) Yes.
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