[转贴]：ASCE关于世贸大厦倒塌所做的说明（e文）

liuyu49

以下内容转帖自ASCE网站
The Collapse of the World Trade Center Towers
Frequently Asked Questions

1. Why didn't the towers collapse upon impact?

The towers did not collapse upon impact because they were originally
designed to withstand enormous loads. The impact of the aircraft did not take
the buildings down. In fact one tower stood for about an hour after it was
hit and the other stood for an hour and forty-five minutes after impact.
When engineers design a building they calculate the loads and forces
to which a building will be subjected over its lifetime. Typically, these
include the effects of hurricanes, blizzards, floods, or earthquakes. These
events are far from common. For a hurricane, the loads used by the structural
engineering profession are those from a hurricane that has only a two percent
chance of occurring in a year.

For New York City this means structural engineers design buildings that can
withstand winds gusting up to 100 mph, possibly coming from multiple directions
, and lasting for hours as the hurricane passes. This approach to
structural design produces tremendously strong buildings. Nevertheless,
structural engineers are constantly striving to improve their ability to predict
the loads their structures must withstand.

The forces from the impact of the airliners alone did not collapse the towers
because those forces, as extreme as they were, did not exceed the capacity
of the overall structural system.

2. Why did the towers collapse?

We do not yet know the answer to this and, in reality, we might never know
with total precision. However the following appears to be what occurred:
The weight of a typical high-rise building is supported vertically by its
columns. These columns commonly extend for the entire height of the
building.
The weight of each floor is transferred to the columns by a complex network
of beams and slabs connecting to and spanning between the columns. Structural
engineers design the beams, the columns, the slabs, and their connections
to resist the anticipated loads.
When the airplanes were flown into the World Trade Center towers a number of
columns were severely damaged. The damage from the impact, though significant
, weakened the structure but did not cause it to collapse. Rather, the intense
heat of the resulting fire fed by great quantities of jet fuel further
weakened the already damaged structural system. This is what is believed to
have led to the complete collapse of the crucial structural elements in the
impact area. The failure of these elements caused the portion of the building
above to drop, touching off a progressive failure as the entire structure
collapsed onto itself.

3. How vulnerable is a typical high-rise building to collapse?

The typical high-rise building has a very low degree of vulnerability to
collapse.The disaster at the World Trade Center was unprecedented in the
devastating combination of forces produced by the impact of the airliner
crashes and the burning jet fuel. Yet, the towers were able to stand long
enough for tens of thousands of people to escape. Unfortunately, it appears
that several thousand people did not escape the subsequent collapse of the
towers.
As it usually does in disaster situations, ASCE is dispatching teams of
engineering experts to the World Trade Center and to the Pentagon in Washington
to conduct studies of the sites. These studies may further our knowledge and
help to enhance structural design practices in the future.

4. Can engineers design buildings that are terrorist-resistant?

Yes, but they would likely resemble fortresses and people probably would not
use them and might not be able to afford them.
Engineers can, and do, use many methods to enhance the security of buildings
and other structures. These approaches include structural elements such as
reinforced frames and perimeters, the use of Kevlar-curtains or bullet-proof
glass, or designs that eliminate or restrict vehicle access or parking,
minimize windows, or secure entrances.
In designing buildings to withstand threats a balance must be reached between
safety and freedom and the effect of each upon our nation's infrastructure.
Events such as those of September 11 may well challenge America's commonly
held feelings about that balance.