| The
Commerce Department’s
National Institute of Standards and Technology (NIST) reported
today that it has done additional analysis of the wind “loads” that
the World Trade Center (WTC) towers were originally designed
to resist and that this information will help NIST investigators
more accurately assess the overall strengths and baseline
performance of the two buildings before they were brought
down by the terrorist attacks of Sept. 11, 2001. The work
is being conducted as part of NIST’s federal building
and fire safety investigation of the WTC disaster.
WTC lead
investigator Shyam Sunder said that NIST—in collaboration
with Leslie E. Robertson Associates (LERA), the structural
engineering firm for the
WTC towers—recently completed its review of the original 1960s-era source
documents containing wind tunnel test data and wind load estimation methods used
for the towers, calculated the wind load estimates based on a clearer interpretation
of this information, and determined the values actually used in the design of
the buildings. These values are higher than the preliminary ones announced in
a WTC investigation interim report this past June that were based on information
publicly reported in 1971. LERA is under contract to NIST; all of its work has
been independently reviewed by NIST and contracted third-party experts. As reported today, the clarified original design wind load
estimates all exceed those established by the New York City
building code prior to 1968 (when the WTC towers were designed)
and through 2001 (when the towers were destroyed). The values
also are higher than those required by other selected building
codes of the era, including the relevant national model building
code.
Sunder also announced
that the NIST investigation team has completed an independent
analysis to determine the wind
loads that would be appropriate for use in designing the
towers in accordance with the current state of the art—rather
than the building codes in effect in the 1960s. This is being
done to better understand and assess the effects of successive
changes in standards,
codes and practices. These NIST “best estimate” wind
load values—based on two sets of wind tunnel test data
collected by independent laboratories in 2002 as part of
industry studies (unrelated to the NIST investigation) and
refined by NIST experts in wind science and engineering—are
within 10 percent to 15 percent of the “most unfavorable” (maximum)
wind load estimates used in the design of the WTC towers.
Wind load capacity is a key factor in determining the overall
strength of a tall building and is important in determining
not only its ability to withstand winds but also its reserve
capacity to withstand unanticipated events such as a major
fire or impact damage.
The NIST investigation
team’s final report is scheduled
for release as a draft document in December 2004. NIST is
not making any recommendations at this time. All recommendations
will be made in the final report. The final report also will
document the details of the technical calculations used to
estimate the wind loads and the structural response of the
WTC towers to those wind loads.
The goal
of the NIST WTC investigation goal is to recommend improvements
in the way people design, construct, maintain
and use buildings, especially high-rises. More information
may be found at http://wtc.nist.gov.
As a non-regulatory
agency of the U.S. Department of Commerce’s
Technology Administration, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
NOTE:
Charts showing the wind load data described in this release
my be found at www.nist.gov/public_affairs/releases/wtc_wind_release_graphics.pdf.
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