Beyond the Towers: Performance of Masonry
By David T. Biggs, P.E., Ryan-Biggs Associates
Masonry Home > Products
and Properties > Beyond the Towers
Comparing the present with the past in the world around us can
be an important learning experience. Such was the case for the Federal
Emergency Management Agency (FEMA) and the American Society of Civil
Engineers (ASCE), in the difficult task of conducting an evaluation
of the World Trade Center (WTC) and surrounding buildings.
The Event
On September 11, 2001, airplanes struck two 110-story office towers
in New York and the Pentagon in Washington, D.C. The towers (WTC
1 and WTC 2) collapsed in less than two hours, and another building
in the complex (WTC 7) collapsed later in the afternoon. Thesebuildings
had few or no masonry components. All of the surrounding buildings
suffered damage from falling debris, wreckage, and fire fromthe
towers. While the impact of portions of the
collapsing buildings did the majority of harm, there was also damage
from flying debris and air to the masonry used in their construction.
The Investigation
Six days after the attacks, teams of engineers, notably from the
Structural Engineering Association of New York and the New York
City Department of Buildings, began evaluating the structural integrity
of over 400 buildings surrounding the WTC site. Using a rapid visual
evaluation system, they performed immediate assessments and made
decisions regarding continued use and occupancy of buildings in
lower Manhattan. Damage was described as either structural or collateral.
The buildings with structural damage were inspected in detail.
Buildings immediately surrounding the plaza were most heavily affected
by the disaster. They weregenerally constructed from 1906 through
the 1980s; most had some masonry components.
Masonry Performance: Verizon Building, 140
West Street
One of the closest neighbors to the WTC site, the 30-story Verizon
building, is a steel-framed brick building constructed circa 1924.
The typical floors, composed of concrete-encased steel beams and
girders, are redundant and robust. The exterior face of the perimeter
framing is encased in brick and serves as both exterior wall and
infill. Header bricks connect the exterior brick to the backup and
support the weight of the exterior wythe. Columns are also brick
encased.
 The
façade, floors, and framing of the south and east sides of
the building sustained heavy damage, as did several exterior columns.
Although the framing deflected as much as 2 ft into the building,
the masonry infill restrained the columns from collapse. The steel
structure was not affected. None of the damage threatened the structural
integrity of the building. And although there was a substantial
fire in WTC 7, there were no fires reported in this building. It
was never out of service. The structure was shored and repaired.
Masonry improved the performance of the Verizon building because:
- Perimeter brick masonry walls absorbed much of the impact
from both WTC 1 and WTC 7, resulting in less damage to the steel structure.
- Masonry infill of the exterior wall provided a redundant
load path and
helped prevent collapse.
- Framing damage was localized, partially due to masonry infill.
- Exterior brick headers held the upper brick from collapsing
above a
damaged section.
- Brick encasement of the columns and concrete encasement
of the steel
framing provided fire and impact resistance.
- Built-up sections appeared to be more ductile and better
able to absorb
energy along with the masonry infill.
- The building did not experience fire damage—masonry
and (safety glass)
windows limited penetrations through exterior walls.
Summary
In the towers themselves, the stairwell and elevator walls were
constructed
of gypsum products. Evidence indicates most of the floors of impact
were
damaged and rendered unusable. While it is presumptuous to assume
that
masonry enclosures would have survived the attacks, it is obvious
that more
durable wall systems would have improved chances for survival for
occupants
above the level of impact. Future research should be devoted to
evaluating
and developing durable, fire-rated egress enclosures for high-rise
buildings.
Reinforced masonry and concrete are two effective solutions that
can be used
without further development.
Example after example demonstrates how masonry helped prevent greater
destruction during the World Trade Center disaster. Some of the
lessons learned:
- Older framed buildings with masonry components performed
generally
better than newer buildings with lightweight curtain wall construction.
- Masonry (walls, beams, partitions, infill) served as fireproofing
and provided
significant structural redundancy.
- Masonry infill absorbed impact energy to minimize damage
locally.
- Masonry veneers and panelized systems are readily repaired.
Masonry proved in this event that it does more than simply enclose
space; it
provides fire protection, structural capacity, and even structural
redundancy.
It can provide safer enclosures for stairways or other exit routes,
affording egress in high-rise buildings during emergencies.
 This
article is excerpted from: Masonry
Aspects of the World Trade Center Disaster, September 11, 2001,
by David T. Biggs, P.E., The Masonry Society,
Boulder, Colorado, 2003. (Available from PCA as LT277.)
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