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Design for Tornadoes
and High Winds
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Are the
buildings we occupy safe when overly strong winds or tornadoes strike?
By Ruth Wood-Steed
After our recent turbulent spring, it is natural to wonder how safe
we are in the buildings in which we live and work. Although this
year brought a seemingly large number of storms, the National Weather
Service attributes only 11 significant weather events or an average
of 1.5 per year to the area since 1994. Only four of those were
attributed to severe weather, tornadoes or severe thunderstorms;
the rest were floods.
But the question still remains. Are the buildings we occupy safe
when overly strong winds or tornadoes do strike? Jerry Rombach,
executive officer of the Home Builders Association of Greater Southwest
Illinois, says that residential buildings constructed since adoption
of the 1996 National Building Code are fairly safe. Very few injuries
or deaths have occurred in new residential construction.
Residential and small commercial buildings are similar in level
of safety; larger commercial buildings are a bit safer. But the
greatest degree of safety is found within hospitals and data centers—facilities
that must remain functional at all times—for they are specifically
designed to stand up to tornadic winds.
According to Jim Adkins, director of structural engineering for
Hellmuth Obata + Kassabaum’s St. Louis office, “Residential buildings
generally use wood framing, stud walls and trusses, with plywood.
While some commercial buildings also use wood framing, larger buildings
are more likely to use steel or concrete framing systems, which
provide better protection against heavy winds.”
Most buildings are designed for 70- to 80-mile-per-hour winds, with
90-mile-per-hour gusts. But severe tornadoes, such as some Missouri
and Illinois communities experienced this spring, can have 150-mile-per-hour
winds.
Modern wind design didn’t begin until the 1970s, when firms began
testing mock-ups for high-rise buildings, such as Chicago’s Sears
Tower, in wind tunnels. Unlike seismic design, which has seen frequent
code changes over the past 30 years, wind design has evolved very
slowly.
Adkins says designing for wind takes a two-phased approach. First,
engineers design the skeletal frame for gross strength, to prevent
collapse or excessive sway, either of which could cause injuries
or death. Second, individual building elements, such as walls, roofing
materials, and windows are designed. “Failures often start and progress
due to a failure of the attachment between individual building elements
and between these elements and the skeletal frame. Accordingly,
connections must be carefully considered.”
Jacobs Engineering’s Phillip Shinn agrees, adding “tornadic or high
winds generally cause structural damage in two ways. The first is
from projectiles hurled by the wind; the second is partial or complete
removal of the roof.”
Shinn took all of this into consideration when designing structural
systems for Edward Jones’ new St. Louis data center. This building
must do more than just protect its occupants until they are safely
out; it must remain operational under all circumstances. The building’s
fully grouted, fully reinforced concrete masonry walls can withstand
150-mile-per-hour projectiles shot directly at them. Its composite-steel-beam
and concrete-slab roof is adequately heavy and strong to resist
high winds, even if a tornado lifts the roofing membrane.
Structural systems aren’t the only building elements, however, that
fail during tornadoes. So do mechanical and electrical systems.
Most buildings, whether residential or commercial, are dependent
on local utilities for their power supply. These are very reliable—when
not, we either live without until service can be restored or rely
on emergency generators for power.
Critical buildings, such as those mentioned above, however, must
be able to continue without interruption in service. Richard Janis,
president of William Tao & Associates, considers what could suffer
wind damage and finds a way around it.
For instance, roof-mounted equipment is in obvious danger, as are
outdoor cooling towers. He says, “There are three main ways of preventing
disruption: first, place the equipment inside; second, harden it,
for instance, by enclosing it in concrete, or third, provide redundancy
for the system.” Frequently when designing hospitals or data centers,
Janis uses a combination of these.
Could an existing building be modified to protect against wind damage?
HOK’s Adkins says he has never been asked to retrofit an existing
building for this purpose, but it could be done, at a fairly sizeable
expense. “On a brick veneer building, you could install steel framing
on the building’s exterior, along with another wall, or you could
remove and replace the existing wall. If the bracing for the new
wall is different from the original, you could add diagonal bracing
or shear walls.”
Jacobs’ Shinn mentions another way to add wind protection. “Some
buildings, such as schools and residences, are adding tornado shelters.
These rooms are about the size of a closet or bathroom, and are
constructed of fully grouted and fully reinforced concrete. They
use metal sliding doors that are more resistant to wind than typical
hinged wood doors.”
HBA of Southwest Illinois’ Rombach adds that the Federal Emergency
Management Agency (FEMA) is now recommending that residences and
commercial buildings add safe rooms, although not currently required
by code. He adds that these rooms can serve other purposes, such
as a laundry room or bedroom, somewhat mitigating the additional
cost required to build them.
So after all is said and done, are the buildings we occupy safe?
It depends. More now than ever before, but finding a safe place
in the bowels of the building is probably still a good idea when
the alarm is sounded.
Ruth Wood-Steed is owner of CommPlus, a St. Louis-based marketing
communications firm. |
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