Page 234 - Forensic Structural Engineering Handbook

7.18 CAUSES OF FAILURES

an escarpment. These topological features act, in a sense, as airfoils in moving air masses.
Winds that approach an incline must accelerate to pass up and over the crest. In doing so,
wind speeds near the crest exceed speeds at similar heights above level ground.
One approach for adjusting wind pressures for hills and escarpments is found in ASCE
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7-05. This method gives coefficients, calculated based on the geometry and size of the
topological feature, that are applied to the pressures calculated as above from wind veloc-
ities. According to this procedure, wind pressure adjustments are needed for structures
located above the mid-height of “steep” hills or escarpments when the topological feature
exceeds certain heights, ranging from 15 ft (4.5 m) near open water to 60 ft (18 m) for urban
areas. A “steep” feature is one with the ratio of the horizontal dimension to vertical dimen-
sion of the upper half of the windward slope of less than 5.
It should be obvious that significant inaccuracies and uncertainties will result whenever
the wind speed at a specific site is estimated from wind speeds recorded elsewhere. Each
adjustment factor used in the process includes some uncertainty. When several are applied
in the calculation, the uncertainty increases. Furthermore, some of the adjustment factors
are applied to wind speed, a term that is squared for the calculation of wind pressure.
To address these uncertainties, the investigator should obtain and analyze wind-speed
data from as many nearby reliable sources as reasonably possible. With multiple, indepen-
dent paths to the determination of basic wind pressures at a site, the investigator will
improve confidence in the values to be used to calculate surface pressures on a structure.
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Pressure Calculations. ASCE 7-05 contains an authoritative procedure for using the
basic wind pressure calculated for the local wind speed to calculate the wind pressures on
various surfaces of a building. The procedure involves the application of coefficients to the
basic wind pressures to estimate actual pressures at all locations on a structure. The proce-
dure has been developed to provide engineers with guidance on wind pressures that may be
used for the design of structures. As such, there are elements of conservatism in the
approach as well as significant simplifications of the complicated phenomena of airflow
around objects. Furthermore, pressure coefficients used in common design procedures gen-
erally represent the worst-case “enveloped” pressure distributions, considering that wind
can approach a structure from any direction. Therefore, any set of pressure coefficients
intended to be used for design likely includes coefficients that do not, in any way, represent
the pressure distributions on some surfaces for specific storms with wind that approaches
from specific directions. Investigators need to evaluate carefully how best to employ coef-
ficients intended for design, considering these potential shortcomings in the applicability
of these coefficients for the investigation of specific wind-related problems.
In analytical approaches, loads on the overall building systems that resist lateral loads
(e.g., braced frames, moment frames, or shear walls) are calculated using coefficients that
are applied to the basic wind pressures to determine wind pressures on windward and lee-
ward faces of a structure. When buildings have sloped roofs, horizontal components of the
wind forces of these elements need to be considered.
It is important to consider the possibility that nonsymmetric buildings, particularly those
with curved surfaces, will have unbalanced transverse wind loads. A curved exterior wall can
act as an airfoil during a storm, thereby generating larger suction pressures than will flat sur-
faces. In these cases, there can be a net transverse wind force on the building as a whole.
Wind pressures on components of buildings can be calculated based on tabulated fac-
tors as well. For these elements, effective pressures usually are larger than those for the
overall building system because components respond to shorter-duration, higher-intensity
gusts than do complete buildings. Also, spatial averaging of time-varying pressures over
the relatively small areas of components as compared to entire structures results in higher
effective pressures for components.
Components need to be evaluated for net pressures, considering both internal and exter-
nal pressures. Depending on the relative air permeability of the several layers of a particular
roof or the wythes of a wall, one layer or wythe may support essentially all the differential