Page 235 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.19

pressure, or several layers or wythes may share load. One layer might support most of the
load if it is relatively airtight and the other layers of the system are permeable. In this case,
pressure in internal voids in the system will tend to equalize pressure on the opposite side of
the wall surface that is permeable, causing essentially all the net pressure difference to be
supported by the impermeable layer. If both layers are relatively impermeable, the air pres-
sure in interior voids will be relatively unaffected by rapid pressure changes on either exte-
rior surface. As such, the air in the void will tend to transmit movement of one layer to the
other, and both impermeable layers will share the total differential pressure.
Structural components near edges of buildings have higher wind pressure factors than do
components located in the field of a building surface to account for wind speed-up, wind sep-
aration, and vortices that develop as wind passes over the sudden geometric changes of a
building edge.
Special consideration needs to be given to buildings that have a disproportionate open
area on one wall. These buildings have unusual interior pressurization, because wind on an
open windward wall can cause positive stagnation pressures inside the building. Conversely,
a disproportionate area of openings on a sidewall or leeward wall can increase negative inte-
rior pressures. Canopies, overhangs, and alcoves have similar wind load characteristics.
Frames that are fully open, signs, chimneys, and other linear structures must be evaluated on
an element-by-element basis. Loads can be calculated from drag factors that have been deter-
mined theoretically or by testing. 1,11–14 In linear structures with relatively dense and repeating pat-
terns of elements (e.g., laced members), allowance should be made for shielding that windward
elements provide for leeward elements. 11,13 For shielding to occur, there must be regularity of the
member patterns and the along-wind distance between planes of similar geometry must be rela-
tively short. However, care must be used when evaluating the potential that shielding reduces the
total load on a structure. Small angles of attack between the wind and the normal to the structural
plane can eliminate the potential for one plane to shield another. In these cases, wind loads often
can be calculated as the sum of the loads on all members subjected to wind.
Analyses of aeroelastic phenomena usually should follow detailed review of available
literature. Useful approaches and thorough bibliographies are contained in Refs. 10 and 15.

Wind Tunnel Studies. Without question, the best way to estimate surface pressures and
wind speed distributions in the vicinity of structures, particularly in urban environments
and with irregularly shaped structures, is through wind tunnel studies. Wind tunnel studies,
if properly designed and executed, can account accurately for the effects of general surface
roughness over the fetch and specific buildings upwind and immediately adjacent to a site.
In addition, wind tunnel studies perhaps are the only way currently available to estimate the
dynamic response of low-frequency (less than 1-Hz primary frequency) structures that are
not of the simplest prismatic shapes.
Properly designed and executed wind tunnel studies can model irregular building shapes
that are difficult to analyze otherwise for local pressures on walls and roofs, and buffeting
by vortices shed by upwind features, channeling of winds, across-wind effects, periodic
loads due to vortex shedding from the structure of interest, flutter and galloping instabilities,
and complicated torsional loads.
Properly designed and executed wind tunnel studies can be substantially more reliable
than analytical approaches by revealing the pressure coefficients associated with specific
wind directions.
To be suitable for such studies, boundary-layer wind tunnels and their models should be
constructed and operated to properly represent the atmospheric boundary layer wind profile
with height, and the modeled building and surrounding structures and features must be rep-
resented in relative detail and to scale. The macro- and micro-length scales of the atmospheric
turbulence must be modeled to approximately the same scale as the physical model; and the
longitudinal pressure gradient that exists in the tunnel, but not necessarily in the actual wind
environment, must be considered when collecting and analyzing data. It is best to minimize