Page 312 - Civil Engineering Formulas

246 CHAPTER NINE

where K velocity exposure coefficient evaluated at height z
z
K topographic factor
zt
K wind directionality factor
d
I importance factor
V basic wind speed corresponding to a 3-s gust speed at 33 ft above
the ground in exposure C
Velocity pressures due to wind to be used in building design vary with type
of terrain, distance above ground level, importance of building, likelihood of
hurricanes, and basic wind speed recorded near the building site. The wind
pressures are assumed to act horizontally on the building area projected on a
vertical plane normal to the wind direction.
ASCE 7 permits the use of either Method I or Method II to define the design
wind loads. Method I is a simplified procedure and may be used for enclosed or
partially enclosed buildings.
ASCE 7 Method II is a rigorous computation procedure that accounts for the
external, and internal pressure variation as well as gust effects. The following is
the general equation for computing the design wind pressure, p:
p qGC p q i (GC pt ) (9.138)
where q and q velocity pressure as given by ASCE 7
i
G gust effect factor as given by ASCE 7
C external pressure coefficient as given by ASCE 7
p
GC internal pressure coefficient as given by ASCE 7
pt
Codes and standards may present the gust factors and pressure coefficients
in different formats. Coefficients from different codes and standards should not
be mixed.

Seismic Loads
The engineering approach to seismic design differs from that for other load types.
For live, wind or snow loads, the intent of a structural design is to preclude struc-
tural damage. However, to achieve an economical seismic design, codes and stan-
dards permit local yielding of a structure during a major earthquake. Local yield-
ing absorbs energy but results in permanent deformations of structures. Thus
seismic design incorporates not only application of anticipated seismic forces but
also use of structural details that ensure adequate ductility to absorb the seismic
forces without compromising the stability of structures. Provisions for this are
included in the AISC specifications for structural steel for buildings.
The forces transmitted by an earthquake to a structure result from vibratory
excitation of the ground. The vibration has both vertical and horizontal compo-
nents. However, it is customary for building design to neglect the vertical com-
ponent because most structures have reserve strength in the vertical direction
due to gravity-load design requirements.
Seismic requirements in building codes and standards attempt to translate
the complicated dynamic phenomenon of earthquake force into a simplified
equivalent static force to be applied to a structure for design purposes. For