Page 193 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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BUILDING CODES, LOADS, AND FIRE PROTECTION*

BUILDING CODES, LOADS, AND FIRE PROTECTION 4.21

the wake of upwind obstructions should be considered in design. Wind-tunnel tests (SEI/ASCE 7-02,
Method 3) on a model of the structure and its neighborhood may be helpful in supplying design data
as an alternative to Methods 1 and 2.

4.7 SEISMIC LOADS

Earthquakes have occurred in many states. Figures 4.4 and 4.5 show current contour maps of the
United States that reflect the severity of seismic ground motion, as indicated in the ASCE standard,
“Minimum Design Loads for Buildings and Other Structures” (SEI/ASCE 7-02).
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 structural damage. However, to achieve
an economical seismic design, codes and standards permit local yielding of a structure during a
major earthquake. Local yielding absorbs energy but results in permanent deformations of struc-
tures. 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 compro-
mising the stability of structures. Requirements for this approach are included in the AISC standard,
“Seismic Provisions for Structural Steel Buildings.”
The forces transmitted by an earthquake to a structure result from vibratory excitation of the
ground. The vibration has both vertical and horizontal components. However, it is customary for
building design to neglect the vertical component 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 lateral static force to be
applied to structure of regular geometry for design purposes. For example, SEI/ASCE 7-02 stipulates
that the total lateral force, or base shear, V (kips) acting in the direction of each of the principal axes
of the main structural system should be computed from
V = C s W (4.7)

where C s = seismic response coefficient
W = total dead load and applicable portions of other loads, kips

Applicable portions of other loads are considered to be as follows:
1. In areas for storage, a minimum of 25% of the floor live load is applicable. The floor live load in
parking garages and open parking structures need not be considered.
2. Where an allowance for partition load is included in the floor load design, the actual partition
2
weight or a minimum weight of 10 lb/ft of floor area, whichever is greater, is applicable.
3. Total operating weight of permanent equipment.
2
4. Where the flat roof snow load exceeds 30 lb/ft , the design snow load should be included in W.
Where the authority having jurisdiction approves, the amount of snow load included in W may be
reduced to no less than 20% of the design snow load.
From the intended building occupancy classification (Table 4.10), the appropriate Seismic
Use Group (I, II, or III) and its corresponding importance factor, I s , is established by means of
Tables 4.11 and 4.12, respectively. The site classification (A–F) must be ascertained in accordance
with Table 4.13. Tables 4.14 and 4.15 can then provide the site coefficients, F a and F v , for the short and
1-s period maximum considered earthquake (MCE) spectral acceleration, respectively. The MCE
maps given in Figs. 4.4 and 4.5 can be used to read the spectral response values of S s and S 1 for the
selected construction site. The seismic design category (A–F) and response modification factor R for
the basic seismic force-resisting structural system must then be identified per SEI/ASCE 7-02, or as
required by the applicable building code. The R-factor value is proportional to the amount of ductility,

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