Page 347 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.1

Source: STRUCTURAL STEEL DESIGNER'S HANDBOOK

CHAPTER 8

LATERAL-FORCE DESIGN

Charles W. Roeder, Ph.D., P.E.
Professor of Civil Engineering
University of Washington
Seattle, Washington

Design of buildings for lateral forces requires a greater understanding of the load mechanism than
many other aspects of structural design. To fulfill this need, this section provides a basic overview of
current practice in seismic and wind design. It also discusses recent changes in design provisions and
recent developments that will have an impact on future design.
There are fundamental differences between design methods for wind and earthquake loading.
Wind-loading design is concerned with safety, but occupant comfort and serviceability is a domi-
nant concern. Wind loading does not require any greater understanding of structural behavior beyond
that required for gravity and other loading, although it is noted that complex, large, or aerodynam-
ically sensitive structures frequently require wind-tunnel testing or more sophisticated dynamic
analysis to assure occupant comfort during wind storms. As a result, the primary emphasis of the
treatment of wind loading in this chapter is on the loading and the distribution of loading.
Design for seismic loading also is primarily concerned with structural safety during major earth-
quakes, but increasing emphasis is placed on economic loss and serviceability through performance-
based design. These different design goals are achieved by permitting a range of different structural
performance levels. During large infrequent seismic events, collapse prevention and life-safety per-
formance limits are economically achieved by permitting large but controlled inelastic deformations
of the structure. Inelastic deformation of the structure during severe earthquakes results in more
detailed structural design requirements, which are needed to assure system ductility and perfor-
mance. Therefore, discussion of seismic design also requires discussion of the inelastic behavior of
steel structures and design requirements needed to achieve acceptable inelastic performance.
Serviceability and economic loss limitations are assured by requiring smaller elastic deformations
during the appropriate design events. As a consequence of these differences, seismic design requires
a more detailed understanding of elastic and inelastic dynamic analysis and evaluation of a wider
range of structural behaviors than are required for most other design loads.
Refer to Chap. 4 for further information on wind and seismic loadings.

8.1 DESCRIPTION OF WIND FORCES

The magnitude and distribution of wind velocity are the key elements in determining wind design forces.
Mountainous or highly developed urban areas provide a rough surface, which slows wind velocity near
the surface of the earth and causes wind velocity to increase rapidly with height above the earth’s
surface. Large, level open areas and bodies of water provide little resistance to the surface wind speed,

8.1
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.