Page 386 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch08.qxd 9/29/05 5:21 PM Page 8.40

LATERAL-FORCE DESIGN

8.40 CHAPTER EIGHT

The NDP uses direct inelastic dynamic analysis of the structure due to specific ground accelera-
tion records, and corrects the major limitations of the NSP. The NDP component deformation
demands are true inelastic dynamic response for the structure under the given excitation. The method
requires accurate and reliable nonlinear models of structural behavior, and these models are not
available for all structural systems. The local component deformation demands obtained by NSP and
NDP are compared to the deformation capacity limits obtained from past experiments on structural
components. Many engineers are faced with the use of nonlinear analysis methods for seismic design
and evaluation, and these procedures will become increasingly common in the future.

8.9 MEMBER AND CONNECTION DESIGN FOR LATERAL LOADS

Wind loads on steel structures are determined by first establishing the pressure distributions on struc-
tures after considering the appropriate design wind velocity, the exposure condition, and the local
variation of wind pressure on the structure (Art. 8.2). Then, the wind loads on frames and structural
elements are determined by distributing the wind pressure in accordance with the tributary areas and
relative stiffness of the various components.
Connections used for wind loading run the full range of unrestrained (pinned), fully restrained
(FR), and partially restrained (PR) connections. PR connections are frequently used for wind-loading
design, because they are economical and are easily fabricated and constructed.
Designs for wind and seismic loading often use floor slabs and other elements to distribute loads
from one part of a structure to another (Fig. 8.21). Under these conditions, the slabs and other ele-
ments act as diaphragms. These may be considered deep beams and are subject to loadings and
behavior quite different from that encountered in gravity-load design. It is important that this behav-
ior be considered, and it is particularly important that the connections between the diaphragms and
the structural elements be carefully designed. These connections often involve a composite connec-
tion between a steel structural member and a concrete slab, wall, or other component. The design
rules for these composite connections are not as well defined as for most steel connections. However,
there is general agreement that the connections should be designed for the largest forces to be trans-
ferred at the interface, and the design should recognize that large groups of shear connectors or other
transfer elements do not necessarily behave as the sum of the individual elements.
Wind-loading design is based on elastic behavior of structures, and strength considerations are
adequate for design of wind connections. Seismic design, in contrast, utilizes inelastic behavior and
ductility of structures, and many design factors must be taken into account, besides the strength of
members and connections. These requirements are intended to assure that inelastic deformations
occur in members capable of developing significant inelastic deformations rather than connections
(Art. 8.7).
Seismic-design loads are determined by the equivalent static-force and dynamic methods. With
the static-force method, the total base shear is determined by Eq. (8.6). It is distributed to bents and
structural elements by simple rules combined with consideration of the distribution of mass and stiff-
ness (Art. 8.4). In the dynamic method, the total range of dynamic modes of vibration are consid-
ered in determination of the base shear. This base shear is then distributed in accordance with the
mode shapes. For both wind and seismic loading, forces and moments in members and connections
can be first estimated by approximate analysis techniques (Art. 8.8.1). They are then usually com-
puted to greater accuracy by linear finite-element analysis methods.
Once member and connection forces and moments are determined, design for wind loading is sim-
ilar to the design methods used for other loading. However, earthquake loading requires greater con-
cern with establishing a ductile element which controls the inelastic resistance and ductility of the
structural system, and balancing the resistance of this ductile element with that of all other elements
in the structural system. The AISC seismic provisions and the discussion in Art. 8.7 are focused pri-
marily on this concern. The balancing effect typically enforces a form of capacity-based design, in
which the capacity of the ductile structural element establishes the minimum resistance for connec-
tions and other, less ductile elements. The consequence of these requirements are that seismic design

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.