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Rho factor for wind and gravity loads?
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 To: <seaint(nospamat)seaint.org>
 Subject: Rho factor for wind and gravity loads?
 From: "Steve Mickelson" <smickelson(nospamat)northstareng.com>
 Date: Wed, 26 Jul 2006 08:01:21 0700
Hello everyone,
Section 1630 of the '97 UBC and section 1617.2 of the 2003
IBC (section 9.5.2.4 of ASCE 702) define the reliability / redundancy factor
rho.
Section C105.1.1.1 of the 1999 Recommended Lateral Force Requirements and
Commentary (Seventeenth Edition; SEAOC), in discussing this factor, states the
following:
"Redundancy is a characteristic of structures in which multiple paths of
resistance to loads are provided. The advantages of incorporating redundancy in
a structure have long been recognized, especially when the structure is
subjected to loads that cause inelastic deformations. A structure with multiple
load paths is less susceptible to problems caused by design and / or
construction errors."
As far as I know, this factor only exists for seismic forces.
My question is this . . . why is there not a rho factor for wind forces or gravity
loads?
Wind forces are similar to seismic forces in that they can occur both
horizontally and vertically (see figure 62 of ASCE 702 and section 1630.1.1,
equation 301 of the '97 UBC) and are both short duration loads (see table
2.3.2 of the 2001 NDS; also see load combinations 126, 129, 1211 and 1213
of the '97 UBC).
I realize / understand that wind is a monotonic force and seismic is
nonmonotonic (causing cyclical, racking motions). It is tempting to reason
that the inelastic deformations caused by seismic forces necessitate the rho
factor.
Oftentimes even though wind force may govern the design of a structure, the
seismic forces are not much less (i.e. the structure is still subjected to
seismic forces, albeit less than those produced by wind). My point here is that
a structure doesn't always see either wind or seismic forces, but many times
both during its life.
With respect to gravity loading, does anyone desire to design a structure where
the failure of a single column may mean the failure of the entire structure? In
general this behavior is undesirable and comes to the forefront of many
discussions regarding blastresistant structures.
Is redundancy not equally important in a structure in designing for wind forces
and gravity loads? If so, why doesn't the code specifically recognize this by
way of a rho factor for wind and a rho factor for gravity?
Thanks in advance for your assistance.
Steve
