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Re: Rho factor for wind and gravity loads?

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Steve,

Redundancy is always a good goal no matter what the design.  However one of the big differences between all other loads than seismic is that you are designing your members to stay in their elastic range.  Seismic , based on the applicable R factor, means that some or many of your members will end up in their inelastic range but not beyond (hopefully) their tensile strength.  That is, you are designing right up to the edge of total building collapse so the code wants alternate load paths and additional lateral system to insure the safe egress of the people inside.

Thomas Hunt, S.E.
Fluor



"Steve Mickelson" <smickelson(--nospam--at)northstareng.com>
07/26/2006 08:01 AM
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Rho factor for wind and gravity loads?





Hello everyone,
 
Section 1630 of the '97 UBC and section 1617.2 of the 2003 IBC (section 9.5.2.4 of ASCE 7-02) 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 6-2 of ASCE 7-02 and section 1630.1.1, equation 30-1 of the '97 UBC) and are both short duration loads (see table 2.3.2 of the 2001 NDS; also see load combinations 12-6, 12-9, 12-11 and 12-13 of the '97 UBC).

I realize / understand that wind is a monotonic force and seismic is non-monotonic (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 blast-resistant 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
 


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