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RE: Rho factor for wind and gravity loads?

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Steve,

 

Under current code, even when wind governs you still must meet the basic requirements and detailing for the lower bound seismic, and this includes the rho factor.  Wind loading is very different from Seismic loading in behavior and demand, regardless of which happens to have the higher force level in a given static analysis.  If you believe you have a less than acceptable level of redundancy, fix it.

 

The rho factor itself has had a less than exemplary existence. There are inconsistencies and material issues in trying to apply the rho factor.  Though I agree 100% with the need for redundancy, I do not agree with the idea of trying to further codify a less than clearly designed value.

 

The need for redundancy is one of the many areas to be considered as an engineer during design.  We need more thinking engineers with proper mentoring by the older in our profession to create quality balanced designs.  We do not need to try and reduce all engineering decisions to some form of less than adequate code equation.  Every building and structure is different with different needs and requirements, good engineering is creating the elegant solution; safe, effective, functional, easy to construct, and cost effective while achieving the program goals.

 

If, as the engineer, you believe that your particular problem has key elements that require a greater concern for their loss as it relates to the structure as a whole, then it is your responsibility to consider and account for this.  You don’t need some code provision to attempt to tell you when and how to consider the problem.  Any gravity Rho factor that was developed probably would not apply to your particular condition anyway.

 


From: Steve Mickelson [mailto:smickelson(--nospam--at)northstareng.com]
Sent: Wednesday, July 26, 2006 8:01 AM
To: seaint(--nospam--at)seaint.org
Subject: 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