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Re: Section 1633.2.6 and 1612.4

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Dear Mike,

The original question by ECVAl3(--nospam--at) (on 9/3/99) 
was regarding COLLECTORS, specifically Section 1633.2.6
of the 1997 UBC, which introduces a new concept regarding
demand forces for collectors.  It is true that the 
definition of Eh refers to Section 1632, but that was 
not the topic of discussion.  Section 1632 is for 
Elements of Structures which is not the same as collectors.

Section 1630.1.1, which defines Eh, also refers to Section 
1630.2, Static Force Procedure, where the design base shear,
V, is defined.  It is from V or V/1.4 that the "unmodified" 
collector force is derived.  My only caution regarding 
"putting the 1.4 factor back in" was that IF one is using 
ASD, then the "unmodified" collector force is typically 
based on E/1.4=Eh/1.4=V/1.4  (only for rho=1), but the 
Special Seismic Load Combinations of Section 1612.4 are 
based on Em=(omega)xEh.

Thus the "unmodified" collector force IN THIS CASE (using 
ASD forces) must be multiplied by (1.4)(omega) to arrive 
at Em.

Regarding your second point, I believe it DOES make 
sense to design a member or connection to the same 
demand for both ASD and Strength Design for the following 
reason: The "capacity" to which the demand is to be compared 
is nearly identical for the two cases.  The "capacity" for 
the ASD case is 1.7 times the allowable stress or force 
(per 1633.2.6), and the "capacity" for the Strength 
Design case is phi times the yield or buckling capacity, 
and these two quantities will be nearly equal for all 
typical materials.

I welcome all comments and corrections.

Mark Swingle, SE
Oakland, CA

Disclaimer:  These are my own opinions.  They are
subject to change due to the ravages of time and 
after being subjected to reasoned criticism.


Mike O'Brien wrote:

<Now I understand where you are coming from but 
<I still disagree.  Eh is defined in section 1630.1.1 
<and in its definition it refers to Section 1632.
<The bottom of Section 1632 refers to back to 1612.2 
<and 1612.3.  1612.3 is for ASD and allows E/1.4.

<Furthermore, it makes no sense to have to design a 
<memer or connection for exactly the same for whether 
<you use ASD or Ultimate.

<Mike O'Brien