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RE: ASD load combinations in UBC '97, vol. II

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Title: RE: ASD load combinations in UBC '97, vol. II

I don't think you are supposed to divide Eh*OMEGA by 1.4 for the ASD checks because the ASD section says to check strength for this load combination and tells you to determine the strength based on 1.7 times the ASD capacity without a 1.33 increase.

If you can divide by 1.4 for ASD then there is not parity with the LRFD specification which has you check strength and does not have you divide by 1.4.

The safety factor for an ASD axial member is 1.92 + - for a slender column and less for a short column.  To determine the design strength when doing the ASD seismic stuff 1.7*nominal strength/1.92=.88*nominal strength, and the LRFD specification has the design strength equal to [phi=.85]*nominal strength.

.88*nominal = + .85*nominal.  If the code allows you to  divide the ultimate load by 1.4 for ASD seismic, if I was designing something where it mattered I think I might play the code by designing with ASD and reduce the force by 30% but I don't think that is what the code intends.

-----Original Message-----
From: Vasu, Ravi [mailto:Ravi.Vasu(--nospam--at)jacobs.com]
Sent: Friday, October 11, 2002 5:58 AM
To: seaint(--nospam--at)seaint.org
Subject: ASD load combinations in UBC '97, vol. II


I have two questions(UBC 1997, Vol. 2) here. 
Thanks in advance.

1  a. Refer to UBC '97, section 2213.5(Allowable stress
design(ASD)), pg. 2-255(column requirements)
The load combinations have 'omega' to check column compression &
tension.  AISC, '97 'seismic provisions for structural buildings',
pg. 135(section c4.1) says that for all Allowable stress design,
E(from UBC) should be divided by 1.4.
So, in the above UBC section 2213.5, I think, from section
1630.2,(to calculate Pe) should be divided by 1.4(since that section
in in ASD).  In fact, in LRFD(section 2211.6.4), for the same check
on columns, we do not need to divide E by 1.4(from AISC on '
seismic...).

b. Similarly, for bracing connection design
and moment connection(UBC section 2213.8.3.1 & 2213.6 under "ASD"-
pg. 2-258 & 2-256), in the combinations with 'omega' times the
'design seismic force', the E (from section 1630.2 procedure) should
be divided by 1.4 or not?.  Pl. verify.

2.  In UBC section 2213.8.3.1(pg. 2-258), the forces for bracing
connection has 'The maximum force that can be tranferred to the
brace by the system'(3 rd item).  Does it mean, the load
combinations to use to find the force is from section 1612.3(pg.
2-4) ONLY?  or any other load combinations required?    If the load
combinations are only from section 1630.2, then forces from items 1
& 2 in section 2213.8.3.1 may not GOVERN at all.?
  
I appreciate your time & response very much.

Thanks

Ravi
Telephone:832-351-6856


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