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# Re: 2nd try - Subject of Code Clarification - Non-Building Structures

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: 2nd try - Subject of Code Clarification - Non-Building Structures
• From: ErnieNSE(--nospam--at)aol.com
• Date: Sat, 21 Feb 1998 14:19:44 EST

Nobody seems interested in commenting on this so here is my best shot!

I don't have the "California" edition of the UBC, but "If" there is nothing in
the code or in reference materials, notes or commentaries on how this is
applied, this is how I will do it based on my judgement:

1. Use the most conservative case of Horiz.(Fp) and Vert.(1/3Fp) acting
together(assuming nothing in the code says that they do not have to be
combined). This is based on  my judgement that earthquake can cause a diagonal
up and down movement of a structure. Case 1 is Horiz. Fp combined with Upward
Vert. 1/3Fp for OTM analysis and Case 2 is Horiz. Fp combined with Downward
Vert. 1/3Fp for vertical reaction analysis.

2. Vertical seismic force is not for anchorage only but for the whole
structure. For example, a bridge or a tower or a tank on legs will have to be
designed for DL + LL(or contents) + Vert. Fp(up or down whichever governs).

3. When calculating Overturning Moments(OTM) and Resisting Moments(RM), use
two cases: Case 1 is OTM  due to Fp (DL only, when structure has no LL, or
tank is empty), plus Upward 1/3Fp, combined with RM due to .85 DL only. Case 2
is OTM due to Fp (DL plus LL(or contents), plus upward 1/3Fp, combined with RM
due to .85(DL + LL).

Unless, there is a possibility that the live load on the structure contrubutes
to the OTM but at the same instant this LL does not contribute to the RM.
Then, Case 3 is OTM = Fp (DL + LL) and RM = .85 DL. For example, a steel tank
connected to the ground at the base of the side walls but without a bottom
plate. The liquid inside the tank is inside a soft plastic container. bearing
directly on the ground and the side walls of the tank.

4. I don't really know where the .85 factor came from but I consider it a
factor of safety against overturning. If you get  OTM  (due to Fp) - RM (due
to 1.0 DL) = 0, no uplift, no connections or holddown required, then, there is
no factor of safety against overturning. Remember that sometimes our DL
assumptions are wrong. I will use .85(DL+LL) in my RM. Except for case 3
above(.85DL).