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RE: ASCE 7-05 Cantilevered Column System vs. Inverted Pendulum-Type Structures

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Bill,
 
I am designing the type of structures you are talking about, (cantilevered steel columns embedded in a grade beam and supporting light framing above), using an R = 2.5 per Item G-1 of Table 12.2-1, (special steel moment frames). Since it is a "Cantilevered Column system"- the columns don't know what kind of connection is up there- SMRF, OMF, etc and the R of 2.5 closesly approximates the old R of 2.2 per the '97 UBC. If you wanted to be conservative, you could use the R of 2.0 for a non-building structure.
 
Hope this helps,
 
Larry Hauer, S.E.



From: t.w.allen(--nospam--at)cox.net
To: seaint(--nospam--at)seaint.org
Subject: ASCE 7-05 Cantilevered Column System vs. Inverted Pendulum-Type Structures
Date: Tue, 11 Mar 2008 11:58:41 -0700

What’s the difference?

 

My (current) situation is a two story SFR. One edge of the second floor is supported by a beam supported by three steel columns. This allows room for a car port. Yes, it sounds like the typical tuck-under scenario.

 

I’m not sure I can take the loads back to the next SW (only 10’-4” away) due to the second floor condition. I would rather take the tributary load out in the three steel columns and design them accordingly. However, I’m in Seismic Category D and it appears Cantilevered Columns is NP. There are no seismic coefficients for Inverted Pendulums in Table 12.2-1 but there are in Table 15.4-2. However 15.4-2 refers to “nonbuilding structures not similar to buildings”.

 

Can someone clear this up for me?

 

TIA,

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509

 



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