From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Thu, 03 Feb 2000 23:43:43 -0800
I have been following this thread with interest. All the replies but one
have assumed the concern is only about overturning (or "capsizing" in
nautical terms) of the entire structure as a body. But the questioner did
not so limit the question. He wrote:
>Per UBC 1630.8, "every structure shall be designed to resist the
overturning >effects.... See Section 1612 for combining gravity and seismic
>Do you use the Strength Design combinations or the Allowable Stress Design
>combinations? Is overturning dependent of the design method, or is it a
>serviceability issue and therefore the service combinations should be used?
Sec 1630.8.1 is where his quoted provision came from, and it is about the
EFFECTS of overturning, and at EVERY level of the structure, not peculiarly
at the soil interface, which is addressed elsewhere at Sec 1630.8.3. The
question is not about stability per se.
Based on the question and code section quoted, it appears that the internal
effects on structural members would be designed for Strength or Allowable
Stress according to the design method chosen, and using the load
combinations in the corresponding part of Sec 1612.
Where there is an absence of positive connection to resist an
overturning-induced separation of parts that gravity normally holds together
in a kind of "preloading", then the "effect" of overturning is that the
parts separate. One does not design the thin air between the separated parts
by either strength or allowable stress methods.
But _whether_ such a separation will be calculated to happen apparently
depends on which set of design loads is chosen. Since Sec 1630.8.1 says the
structure shall be designed to "resist" the overturning effects, I presume
that the separation of structural parts due to overturning is something that
is to be resisted, (ie, prevented from happening) as a definite design
So it would matter which system of loading is chosen, at every level where
there is a possibility of abutting parts separating due to seismic
To expand on the question: Since the 1982 UBC, there has been a similarly
worded provision very early in the chapter, and it refers to provisions in
wind and seismic sections, and now to retaining walls. In some of those
locations the focus is on bodily overturning, but not in the current seismic
section cited in the posted question, or in its predecessors.
I think the answers already posted should be reconsidered to see if they fit
the question actually asked.
Charles O. Greenlaw SE Sacramento CA