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Re: Overturning check

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Charles 

I believe that the situation that you are refering to often occurs when
dealing with wind uplift on light roofs and when using working stress
methods.  If the dead load is equal to the wind uplift load you might
assume that you did not have to provide any ties to attach the roof to the
building.  Such an approach would leave you with no factor of safety
against uplift.  The problem is that  In such cases I use the following
load combination to insure that there is a factor of safety at the
interface.

( (1.4 *  W)  + (0.85 * DL) ) / 1.4

The strap or other element is then designed to resist this load using
typical allowable stress capacities.

In the case of earthquake loading I  replace W with E with the exception
that the result would not exceed the total availible dead load below the
level under consideration.

If one is using an LRFD approach then this load case would not be
necessary.  But in the case of seismic loads where you are checking
overturning stability you would probably want to limit the resultant value
to not exceed 1.4 times the total availible dead load below the level under
consideration.

Mark Gilligan

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Message text written by INTERNET:seaint(--nospam--at)seaint.org
>..........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
objective. 

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
overturning effects.

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