Let me throw another problem in here. If you use the LRFD loading for
seismic, you end up with larger overturning. In many cases an ASD
approach would work out with no net overturning (stable) and with the
LRFD the system is unstable.
Case in point: say you have a 10 ft wide by 10 ft tall X-brace. Apply
10 kips service shear horizontally at the top with 25 kips dead load
centered on the brace. The service O.T. moment is 100*kip*ft and the
dead load R.M. is 125*kip*ft, so (0.90)(125*kip*ft)-(1.0)(100*kip*ft) =
12.5*kip*ft (stable). In LRFD: (0.90)*(125*kip*ft)-1.4(100*kip*ft) =
-27.5*kip*ft (unstable). And this doesn't even include the vertical
acceleration reductions for LRFD that will generally place 0.90 DL
factor under 0.80.
So my point is, how do you design footing soil pressure with ASD and
the footing with LRFD when the footing is no longer stable? Note this
only applies to seismic conditions, wind is a whole other topic. Also,
for those of you looking to move to LRFD wood, you will end up with many
more holdowns. When holdowns are used they will probably be smaller
(ultimate values from the anchors). In know LRFD is a four letter word,
but in this case it appears much more conservative than the ASD
approach. Why are we being penalized by overturing? Were there a lot
of overturning failures in Northridge?
One last thought. The material that would best be served by LRFD
approach is probably soil, yet I haven't heard a rumor on for it. Why?
Jake Watson, E.I.T.
Salt Lake City, UT
Mark Gilligan wrote:
> 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
> ( (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
> 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
> 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
> 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
> 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
> 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
> section cited in the posted question, or in its predecessors.
> I think the answers already posted should be reconsidered to see if they
> the question actually asked.
> Charles O. Greenlaw SE Sacramento CA