From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 11 Feb 2000 09:57:40 -0500
You have just described one of the *major* disparities with LRFD; the unequal
load factor effect!
This is even more pronounced with footings subject to partial uplift under
service loading, i.e., the heel is starting to uplift. It was exactly this
situation that caused me to start questioning the validity of LRFD,
particularly the use of unequal load factors.
The situation that I repeatedly experienced was a footing subjected to moment
in which there was uplift on the heel when subjected to service loading,
however, when the load factors were applied, there was *no* uplift! This is
impossible if, under service loads, there is zero soil pressure at one point,
there should be should be zero soil pressure under factored loads. This
completely revised soil pressure loading could adversely change the
structural design of the footing.
A number of years ago, there was a paper published about the collapse of an
underground and above ground parking plaza in Canada (BC, IIRC). It appeared
to me that the authors were grasping at straws to show why the collapse
occurred, i.e., there were fractions of an inch difference in the thickness
of the concrete, the soil in the above ground planting areas was different
than assumed in design, etc. The photograph of the collapse, IIRC, showed
the collapsed area almost completely devoid of cars! When I get a chance, I
would like to look further into that collapse, which I believe was designed
by LRFD, to see if the unequal load factors caused the design to be unsafe.
A. Roger Turk, P.E.(Structural)
Jake Watson wrote:
>> 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?<<