From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Thu, 17 Jun 1999 13:53:42 -0400
Jake Watson wrote:
. > I am in a similar situation, but I am also very new to the engineering
. > profession. So I like to sit back, watch and learned from the experience
. > of the people on this list.
Hi, Jake! Welcome to the list!
. > LRFD does not profess to give an "exact" answer for all loading
. > conditions. Instead it is based on the assumption of an
. > "envelope" answer.
But even with considering the envelope, LRFD will give you problems. If we
apply the load factors to the loads and consider the envelope from that we
will get one thing. If we draw, say, separate moment diagrams for factored
DL and factored LL, we can create an envelope from these. But, what if the
factored moment due to LL at a particular point is, say, negative, and the
factored moment due to DL at that point is positive, what envelope do we
use? The load factor for DL alone is 1.4; the load factor for DL when used
in combination with LL is 1.2. What load factor was used to draw the moment
diagram for the DL?
Let us also consider in the same situation that the service load DL and LL
moments at that same point are both positive. But, when we apply the load
factor to the live load, the live load moment at that point becomes negative
(entirely possible particularly for a continuous beam with the live load in
an adjacent span), what do we do? Do we just neglect the negative factored
LL moment and use only the factored DL? With ASD, we would consider the sum
of the service load moments at that point because both were of the same
sign. Do we apply the load factors to the service load moments (or whatever
we are considering), but that has caused me much consternation in designing
concrete footings subject to overturning with partial uplift.
Drawing shear and moment envelopes for service loads and factored loads is no
problem. What is the problem is that the maximum conditions for factored
loads do not have to occur at the same point as the maximum conditions under
service loads. Since service loads represent the loads that can be expected
on the structure, the factor of safety should be applied to service load
conditions. Applying factors of safety (load factors) to loads merely
represents an analysis for an overload condition, which may not provide
adequate safety for service load conditions.
A. Roger Turk, P.E.(Structural)