Roger, I appreciate your patience. There are some on this list who are so
enamoured with smoke, mirrors, and blind (pro LRFD) prejudice that their
opinions have degenerated to the name-calling level.
Naturally, none of us hard core conservative, radical right-wing ASD
fanatics have a prejudiced bone in our bodies :-)
Fountain E. Conner, P.E.
Gulf Breeze, Fl. 32561
> From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
> To: seaint(--nospam--at)seaint.org
> Subject: Re: ASD vs. LRFD
> Date: Wednesday, June 16, 1999 2:16 PM
> Charley Hamilton wrote:
> . > I think (note the disclaimer!) that the intent of LRFD was to take a
> . > step closer to reflecting the "actual" state of uncertainty
> . > due to uncertainty of loads, partially due to uncertainty of
> . > from ASD (which seems to be based on empirical design stress limits?
> . > What is the basis of ASD?
> ASD is not based on "empirical" or "arbitrary" stress limits. ASD is
> on a factor of safety with respect to first yield. LRFD is based on a
> of safety with respect to a fully plastic (yielded) section.
> If we look at a member in pure tension, we can compare the factors of
> of the two methods rather easily as first yield and a fully plastic
> section occur at the same load levels.
> ASD says that you have a factor of safety of 1.67 wrt yield on the gross
> F(t) = .60F(y)
> where F(t) is the allowable tension stress.
> or a factor of safety of 2.0 wrt ultimate on the effective net area:
> F(t) = .50F(u)
> With LRFD, figuring out the factor of safety is a little more complex as
> there are different load factors for different loadings, however, for
> load alone, you have a factor of safety of 1.56 wrt yield on the gross
> phi*P(n)/A(g) = F(y)
> where phi = 0.9
> phi*P(n) >= 1.4*DL
> P(n) >= 1.4*DL/0.9
> or a factor of safety of 1.87 wrt ultimate on the net section:
> phi*P(n)/A(e) = F(u)
> where phi = 0.75
> phi*P(n) >= 1.4*DL
> P(n) >= 1.4*DL/0.75
> For both dead load and live load, you have to consider the relationship
> both to the total load (TL). If both the dead and live loads are each
> one-half of the total load, then;
> phi*P(n) >= 1.2*TL/2 + 1.6*TL/2
> = 1.4 TL
> and the factor of safety wrt yield would be 1.56 again, and wrt ultimate
> would again be 1.87.
> For bending, the ASD factor of safety wrt first yield on the extreme
> varies depending on the shape, but for a compact shape, the FS is 1.52:
> F(b) = 0.66 F(y)
> For bending in LRFD, I'll let someone else [try to] figure out the factor
> Hope this helps.
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona