Return to index: [Subject] [Thread] [Date] [Author]
Re: ASD vs. LRFD
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint(--nospam--at)seaint.org
- Subject: Re: ASD vs. LRFD
- From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
- Date: Wed, 16 Jun 1999 15:16:28 -0400
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 (partially . > due to uncertainty of loads, partially due to uncertainty of capacity) . > from ASD (which seems to be based on empirical design stress limits? . > What is the basis of ASD? Charley, ASD is not based on "empirical" or "arbitrary" stress limits. ASD is based on a factor of safety with respect to first yield. LRFD is based on a factor 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 safety of the two methods rather easily as first yield and a fully plastic (yielded) section occur at the same load levels. ASD says that you have a factor of safety of 1.67 wrt yield on the gross area: 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 dead load alone, you have a factor of safety of 1.56 wrt yield on the gross area: 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 of 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 it would again be 1.87. For bending, the ASD factor of safety wrt first yield on the extreme fiber 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 of safety. Hope this helps. A. Roger Turk, P.E.(Structural) Tucson, Arizona
- Follow-Ups:
- Re: ASD vs. LRFD
- From: Paul Crocker
- Re: ASD vs. LRFD
- Prev by Subject: Re: ASD vs. LRFD
- Next by Subject: Re: ASD vs. LRFD
- Previous by thread: RE: ASD vs. LRFD
- Next by thread: Re: ASD vs. LRFD
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]