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Re: ASD vs. LRFD

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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
(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