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RE: STRESS AND FORCE

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O.K. Steve, I'll take the bait.

 

While stress level analysis is a good indicator of service, it is not a good indicator of failure. I mean, really, do you believe Mc/I to failure?

 

That being said, as a design engineer it only started to make sense using LRFD when I was no longer permitted to use a 1/3 increase in allowable stresses / allowable loads.

 

I think I was one of the last engineers to stop using ASD for concrete. My rationalization (a high falootin' term for excuse) was that all the concrete I design (mostly residential foundations) remain in the elastic range anyway and crack control was paramount. But, hey, what do I know? I just wonder why all those two way slabs designed in the 80s and 90s have such a high rate of service problems due to deflection. Somebody tell me because I have no idea!

 

Why are you being such a trouble maker? You must be bored.

 

:o)

 

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: SGE Structural [mailto:sgordin(--nospam--at)sgeconsulting.com]
Sent: Thursday, November 13, 2008 9:31 AM
To: seaint(--nospam--at)seaint.org
Subject: STRESS AND FORCE

 

Struggling through the 2007 AASHTO BDS, I fail to understand one very basic thing.  There was a good thing invented once, called stress, why the new codes use forces instead? 

 

The use of forces - as those of resistance or loading - is so less informative, and does not give the same feel for the performance of material/component.

 

Of course, all new equations allow conversion into stress, but why the change? 

 

V. Steve Gordin, SE
Irvine CA