Ultimate Stresses in Design Codes
Ultimate Stresses in Design Codes
(OP)
This may be a novice question, but are ultimate stresses (tensile, compressive, etc...) that are listed for materials in design codes (such as the Aluminum Design Manual, Timber Design Manual, etc...) engineering stresses? I have always assumed that they are engineering stress because that is generally how we calculate stresses on members to compare to the design values, as opposed to using true or "actual" stresses. Thank you for any help.






RE: Ultimate Stresses in Design Codes
DaveAtkins
RE: Ultimate Stresses in Design Codes
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RE: Ultimate Stresses in Design Codes
RE: Ultimate Stresses in Design Codes
RE: Ultimate Stresses in Design Codes
As a sample is tested, in regards to a stress-strain curve (think of a simple tensile test specimen), usually all we are interested in is the engineering stress--even though the material deforms throughout its load cycle until failure. We do this by taking the force divided by the original cross-sectional area, and likewise, the strain is a measure of some change in length divided by original length. However, true stress is a measure of force over actual area of the necking part and true strain is the rate of increase in length divided by the instantaneous length. I have attached an example curve (I found this on academic.uprm.edu)
I think I have answered my own question, but had a moment of doubt concerning whether or not design codes use TRUE STRESS values for ultimate strengths or ENGINEERING stress values. I believe they use engineering stress values.
I am sorry for the confusion and thanks for the interest.
RE: Ultimate Stresses in Design Codes
RE: Ultimate Stresses in Design Codes
BA
RE: Ultimate Stresses in Design Codes
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RE: Ultimate Stresses in Design Codes
BA
RE: Ultimate Stresses in Design Codes
1. The Actual stress the member sees under a specified load,
2. The Allowable design stress for the material, and
3. The Ultimate yield stress for the material.
And JAE... the OP was probably referring to the stress engineers feel in their jobs at times.
Mike McCann, PE, SE (WA)
RE: Ultimate Stresses in Design Codes
I think the OP meant why don't we use the reduction in area for calculating ultimate stress. That would introduce second statistical variation, one from the statistical variation of the force required to break the object and one from the statistical variation of the area at breakage. For practical purposes we can ignore the reduction of area and base everything on the original area. One less statistical variation.
In seismic design of refueling wharves in California (governed by MOTEMS), concrete piles which are the primary lateral force resisting system are now designed using computer programs based on actual force-elongation to failure diagrams provided with mill certs for reinforcing and prestressing steel so we do effectively allow for reduction in area in our ultimate strength design.