Allowable Bending Stress in Plate
Allowable Bending Stress in Plate
(OP)
AISC - ASD 9th Ed. allows a stress increase for bending in plates of 25% (or .75 Fy, Eq. F2-1). Is there a similar increase in the AASHTO code?
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Allowable Bending Stress in Plate
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Allowable Bending Stress in PlateAllowable Bending Stress in Plate(OP)
AISC - ASD 9th Ed. allows a stress increase for bending in plates of 25% (or .75 Fy, Eq. F2-1). Is there a similar increase in the AASHTO code?
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RE: Allowable Bending Stress in Plate
My limited experience in AASHTO is they run a little more conservative than AISC but they are not that far apart.
I would be interested to hear from other colleagues on this as well.
RE: Allowable Bending Stress in Plate
When a typical I-beam or Channel section (or similar) reaches first yield in the flange, you are quite close to the full plastic moment capacity of the section. This is because a large part of the total moment-carrying capacity is provided by the flanges, and only a small part by the web. Once the flanges start to yield, you have just about reached the section's ultimate capacity.
Specifically, if you check the ratio of Elastic Moment Capacity (i.e. moment at first yield) to Plastic Moment Capacity (i.e. ultimate moment at which a plastic hinge is fully formed) for typical I-beams, you will see the full plastic moment capacity is only about 15% higher than the moment at first yield. (The actual ratio varies according to the geometry of the cross-section, but usually falls in the range of approximately 10% to 20%.)
However, for a flat plate, the moment at full yield is actually 50% higher than the moment at first yield. This is because the full depth of the plate contributes to the moment resistance, not just the top and bottom surfaces of the plate.
Accordingly, it makes sense to allow a higher bending stress in flat plate than in a typical rolled beam, as this will provide a comparable factor of safety on collapse by bending stress.
Hope this helps!
RE: Allowable Bending Stress in Plate
If someone knows of an AASHTO section that lets us design flat plates (such as column base plates) to a higher allowable stress, please cite the reference. Most highway design for permanent structures now seems to be done using AASHTO's LRFD method. This adds more confusion to the plate design. Shouldn't you be comparing plate design by AISC LRFD to AASHTO LRFD?
RE: Allowable Bending Stress in Plate
Regards
VOD