The increase in the allowable bending stress in plates is due the mode of failure of plates. They do not fail, when subjected to bending, by buckling. They fail by yielding!
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.
I am not familiar with AISC & AASHTO specifically, but most codes that I have dealt with have a similar provision. However, my understanding is a little different to Lutfi's. The maximum allowable bending stress in a beam assumes that the beam is restrained against lateral torsional buckling etc. For slender beams, the allowable bending stress will be a lot lower.
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.
I've done a lot of design for temporary (and some permanent) structures on highway projects. I have never found anything in AASHTO's Standard Specifications for Highway Bridge Design that lets me use more than Fb = 0.55 x Fy. For temporary structures, I will usually use a temporary allowable overstress of 20% to 25% on top of AASHTO's 0.55 x Fy allowable stress. This gets me a little closer to AISC's 0.75 x Fy.
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?
The only thing I can link to an allowable overstress in AASHTO is under Load Combinations where different groups of loads allowed different overstresses.
