huangyhg

nominal moment strength & plastic moment capacity
hi,
how is that for a rectangular cross-section bent about the minor axis, the nominal moment strength is equal to plastic moment capacity??
isn't nominal moment strength = stress x b x d^2/6
plastic moment capacity = stress x b x d^2/4
rmc,
this is an international forum. which country and code are you using?
also how do you come to your original conclusion.
regards
csd
this is a sentence that i came across and wasn't able to understand while reading the beam base plate design in the steel design text book by segui
post-1989 aisc, the nominal strength has been based on the plastic moment.
i am sorry but i didn't quite clearly understand what 271828 said..is there any explanation for my doubt based on mechanics of rectangular section?
is it because d<<<b that when the section bends about the minor axis there is not much difference between its nominal moment capacity and plastic moment capacity??
i'd say that the plastic bending form factor for a rectangle is 1.5, as you've applied.
possibly, reading your last post, if d <<< b then the nominal bending moment is so low, that 1.5x is still very low, so why bother ...
it doesn't have anything to do with b vs d.
in aisc steel design post-1989, in almost all cases (bars, circles, i-shapes, etc.), the nominal flexural strength is the plastic moment. the excepts are usually sections for which stability controls--such as local buckling of a beam flange, lateral-torsional buckling of an i-shape bent about its strong axis, etc.
i think you are getting confused with the terminology. the nominal moment strength is whatever we choose; we define it to be what we want, feel safe using, calculated or arrived at empirically. so in this case we're saying that the highest load the rectangular section bent about its minor axis can take is equal to the plastic moment. that is being defined as the nominal moment. perhaps this definition of nominal moment has changed since you last investigated this particular bit of engineering.
rmc1980-
the reason that a rectangular section has a nominal moment capacity = stress*b*d^2/4 is that there are no buckling concerns for this type of shape loaded in this manner. because of this, you can achieve the full plastic moment capacity of the beam before any buckling would occur.
that being said, you would use the plastic section modulus instead of the elastic section modulus.
the plastic section modulus for a rectangular shape is bd^2/4 as opposed to bd^2/6 for the elastic section modulus.
this is the same reason that you can use 0.75fy as an allowable bending stress instead of 0.6fy or 0.66fy in the green book.
i don't know your specific structural business as well as i know airplane structure, but i would have thought that bending the weak axis of a rectangle (with d<<<b) would be susceptable to local buckling, and that the maximum compression stresses would be limited by fcy (often less than fty) but then you carry a factor of safety (by limiting your allowable to .75fy), but i'd query using a higher allowable for plastic bending; possibly i'd query how the relatively large deformation required to generate these stresses would be allowed by the strcuture around the item in question.
just my 2c worth ...
rb1957-
the difference between lrfd and asd is that you wouldn't consider the deflections that would cause the "ultimate" or plastic moment capacity of the beam. you check deflections with the service loads. at ultimate strength, you are concerned with strength not serviceability. serviceability is based on service loads.
also, bending about the weak axis reduces the suceptibility to buckling, locally or otherwise. buckling is resisted by, among other things, the stiffness of the

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