huangyhg

thicker flange steel beam
a steel beam that has a steel plate welded to one of the flanges would have two different sx. if used as a simply supported beam, would you put the thickened flange on the top (comp) or the bottom (tension). does one of those orientations allow you to use the larger section modulus when calculating bending stress, or are you stuck using the smaller of the two sx?
with the thickened flange, there is only one ix for the deflection calculation so it shouldn't matter if the thickened flange is on the top or the bottom.
assume that unbraced length is not a problem and it is just a beam (no concrete slab on top for compression).
any help will be greatly appreciated.
thanks
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in this case, i would put it on the top (to provide a greater stop and subsequently a lower compressiove bending stress). you will have to check the allowable bending stresses based on local buckling (since you stated lateral-torsional buckling is not a concern) and yielding.
the value of s calculated using the distance from the neutral axis to the top is the one you use to calculate the top stress and vice versa (look back at your materials texts).
keep in mind that the allowable stress of the plate may be lower than the beam.
csd
i would put the plate on the top to add to the strength in the ltb failure mode. being more thorough, you would calculate strength based on both sx's and use the least value. you could also realize that using the smaller value will give you more stress and just check that. using both values, you could find the stress in the compression and tension flanges and compare these to allowables. the allowable will not be the same for both.
it all depends on how you want to work the problem and what you are interested in investigating.
ucfse,
if its a 50ksi beam and 36ksi steel you could get the critical stress being in the plate rather then the beam flange.
csd
without ltb as an issue, i'd put it on the bottom
if
1. the plate is added in the field and,
2. the plate is wider than the beam flange.
this is because overhead welding is more difficult.
if the plate is narrower than the beam flange, then putting it on the top would make the welding easier.
if ltb is and issue:
i'd agree with ucfse and put it on the top to minimize ltb buckling.
if the top flange is fully laterally supported, i would put it on the bottom to help with lateral buckling.
mike mccann
mccann engineering
csd, i know that. i wasn't trying to write an exhaustive post.
i believe if you check your text books for the definition of s, you'll see they all include "distance to extreme fiber". some books are vague, but others say that is what you use for design, regardless if you have two different values for stop and sbottom.
very true lppe. and is normal for composite design to have an st and sb due to the concrete overlay, assuming composite action, of course.
mike mccann
mccann engineering
i think you have two votes for top and two votes for bottom. as long as you know that the stress is different at the two flanges, you have all the information you need to make a decision.

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