depth reduction in existing beam

huangyhg

depth reduction in existing beam
i have 40 in deep existing steel beam 40 feet long span. for sure beam is overly designed. i need to cut out 20" deep
and 5' long piece of beam to provide clearance. at the new short section i can add plates to  mock reduced size w beam. but any other thing i need to know in design which can affect especially due to abrupt change in section.
you need to analyze the beam with its reduced section properties for the loading which it carries.  then if the reduced section is adequate, the flange plates need to be carried a generous distance past the termination of the old flange.  nothing mysterious, just do the numbers.
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due to the reduced section, regardless of the load seen, there will be new deflection not seen previously by what it supports.  better ask yourself what the structure can tolerate.
mike mccann
mccann engineering
hokie66
how long i should extend the new flanges?
"for sure beam is overly designed."  how do you know?  i would analyze the beam for the loads it carries to be sure.  if it is part of a portal frame, don't forget to check it for lateral loads in addition to gravity loads.
the aisc manual of steel construction discusses how far to extend cover plates beyond where they are needed--your new flange is not exactly a cover plate, but i think these rules would apply.
daveatkins
"how long i should extend the new flanges?"
they should overlap far enough that the load can shear into the new flange ... practically, i'd see this being done by angles eteneding over the new flange and projecting back into the original beam ... calcs will tell you how far this needs to be
with the decrease in depth, i would check the web shear at the reduced section too.  might need web stiffeners.
mike mccann
mccann engineering
dave is on the right track for length of the new flanges, but i would be a bit more conservative because the force in the flange has to go through the web to get to the existing flange.  you actually will transition between three shapes; the cut section with new flanges, the original section with added plates on the side, and the original section.
as it has been mentioned before, by cutting the section at that location, increased deflections will surely follow. not only that dgkhan, you do not mention if this beam is isolated or not, but if it is part of a structural frame, basically you might be changing the structural model.
even if the reduced section can take the load, in effect, reduced stiffness is almost creating a pin connection, which would change the way the frame reacts to loads (wind, earthquake,...) moving stresses from one part of the structure to another part, which might not have been designed for it.
i don't know......... reducing the section depth by 1/2?  i guess it is good that it is on the tension side, but it seems to me like there would be some serious stress concentrations at the abrupt (and large) section change that might need to be considered in addition to the strength issues.