huangyhg

increasing beam capacity
at work we are looking at changing the loading of a room. the dead load on the floor will be dramatically increased. i have checked and the primary steel beams supporting the load will fail in flexure. however, the beams are sufficent in block fracture, shear, etc. an initial quick check of the secondary steel beams will support the added load.
i'm looking for a good method to increase the flexural capacity of the primary steel beams.
what do you guys recommend??
there is a bit of room below the slab to increase the depth of the beams. i'm thinking about welding on a plate (perpindicular) to increase the depth (web). then weld a plate at the bottom to create another flange. is this a viable option?? would the flange plate be necessary, use a tee instead?? what possible problems would this option bring on??
using the above example, what if the additional plates had to be added in sections due to space limitations (say four 5' sections to make up the 20' span)?? what additional concerns would this bring up??
p.s. removing the existing beams and installing new ones are not an option in this situation.
any advice appreciated!!
---
andrew

i would go with the wt shape to extend the depth. that is the key: extend the depth. you could also use a tube section as well.
welds between should take the q = vq/i horizontal shear. with 5' pieces, i would provide spliced connections capable of resisting the full tensile force developed in the added steel.
it sounds like you have static loading. if it is dynamic/repetitive you need to check fatigue on all these connections.
you have calculated that the beam will fail in flexure.
has this calculation been based on the full moment capacity of the section, or a reduced moment capacity due to effective length?
it may be possible to reduce the effective length of the section, hence increasing the moment capacity of the section, by adding additional restraints to the primary beams.
just a thought.
valleyboy,
good point!! that is something that i completely overlooked. i was being very conservative and assuming the whole span as being unrestrained.
looking again, the beam (w16x31) may already be "restrained" by the 6" concrete slab that it is supporting. the problem is that i'm unsure if the metal decking is welded to the beam (and if so at what spacing). if the decking is attached, is it proper to assume the slab provides adequate lateral restaint??
if not, what is the most common way to provide lateral support?? i need lateral support at 4' spacings to get the allowable moment i need.
sorry, i don't work with steel very often.
tia!!
---
andrew
i'm not familiar with lrfd, but it should allow greater capacity for the beam, however deflection may still be a problem. however "wt" welded to the bottom does work well.
if the deck is welded to the beam, which it most likely is, then you should have sufficient lateral bracing. also, try what lsmfse suggested with lrfd. because you are only increasing dead load, you may get the result you are looking for.
if not, another gentleman in our office did the same thing jae suggested and used a wt to extend the depth. i never heard from the contractor whether or not it was easy to do. i would suspect welding a wt to the bottom flange of an existing beam would be rather difficult. make sure welder is aws certified.
good point pylko...welding a wt is an overhead weld while adding a tube shape that is wider than the flange allows for downward welding on each bottom flange tip.
agree that the deck should be adequate for lateral bracing (lb = 0).
i like to remind that lateral restraints only work if the design is controlled by the
jae pylko good points!
dmwwengr,
by me solutions for this problem are:
1. increasing the depth.
2. incresing the with of bottom flange.
why not trying to ad a wide plate at the bottom flange.
this will increase the moment resistance of the beam and also it's lateral stability.
for the good welder with corect equipment it will not be a problem, as the weld will be added downward.
here where i am from it is not restricted by codes.
it will help if you have minor problem with moment capacity.
in other case jae and pylko-the best!
dmwwengr...i would use the tube or plate approach..much easier to implement. lateral support is likely there.
try to use longest pieces possible...otherwise, since your addition will be in direct tension, the splice weld becomes a bit more critical.
good comments..all. are these forums great or what!!?
dmwwengr---i certainly won't disagree with the comments so far. all the solutions put forward here should work, but at what cost? we must keep an eye on the bottom line, i suppose. as to the difficulty in welding, the wt approach is by far the easiest to impliment in the field. overhead welding is one of the easiest welds to do correctly. a qualified welder will have no problems. continuous welds are not necessary to achieve you goals (alternate and stagger your welds to control warpage).
to weld on a sufficiently large plate to the bottom flange(or alternately, two smaller plates on the top of the bottom flange, both sides) is my next choice, but weight and handlling difficulties come into play. ts is not a good choice from the welding standpoint, although it would probably be the lightest option(if the ts section used exceeds the width of the flange, welding becomes much easier). what ever option you choose, make certain that you unload the beam to be welded on sufficiently (falsework or such added temp supports, watch the camber!!!) never weld to a loaded beam like this!
for safety sake always use qualified union ironworkers whenever possible. when in doubt, ere to the 'too strong' side.
rod

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