strengthening existing steel members

huangyhg

strengthening existing steel members
i have an existing 2-storey steel structure which is to be extended upward, making 3 floor levels. upon analysis of the existing members to support the new loads, i have found that a few of the beams and columns are inadequate - there are either deflection issues or the member fails in bending moment capacity.
i have been told that the way to strengthen the members is to just determine what the optimum member size would be, find the section modulus of that optimum size and then add steel plates to the flanges and webs of the existing beams/columns in question to increase the section modulus accordingly. i just wanted to know if this is all that's required....matching the section modulus.
any help would be greatly appreciated. thanks so much.
increasing section modulus increases strength, increasing moment of inertia decreases deflection.  adding plates will help both, but you will need to check both.  welding steel plates to existing members may or may not be the most efficient way of reinforcing your structure.  field welding is tedious and expensive.
have you checked the columns?  footings?  lateral system?  are these beams that you are proposing to strengthen now roof beams?  if so, what about the secondary   
thanks hokie66 for your response.
i have modeled and analyzed the entire structure using ram structural systems. most of the columns are ok as is but some of them require strengthening. the footings are ok to support the new loads and braced frames are used to resist lateral loads.
the beams that need strenghtening are currently roof beams but the structure was originally designed with the intention of going up, so there is currently composite metal decking with concrete topping on the roof which will now become the third floor level.
then why are the beams and columns overstressed?  increased load due to different useage, or code changes?
mike mccann
mmc engineering
have you considered llr for the major beams and columns?
mike mccann
mmc engineering
i would say that it is more involved than that.  i've done strengthening of existing members and, at least in my experience, it's not that simple unless you're jacking the structure to relieve the existing stresses.  if you jack the structure to relieve existing stresses, then yes, you're procedure is fine.  if you're not, which in my experience is more likely, then you have a little more work on your hands, but not too much.  additionally, plates usually provide only a marginal benefit.  i've always ended up having to use a wt section.
here's the procedure i've used.  first, find out the fy of the existing beam.  next, i determine the dl stresses in the member.  i typically will select a wt size that seems reasonable to me - i wish i could offer more guidance on selecting it, but you'll get a feel for a good place to start as you do more of them.  i find the new section properties of the built-up member.  now i check stresses at 3 places: the top of the beam, the bottom of the existing beam, and the bottom of the wt section.  in my opinion the only one that is really necessary is the top of the beam - for most cases.  i say most cases, because under typical loading for a simply supported   
to add to strleit's post, i have used the attached spreadsheet to aid in my analysis / design.  it really does a nice job of delineating your stress checks.  i have a modified version of that spreadsheet, so it better fits my specific needs (different types of reinforcing schemes and so on).
otherwise, like strleit, i have also done numerous reinforcing of existing steel beams and his procedure is the same as mine.  one of the "benefits" of retrofitting older buildings, right?
joel berg
hokie66...that's a formulaic consideration.  with some minor exceptions, you can't increase one without increasing the other, though admittedly not necessarily at the same rate (i.e. adding plates to the bottom flange only).
"...there is currently composite metal decking with concrete topping on the roof which will now become the third floor level."
it's not just a matter of adding plates here, especially if there are studs in addition to the metal deck.  is the floor fully composite now, or partially composite?   
mike mccann
mmc engineering
connections !
check them as well
or not composite at all?
it's the decking which is described as composite, not the beams.