Composite Beam Action? 1

[GoBizon]

I’m looking for feedback on the analysis/common practices for reinforcing an existing W-flange beam. I’ reviewing an existing W-flange beam supporting some mechanical equipment. The owner wants to add equipment to the beam doubling the load on the existing beam and over stressing the beam. I would like to reinforce the beam with a WT section welded to the bottom flange of the existing W-flange beam. The existing load cannot be removed during the installation of the WT. My question is this: Do I treat the design of the built-up section similar to a composite steel/concrete section (simplified: Moment_existing/Section_existing + Moment_additional/Section_built-up)?

 

[lkjh345]

Thats the way I handle these situations. My thought is the existing stress is already present in the existing member, and the new stresses are additive to that existing stress.

The sum of the new stress plus the old stress must be below allowable. ( Here I go still thinking in ASD!!)

For full disclosure, this is a point of disbute in our office.

Is there anyway to shore the beam up from below to relieve the existing stress on the beam while the new WT section is added? Then the built up section would be effective (in my mind) for the whole load when the shoring is removed.

JMHO.

 

[PEinVA]

I've done it that way in the past for situations like that. The problem I've always had was adding enough of a WT to help, but not more steel that would make the composite beam drop below 25% thus negating any composite effect from the concrete that I could design with.

Some other thoughts:

Is there anyway you can cut the tributary area of the beams to lessen the load instead of installing a large WT section. Also, can you add studs to this beam? Maybe do some studs with a smaller WT section.

Just some ideas.

Is it failing in bending and shear?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[MiketheEngineer]

Can you sister the existing beam?? Or add columns

 

[GoBizon]

To follow up with some of the questions: The existing beam is part of a frame built on top of a c.i.p. concrete roof. The 4 corners of the frame sit on stub columns centered on the existing concrete columns (the existing roof as not able to carry the mechanical unit and snow drifting). Shoring would be difficult, in my opinion. The beam is failing in bending only, shear and deflection are o.k.

 

[kslee1000]

Instead of WT at the bottom, can you add two channels, or angles to the web? The way the NA will not shifting too much.

 

[civilperson]

A stub column similar to the corners at midspan is my suggestion. Simpler than welding continuously to beam.

 

[gwynn]

Analysing the strengthened section as suggested is generally acceptable. It may be conservative if designing for the full plastic moment, but that may not make much difference anyway.

There are a large number of ways to strengthen such a frame. Anything from changing fixity condtions at the ends of the beam to adding kickers to kingposting the beam may work depending on the actual geometry and loads, but if you have determined that welding a WT to the bottom flange is the best bang for your buck then good luck with it.

kslee
I don't see the benifit of adding to the sides of the web. You would need to add far more steel by weight to get the same increase in bending strength and require a much longer length of field welding.

 

[Lion06]

I posted an example in this thread
thread507-221554

Check it out.
Read through the thread, too, it will be helpful

 

[csd72]

A tee is very hard to weld on site as it cannot be easily clamped to the existing beam and has a tendency to twist/warp.

I have used a small but heavy W shape to do the same job.

 

[dougantholz]

If you have the clearance under the beam, how about a king post truss? Less material - not too many man hours. Look at it with only a an additional 12 or 16" of depth as a shallow truss an you can be amazed at the additional flexural strength. Not to mention the boost in deflection capacity. This doesn't help your shear capacity, but hey - you can't solve that with a T on the bottom anyway.