Make an Existing non-composite beam act composite?

[tweedledee]

Have any of you guys modified an existing non composite steel beam with existing concrete slab on it so as to make the beam act Composite with the concrete??

I have heard of a way of coring out the concrete down to the top of the beam flange, welding on studs, and placing non-shrink grout in the core??

Has anyone had experience with this??

 

[Lutfi]

I have read and heard of other engineers utilizing this method. I think it works and I would utilize it if I need to increase steel beam capacity due to new requirements to increase floor loading.



Regards,
Lutfi

 

[jloo73]

we have not cored from the top before although that is a possiblity. We have added plates to the top flange and expansion bolted or epoxied threaded rods to the underside of the slab to produce partial composite action. We did this because we were adding additional beams and we had access to the underside of the slab.

 

[RRPC]

In older structures I have sometimes considered concrete encasement for fireproofing to act compositely if it meets the requirements. Check slab thickness / steel coverage, concrete reinforcing or mesh. Don't forget that the composite section resists only the live load so sometimes its hard to achieve effective use of the composite section since the steel already has a lot of DL stress.

 

[Ingenuity]

tweedleldee,

About 7 years ago I designed and constructed NEW composite steel beams to an EXISTING PT slab that required strengthening for a new dynamic load. There was very limited head room available underslab - hence why we needed composite action.

We cored the PT slab (scanned it first to miss the tendons), then used a paper template to mark the location of cored holes to proposed stud locations. Welded the studs to the top flange of the beam on the ground, then lifted the beams up with the studs passing though the cored holes.

We elected to use low-exothermic 2 component epoxy resin mixed with silica sand for the grout of the annulus between the stud and cored hole - and also epoxy injected the small space/gap between the top flange and underside slab.

A pre-bagged cementitious grout would have also worked to fill the annulus.

For your case with an EXISTING steel beam in place to be made composite, the coring will work the same but you need to make sure the core hole is large enough to enable the stud welding machine access. I guess you cannot undertake a 'bend test' of a welded stud either, unless a sample area be chipped out.

HTH

 

[VoyageofDiscovery]

To get more "composite" action, if possible jack the beams along their length prior to coring. Another option is to drill from bottom up and use epoxy anchors.

Regards

VOD

 

[frogit22]

I have used this method before, of course, remember that any capacity you add will only be capable of carrying live load (the existing beam is already carrying the dead load so the composite action of the slab will never see the load).

 

[kxa]

Is this a beam that is overstressed or sagging already or you just want to increase its strength for LL. If you have no shear problem, you may be able to jack it enough to release some of the stresses while not inducing any tension in the concrete. You can then attach a cover plate to the bottom before removing the jack. The length and size of cover plate and amount of jacking would depend on the beam condition and the loading.

 

[Taro]

Jacking up the beam to relieve the dead load is not really required for strength design (although it might help if deflections are excessive). The old ASD superposition of stresses method is outdated for composite beam design.

 

[tweedledee]

kxa,
The beam is not overstressed at the moment, but it will be.
The beam carries a dead load of 0.36klf. I will be adding the same amount of dead load to it (new dead load =0.72klf), + designing for the existing live load of 0.6klf.

tweedledee

 

[kxa]

Looks like it will be overstressed due to the additional DL=0.36 klf or perhaps a little more. Calculate the existing load carrying capacity of the beam (find Moment) and draw the moment diagram for the proposed loading. You will need a cover plate for the protion of the beam that the moment diagram exceeds the existing moment capasity of the beam. Also calculate the deflection for the additional load and jack the beam by that much. Check stress in concrete, shear in steel beam and the connections at the ends. This is kinda like post-tensioning the beam. Hope it helps.