Beam Strengthening with Web Plates

[peru4444]

I have an issue on a job I'm working on where it is proposed to increase the loads on the structure. I have assessed the capacity of the existing beams (recent steelwork, Grade S275) and one of the beams is failing by approx. 8%. The architect is not amenable to the use of flange plates to strengthen the beam as head height is already tight. Therefore, I am proposing web plates, site welded to each side of the web. This would increase the section modulus and to ensure that the beam has sufficient capacity. The steel fabricator has queried this as he is concerned that the effect of heat from welding may weaken the beam. He has suggested bolting web plates instead.

1. Is there a risk of site welding weakening the beam?
2. Will a bolted web plate solution in fact increase the section modulus?
3. Would the bolts have to be HSFG?
4. How would I design the bolts?


Thanks

 

[hokie66]

Site welding would not "weaken" the beam if done appropriately. However, if your fabricator/builder wants to bolt on reinforcement, that will work as well. Do the numbers on the deflection compatibility of the two (or three) members, then bolt the reinforcement on with enough bolts to transfer the load. I might use a channel rather than plates, as it is more efficient in resisting moment.

 

[shobroco]

I'd suggest angles rather than plates site welded to the beam; usually it is the compression flange that needs the reinforcement to increase the capacity and an angle open up & in is easy to weld to a beam top flange & web (both sides of the beam). The analysis is easy & the welds will be intermittent, not continuous so there is no chance of weakening the beam with heat.

 

[msquared48]

I would suggest adding interior flat plates to the either side of the web, top and bottom if necessary. No head height reduction with this.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Failing by 8% is probably not much of a concern, but if you are concerned, why not add one 5/8" dia. round bar top and bottom each side of the web welded to the beam? That should provide the necessary strength and is not difficult to accomplish.

If you are concerned about the welding weakening the beam, provide a prop at midspan, just a wood post or a telepost.

BA

 

[macmet]

I am not a structural guy, but I find this forum quite interesting. Would some of you be willing to post sketches of your suggestions so us less familiar can follow along?

If not, not a big deal, but would be interesting to see as well.

Cheers.

 

[BAretired]

If you want sketches, you supply them along with your question. We can modify your sketch as required.


BA

 

[corus]

I think the fabricator is more concerned with deformation of the beam with welding, rather than strength, hence the alternative option of bolting on the plates.

The problem with both welding and bolts is the increased likelihood of fatigue damage. If you welded plates on then the welds may be near the flanges and so see higher stresses. With bolts the concern may be with the stress concentration from the bolt hole, but if these are near the neutral axis of the beam, less of a concern.

For the bolted connection you have to consider the horizontal shear forces in the beam set against the shear capacity of the joint. Have a look at this site

http://pages.uoregon.edu/struct/courseware/461/461_lectures/461_lecture39/461_lecture39.html

 

[DST148]

@peru4444 - As suggested by MiketheEngineer, welding steel bars to the top of the bottom flanges could be one option. Your OP mentions that the head room is tight. Given 8% deficiency in flexure, welding a thin plate to the underside of the beam may work. Moreover, how is the fire-proofing of the steel members achieved? Is there a false ceiling? Speaking to the architect may help. Welding options could be tricky and expensive for over-head applications in the field.

As a side note: If the area is accessible from above, and only flexure and deflection is a concern, then increasing the capacity as a composite beam by field welding shear studs to the top flange could also be considered. This also requires saw cutting of the existing slab and field repairs.

 

[msquared48]

DST148:

I am not MikeTheEngineer, but thank you for the complement.

That being said, if you do intend to take this approach to strengthen the beam, remember to deload the beam with shoring prior to any strengthening, or the beam will have initial embedded stresses that will not be relieved by the strengthening process.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DST148]

@Mike McCann - I am sorry Mike. It was BAretired who had suggested welding round bar.
I fully agree with you about relieving the member of the load prior to strengthening measures.

 

[bridgebuster]

I agree with BA, 8% is nothing to lose sleep over. However, if your beam is failing in flexure adding to the flanges is more economical than adding to the web. Do you need to reinforce the entire beam or just a portion?

 

[ToadJones]

If you decide to weld...
go to:

http://www.steeltools.org

and search "BEREIN13"

Tomanovich has made a spreadsheet for just about every way you can imagine reinforcing a wide flange.