Strenghtening a bottom flange of a steel section with a steel plate 2

[mats12]

I need to strenghten a bottom flange of a steel profile (IPE 270).
Flange is 135 mm wide and 10 mm thick, my steel plate has to be 15 mm thick and 120 mm wide. I cant use wider steel plate.
I want to connect a steel plate to bottom flange with fillet welds on bot sides as shown in attached image.

Thats not really my cup of tea so Im wondering is it OK that steel plate is that thick - fillet weld is not made to the bottom of a steel plate but only to about 1/3 of its thickness. Is that OK?

I have to calculate a shear flow between a steel profile and a steel plate and consider this while designing a weld. Right? Any other forces acting on a weld?

Sorry for bad English.

TNX

 

[Shu Jiang PEng SE]

You need to calculate the shear flow along the plane of the interface between steel plate and steel section, then design your weld.

 

[JAE]

And the weld can be intermittent welds (stitch welds) but you should use continuous welds at the ends of the steel plate.
We like to use continuous welds that are 2 x the width of your plate.

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[Celt83]

along with what everyone else noted I like to make the plate wider than beam flange to avoid as much overhead welding as I can.

Edit: my reading comprehension is awful, didn't see you noted you can't use a wider plate.

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[JAE]

They stated that they couldn't use a wider plate but agree - top down welding is preferred.

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[TehMightyEngineer]

I'd put two thicker plates on top of the bottom flange, rather than one plate with welding overhead. Make sure you get the same grade/tensile strength plate as your beam, likely no cost difference but it's a HUGE hassle to have differing strengths in a built-up beam.

You need to unload the beam for this strengthening to be effective. Doubly so if you're welding to anything but the tips of the flanges as welding reduces the strength of the cross-section during heating.

You'll have an unsymetric beam; make sure you're calculating your strength properly for a singly symmetric beam.

AISC specification section F13 for cover plate and built-up beam details, see if you have similar details for your governing code.

You have to "develop" the plate just like you develop rebar; if you have high end moments then you need heavy welds at the end to ensure it's composite where you need it (similar to hooks at ends of rebar).

Lots of AISC papers on strengthening beams if you have an AISC membership.

Use stitch welds of a reasonable size and spacing; no sense having a continuous weld. You'll still have small fillet welds though; doesn't take much to make them composite. Make sure you meet minimum weld thickness requirements.

Plenty of topics on this on eng-tips, try a search for other details.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

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[WARose]

If we are just talking flexural/shear loads......yes: just shear flow. (Covered by a stitch weld.)

 

[JLNJ]

Adding a plate to the bottom only is terribly ineffective. Are you sure this will work for you?

 

[CANPRO]

Putting two smaller plates on the top of the bottom flange is a good idea to avoid the overhead welding. Just be careful of the radius between the flange and the web...you will need to chamfer the corner of your plate to clear the radius. The radius on your beam is 15mm, so most of your plate will be chamfered away...might need to increase the plate thickness to accommodate the weld, which isn't a problem because material costs here will be minor compared to labour.

For on site reinforcement, I suggest drawing everything as accurately as reasonably possible (include the radius in the corners as opposed to the square corners you show).

 

[mats12]

thanks for help.

I have made a spreadsheat.

If you see any mistake please let me know!

 

[JLNJ]

Your stress without reinforcement is: 90 x 100 x 13.5 / 5790 = 21 kN/cm^2, do I have that correct? If this is under your allowable of 23.50, why reinforce?

All that effort reduces your maximum stresses just 10%. And that's not considering residual stresses from the in situ condition (a debate all its own).

 

[Celt83]

looks like your spreadsheet assumes the plate and I section have the same yield strengths, maybe the case for you but I'm used to them being different.

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[TehMightyEngineer]

I recommended that to him Celt. In my experience it's generally not worth it to mix grades in cover plate reinforcement.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

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[mats12]

Yes, stresses are not that different, but deflection is reduced for about 40%.
BTW this beams are not part of any structure YET. So there is no overhand welding.
But they has to be used because they are already bought.
The span is pretty large and deflection is a problem.
By adding a plate in the bottom flange I reduce them.

I should mention this in my 1st post - deflections are my main issue.

 

[TehMightyEngineer]

BTW this beams are not part of any structure YET. So there is no overhand welding.

Unless your labor is free I would say it's almost certainly that buying a new beam is cheaper, you must have a very odd situation if this is feasible. Especially to only reinforce one flange and not both.

If deflections are your issue you're wasting money by only using a cover plate; what you need is more depth. Weld a WT onto the bottom and you'll have much less material.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[CANPRO]

Great advice from TME. Unless you absolutely have to use the beams on hand, just buy new ones and save the lighter ones for the next job.

 

[wannabeSE]

TME
My first thought is the beam is composite with a concrete slab. A plate on the bottom flange only is not uncommon in older buildings with composite beams

 

[TehMightyEngineer]

wannabeSE: I could see that but OP's spreadsheet doesn't show any composite action so I presume that's not the case here. I've also performed plenty of designs with only the bottom flange available as it was composite; but if the beam is loose I see no reason they can't keep a symmetrical section.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BridgeSmith]

TME is right on - if you have to pay a qualified/certified welder to attach the cover plate, you'll come out ahead by just buying a heavier (or deeper if you have the room for it) beam.

 

[mats12]

TME why would you say that my spreadsheet doesn't show any composite action? Because its innefective?

It lowers stresses in lower flange but since the center of gravity of composite is so low, it doesnt help much because we still have larger stresses in top (compression) flange.

Also moment of inertia (I) is significantly larger for a composite section (8418,89 cm4) compared to IPE 270 alone (5790 cm4) which means that it helps with deflection a lot.