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Adding Plate on I section

Adding Plate on I section

(OP)
We already fabricate an I beam 150x75x14 (tw=5, tf=7).suddenly the client want us to increase the design loads. The solution we came up is to add a plate at the bottom flange. and the thickness of the plate is 9mm. The question is it ok to have a thicker additional plate than the thickness of the flange. or is there any other restriction on adding plate on I beam section. Thank you in advance

RE: Adding Plate on I section

Interesting question. I believe you can if you check the load path through the weld and don't find you are causing the web to be overloaded and cause it to buckle.

Best regards - Al

RE: Adding Plate on I section

Design check per your applicable steel code. And weld distortion may be an issue- the whole beam could be curved or warped when finished.

RE: Adding Plate on I section

Unless highly loaded or subject to repetitive loading, you may be able to space intermittent welds stitching the plate to the beam. Unless fabricated, it may be less costly to just use a different beam.

Dik

RE: Adding Plate on I section

Will increased deflection be an issue? Would it be better to add the plate to the top flange to reduce LTB.

RE: Adding Plate on I section

What's the primary aim of the reinforcement? Deflection control? Plastic moment capacity? If it's the former, you should be fine. The elastic neutral axis will shift downwards but not too much. If it's the latter, consider that your plastic neutral axis will land within your bottom flange. That would imply:

1) A web that is uniformly-ish compressed rather than flexed with a T/C strain gradient.
2) A top flange that may be strained to a unreasonable degree.

I'd be tempted to design for flexure using elastic capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Adding Plate on I section

(OP)
all your answer/opinion are very much appreciated. Its the plastic moment capacity that governs. thank you all

RE: Adding Plate on I section

Jandra,

Did you mean Elastic Moment Capacity instead of plastic? See Koot's last response. I'm sure you're familiar with the process for checking the weld since you should have done it when you designed the original built-up beam, but V*Q/(I*b) is a great composite beam equation.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

RE: Adding Plate on I section

Adding a reinforcing plate that is thicker than the original plate is done quite often, especially in truss reinforcing. Just be careful with the welds. You don't want to put too much heat into the thinner plate, otherwise you may get distortion.

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