Adding reinforcing plate

[slickdeals]

thread507-199450

I am evaluating an existing W6x12 beam in a warehouse that does not work for flexure based on a 125 psf storage live load that it might get subjected to. The beam has simple spans and is supporting metal grating welded to the top flange (assumed to brace compression flange).

Adding a plate to the bottom increases the section modulus of the bottom face significantly, but only marginally increases that of the top face. So a straight up, M/Stop fails.

However, the plastic section modulus of the beam with reinforcing plate exceeds the required section modulus.

I am calculating the stresses based on a 36 ksi yield stress to be conservative.

Would it be acceptable to compute the design capacity of the beam based on a plastic section modulus of the composite section (W6+plate)?

We are Virginia Tech
Go HOKIES

 

[nutte]

You wind up with a singly symmetric I shape, which is covered in section F4 of the AISC 2005 specification. Here, you're allowed to use Z (not to exceed 1.6 Sxc) if your web is compact.

 

[jaytee01]

If you are adding the reinforcing plate while there is still some load on the existing beam, the reinforcement is only carrying the additional load. You can't just look at the total load supported by the total section modulus to get the bending stress in the beam.

Determine the stress (f1) in the original beam due to the load present in the beam at the time of reinforcing. Determine the allowable stress for the beam. If the top flange is continuously supported and compact, then Fb=0.66Fy or 23.8 ksi. Subtract the existing stress, f1, from the allowable stress. This stress is the amount remaining for the reinforced beam before you exceed the allowable stress. So, to calculate the required section modulus of the reinforced beam, divide the moment due to the loading imposed on the beam after reinforcement by the remaining stress. Add cover plates or shapes to your existing beam so that you meet or exceed this required modulus.

When your reinforced beam is now fully loaded, the maximum stress will stay below the allowable values.

As an example, if you remove all load from the beam before reinforcement, then add the plate, then add the load, the entire reinforced section will carry the whole load. Therefore, it is advantageous to remove as much load as possible in order to get the smallest reinforcement section.

 

[msquared48]

Can you make the section fixed over the existing column to decrease the section modulus needed?

Will need to shore if this is done in order to load the beam properly when the shoring is removed.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[slickdeals]

@mike,
I think this was an error in design. This mezzanine was built in 1987 without a permit. We are now doing as-builts and trying to get it "legalized".

All beams in adjacent bays with similar tributaries were W6x20. Making it fixed might end up being more expensive than adding reinforcing plates.

@jaytee,
Valid points. The Allowable stress method is definitely going to yield me large plates. I am going to try to go the plastic section modulus route in order to get lesser reinforcing, without having to jack up the beam and relieve stress. I will have to work out the existing stresses and then figure out how to work it.

We are Virginia Tech
Go HOKIES

 

[jberg]

I would recommend looking at one of Alex Tomanovich's spreadsheets, BMREINF9.xls.

http://www.steeltools.org/STEELTOOL...umentKey=8fa14250-9c08-4fbf-91db-658dd555fbc4

JWB

 

[slickdeals]

I found this presentation that discusses LRFD vs ASD approach in reinforcing existing beams.


We are Virginia Tech
Go HOKIES

 

[VoyageofDiscovery]

You may have to look at the structure through LRFD.

HTH

VoD