Modifying Existing Roof Beam

[mmarlow]

Our Customer has an existing roof beam in their garage that they would like to modify to allow a larger piece of excavating machinery into the Garage. I have attached a sketch of the existing beam, and the area that they would like to remove (6" x 14'-0). I've done some field work and found the beam to be a W24x68. It is only 38% stressed and is SL/1075 for deflection (flat roof).

I have done some checks to add a new 5/8"x10" plate to the bottom of the modified section. The Ix and Sx of the modified section satisfy the flexural requirements (even with 36ksi steel) and serviceability requirements . I have checked with an FEA to confirm deflection calcs. I am thinking I will groove weld new plates to the end of this new bottom flange and fillet weld those to the existing web to transfer the tension into the web of the existing section.

The shear in the Web of the new modified 18" beam is also OK.

My questions and concerns are:
1. What is a reasonable way to transfer the tension from the new plates to the existing flange? Other than plating it solid
2. Can I just check the web to transfer the tension if the upper portion of the web is Ok in shear? The worst area of tension transfer is at the center of the beam, where the shear is very low (2kip.)
3. I feel like I should be worried about the corner of the 6" cut and the 14' cut. Where there will be a 90 deg. cut and a lot of tension might want to transfer through that area.

Thank you,
Morgan


-MMARLOW EIT

 

[CANPRO]

A couple comments:

- are you sure you've identified all sources of load? If I checked an existing beam and it appeared to be way over designed like the one in your case, I'd be digging deeper to make sure I'm not missing anything
- your section C shows a single plate centered on the web, section B shows a plate each side. So the single plate will have to be spliced with the two side plats on site in order to get this installed. Not ideal. I'd consider using continuous angles on each side so they can be installed separately. and the angle will hold the new "flange" (horizontal leg of the angle) at 90 deg to the web.
-I assume the beam is going to be shored up during the modifications
- The tension between the existing flange and your new "flange" will be transferred via panel shear in the beam web. This is similar to when you have a beam moment connected to a column where the column has horizontal stiffeners to match the beam flanges.
- Any uplift on this beam? Might explain why your gravity load check shows the beam way oversized. Breaking the bottom flange like this will obviously have an impact on the stability of this beam if/when the bottom flange goes into compression.

 

[Ron247]

I see what CANPRO mentioned about uplift. Is the bottom flange braced currently at all?

I guess your question about the transition from C to B has to do with how long a distance to apply B to the beam. The extra flange material in B makes the neutral axis shift upwards so I have never been sure of how far to overlap. I do not recall a reference on the topic. I have done this years ago and as I recall my minimum lap was the original depth of the beam or 4xcut height which is 24" for your situation. I do not recall my final lap but I know those were my "starting minimums".

 

[mmarlow]

CANPRO,

-Thanks for the feedback. I have looked into the field work closely.. Its the smallest 24x9" beam AISC lists, and the flange thickness checks. My only thought is that this is over a working space and may have been designed to rig off? I will callout to have something painted on the modified beam.

-I like you idea of angles.

-It will b shored to get DL stresses out of the beam

-Its a simple span beam, I cannot see a situation where uplift is a concern. There is suction on the roof, but the beam is still extremely over sized for those loads

-I agree, that the beam is very large considering its use. But its a flat roof with light framing. I will look more into it, and confirm my field work.

Ron,

The bottom flange is not braced. See above to my response to CANPRO

Thank you,
Morgan

-MMARLOW EIT

 

[Ron247]

Marlow,

The comment you made about rigging off makes more sense as to why the beam appears oversized. You need to make sure about that point. If they are using it to lift/hoist things, even if you are still supplying the identical Ix, Sx etc, you step into the liability pool. I would not trust the Client to do what I stated, I would prefer it was outright stated somewhere.

 

[CANPRO]

-Its a simple span beam, I cannot see a situation where uplift is a concern. There is suction on the roof, but the beam is still extremely over sized for those loads

So how are you addressing the bending capacity of this beam in the uplift scenario? If there is suction on the roof and it overcomes the dead load, you'll have compression in the bottom flange. Your beam scales out to be about 40' long, which reduces your bending capacity of the beam by about 80% in the unbraced condition. The unbraced/uplift condition may have been the governing criteria in the original beam design.

I'm not sure how to approach the unbraced capacity of the beam with the notch in the compression flange, but if you have net uplift on the beam then this condition must be addressed. In this situation, I don't think it is sufficient to just bring the reduced section back to an equivalent Sx - you need to look at compression flange stability.

 

[mmarlow]

CANPRO,

Now I understand what you're saying... Ill be sure to check that. I can brace the bottom flange at a few locations too. Maybe either side of the modified section.

Thank you!

-MMARLOW EIT

 

[Ron247]

Also, remember that the 2nd Fb equation check for bending has a caveat about the compression flange cannot be less than the tension flange. The formula we used to call the Channel Formula. I am not sure about how much variation they mean, they just make the statement. I would guess a minor infraction is not that big a deal but I have never heard any numbers on the requirement. Since you used the largest of these 2 formulas, take that into account.

 

[KootK]

The flange in tension can be handled fairly simply. The sketch below shows what I do when I don't want to add vertical stiffeners at the beginning and end of the overlap. We can talk about the flange in compression case seperately if you feel that's a meaningful issue on this.

 

[mmarlow]

Thank you CANPRO

Kootk,

Great, I appreciate the sketch. I calc'd some wind loads and the uplift is very low. Even with the Unbraced length, we're talking 10% stressed. I will add some bracing to the btm. flange, but tension transfer is more my concern.

Thanks for the help everyone.

-MMARLOW EIT

 

[Ron247]

I have not had to do anything related to a tension field in years. The 30 degree in your sketch, is that a common value to consider for the angle or is that somehow related to the geometry of this situation?

Also, do you have any practical minimums or reduced allowables you use when doing one of these?

 

[KootK]

The 30 degree in your sketch, is that a common value to consider for the angle or is that somehow related to the geometry of this situation?

It's a common value not specific to the geometry. It pays homage to the classic Whitmore section load spread and the tension lag phenomenon. Based on the load spread assumed in web crippling calcs, I feel that one could even make an argument for a 1:2.5 spread.

Also, do you have any practical minimums or reduced allowables you use when doing one of these?

- I start at 1.5 x H + 6" and work up from there. The beauty of having a rational procedure is that one can mostly just let the numbers be your guide.

- When checking the tension field, I will use a combined stress approach to account for the shear that also resides in the web concurrently. Usually doesn't matter much.

- I like smallish fillet welds, to the tune of 5/8 x t on the beam web. This alone will will lengthen the connection considerably for a fat, heavily taxed flange.

- For serious things, like a notch in a transfer girder below a 20 story building, I'll not do this at all. Instead, I'll go straight to the arrangement with vertical stiffeners and study the shear panels bound by those stiffeners, similar to what CANPRO had in mind. In my mind, this is the only method that doesn't rely on my own judgement with regard to procedure. Above a certain scale, I don't like to be making stuff up from first principals if I don't have to.

 

[Ron247]

Gotcha, Thanks
Modifications like this you rarely find any real documentation on. Useful minimums, practical boundaries and starting values tend to be helpful.

 

[CANPRO]

KootK, I like that method for dealing with the tension transfer, nice and simple. Just wondering if it is appropriate to distribute the tension upwards towards the compression flange or if all the tension should be shown going to the tension flange - that is my gut feel, haven't put much thought into it beyond that...I usually think about these kinds of things while I'm stuck in traffic...maybe I'll answer my own question on the way home today.

 

[KootK]

Just wondering if it is appropriate to distribute the tension upwards towards the compression flange or if all the tension should be shown going to the tension flange

There's really no way to distribute the load 100% to the bottom flange and still comply with statics / mechanics of materials. After all, as you move into the deeper section, the axial force in both flanges must decrease. That said, for typical step geometries, most of the load goes to the nearby flange. I'll often calc it as though 100% goes to the near flange and just ignore the far if I deem it appropriate.

There, now you can relax and enjoy some top 40 on your way home.

 

[CANPRO]

There is no relaxing on the way home, too many people trying to run you off the road around here. During a moment of road rage, the tension fields you drew made perfect sense to me viewed as a truss - below the reinforcing plate, you're basically just transferring the bottom chord tension, and the tension field above is just a web in tension working to reduce the axial forces in the chords. I feel like that should have been obvious to me know that I see it.

 

[jayrod12]

I feel like that should have been obvious to me know that I see it.

Same here.