OWSJ Intermediate Support

[BSVBD]

Please see attached...

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1481752061/tips/OWSJ_1_fikez5.pdf[/url]

Existing Pre-Engineered Metal Building (PEMB) Open Web Steel Joist (OWSJ).

Newly imposed HVAC RTU

I reinforce steel joists on a regular basis. But, I’ve never provided intermediate support for an OWSJ.

The client refuses to allow us to perform the necessary field work to reinforce the joist on the right end (17’-0”) of the joist as it is directly above their engineering offices.

While investigating the Sx and MOI to analyze whether we really need to reinforce, we had an idea to provide intermediate top chord support at the 23’-0” location.

Is there any reason this would not be acceptable?

If we follow through, do we sever the bottom chord below the intermediate support?

Other recommendations / cautions?

Thank you!

 

[KootK]

Thoughts:

- you may indeed want to sever the bottom chord if it could not sustain the compression implied by continuity.

- you may create a condition where webs previously designed to be in tension are now in compression.

- you've gotta thread that beam in there somehow.

- if at all possible, I'd run new webs up to the bearing point. It's hard to imagine that your T/C would survive otherwise.

- you'll need to re-evaluate the joist bracing scheme.



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.

 

[KootK]

This is a permutation that has worked for me well in the past. The use of slotted flat bars for the new webs can have some advantages. Depending on he webbing style, it may take a little detailing voodoo to avoid the need for temporary shoring. Although, given my druthers, I'd prefer to see temporary shoring anyhow.

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.

 

[SlideRuleEra]

BSVBD - I believe that an added support can be made to work, but I would not leave the top chord continuous. With the bottom cord cut, as it should be, both parts of the original joist will act a simple spans to some extent... and that's my concern. As the two halves deflect independently, what happens to that continuous top cord as time goes by?

Choose a WF with a flange wide enough (say, 8") to provide adequate bearing length for each part of the original bar joist. Sever both the top and bottom cords to make two truly simply supported bar joists.

Extend the added bearing angles past the closest remaining bar joist web member. Use web stiffeners at the loading points to help deal with eccentric loading on the WF. Not a perfect solution, but I believe it will work.

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[Once20036]

As KootK described above, I think the biggest issue is going to be members which were previously in tension now expected to act as compression members. This will happen on either side of your new support point and still required you to do work over the existing offices.

Personally, I`m not sure that I'd worry about the top chord continuous over the support. It will be overloaded, yield, rotate, and behave as though you had two simple spans - same as if you cut it. I`m not sure how you'd be able to get in there and do the cutting without coming in from the top, and any procedure risks damaging your support beam.

You may also have trouble when it comes time to transfer the load to your new beam. It isn't clear how that beam will be supported, but I suspect new posts. So you're taking a beam that's never seen load, carrying the load to posts that have never seen load, and transferring that to footings that have never seen load. You`re going to end up with a low spot in your roof. If you're not careful - you`ll end up with a very low spot in your roof.
Be sure to think through drainage/ponding/roofing issues.