Modify existing open web joists

[Moe Baban]

Hi all,
I'm working on steel structure building restoration and change of use, and facing the following situation:
The existing open web steel joists (22H6) are not good to carry the new live load and I have to add steel beam in the middle to cut the span. please see sketch.

My question, these joist are supported by the top chord and I'm installing the beam under the bottom chord, is a double angle sufficient to support the top chord at the beam location as shown on the sketch.

Your response and suggestions much appreciated.

 

[r13]

Is there a deck above, roof or floor? Have you analyzed the truss with new support but without the double angle?

 

[KootK]

I don't see that you need the double angle vertical at all. What you do need is to check the rest of the truss under the new boundary condition arrangement. Key elements are likely to be the unbraced length of the bottom chord where it will be newly in compression and any web members that will undergo stress reversals.

 

[dik]

With normal pinned frame connections, the double angle would be what you would call a "zero member", that is, it would carry no load. The diagonals from the point load on the bottom to the top panel points would be doing all the work.

Dik

 

[StructEngBrah]

I would be more inclined to reinforce the chords and web members as required under the new loads. This wouldn't result in a new column and beams having to be added. SJI has published some nice guides and presentations which discuss reinforcing OWSJs.

https://steeljoist.org/wp-content/uploads/NASCC2004-JMFisher-Joist-Reinforcing.pdf

As others have pointed out, with your current scheme, you are going to potentially run into problems with the bottom chord buckling. You will still also need to make sure your web members can handle the shear demands as well. I'm surprised the building owner is OK with the idea of having a new columns installed in the middle of a bay.

 

[BAretired]

I agree that the joist needs to be re-checked as some members will have a stress reversal. The double angle proposed between beam and top chord is limited by the flexural capacity of the top chord spanning between panel points of the joist, rendering it almost useless as a support.

Reinforcing the existing joists as suggested by StructEngBrah is an idea worth exploring, but, depending on the magnitude of the additional live load, this may entail reinforcing the existing beams, columns and foundation as well.



BA

 

[Moe Baban]

retired13: Yes, I have a floor above. I did analyze the joist and with the existing span it will not hold the new live load.

 

[Moe Baban]

KootK: these joists are supported by the top chord and I'm trying to transfer the load to the beam below with out affecting the bottom chord.

 

[Moe Baban]

dik: agree.

 

[Moe Baban]

StructEngBrah: the new columns are located in side partition walls. thank you for the attachment.

I did analyze the existing beams and columns and they failed when I applied the new load. The new beams will help reduce the load of the existing.

 

[Moe Baban]

BAretired: noted. thank you.

 

[KootK]

KootK: these joists are supported by the top chord and I'm trying to transfer the load to the beam below with out affecting the bottom chord.

Respectfully, I think that you're out to lunch on that. Just because the joist is supported at the top chord at the ends, that doesn't mean that it also needs to be supported from the top chord at the new support in the middle. It's actually an error to think of the joist as being vertically supported by either chord in any location. Everywhere, the joist will be vertically supported by the webs unless you hazzard a support point between webbing joints.

 

[r13]

Moe,

I did a quick check with your scheme, with middle beam and column but without the vertical angles. The truss is fine if the additional load is kept within 50% of the original load. You can do a quick calculation to verify my claim. You should provide a guide support in direction of the truss span to avoid drawing horizontal load into the support, and a bearing pad below the bottom chord to eliminate the squeaking noise from metal against metal.

 

[BAretired]

BAretired: noted. thank you.

Perhaps you missed the part where I said that the two angles you are proposing at midspan are rendered useless as a support. As KootK says, you are out to lunch on that.

@retired13,

I have no idea how you can make that claim. You must have information that I don't have.

BA

 

[r13]

This is a simple short span truss, not very difficult to verify. I did use RISA to play with the two models though.