Modification of existing bar joists

[knfeng]

Need some help/advice with the following situation proposed an architect. A new elevator shaft and stairs are intended to be installed within an existing 3-story, 25 year old building. Framing system is 18" 'H' series bar joists @ 24" o.c. supported on exterior 4" brick/4" block (I'm assuming composite) w/ 4-1/2" concrete on decking.

The plan at the elevator is to build the shaft up thru the vertical space while the joists are left in place (removing the concrete and decking within the shaft space) and then provide some sort of connection/stiffening of the joist at the masonry walls. After the joist is supported the joist within the space can be removed.

The stair would be supported on new steel beams which would also support the existing bar joists, also cut back.

Has anybody done anything similar to this before. I do plan on contacting some joist manufactures and possibly SJI but wanted to get some other opinions, too.

Thanks in advance.

 

[msquared48]

If you cut or change the joists in any fashion, you will void the manufacturer's warranty on the product.

I strongly suggest going to the original manufacturer, if available, and involve them in the changes. If they are out of business or not available, the SJI is a good start.



Mike McCann
McCann Engineering

 

[JAE]

msquared48, I doubt that a 25 year old building still has a warranty of any kind, wouldn't you think?

We've done this numerous times. Fairly straightforward.

Plot the joists shear and moment capacity diagrams (remembering that the center of the joist carries either 25% or 50% of the end shear capacity depending on the year the joist was made).

Then replot the shear and moment diagrams with the new spans and loading and compare with the original - this will indicate where strengthening will be required.

Strengthening webs of H or K joists can usually be done with small angles welded on either side. For chord strengthening, use of plates or round bars can work.

See the guide by SJI on strengthening joists:

[red]TECHNICAL DIGEST #12 - EVALUATION & MODIFICATION OF OPEN WEB STEEL JOISTS & JOIST GIRDERS 2007 (February 2007)
[/red] ($25)

found at:

sji link

 

[msquared48]

JAE:

Thanks for the reference.

More than likely you are right, but I would still check it out first. Strange things do happen...

Mike McCann
McCann Engineering

 

[jike]

As JAE said, we have done this sucessfully numerous times. I recommend a site visit to field measure the joists including panel points locations. This may affect how the reinforcement is detailed since all joists are not manufactured the same.

 

[JKW05]

I've run into this situation a number of times....

Field measure the joist.
Analyze the existing joist as a truss
Analyze the new configuration as a truss.
Check the stresses in the chord and web members vs. allowable.
If there is a concern about the welds from the web to the chords, you could compare the new web forces with the original. If they don't increase, I would say it's o.k.

I can't recall finding any problems with the ones I've checked, since the span is being reduced. If you're a real idealist, you may want to separate the bottom chord from the new wall and provide a new diagonal to the new "top seat" location to avoid a "fixed-end" condition.

 

[knfeng]

Thanks for the replies, you've all reinforced what I was thinking. It all seemed fairly straightforward, I just wanted to make sure there wasn't anything obvious that I was overlooking.