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Re: Joist Modification

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I went through a similar exercise about five years ago.  My joists 
were parallel chord joists deeper than yours.  I had the joists 
shored, cut out the bottom chord and installed a new bottom chord 
over the desired length (it was near the end of the joist).  I left 
the old web diagonals in place (round bars) welded them to the new 
bottom chord and then installed new diagonals on the outside of the 
chord angles.  

The only problem arose on some long span steel joists which consisted 
of double angle top and bottom chords, vertical double angles and 
flat bar diagonals.  Went through similar exercise.  Making the 
attachments was more of a problem but we succeeded.  

The problem area was attaching the new bottom chord to the old.  I 
provided a sloping transition bottom chord and weld the 2 chords 
(full penetration) on to back-up bars and then added diagonals to 
resist the forces caused by the change in direction at the chord 
splices.  I did not modify the end.  I just re-connected everything.  
They're still standing thanks to the Great Engineer in the sky.  

Gary Hodgson, P.Eng.
Niagara Falls, ON

On 21 Jul 2003 at 15:54, Meyer, Jonathan wrote:

> I have encountered a situation where a joist bottom chord prevents
> proper head room clearance. The joists (24LH 09 - Bottom of metal deck
> ~30' AFF) are in a controlled atmosphere building built in the 60's. A
> platform was installed to provide a means of egress from the upper
> walkways in the building. This platform is partially hung from the
> roof steel and supported by the existing columns and the exterior
> tilt-up wall panels. Joists span 45 feet and support the roof (EPDM,
> insul & metal deck). As fate would have it there is only 6'-0" clear
> below the joist to the platform (min required is 6'-8") One suggestion
> is to weld a beam to the bottom of the top chord for a distance of 8
> feet at the end of the joist. Then  cut the joist bottom chord and
> attach it via members sloped to tie into the bottom flange of the beam
> and the bottom chord of the joist. The webs would also be modified to
> attach to the beam.
> I have never tried such a modification nor seen one like this, which
> leads me to believe that it has rarely (if ever) been done. I am not
> enthusiastic about such a modification, but I would like to ask if
> anyone has ever designed anything similar or seen such an animal. If
> so I would appreciate your input. If anyone has any suggestions for
> modifying the end of a joist I would also be interested in hearing
> them. Thanks for any advice that you may provide.
> Jon Meyer
> Webber Smith Associates
> Lancaster, PA

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