Construction Sequence for Joist Reinforcing

[GalileoG]

I have a few OWSJ that are supporting existing roof top units. These roof top units will be removed and replaced with heavier units and so the OWSJ require reinforcing. What is the required sequence of construction here? The joists are to be shored during reinforcing work but can the existing units remain while the reinforcing work is conducted? I have designed the reinforcing to resist the difference of load between the original design load and the new design load, not for the total load acting on the joists. Your suggestions would be appreciated.

 

[Triangled]

May we assume your reinforcement scheme is essentially the welding of new material such as rods and angles to the existing OWSJ?

 

[GalileoG]

That is correct. New angles will be welded to the top and bottom chords, and the web members.

 

[GalileoG]

That is correct. New angles will be welded to the top and bottom chords, and the web members.

 

[Triangled]

understanding that, as you said, you will be shoring first,
I think you could leave the equipment in place during the welding operation and then either a) remove the shoring after the welding and before the removal and replacement of the hvac, or b) wait until after the removal and replacement and then removing the shoring.
I would just advise a consistent logic through the design and construction process based upon the anticipated load to be carried by your reinforcements, understanding that the the existing truss element and that element's reinforcements will share any loads beyond the forces residual in the truss element at the time of the installation of the reinforcement, in proportion to their E*A.

assuming your shoring program includes a calculated jacking operation so that there is zero force in the truss members when the shoring is completed, then your reinforcements (properly welded and developed) as the shores are removed, will (theoretically) share the induced truss element forces in proportion to their Area * E, up to the loads imposed by the existing roof dl + hvac, then down to the loads imposed by the existing roof dl after the existing hvac is lifted, then up to the loads imposed by the existing roof dl + new hvac, then up to the whole plus snow.

if your shoring program does not include a jacking operation, but merely prevents the existing joists from deflecting any more than their current deflection under the influence of the roof dl + existing hvac, then there is a residual force in the truss members when the shoring is complete. Then your reinforcements (properly welded and developed) as the shores are removed, will still (theoretically) share the induced truss element forces in proportion to their Area * E, but in this case, assuming the shores are removed after the welding is done but before the hvac is removed and replaced (and before any other different loads are imposed such as snow or live load), the force in the original truss element will be the same as it was before reinforcing and the force in the reinforcement will be zero.

 

[chicopee]

I would remove the roof top units before work commenced because you have no idea how much the existing OWSJ have deflected. Also have you contacted the OWSJ manufacturer about your proposal to reinforce the joists? You may also want to consider spreading the load of the new roof top units over additional joists. I personally would be more inclined at installing new OWSJ's to reinforce the existing joists instead of welding reinforcing members because I think that the manufacturer will not approve your plan.