Cr repetitve factor for a floor joist with a point load

[samdamon]

Gents-

I have an existing 2x12 @12" o/c framed floor with diagonal tongue and groove plank deck that is 1" thick. I am checking a few of the floor joists for some new point loads arising from proposed mep equipment. The point loads occur every 4' each way in plan. Can anyone agree that it is ok to use the Cr factor of 1.15 on Fb for the joists in this situation? The deck is too thin to perform a lateral distibution of load for moment per the old NDS section 15.1, but my understanding is that it is ok instead to use Cr=1.15. Any thoughts?

 

[JAE]

The decking will still draw down adjacent joists to a degree so some distribution of concentrated loads happens.

Do you also have bridging? This also helps to distribute the loading.

 

[samdamon]

Thanks for reply. Yes bridging is present, it is nailed x-bridging which will probably spread the load a little. I'm thinking of adding additional bridging at the new point loads in the form of solid blocks perpendicular to the joists with a light gauge strap along the bottom.

 

[JAE]

There's nothing wrong with adding the solid blocks - especially if you have a distinct concentrated load coming onto the floor in small areas or feet.

 

[COEngineeer]

My understanding was the repetitive factor is used because lumber beams are not perfect. So if you have more than 2 plys, the chance for all 3 have defect/knot at same location is a lot smaller. If you are designing the point load to be taken by only 1 2x12, then I would not use the repetitive factor.

Never, but never question engineer's judgement

 

[MiketheEngineer]

But don't forget that dead loads need a .9 factor (reducing capacity)

 

[ARLORD]

MiketheEngineer

You only use one load duration factor per load combination. You use the one corresponding to the shortest load duration. See the NDS commentary.