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Cr repetitve factor for a floor joist with a point load

Cr repetitve factor for a floor joist with a point load

Cr repetitve factor for a floor joist with a point load

(OP)
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?   

RE: Cr repetitve factor for a floor joist with a point load

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.

 

RE: Cr repetitve factor for a floor joist with a point load

(OP)
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.  

 

RE: Cr repetitve factor for a floor joist with a point load

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.

 

RE: Cr repetitve factor for a floor joist with a point load

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

RE: Cr repetitve factor for a floor joist with a point load

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

RE: Cr repetitve factor for a floor joist with a point load

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.

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