existing roof joists overstressed from new added load
existing roof joists overstressed from new added load
(OP)
I'd like to get other structural's opinion with the existing roof joists that I'm reviewing.
The roof joists were originally designed for 16PSF of LL (reduced from 20 PSF due to the pitch - per the old UBC) with no extra capacity. Since the IBC no longer recognizes the LL reduction per roof slope (it's now based on the given area a particular member is subjected to), I cannot make the existing roof joists to calc out for 20 PSF LL with a new additional point load of 125 lbs (more or less from a new equipment). The Bldg. Dept. wants to see calculations to show that the joists are good for 20 PSF + the 125 lbs. point load.
No work is being done to the roof, we're just attaching new equipment that adds not even 125 lbs. per joist. My argument is that 125 lbs. does not even approximate an average sized man walking on the roof (and the area of the roof with the new equipment footprint would no longer be accessible for someone to walk on).
Is this a plausible argument to make and also, since we're not altering the roof, the design LL from the old code should still be valid?
Thanks for your input.






RE: existing roof joists overstressed from new added load
RE: existing roof joists overstressed from new added load
I have used that argument if I could prove to myself that the live loads could not exist where the new equipment was located and the 125 lb load was still acceptable.
Good luck
RE: existing roof joists overstressed from new added load
You don't say where the building is located, but 20 psf seems light if you get significant snowfall in your area.
BA
RE: existing roof joists overstressed from new added load
RE: existing roof joists overstressed from new added load
Why do engineers want to take on liability for a situation which is beyond their control? Report what you find to the authorities and let them decide if further action is needed.
BA
RE: existing roof joists overstressed from new added load
RE: existing roof joists overstressed from new added load
RE: existing roof joists overstressed from new added load
FYI, you can take the reduced load for slope, and apply the load, reduced again normal to the joists, but with a greater span. It will make a difference where deflection controls in longer spans, and is more realistic. Think about it.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: existing roof joists overstressed from new added load
So what is the roof slope in this case?
If the joist design satisfies an earlier code but does not meet the present code, the authority having jurisdiction will not normally require remedial action unless a dangerous situation has been identified. However, it is not permissible to add any new load to the joists.
BA
RE: existing roof joists overstressed from new added load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: existing roof joists overstressed from new added load
RE: existing roof joists overstressed from new added load
I still believe, however that you should be open about your findings with the Building Department. If you can convince them to accept the roof as built and to allow the additional 125# load per joist even if it doesn't quite satisfy the current code, then it will be a mutual decision and you will not be hung out to dry at some later time.
BA
RE: existing roof joists overstressed from new added load
The other portion of the IBC which can be looked at is chapter 17 - there are in-situ load tests that can be run to verify strength under new loading.
Short of this, I would usually allow a 5% overstress in a member for something like this. For wood I am a little more conservative (2 to 3%) than with steel joists.