existing roof joists overstressed from new added load

[RacingAZ]

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.

 

[steellion]

I would add web reinforcement (e.g. a L1x1) at the point load and call it a day. There is no change to the occupancy or load requirements of the roof, therefore, the old code still applies. The IBC existing building provisions will back you up on this. If it were more than 125#, say over 300#, I'd reconsider, but you're exactly right, that's well less than an average man walking around on the roof.

 

[MiketheEngineer]

First - look at IBC 1607.11.2

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

 

[BAretired]

If you are reviewing the design of existing joists and cannot make them work, that is what you should report to the authority having jurisdiction. Tell them how much the joists are overstressed and let them decide what is to be done.

You don't say where the building is located, but 20 psf seems light if you get significant snowfall in your area.

BA

 

[slickdeals]

I believe that unless you are not making substantial changes to a building, you don't need to bring it up to code of current day. I don't think adding a 125 lb. load would quantify as a substantial change to the building.

 

[BAretired]

If we are talking about wood joists closely spaced, 125 pounds per joist is substantial. You are not justified in adding any load to a roof which does not meet the current code.

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

 

[JAE]

Doesn't section 4.9.1 in ASCE 7-05 provide for reduction with pitched roofs?

 

[JAE]

...also equations (16-31), (16-32), and (16-33) in the IBC 2006.

 

[msquared48]

Exactly JAE. The slope still applies for joists, a well as the area reduction (not so area-wise for the joists though).

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

 

[BAretired]

The slope of the roof has not been given (unless I've missed something along the way in this discussion). If the slope is 30 degrees or less, there is no slope reduction for snow load according to the National Building Code of Canada (NBC) unless the roof is 'slippery'. For greater slopes, an equation is given to determine a slope factor less than 1.

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

 

[msquared48]

True that the greater the slope, the greater the ultimate positive impact on solving the problem without modificaton.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RacingAZ]

This is in a location without snow load consideration, standard roof LL of 20 psf (reducible). The pitch is 4:12 and in the old UBC (which the building was under when permitted), the 20psf LL was reduced to 16psf. For IBC, the reduction factor for a 4:12 roof pitch sums up to be 1 per eq. 16-32 so there is really no reduction.

 

[BAretired]

If there is no snow load, then 20 psf is simply a minimum live load to allow for occasional access. The whole exercise is somewhat academic.

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

 

[JAE]

Whether it is snow load .... or live load...it is still a load mandated by the code.

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.