Adding new roof top units to existing roof

[GalileoG]

Dear eng-tippers,

When adding new roof top units to an existing roof, what analysis/design methodology do you follow?

I understand that the general consensus would be to check if the stress increase is 5% or less as per the IBC, and if so, no member evaluation/strengthening would be required. Should the applied stress increase by more than 5%, member evaluation/strengthening would be required.

However, I am following the Canadian code and I cannot seem to find a similar allowance or relaxation. My strict interpretation of the Canadian code is that ANY increase, however small, requires a full evaluation. This seems rather impractical and unreasonable, especially if one is required to evaluate an existing building’s lateral load resisting system.

Any help or suggestions would be appreciated. Thank you.

 

[BAretired]

I doubt that anyone would be too worried about a five percent increase in stress whether or not it is stipulated by the code. The usual problem is that the roof top units, together with increased snow load caused by the units, results in substantially more than five percent increase in stress.

BA

 

[IsaacStructural]

I follow what I think is a more logical approach of checking the local member being loaded, the bar joist, steel beam, slab etc. At some point, usually either at the girder, wall or column level, the load increase becomes so minute, that any further check is written off with a simple comment. This shows that I consider the effect on these elements, but deemed it unnecessary to verify the capacity further. I've never had a problem with this approach, and in general, verifying the capacity of a couple beams doesn't take much effort at all.



M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[nac521]

I agree with BA and Issac. This is highly dependent on the size/location of the units and the roof construction. Adding a 5000 lb unit on the center on a bar joist roof is probably not a good idea and would most likely require structural modifications, but a 650 lb unit on a steel girder wouldn't require much of an analysis. I am not familiar with the Canadian code to comment on that issue.

 

[MiketheEngineer]

Agree w/ all

 

[JedClampett]

Aside from the change in snow loads and drifts, for roof mounted equipment I usually try to see if the equipment footprint times the design live load is less than the equipment weight. So for the 650 lb load cited above and a 20 psf live load, if the equipment is 6'-0" square, there's no load increase.
Sorry to state the obvious.

 

[LonnieP]

Might consider checking below the roof to make sure nothing unsual had been attached to the same member you want to carry the new A/C unit. A previous engineer may have already used whatever 'redundant' capacity existed.

 

[IsaacStructural]

JedClampett, are you assuming the roof live load isn't snow? if the 20psf is really snow load, 20psf that can pile on the roof or on the equipment, can't it? depends on the climate I suppose.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[JedClampett]

As I stated, the snow load needs to be considered separately. Snow will fall on the equipment and drifts might form around it. In Canada, it's likely to be a big deal. In Arizona, not so much.

 

[spats]

When possible, I like to put roof-top units on a raised platform above the height of flat roof snow load to preclude any drifting.

 

[msquared48]

I like to convince Architects to put any rooftop units on the ground wherever possible - solves a lot of problems if the space is available and the roof is weak.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com