Wind and seismic load evaluations for existing roof rafters for commercial building 2

[H Bui]

Good day,

I am working on a project that requires to have a new Makeup Air Unit (MAU)installed on the roof top of an existing commercial building. The local Building Department requests to show the existing roof structure can support the weight of the new MAU (under 500 lbs.) as well as evaluating both wind and seismic load for the roof. My question is, seismic analysis of only the roof structure would not be sufficient enough because the force would need to be transferred to the bearing walls and to foundation, which seems to me a full seismic analysis of the entire commercial building. Since the project is only a tenant improvement of a suite within the entire building, should any seismic analysis be "overkilled" for such a small project?

Please share any experience that you may have working on tenant improvement project that require to have a new MAU installed on the roof top. My project is in Northern California.

Thank you.

 

[milkshakelake]

500 lb lateral load is like 50 lb or less, assuming a high Cs of 0.1. (Please don't take this as a fact; it's just an illustrative concept, and the actual lateral load might be very different.) It is absolutely ridiculous to require a full building assessment for this tiny load. I have done much heavier rooftop unit installations that didn't require a full building seismic analysis. But some things like adding a bulkhead would require it.

This is my go-to for things like this, ASCE 7-10:

 

[strucbells]

Agreed with MSL, that would be an absurd ask for this type of project.

Sounds like you are putting in a new unit, not replacing an old unit, so if your jurisdiction has adopted some form of the International Existing Building Code (IEBC), your project would be a Level 2 Alteration, see 2018 IEBC 806.2 and 806.3 for the gravity and lateral load requirements. Check your actual local jurisdiction to determine what code may apply and what amendments they may have adopted.

I would expect to do a quick gravity load check to distribute the unit's load (and any addt'l snow drift load if any side of the unit >15ft, probably not based on the MAU weight and depending on where you are in northern CA) to the primary gravity resisting system. I would calc wind/seismic loads for design of the anchorage of the unit to the roof but say the building frame itself is ok by inspection per the 10% rule if the IEBC applies(see 806.3 above) or by engineering judgement. The additional wind or seismic load on the building itself due to a new rooftop unit is like a gnat on an elephant for any remotely normal situation.

 

[H Bui]

Thank you MSL and SB for your answers. I think utilizing 11B.2.2 as an exception to avoid conducting seismic analysis on the existing roof is a sound approach to present to the local B.D.. Additionally, I concur that calculating the dead load of the MAU to existing roof rafters to ensure their adequacy is imperative.

Thank you both very much. Your help is greatly appreciated.

 

[lexpatrie]

Is it the building official that is asking this? Perhaps they've made some general comments and you're wondering how to interpret that?

The general approach is that one performs the analysis for the changed loads until they become not significant in the eye of the professional engineer (that sounds a bit nerve-wracking, perhaps), but for a light RTU, you'd probably see most add some support angles for the roof deck around the unit perimeter, then checking the load on the joists or purlins, (probably some reinforcement) perhaps check the beams or girders that support it, and fairly unlikely they'd consider the load on a splice (for a cantilever beam roof system, for example), and unlikely they'd be checking the column and footing for the added loads, because they become pretty small as a percentage of the existing load the farther and farther you go into the load path to ground.

Seismic would increase the lateral load due to the added weight of the RTU, but for a fairly large roof, you could either a) have the minimum forces govern, b) the maximum forces govern, or c) the say 10% from the RTU would increase the diaphragm load maybe 1%?

You may not have the original design to work from, at which point probably some measure on on site work to determine if it's something fairly normal for the time of construction, has a load path, locate the seismic frames/walls and their distance from the RTU, and compare the currently required forces versus what you think was done at the time it was originally built, plus any weakening you've diagnosed after construction.

This isn't an addition, though, it's an alteration.

Wind load shouldn't change all that much, the wind guidance on rooftop units appeared pretty recently in ASCE 7.