Wind force on Rooftop Equipment

[StructCA]

I'm currently working on project checking the capacity of an existing building to support a new piece of equipment being added to the rooftop. In this situation wind is the governing lateral force, the equipment area is relatively small when compared to the building area, and the height is less than 60 feet total. The existing building is from the 1960s and is a non-code complying lateral system so my first thought was to compare the buildings original wind design load based on old code to the new equipment's wind load based on ASCE7-10 and see what the impact is.

My question is what section of ASCE to use to determine the new equipment's force on the building? It appears to me that ASCE Section 29.5.1 is providing a force for design of the equipment itself and/or the connection to the main building, but not a force for comparison to the building wind design force. Any thoughts? FYI, I'm not responsible for the design the equipment.

I've also thought that perhaps I would calculated the building main wind force based on Chapter 27 and apply the pressure over the equipment for a comparison. Does this seem reasonable?

Any input would be great.

 

[Nor Cal SE]

This is one of those judgment calls where you could talk with 10 different SE's and likely get 10 different answers. Here is my thought:

Perhaps begin by looking at the projected wind area of the whole building in comparison to the added projected wind area of just the equipment. The precise load won't matter yet as you're just checking an area comparison. I would say if the ratio of projected wind area of equipment divided by whole structure is 0.05 or less, than your building modification could be considered "incidental" and you could forego further analysis on the whole building.

You would still want to check equipment anchorage to the roof for the ASCE 7-10 wind loads on just the equipment, ensuring that the anchorage can withstand those loads and that the loads can adequate transfer to the roof. Furthermore, you'll want to ensure the structure can resist the added weight strictly from a vertical load standpoint. In my opinion this would suffice.

 

[CBSE]

Don't forget about seismic loading either.

 

[EngineeringEric]

I approach as follows.... Look at the equipment as if it was a free standing sign or a parapet on the MWFRS, record these numbers. Look at your C&C calcs for the equipment and compare to C&C of the parapet. Then with all this information I apply a load that i think works... Something to show a judge is all you need

I believe the chapter for roof top equit and objects does have some good information and is worth digging through, it is usually at the end of the chapters.

 

[CBlaker114]

I would think this is mainly an attachment check and local framing check, yes?

I would reference the Existing Building code here. For any members where gravity loads aren't increased by more than 5% and where lateral loads aren't increased by 10%, you don't even need to check them. You may need to track these loads a ways into the framing but I doubt you're increasing your MWFRS loading by 10%. I'd just check the C&C loads for the rooftop equipment vs. the MWFRS ofthe existing building and see what members are going to be affected.