Roof Top Equipment Platform Live Load

Question

Roof Top Equipment Platform Live Load

jcali (Structural)

(OP)

16 Feb 06 14:01

When providing a maintenance platform for a rooftop HVAC unit, what is the proper live load for this platform? The platform that I have in mind would be a steel frame supported by posts a couple feet above the roof with an open grating walking surface(In this case it would be added to an existing roof). The walkway would be used for equipment maintenance. The IBC LL of 60psf for "walkways and platforms" seems very high when considering the platform load as a whole applied to the existing roof. I could see using a localized load of 100psf for sizing the grating, but what is the realistic load on the whole platform when determining post loads applied to the roof structure below?

I'm thinking that the code determined "minimum roof live load" (not snow load) in the neighborhood of 20 psf would apply to the platform as well. The snow load would also be applied to the roof around and under the platfrom. What do you think?

-JCali

JAE (Structural) · Answer 1 · 2006-02-16 14:50:50

There is an ASCE 7 live load called "catwalks" that uses 25 psf. We use that for maintenance access-only platforms and also compare with required snow loads if outdoors.

jcali (Structural) · Answer 2 · 2006-02-16 14:54:43

Actually, ASCE 7 now says 40 PSF for catwalks. Which still seems high if applied to the entire catwalk surrounding a large piece of equipment.

-JCali

JAE (Structural) · Answer 3 · 2006-02-16 15:39:19

jcali - you're right - for the 98 and 02 versions - looks like they added "Catwalks for Maintenance access" at 40 psf with a 300 lbs concentrated load. The appendix for both still has the 25 psf for "catwalks".

jcali (Structural) · Answer 4 · 2006-02-16 16:03:48

Interesting. You're right, the commentary says 25psf. I wonder which governs?

40 psf doesn't make sense to me in some cases... For instance, if the equipment is sitting directly on the roof, you could use the minimum roof live load to cover the load from maintenance personnel. In many cases the roof LL is 20 psf or less. But as soon as you raise the same piece of equipment onto a platform, the maintenance LL increases to 40 PSF. It's the same maintenance person working on the eqpt whether it's on the roof or on a platform, yet the design load is different. Locally, I'd design the grating for at least 100 psf, but would want to use a much smaller LL to determine post loads applied to the roof structure.

Thoughts?

-JC

JAE (Structural) · Answer 5 · 2006-02-16 16:27:10

Maintenance sometimes involves combinations of multiple workers, tool boxes, and portions of equipment removed from the unit and placed on the catwalk to allow hands-on work? That could add up a bit. With 40 psf, you can still reduce the live load for trib/influence areas but for catwalks that wouldn't be a whole lot.

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Roof Top Equipment Platform Live Load