minimum live roof loads

[Lomarandil]

Looking to glean the experience of the group here.

I practice in a country with no building code. That said, I try to strive for rationable, defensible designs.

It is clear to me that sheet metal roofs here are not designed for 20psf/1kPa live load. But, they stand just fine under the minimal dead and wind loading we have.

So, are there any published justifications that allow for a design roof live load less than the standard ASCE 7 20psf (and applicable slope/trib area reductions) without neglecting it entirely and hoping no one goes up there?

Sheet metal and light-gage cold formed steel roof, 70mph wind, no mechanical equipment of note. Most are under 20* slope.

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The name is a long story -- just call me Lo.

 

[Settingsun]

Australian warehouse rooves are designed for 0.25 kPa. Although be aware that this may have caused/contributed to a half dozen collapsing during a hail storm in 2015. Search for 'Huntingwood warehouse collapse' if interested. Happened on a national public holiday so no-one inside any of them iirc.

 

[patswfc]

Lo,

In the UK roofs used to be designed for a min of 0.6kN/m2 if only accessed for general maintenance, otherwise 1.5kN/m2 would be used. Point loads were also considered but cant remember these of the top of my head.

In the Eurocodes the main code states that the roof can be designed for an imposed load ranging from 0.0 - 1.0kN/m2 and point loads from 0.9 - 1.5kN.
It recommended values are 0.4kN/m2 and 1kN.
Each national annex can then given different recommendations. The UK NA recommends 0.6kN/m2 for a pitch <30 and 0.0kN/m2 if >60, with an equation to calc the udl for slopes in between. All roof pitches should be checked for a 0.9kN point load.

Assuming your roof pitch is < 30 then the UK min roof imposed udl would be 0.6kN/m2. I dont know about the other countries but would presume that quite a few would use 0.4kN/m2 (about 8psf).

Obviously all roofs need to be designed for whatever wind and snow loads, however these dont need to be considered with the min roof imposed load.
Is it likely to be the case that these loads are more onerous in your location?

 

[ajh1]

Keep in mind also that the ASCE requirements are area dependent. 20 psf for members supporting less than 200 square feet, 12 psf for members supporting more than 600 square feet, with a sliding scale between those two points. The downside to working with the 12 psf values is if you have continuous secondary members and don't use the 20 psf value you are required to check patterned live load conditions. Many times it is easier to simply throw the 20 psf in and be down with it. Typically what you could end up with is secondary members at 20 psf and primary support frames at 12 psf.