Ponding Load Case 1

[kipfoot]

A basic ponding question: As I interpret ASCE 7-16, the ponding load is treated as Rain (R in load combinations). This suggests that it does not act concurrently with Snow load. However, the commentary on sections 7.11 (Ponding Instability) states:

It is very important to consider roof deflections caused by snow loads when determining the likelihood of ponding instability from rain-on-snow or snow meltwater

So, when determining the capacity of existing open web roof joists on a flat roof, are Rain and Snow considered separately (i.e. D+[Lr or S or R]) or must I consider that the deflected roof shape from snow load will fill with water?

This is an existing roof, 44" LH trusses spanning 88', built in 1961, with a dead flat roof, no parapet. My task is to determine its adequacy to carry a sizable new rooftop mechanical unit, which it cannot, so the roof will need to be reinforced in some way to carry the load from the RTU, additional drift, and possibly ponding.

 

[phamENG]

I think you do them separately (i.e. a ponding case for rain and a ponding case for snow/rain-on-snow surcharge). For the snow case, I think it would be a case of checking the roof to ensure it doesn't reverse slope. If it does, then you'd need to consider the full snow weight or snow+rain on snow surcharge as if it melted, ran back, and ponded in the middle of the roof (or where ever).

 

[le99]

While snow load is clearly specified by the codes, the rain load is not so provided and is difficult to estimate/determine. The solution is to provide an adequate slope as suggested by ASCE7, which requires “susceptible bays” be investigated to ensure adequate member stiffness is present to prevent progressive deflection. Specifically, “Bays with a roof slope less than ¼ in./ft. …shall be designated as susceptible bays. Roof surfaces with a slope of at least ¼ inch per foot (1.19°) toward points of free drainage need not be considered a susceptible bay.”

Structure Mag.

 

[phamENG]

le99- pretty hard to slope a roof that has been flat for 61 years. And rain loads are very clearly defined in the code. IBC 1611.1 and figure 1611.1 have all you need. Throw in some ASCE for more background and you're good to go. It's ponding that takes a bit more consideration regarding the actual behavior of the structure.

 

[le99]

Admittedly, I was ignorant of the rain load data provided by IBC. But as for this case, I would put my efforts to strengthen the bay to take out the deflection expected from the RTU unit.

 

[kipfoot]

Thanks. I think the gist of what I was looking for is in section 7.10 of ASCE 7: The snow on rain is already baked into the snow load for ground snow loads above 20 psf. (Though, I just learned about the surcharge for 20 psf loads today.) :/

I'll definitely want to stiffen the existing trusses to match the existing deformation....and possibly have them add a roof drain. Honestly, the other thread has me thinking about adding a couple locally fabricated trusses to carry the RTU.

 

[DaveAtkins]

And remember, designing for blocked drains is a separate analysis from checking ponding. I agree that rain and snow are checked together, except as mentioned above.

The tricky part for you will be, do you just upgrade the roof for the new mechanical unit, or do you fix deficiencies in the entire roof (if you find any). Your scope of work probably does not include fixing deficiencies in a 60 year old roof.

DaveAtkins

 

[EcoGen]

What is the maximum width of the RTU on plan? If it is less than 15 feet then you don't need to consider any additional snow drift.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[kipfoot]

If it is less than 15 feet then you don't need to consider any additional snow drift.

Thanks for pointing this out, because I wasn't thinking of it. (ref: ASCE 7 7.8) My unit is 19 feet wide.

 

[structSU10]

For reference SJI put together an excel program that is very useful for ponding analysis. It covers both joist framing and steel framing directly.

https://steeljoist.org/product/roof-bay-analysis-tool-with-ponding-analysis/

there is a webinar on how its used:

https://www.youtube.com/watch?v=TLaeK_JjYOQ