Roof Loading

[trishd]

I am checking the design of an existing roof system that is comprised of steel OWSJ, decking and roofing. The roof has to be designed to store an average depth of 4" of rain water.

Using the Ontario Building Code, we generally check a roof for 1.5x (snow plus factored rain). In this case, we are trying to decide if the roof needs to be checked for 1.5x (snow plus factored rain) plus 1.5x(retained water).

Any ideas?

 

[JAE]

This is a ponding check - not a simple load check. You have to determine if the stiffness of the joist system can resist a sequential (i.e. multiple order) change in the loading as the joist bends and water progressively flows downhill to the center of the span.

 

[hokie66]

What do you mean by "store" rain water?

 

[msquared48]

As in a stormwater detention system Hokie.

Mike McCann
MMC Engineering

 

[hokie66]

I thought so. For how long? Who would try to do that on an existing roof framed with bar joists?

 

[JAE]

I had a project last year which I was checking and posted this about the issue (disagreement) we were having about the intent of AISC's ponding check:

thread507-228525

 

[msquared48]

To ddo this, you have to have interior drain lines and either exterior or interior overvflow skuppers. To me, it represents a long term maintenance heacache in addition to the negative structural implications.


Professionally, I would never do it where I live. But, I don't live everywhere. Cheers.


Mike McCann
MMC Engineering

 

[onteng]

This is pretty common in Ontario, particularly in urban areas. The local municipality governs whether storm water management is required on a site. Although there are site related solutions, typically we are asked to have stored water on the roof. The Ontario Building Code has a checklist to follow regarding stored water. If the conditions of this checklist can not be followed, then you will need to design for snow load in combination with rain load.