ASCE 7-16 vs Building Code Rain Loads

[wallerdf]

ASCE 7-16 section 8.2 states the design flow rate of the secondary drains shall be 15-min duration / 100 year. The previous sentence states in accordance with the provisions of the code that has jurisdiction which is what previous years of the ASCE said. Florida Building Code (1611) & Florida Mechanical Code (1108) both state 60-min duration / 100 year. I'm getting major push back from a plumbing engineer that say both codes state 60-min duration and therefore that is what we are going to design towards. I live in Florida so I get the amount of water that comes down in a short duration. I guess I'll just calculate dh at 15-min and design for that. Which may very well be larger than the 20psf roof live load. Any thoughts?

 

[r13]

The accumulation of water caused by the deflection of the roof structure is called ponding, and, as it results in added load, it can lead to roof instability. The key to managing rain loads in buildings is roof drainage, including a primary drainage system and a secondary drainage system, which is set at an elevation higher than the primary system to drain any water that builds up after the lower system is blocked or not working.

Guidance for roof drainage systems can be found in Chapter 8 of ASCE/SEI 7-2016: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. This chapter has a general focus on rain loads, and uses the same 100-year return period for determining the design flow rate, with different frequencies for the secondary and primary drainage systems.

Read more at the ANSI Blog: Rain Loads: Design Guidance in ASCE/SEI 7-2016 and the International Building Code

https://blog.ansi.org/?p=6906

I think the quote above explain the reason for using different duration for the primary and secondary roof drainages. Link