Note that the 2021 International Plumbing Code has not yet been updated to reflect the 100-year/15-minute (or twice the 100-year hourly) duration rainfall event design requirement for secondary drainage systems the structural engineer would be advised to coordinate with the plumbing engineer to ensure that the secondary drainage systems are designed for the higher rainfall rate. However, the 2021 IBC, by giving two options – the 15-minute duration or twice the 60-minute duration – results in values similar to ASCE 7 when drainage pipe or scupper flow rates are calculated. Also, note that 2021 IPC and 2021 IBC rainfall maps (Figures 1106.1, respectively) both include a 60-minute duration rather than the 15-minute storm duration. Note that using twice the 60-minute duration is close to the 15-minute duration rainfall rate for many regions. In the 2018 IBC and International Plumbing Code (IPC), a 100-year/60-minute duration is used for both the primary and secondary systems. The 1995 Standard Plumbing Code used the 100-year/60-minute duration for primary drainage system design and a 100-year/15-minute duration storm for the secondary drainage system. Legacy ASCE load standards’ design rainfall durations for plumbing systems were between 15- and 60-minutes for a 100-year mean recurrence interval. Design for ponding instability is based on secondary drainage system rain loads. Susceptible bays of roofs shall be evaluated for ponding instability in accordance with Section 8.4 Chapters 7 and 8 of ASCE 7.įigure 2. Where the phrase “undeflected roof” is used, deflections from loads (including dead loads) shall not be considered when determining the amount of rain on the roof.ġ611.2 Ponding instability. R = Rain load on the undeflected roof, in psf. Alternatively, a design rainfall of twice the 100-year hourly rainfall rate indicated in Figures 1611.1(1) through 1611.1(5) shall be permitted.ĭ h = Additional depth of water on the undeflected roof above the inlet of secondary drainage system at its design flow (in other words, the hydraulic head), in inches.ĭ s = Depth of water on the undeflected roof up to the inlet of secondary drainage system when the primary drainage system is blocked (in other words, the static head), in inches. The design rainfall shall be based on the 100-year hourly rainfall rate indicated in Figure 1611.1 15-minute duration event, or on other rainfall rates determined from approved local weather data. as per the requirements of Chapter 8 of ASCE 7. Each portion of a roof shall be designed to sustain the load of rainwater that will accumulate on it if the primary drainage system for that portion is blocked plus the uniform load caused by water that rises above the inlet of the secondary drainage system at its design flow. Strike-through indicates deleted text and underlined denotes new text.ġ611.1 Design rain loads. The following text shows the specific change in the 2021 IBC. Secondary drainage system rain loads have been updated in the 2021 International Building Code® (IBC) to be consistent with ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures.
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