Roof ponding
Flat roof framing under rain or snow: the ponding stability and strength analysis that AISC 360-22 B3.10 requires of a roof that can retain water. AISC 360-22 removed the former Appendix 2 and gives no method of its own (its Commentary points to SJI Technical Digest 3); this type applies the methods of AISC 360-16 Appendix 2 and its Commentary, valid for flat roofs of rectangular bays with uniformly spaced secondary members and uniformly loaded primary members.
Inputs of the ponding check #
- Primary members: a rolled shape, or I and S entered (joist girders, trusses), and the span Lp; or none, for secondary beams bearing on walls (Cp = 0).
- Secondary members: a rolled shape or I and S entered (joists), the span Ls and the spacing S; for an entered joist or truss I taken from the chords only, the 15 % reduction for web strain (former Appendix 2.1 User Note). The deck's moment of inertia Id per foot.
- Loads: the roof load at the onset of ponding and the rain or snow load without the ponding contribution (nominal; the rain to the secondary drains with the primary drains blocked, ASCE 7-22 Chapter 8, is your entry), and Fy.
- Method: improved (stress index) or simplified; either satisfies the former appendix.
Auto-design varies the secondary shape, lightest first within its family; the primary shape too when you unlock it.
Checks performed #
| Check | Basis |
|---|---|
| Cp + 0.9 Cs ≤ 0.25 (simplified) | 360-16 Eq. A-2-1, A-2-3, A-2-4 |
| Deck Id ≥ 25 S⁴/10⁶ | 360-16 Eq. A-2-2 |
| Bounded ponding deflections, (π/4) αp αs < 1 | 360-16 Commentary Eq. C-A-2-4 |
| Primary: Δw/Δo within Up = (0.8 Fy − fo)/fo | 360-16 Eq. A-2-5, Commentary Eq. C-A-2-1 |
| Secondary: δw/δo within Us | 360-16 Eq. A-2-6, Commentary Eq. C-A-2-2 |
The improved method's Figures A-2.1 and A-2.2 are drawn from the Commentary's Marino (1966) equations: the type computes the ratios directly instead of reading the charts. fo is the bending stress of each member under the load at the onset of ponding plus the rain or snow, the secondary as a simple span under q S, the primary uniformly loaded by q Ls.
Limitations #
- Rectangular flat bays only; roof slope, camber, cantilevers, continuous framing and non-uniform drifts are not modeled (SJI Technical Digest 3 gives fuller methods).
- The members' strength under the full load is designed by their own calculations.
- The last term of Eq. C-A-2-2 is read from a poor text layer (to confirm against the code text).
Verification examples #
Verification status: 8 values compared in 3 examples (0 published, 3 hand calculations from the cited equations): 8 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: the provisions it uses were read in the target edition.
No published ponding example is in the free library: the values are hand calculations of the former Appendix 2 and its Commentary equations. SJI Technical Digest 3 (2018) would add published examples.