Rain load
Rain load on a roof with its ponding head, to ASCE 7-22 Chapter 8: R = 5.2 (ds + dh + dp), with all drainage that 8.2 takes as blocked out of service. The hydraulic head comes from the tables of the commentary or your own data, and the ponding head from your analysis or the amplified method of the commentary. ASCE 7-22 puts ponding inside the rain load, so there is no separate ponding check (AISC 360-22 no longer has the old Appendix 2 method). R is unfactored: copy it into the member calculations as load type R.
Inputs of the rain load #
- Storm and roof area: the risk category, which sets the return period (Table 8.2-1: 100, 200 or 500 years); the rainfall intensity i of the 15 min storm of that return period, from NOAA Atlas 14; and the area served by one secondary outlet A, the tributary roof plus half of any wall that diverts rain onto it.
- Secondary drainage system: the static head ds up to the secondary inlet, at the point you are designing (a low point has the largest). The hydraulic head dh is entered from test data or calculations, or found from the flow Q = 0.0104 A i in Table C8.2-1 (overflow drains), Table C8.2-3 (rectangular scuppers), Table C8.2-5 (circular scuppers) or Eq. C8.2-2 (free overflow along a level roof edge, dh negligible). For a drain you may give another dam diameter (weir flow, Eq. C8.2-3) or bowl depth (orifice flow). Whether the secondary system qualifies at all is your input too: how far its inlet stands above the primary inlet, whether it shares drain lines with the primary, and whether it has controlled flow drains.
- Ponding: enter dp from a structural analysis of the roof under unfactored rain and dead load, or use the amplified method with D, E, the spans, the spacing, and I of the primary and secondary members. Enter I as the deflection calls for it: the commentary says a joist or truss modeled as a beam is commonly reduced 15 % for shear deformation. It says too that all deformations that add to the roof's deflection should count.
- Bay with low slope: whether this bay holds water up to the secondary system (8.2), or holds none: internal (8.3 condition 1) or next to a free-draining edge with secondary members parallel to it (8.3 condition 2); and the roof slope.
Checks performed #
| Check | Basis |
|---|---|
| Flow | Q = 0.0104 A i (Eq. C8.2-1) |
| Hydraulic head | Tables C8.2-1, C8.2-3, C8.2-5, linear between points and scupper widths as their notes allow, Eq. C8.2-3 and the bowl depth rule, kept to 80 % of the table value; Eq. C8.2-2 |
| Past a table | NOT FEASIBLE: their notes say extrapolation is not appropriate. Under a drain table's first flow, its first head |
| Secondary system | 8.2(b): inlet at least 2 in. above the primary; (c) its own drain lines; (d) no controlled flow drains. Otherwise it reads Not adequate: base ds and dh on the next point of discharge |
| Ponding stability | Eq. C8.2-4: 1.15 Cp + Cs under 1.0; at 1.0 or more there is no solution, which is ponding instability, and no R is given |
| Rain load | R = 5.2 (ds + dh + dp) (Eq. 8.2-1). With the amplified method: Bp (D + R without ponding) − D, Eqs. C8.2-4 to C8.2-6, and dp from R |
| Dead load with rain | 1.2D + 1.6R (2.3.1) and D + R (2.4.1), for reference |
| Low slope | 8.3: under 1/4 in. per ft, or under β = (Ls/S + π)/20 next to a free-draining edge, an advisory asks you to investigate the deflected roof |
A result reads Calculated, or Not adequate where the secondary system or the framing fails. The drawing is a section at the drain with ds, dh and dp dimensioned at their true values; it is not to scale along the roof.
Limitations #
- The drainage system is not designed. The tables are for one secondary outlet, and assume free outfall at scuppers (backwater raises dh).
- The amplified method is approximate and applies to rectangular bays of primary members carrying equally spaced secondary members, with sinusoidal shapes behind Cp and Cs. Other framing needs an analysis: enter dp. E is one value for both members.
- The method gives one bay's uniform load. Peak loads over a member's span and the 8.3 investigation of a deflected roof are yours.
- Roofs with internal gutters, large adjacent walls, or high roofs draining onto low ones change the critical duration (C8.2): the 15 min rainfall may not hold. 8.4 on draining onto existing roofs is yours to check.
- No member shape lookup: enter I. That keeps this type independent of the member calculations.
- The SI tables (C8.2-2, C8.2-4, C8.2-6) are not used separately: metric input converts to the inch-pound tables, whose values they round.
Verification examples #
Verification status: 22 values compared in 12 examples (9 published, 3 hand calculations from the cited equations): 22 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.
All published values come from the ASCE 7-22 commentary. Design Guide 40 (rainwater accumulation) is not on disk; it would be the next check.
| Example | Source | Values |
|---|---|---|
| Ponding in a 40 × 40 ft steel bay | C8.2, p. 672 | Cp 0.155, Cs 0.128, Bp 1.44, 41 psf, 59 psf, R 44 psf, factored 88.4 psf |
| Dam adjustment | C8.2 Example 1, p. 670 | dh 2.4 in., total 4.4 in. |
| Bowl depth under orifice flow | C8.2 Example 2, p. 670 | dh 4.0 in., total 6.5 in. |
| Slope limit | C8.3, p. 674 | β 0.76 in. per ft |
| Table lookups | Tables C8.2-1, C8.2-3, C8.2-5, pp. 668 to 671 | Five cells, one interpolated |
Hand calculations cover Eq. 8.2-1, Eq. C8.2-1 and the case with no primary members. The example rounds R to 44 psf before factoring and the program carries 44.1 psf, so its 88.6 psf against the printed 88.4 psf is inside the tolerance. Examples 1 and 2 print one decimal, so they are held to half of that step.