Designing Steel Joists and Girders for ASCE 7-22 Ponding Rain Loads

Q: Which ASCE 7-22 load combinations include the rain load?

In ASCE 7-22 the rain load R, including ponding head, takes the Lr or S slot. It is the principal load in LRFD combination 3a, 1.2D + 1.6R, and ASD combination 3a, D + R, and a companion load in LRFD 2a and 4a and ASD 4a and 6a. No further ponding check is needed once R includes ponding.

An ASCE 7-22 ponding analysis gives you a water profile, not one rain load for the bay. This article covers how to put that water on steel joists and girders without overloading or underloading them.

Under ASCE 7-16, ponding was a separate stability check and never part of the member load.^[1] Under ASCE 7-22 it is part of $R$ , and the usual shortcuts miss in both directions: on the example bay, a uniform load overstates joist moment by 47% and a trapezoid understates it by 21%.

This is the companion to how to calculate ponding head for ASCE 7-22 and uses the same bay. Not every jurisdiction has adopted ASCE 7-22, so confirm the edition first.

Ponding Loads on Joists and Girders at a Glance

Step	What to do	On the example bay
Joist load shape	Point loads from the analysis, or any shape that stays on top of the water	Uniform at the low-point value: +47% moment. Trapezoid at the roof slope: −21%
Load across the joists	Middle joist's load on every joist, or each joist's own	23.4 psf at the columns, 41.2 psf at the middle joist
Girders	Joist end reactions as point loads	Largest at midspan, where the girder sags most
Load combinations	$R$ takes the $L_{r}$ or $S$ slot	LRFD $1.2 D + 1.6 R = 83.9$ psf at the low point
Joist schedule	Minimum shear of 12.5% of the maximum end reaction	Covers shear reversal from water at the low end
Reducing the load	Keep the overflow inlet low, stiffen the girder	A W21×44 girder takes $R$ from 41 to 28 psf

The Example Bay

A 30 × 30 ft interior bay of a single-story retail building in Bentonville, Arkansas, from the companion article on calculating ponding head:

Framing: W16×26 girders, 16K3 joists at 5 ft on center, sized for $1.2 D + 1.6 L_{r}$ at L/240
Slope: 1/4 in. per ft along the joists, low edge along a girder
Dead load: $D = 15$ psf
Water level: 4.5 in. above the low point, so $R_{0} = 23.4$ psf
Iterative ponding analysis: 7.92 in. of water at the low point of the middle joist, so $R = 41.2$ psf and $d_{p} = 3.4$ in.

Every number below comes from that analysis, run in the free rain load and ponding calculator.

Load the Joists as the Water Sits

ASCE 7-22 Section 8.2 requires each portion of the roof to carry the water that accumulates on it.^[2] On a sloped, deflected joist that water is deepest at the low end and runs out before the high end, so whatever load you apply has to stay on top of it along the whole span. Common shapes on the middle joist, against the analysis's own point loads (unfactored rain only):

Load shape on the joist	Moment	Shear
Point loads from the analysis	exact	exact
Trapezoid: 41.2 psf at the low edge, falling at the roof slope	−21%	−15%
41.2 psf uniform over the whole joist	+47%	+25%
41.2 psf for 12 ft from the low edge, then tapering to zero at the high edge	+14%	+10%
27.9 psf uniform (equal moment)	about 0%	−16%

Moment vs. water−21% Shear vs. water−15% Stays on top of the waterNo

Unfactored rain load on the middle joist of the example bay (5 ft tributary width), compared with the analysis's own point loads. A shape drawn in green stays on top of the water everywhere; dashed orange dips below it somewhere along the span.

Trapezoid: ignores the sag, so it drops below the water near the low end.
Equal-moment uniform load: misses shear, because the water is heaviest at the low end.
Uniform $R$ : the simplest safe shape, at 47% extra moment. Flat for 12 ft, then tapered, costs 14%.

Each Joist Carries Its Own Rain Load

The girder doesn't deflect at the columns, so the joists on the column lines carry only $R_{0} = 23.4$ psf at the low end. The load peaks at the middle joist, 41.2 psf, where the girder sags most.

Rain load at the low end of each joist line across the 30-ft girder, psf, from the converged iterative analysis. On the column lines the girder does not deflect, so the load is R₀; it peaks at the middle joist, where the girder sags most.

Using the middle joist's loading for every joist is simple and conservative. The girders take the joist end reactions as point loads, not psf over a tributary width.

Load Combinations: $R$ Takes the $L_{r}$ or $S$ Slot

In ASCE 7-22, $R$ including ponding goes into the basic combinations with no further ponding check. The ponding analysis itself uses unfactored loads (Design Guide 40 Sections 2.2 and 2.3).^[2,3]

ASCE 7-22 combination	Load	At the low point of the middle joist
LRFD 3a (Section 2.3.1)	$1.2 D + 1.6 R$	$1.2 (15) + 1.6 (41.2) = 83.9$ psf
ASD 3a (Section 2.4.1)	$D + R$	$15 + 41.2 = 56.2$ psf

$R$ is also a companion load in LRFD 2a and 4a and ASD 4a and 6a. For ASD $D + R$ , Design Guide 40 Section 2.3 allows the load effects from the ponding analysis to be used directly.^[3]

Specify a 12.5% Minimum Joist Shear

Joists are designed for uniform load. Water piled at the low end can move the peak moment off midspan and reverse shear near midspan, putting tension webs in compression. Design Guide 40 Section 2.5 recommends a minimum shear of 12.5% of the maximum end reaction, which the SJI Roof Bay Analysis Tool also assumes.^[3] Put that note on the joist schedule.

What Reduces the Ponding Rain Load

At the low point, $R$ is the water the drains hold, $R_{0}$ , times what the sag does to it: $23.4 \times 1.76 = 41$ psf. Drainage choices move $R_{0}$ ; framing choices move the multiplier. The figure changes one thing at a time.

Rain load at the low point24 to 85 psf Swing61 psf

Static head, d_s Hydraulic head, d_h = 2.5 in. Ponding head, d_p, below the undeflected roof

Water at the low point of the example bay from the iterative analysis in the free calculator, changing one thing at a time. Inlet 0 to 6 in. above the low point, members unchanged. The as-designed case is highlighted.

Change	Effect on $R$ at the low point
Overflow inlet height	Each inch adds 5.2 psf to $R_{0}$ and 8 to 12 psf to $R$
Bay size, 30 × 30 ft to 40 × 40 ft	41 to 59 psf, with members sized for the same loads
Stiffer girder, W16×26 to W21×44	41 to 28 psf
Stiffer joists (L/360) or more joists	Barely changes

Set the overflow inlet early. It is the largest lever. Design Guide 40 Section 2.9 suggests assuming a 2-in. inlet and 4 in. of hydraulic head until the drains are designed, then confirming by RFI before construction drawings go out.^[3] Moving the inlet during design is a line on the plumbing sheet; after the framing is set, it is heavier steel.

Iterate on long bays. From 30 × 30 ft to 40 × 40 ft, the Design Guide 40 amplification factor $B_{p}$ climbs from 2.4 to 7.2, far faster than the load itself, and Design Guide 40 notes $B_{p}$ loses accuracy as it grows.

Why more joists barely help. For a joist sized exactly to a deflection limit of L/n under a load $q$ , substituting into ASCE 7-22 Commentary Equation C8.2-6 gives:

C_{s} = \frac{384 γ L _{s}}{5 π ^{4} n q} \approx \frac{0.79 γ L _{s}}{n q}

This is my derivation, not a published equation. The spacing $S$ drops out, because closer joists are each sized lighter. The design load $q$ is in the denominator, which is why light roofs in low-snow climates are the most flexible.

The Bottom Line

Put the ponding water on the joists as point loads or a shape that stays on top of it, put the joist end reactions on the girders, and specify the 12.5% minimum joist shear. Before the framing is set, check the overflow inlet height and the girder, because they move the load far more than the joists do. For the rain load itself, Prose carries your ponding head into $R$ for every region and drainage zone on the project, with each step cited in the report.

Frequently Asked Questions

How should ponding rain load be applied to steel joists?

As the water actually sits. ASCE 7-22 Section 8.2 requires each portion of the roof to carry the water that accumulates on it, so the applied load has to stay on top of that water along the whole span. The point loads from the ponding analysis do that exactly. A uniform load at the low-point value is safe but heavy, and a trapezoid falling at the roof slope drops below the water near the low end.

How is ponding rain load applied to girders?

As the joist end reactions, applied as point loads at each joist bearing. The rain load is not uniform across the girder: it is R₀ at the columns, where the girder does not deflect, and largest at midspan, where it sags most.

Which ASCE 7-22 load combinations include the rain load?

In ASCE 7-22 the rain load R, including ponding head, takes the L_r or S slot. It is the principal load in LRFD combination 3a, 1.2D + 1.6R, and ASD combination 3a, D + R, and a companion load in LRFD 2a and 4a and ASD 4a and 6a. No further ponding check is needed once R includes ponding.

Why do joists need a minimum shear for ponding?

Joists are designed for uniform load, and ponding water is heaviest at the low end. That can move the peak moment off midspan and reverse shear near midspan, putting tension webs into compression. AISC/SJI Design Guide 40 Section 2.5 recommends specifying a minimum shear of 12.5% of the maximum end reaction.

What reduces ponding rain load the most?

The height of the overflow inlet, because each inch adds 5.2 psf to the first-order rain load before ponding amplifies it. After that, a stiffer girder. Adding joists or stiffening them barely changes the load, because closer joists are each sized lighter.

How to calculate ponding head for ASCE 7-22: the Design Guide 40 $B_{p}$ method and the iterative analysis worked on this bay, and why they give different answers
Rain load calculation per ASCE 7: the SDSL rules, static and hydraulic head, the design storm by Risk Category, and what changed across editions
Free rain load and ponding calculator: one bay, iterative ponding analysis, ASCE 7-22 and 7-16 rain loads side by side, with a calc report

If you read this provision differently or have run into it applied another way on a real project, reach out at support@prose-eng.com. I'm a practicing engineer, not an infallible one.

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References

American Society of Civil Engineers. Minimum Design Loads and Associated Criteria for Buildings and Other Structures(ASCE/SEI 7-16), Selected provisions and commentary. Reston, VA:ASCE, 2016.
American Society of Civil Engineers. Minimum Design Loads and Associated Criteria for Buildings and Other Structures(ASCE/SEI 7-22), Selected provisions and commentary. Reston, VA:ASCE, 2022.
American Institute of Steel Construction and Steel Joist Institute. AISC/SJI Design Guide 40: Rain Loads and Ponding. Chicago:AISC/SJI, 2024. Replaces AISC 360-16 Appendix 2, which was removed from AISC 360-2022.

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