Twenty seconds of video, one rule: keep a pipe sleeve in the middle third of the depth, away from the high-shear strip near each support. That rule comes from ACI 314R-11's figure for conduits and pipes passing horizontally through beams.
Get it wrong and you get a beam quietly weaker than it looks, or a coring crew after the forms are stripped. The video gives you the rule; this article gives you the four numbers behind it and shows how to check one beam in five minutes.
Watch: the 20-second RHC ENGINEERING explainer this article expands on. Open on YouTube
The video's text carries two instructions, one on screen, one in the description. "The middle third of the depth" is a vertical rule: through the cross-section, soffit to top of slab, the pipe sits near mid-height. "Away from the high-shear support regions" is a horizontal rule; the figure sharpens it: stay out of the first quarter nearest each column and the whole middle third of the span. What is left is a narrow window opening a quarter of the way in and closing a third of the way in.
The same figure caps pipe diameter at one-third of the depth and asks for at least three diameters, centre to centre, between neighbours.
The ACI figure measures l, and the quarter and third marks, from the column centrelines. Site practice, including our companion article, measures l as the clear span between column faces instead, the more conservative reading; the two differ by half a column width at each end. Define l once on the drawing and hold every sleeve to it; the example below uses the clear span from the face.
On a uniformly loaded, simply supported beam, shear peaks at each support and falls in a straight line toward mid-span: a quarter of the way in it has dropped to half the end value, a third of the way in it is down to a third. The first quarter-span stays solid: the web carries its heaviest diagonal compression there.
Moment runs the other way, staying above about 89 percent of its peak across the middle third, so that stretch stays solid too. Through the depth: bottom third for tension steel and cover, top third for the compression zone, middle third, nearest the neutral axis, is where an opening costs least. Horizontal position is a shear decision; vertical position is a flexure decision.
ACI 314R-11 is a guide, not a code, so the window and band are recommendations unless the specification or structural notes adopt them, the cheapest move a designer can make. The ACI code, and NSCP 2015, carry mandatory language for embedded pipes: at most one-third the member thickness, at least three diameters on centre, cover over the pipe, no uncoated aluminium, extra rules for hot or pressurised lines, and never impairing the construction's strength significantly. Strictly, the size and spacing limits cover pipes embedded within a member, not ones merely passing through; most reviewers apply them to transverse sleeves anyway, which is conservative. The strength rule always applies.
Four numbers decide whether a sleeve fits (l clear span, h overall depth, d sleeve outside diameter):
| Check | Formula | Tape-measure version |
|---|---|---|
| Diameter | at most h/3 | Sleeve outside diameter, not nominal size |
| Band | h/3 deep at h/2 | From the soffit: invert above h/3, crown below 2h/3 |
| Window | l/4 to l/3 | From each column face: marks at l/4 and l/3; sleeve edges between them |
| Spacing | at least 3 diameters, centre to centre | n sleeves need (3n - 2)d of window |
Interior beam, 250 mm wide, h = 500 mm, columns 400 mm square, 4.9 m grid. One 75 mm outside-diameter sleeve is proposed; the plumber asks for a second beside it.
Clear span: half a column at each end is 400/2 = 200 mm, so l = 4900 - 200 - 200 = 4500 mm. Maximum diameter: h/3 = 500/3 = 166.7 mm, so 75 mm passes.
Band: h/3 = 166.7 mm deep, centred at h/2 = 250 mm above the soffit, from 250 - 83.3 = 166.7 mm to 250 + 83.3 = 333.3 mm. The sleeve at 250 mm occupies 212.5 mm to 287.5 mm, 45.8 mm of slack each side. Passes.
Window: l/4 = 4500/4 = 1125 mm to l/3 = 4500/3 = 1500 mm from the support face, a width of 1500 - 1125 = 375 mm, which is l/12. The far-end window, still measured from the same face, runs 4500 - 1500 = 3000 mm to 4500 - 1125 = 3375 mm.
With the sleeve centred at the window's midpoint, (1125 + 1500)/2 = 1312.5 mm, its edges land at 1312.5 - 37.5 = 1275 mm and 1312.5 + 37.5 = 1350 mm, 150 mm to spare each side. Passes.
A second sleeve needs at least 3 × 75 = 225 mm centre to centre; two sleeves need (3 × 2 - 2) × 75 = 300 mm, which fits inside 375 mm. Set the centres 112.5 mm either side of 1312.5 mm, at 1312.5 - 112.5 = 1200 mm and 1312.5 + 112.5 = 1425 mm; edges land at 1162.5 mm and 1462.5 mm, 37.5 mm to spare each side. Passes.
A third sleeve would need (3 × 3 - 2) × 75 = 525 mm, which exceeds 375 mm. It goes to the far-end window, 3000 mm to 3375 mm, or reroutes.
Tape-measure summary. Sleeve centre 250 mm up from the form soffit, invert at 212.5 mm, crown at 287.5 mm. First sleeve centre 1200 mm from the column face, second at 1425 mm, 225 mm apart on centre, both inside the 1125 to 1500 mm window from that face.
For the enforceability argument, see the companion article, Sleeves Through Beams: What a Plan Checker Can Actually Enforce.
The free ACI Beam Horizontal Opening Checker, a Windows desktop app from RHCES, checks pipe and conduit openings against the ACI 314R-11 provisions with a live elevation and cross-section that redraws as you type; sign in with Google on first launch, no credit card, no license key. By design its best verdict is PASS WITH REVIEW ITEMS, never a bare PASS, and a FAIL means the simplified provisions no longer apply and the opening needs explicit design.
Twenty seconds is enough to remember the rule; the four numbers confirm it before the formwork closes.