Pipe Support Span Calculator
Maximum pipe support span from bending stress, mid-span deflection, and local support stress, per MSS SP-69 practice — the smallest of the three governs.
When to use this calculator
Use when laying out supports along a straight horizontal run and you need the spacing between them. Three independent limits apply and any of them can govern: the bending stress in the pipe acting as a beam, the mid-span sag, and the local stress where the pipe rests on the support. The tool evaluates all three and returns the smallest, which is the span you can actually use. This is the support-spacing calculation — the horizontal gap between two neighbouring lines on a rack is a different tool, Pipe Spacing.
Required inputs
- Outside diameter D and corroded wall thickness t, in mm
- Second moment of area I, in cm⁴
- Uniform weight per unit length W, in N/m — pipe, contents, insulation
- Young's modulus E, allowable bending stress f_max, allowable bearing stress S, in N/mm²
- Allowable mid-span deflection δ_max, in mm
- Support bearing width b and bearing length d, in mm
Expected outputs
- Span allowed by bending stress
- Span allowed by mid-span deflection
- Span allowed by support bearing
- Governing span — the smallest of the three — and which limit set it
Formula overview
Fixed engineering units with no unit toggle, because the constants 160, 10⁵, and 0.644 already embed the conversions: D, t, b, d, R and δ_max in mm; W in N/m; I in cm⁴; E, S and f_max in N/mm². Every span comes out in metres.
MSS SP-69 practice — the allowable span is the minimum of:
L_stress = √( 160·I·f_max / (D·W) )
L_deflection = ( 384·E·I·δ_max / (5·W·10⁵) )^¼
L_bearing = S·(d + b)·t² / ( 0.644·W·√(R·t) ), R = D/2
L = min( L_stress, L_deflection, L_bearing )Worked example
DN 80 line, bare: D = 88 mm, t = 5.5 mm, I = 125 cm⁴,
W = 110 N/m, E = 210 000 N/mm², f_max = 25 N/mm²,
δ_max = 12.5 mm, S = 67 N/mm², b = 165 mm, d = 100 mm, R = 44 mm
L_stress = √(160 × 125 × 25 / (88 × 110))
= √51.65 = 7.187 m
L_deflection = (384 × 210 000 × 125 × 12.5 / (5 × 110 × 10⁵))^¼
= 2 290.9^¼ = 6.918 m ← governs
L_bearing = 67 × (100 + 165) × 5.5² / (0.644 × 110 × √(44 × 5.5))
= 537 089 / 1 102 = 487.4 m
L_gov = 6.918 m, set by deflection.
Now fill the line with water — W rises to 157 N/m:
L_stress = 6.021 m ← now governs
L_deflection = 6.333 m
The contents both shorten the span and flip which limit controls it.Common mistakes
- Counting only the pipe weight. W must include the contents at operating density and the insulation — a water-filled 6" line weighs about 65% more than the bare pipe, and the span scales with W.
- Applying a straight-run span across fittings. Valves, flanges, and branch connections are concentrated loads and need a support close by, regardless of what the straight-run spacing allows.
- Using the span for a line that also carries thermal movement. The spacing rules assume a beam under gravity — where the run grows, the support type and its ability to let the pipe slide or swing matter as much as the spacing.
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