# Pipe Spanning Supports

## INPUTS

DN CLASS Internal Pressure (MPa) Cover (m) Soil γ (kN/m3) Pipe length (m) Saddle angle (degrees)

## OUTPUTS

Local stress, fr MPa of max
Hoop stress due to internal pressure, fr MPa of max
Flexural stress at centre span, fb MPa of max
Flexural stress at centre span, fb mm of max

## EQUATIONS

Support width

b = (2Yt)1/2

where

b = minimum (axial) saddle width. (mm)

Y = actual outside diameter of pipe. (mm)

t = nominal pipe wall thickness. (mm)

Localised stress at supports

fr = K (wL/a2) ln(Y/2a)

where

MPa

w = unit load per linear metre. (N/m)

L = span length. (m)

a = minimum wall thickness of pipe. (mm)

Y = pipe outside diameter. (mm)

K = 0.03 - 0.00017(b-90o)

b = saddle angle, 90 < b

Hoop stress due to internal pressure

p = 2fa/D    thus    f = pD/2a

where

a = wall thickness. (mm);

D = mean pipe diameter. (mm) = (Y-a) mm

Flexural stress at centre span

fb = 1270YwL2/(Y4-d4)

where

Y = pipe outside diameter. (mm)

w = unit load per linear metre. (N/m)

L = length of span. (m)

d = Y-2a. (mm);

Beam deflection at centre span

To prevent damage to the cement mortar lining, maximum allowable midspan deflection is;

yr = 8.33L

where

yr = maximum allowable deflection at centre span. (mm)

L = length of span. (m)

Beam deflection at centre span for a uniformly distributed load on a simply supported beam

can be calculated using;

y = 265.26x106wL4/(E(Y4-d4))

where

y = deflection at centre span. (mm)

w = unit load per linear metre. (N/m)

L = length of span. (m)

Y = pipe outside diameter, (mm)

d = Y-2a. (mm)

## DESIGN FOR DUCTILE IRON PIPE ON SUPPORTS

Reference: Ductile-Iron Pipe and Fittings, Manual of Water Supply Practices M41, AWWA, 2nd Ed

DESIGN PROCEDURE

DN Internal
Pressure
(MPa)
Cover(m) Soil g (kN/m3)
1 Pipe/trench details
2 Pipe length m
Support width mm
3 Unit load per linear metre. (N/m) w
Pipe Water Soil Total, w
N/m
4 Local Stresses K = 0.03 - 0.00017(b-90o)
K =
fr = K (wL/a 2) ln(Y/2a) fr allowable
fr = MPa
5 Hoop stress due to internal pressure
f = pD/2a f allowable
f = MPa MPa
6 Flexural stress at centre span
fb = 1270YwL2/(Y4-d4) MPa MPa
6 Beam deflection at centre span
y = 265.26x106wL4/E(Y4-d4) yr = 8.33L
mm mm