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Structural Design

 

Thrust Block Design For Fittings

For any rubber ring jointed pipeline system, provision must be made for potentially unrestrained forces at changes of size or direction in the pipeline, that is at bends, tees, reducers, valves and closed ends. In buried installations, fittings are usually restrained by concrete cast in situ. These thrust blocks are formed and sized to distribute the applied force from the fitting to a safe soil pressure at the soil / concrete interface . The resistance which can be provided will depend on the soil type and depth. Where bends are in the vertical plane with a convex profile downwards, the weight of the concrete anchor block alone may be the restraining force.
 
Hydrostatic forces in kilonewtons on rubber ring jointed fittings per ten (10) metres hydrostatic head.
 
Pipe DN
(mm)
Pipe OD
(mm)
90° Bend 45° Bend 22.5° Bend 11.25° Bend Tee/
Closed End/
Valve
100 122 1.62 0.88 0.45 0.22 1.15
150 177 3.41 1.85 0.94 0.47 2.41
200 232 5.86 3.18 1.61 0.81 4.14
225 259 7.31 3.96 2.01 1.01 5.17
250 286 8.91 4.83 2.45 1.23 6.3
300 345 12.96 7.02 3.57 1.79 9.16
375 426 19.76 10.71 5.44 2.72 13.97
 
Note: For concentric reducers the resultant thrust will be the difference between the “closed end“ forces for the two pipe sizes.
 
 
Soil bearing capacities in kPa – apply minimum factor of safety of 1.1
 
Soil Group
Description as per
AS 1786
Minimum Soil Cover Above centre line of
Thrust Block in Meters
0.75 1.0 1.25 1.5
GW, SW 57 76 95 114
GP, SP 48 64 80 97
GM, SM 48 64 80 96
GC, SC 79 92 105 119
CL 74 85 95 106
ML 69 81 93 106
OH 0 0 0 0
 
Thrust blocks must be configured to distribute the hydrostatic force to a “wall” of undisturbed soil which is approximately perpendicular to the imposed load.
 
The equation for this calculation is :
          A = T/ b x f
Where A = area perpendicular to force (m2)
          T = hydrostatic thrust (kN)
          b = soil bearing capacity (kPa)
          f = factor of safety
 
 
 
 
     



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