Design of Thrust Blocks_London_Ted

US Army Corps of Engineers Kansas City District

JOINT RESTRAINT FOR PRESSURIZED PIPE

Presented by: Ted London, PE US Army Corps of Engineers Kansas City District

US Army Corps of Engineers

How Thrust is Generated

Kansas City District

F=P*A




Equilibrium




What changes equilibrium: Operating Pressure Transient Pressure (surge) Velocityy Head Changing direction of flow Changing diameter of pipe Thermal changes




Changes in equilibrium are developed at: Horizontal bends Vertical bends Dead ends Tees Wyes Reducers Fire Hydrants






Bearing g Capacity p y versus Passive Resistance

 BEARING CAPACITY FAILURE: The thrust generated exceeds the bearing capacity of the soil times the bearing area  PASSIVE RESISTANCE FAILURE: The thrust generated slides a wedge of soil


Bearing g Capacity p y


Passive Resistance

thrust block by bearing pressure (AWWA m23)


Known


Internal pressure P  150psi Pipe outside Diamter D  9.050in Deflection angle   45 Depth to bottom of thrust block h  7ft Safety Factor Sf  1.5 Soil Bearing B  1500psf

Solve 2

Internal Area A   

D

4

Unbalanced Force T  2 P A  sin 

 

 

 2 180 

A b  T

Sf B

Ab  Passive resistance thrust block design is required if the height of the thrust block is greater than 1/2 * depth. IIn the h above b example, l thrust h bl block kh has a h height i h off 2 and d llength h off 4 4. 2 fft tallll iis lless than h three (1/2*7)

thrust block by Passive Resistance (AWWA m23)


Known

Internal pressure P  150psi Pipe outside Diamter D  9.050in


Pipe Deflection angle   45 Depth to bottom of thrust block h   4ft Safety Factor Sf  1.5 Soil Bearing B  1500psf g of Soil   110pcf Unit Weight p Soil Internal Friction angle   30 Cs  0psf 2

Solve

Internal Area A   

D

4

Unbalanced Force T  2 P A  sin 

 

   2 180 

   Nd  tan  45     2  180  

A b  T

2

Sf   h  Nd  2 Cs  Nd

.5

Ab  In the above example, a height of 2 and length of 4.5 ft. 2 ft tall is less than three (1/2*4.5)


Tables should have correct bearing Capacity in calculations


Ductile iron mechanical joint fitting


Ductile Iron Bolted


PVC Restrained Bell and Spigot


Types of Restrained Joints and How Theyy Work


Errors and Omissions


 Not considering passive resistance  Over-estimating bearing capacity of soil  Use same table over and over without thinking  Poor installation  Not considering saturated soil  Not considering the sheer SIZE of the thrust block required to resist the force

Results of Errors US Army Corps of Engineers Kansas City District

 Disengagement Di t off pipes i  Displacement of soil Wedges  Higher maintenance costs  Poor construction blamed in lieu of design  Root cause may not be determined

References US Army Corps of Engineers Kansas City District

AWWA M23: PVC Pipe – Design and Installation AWWA M41: Ductile Iron Pipe and Fittings Ductile Iron Pipe Research Association (DIPRA) EBAA Iron American Ductile Iron Pipe Co. (ACIPCO)


Questions

Design of Thrust Blocks_London_Ted

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