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

US Army Corps of Engineers

How Thrust is Generated

Kansas City District

Equilibrium

US Army Corps of Engineers

How Thrust is Generated

Kansas City District

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

US Army Corps of Engineers

How Thrust is Generated

Kansas City District

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

US Army Corps of Engineers Kansas City District

How Thrust is Generated

US Army Corps of Engineers Kansas City District

How Thrust is Generated

US Army Corps of Engineers Kansas City District

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

US Army Corps of Engineers Kansas City District

Bearing g Capacity p y

US Army Corps of Engineers Kansas City District

Passive Resistance

thrust block by bearing pressure (AWWA m23)

US Army Corps of Engineers

Known

Kansas City District

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)

US Army Corps of Engineers

Known

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

Kansas City District

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)

US Army Corps of Engineers Kansas City District

Tables should have correct bearing Capacity in calculations

US Army Corps of Engineers Kansas City District

Ductile iron mechanical joint fitting

US Army Corps of Engineers Kansas City District

Ductile Iron Bolted

US Army Corps of Engineers Kansas City District

PVC Restrained Bell and Spigot

US Army Corps of Engineers Kansas City District

Types of Restrained Joints and How Theyy Work

US Army Corps of Engineers

Errors and Omissions

Kansas City District

 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)

US Army Corps of Engineers Kansas City District

Questions