Thust Block Design

1SHC 310 SHC 310 -- HydraulicsHydraulicsTHRUST BLOCK DESIGNTHRUST BLOCK DESIGN

13 February 200613 February 2006

By: Marco van DijkBy: Marco van Dijk

Department of Civil & Department of Civil & BiosystemsBiosystemsEngineeringEngineering

Thrust Block DesignThrust Block Design Introduction Basic theory Calculation procedures Calculation of resultant force Calculation of restraining method Installation guidelines Examples Software program Hand calculation

2 Piping systems are subject to unbalanced thrust forces resulting from static and dynamic fluid action on the pipe.

These forces must be balanced if the piping system is to maintain its integrity.

Unbalanced thrust forces occur at change in directions of flow such as elbows, tees, reducers, valves and dead ends.

Reactive forces can be provided in the form of thrust blocks, or transmitting forces to the pipe wall by restrained, harnessed, flanged or welded joints (Forces from the pipe shell transferred to the soil).

Introduction

Basic theory

The fundamental equations of fluid dynamics:

leavingflowmassenteringflowmass =1. Conservation of matter (mass)

leavingentering QQ =

2. Conservation of energy

2

222

1

211

22z

gv

gp

zg

vg

p++=++

rr

3. Conservation of momentum

( )12 vvQF -= rddtmdv

F =

lf hh ++

3Basic theory

Calculate the forces:

ApF xPx =Pressure forces

( )1x2xMx VV?QF -=Momentum forces

Reaction forces ( ) += RxPxMx FFF

Calculation procedures

Step 2: Determine the pressures, velocities and flow rates at the specific point

Step 3: Determine the forces using the fundamental equations of fluid dynamics

Step 4: Determine the soil conditions (if underground installation)

Step 5: Calculate thrust block dimensions/weight

Step 1: Select axisX

Y

Z

4Calculation of resultant force

Calculate the magnitude and direction of the force exerted by the T-junction

First find the three velocities by continuity

m/s1.886AQ

V1

11 ==

m/s122.2AQ

V2

22 ==

m/s775.4AQ

V3

33 ==

Determine velocities

Calculation of resultant force


Apply Bernoullis equation to find pressure p2 and p3

2

222

1

211

22z

gV

gpz

gV

gp ++=++

rr

( )2

22

21

12

VVpp

-+=

r

kN/m499.532 =p

Determine pressures (energy equation)



Apply Bernoullis equation to find pressure p2 and p3

3

233

1

211

22z

gV

gpz

gV

gp ++=++

rr

( )2

23

21

13

VVpp

-+=

r

kN/m490.383 =p

Determine pressures (energy equation)



Calculate pressure forces

xPxPxPPx FFFF 321 ++=

FP1

0011 ++= ApF xPxkN79.95=PxF

X-direction:

FP2

Y-direction:

yPyPyPPy FFFF 321 ++=

)()(0 3322 ApApF yyPy +-+=

kN19.96-=PyFFP3

Determine pressures forces



Calculate momentum forces

[ ] [ ]110 V?QFMx -=kN566.0-=MxF

X-direction:

Y-direction:[ ] [ ]03322 --+= )V(?QV?QFMy

kN40.0-=MyF

V1

V3

V2

Determine momentum forces



Calculate reaction forces

PxMxRx FFF -=

kN09.80-=RxF

X-direction:

Y-direction:

PyMyRy FFF +=

kN19.50=RyF

FRY

FRx

kN82.4322 =+= RyRxR FFF

FR

( ) == - 7.13tan 1 RxRy FFq

?

Resultant force = - Reaction forces

Determine resultant force

7Calculation of restraining method

Thrust block (bearing area) Thrust block (friction between thrust

block and soil) Anchor rings / Puddle flange Harnessed joints (friction between soil and

pipe) Combination of these methods

Calculation of restraining method

Thrust block (bearing area)

soilofcapacitybearinghorizontalSafeforceThrust

requiredareaBearing =

Fu = total thrust force (kN)A = bearing area of thrust block (m)Pbearing = bearing capacity of soil (kPa or kN/m)

Safety factor ?

bearing

u

PFA =

8Calculation of restraining method Guidelines for bearing capacities

10040

Loose Uniform Sand

DrySubmerged

200100

Loose Well -graded Sand, Gravel, Sand-gravel mixtures or Dense Uniform Sand

DrySubmerged

400200

Dense Well-graded Sand, Gravel and Sand-gravel mixtureDrySubmerged

NON-COHESIVE SOILS

1 000Very soft rock - Can be peeled with a knife: material crumbles under firm blows with sharp end of a geological pick

2 000Soft rock - Can just be scraped with a knife: indentation of 2 to 4 mm with firm blow of the pick point.

5 000Medium hard rock - Cannot be scraped or peeled with a knife: hand- held specimen breaks with firm blow of the pick.

10 000Hard sound rock - Broken with some difficulty and rig when struck.

ROCK

Bearing capacity (kPA)Soil type


Friction between thrust block and soil( )gMMMMF pswcs +++= m

Fs = total friction resistance between the thrust block and soil (N)

Mc = mass of concrete thrust block (kg)Mw = mass of water in pipe resting on thrust

block (kg)Ms = mass of soil on top of thrust block (kg)Mp = mass of pipe resting on thrust block (kg)

= friction coefficient between soil and thrust block

g = gravitational acceleration (m/s)

9Calculation of restraining method

Friction between thrust block and soil( )gMMMMF pswcs +++= m

Fs = total friction resistance between the thrust block and soil (N)

Mc = mass of concrete thrust block (kg)Mw = mass of water in pipe resting on thrust

block (kg)Ms = mass of soil on top of thrust block (kg)Mp = mass of pipe resting on thrust block (kg)

= friction coefficient between soil and thrust block

g = gravitational acceleration (m/s)


Friction between thrust block and soil

0.40 to 0.50Very firm and hard clay

0.30 to 0.35Medium to hard clay and clay with silt

0.30 to 0.35Fine sand with silt; non-plastic silt

0.35 to 0.45Clean fine sand; fine to medium sand with silt or clay

0.45 to 0.55Clean fine to medium sand, medium to coarse sand with silt, gravel with silt or clay

0.55 to 0.60Clean gravel to coarse sand

0.70Clean hard rock

Friction coefficient ()

Soil

Friction coefficient () is affected by the degree of compaction

and moisture content

10


Anchor rings / puddle flange


Anchor rings / Puddle flange

23396185

103134170210302472679

3.23.23.23.23.23.23.23.23.24.84.8

1.92.02.53.03.33.74.24.75.67.08.4

1010101010101013131616

252538385151517676

102102

168219273324356406457508610762914

Maximum pipe pressure of 1034 kPa (150 Psi)

Permissible load on ring

(kN)

Minimum weld tw(mm)

ty(ty = ts + tr)

(mm)

Ring thickness B

(mm)

Ring width A

(mm)

Pipe OD(mm)

11


Harnessed joints (friction between soil and pipe)

Steel pipe A guide for design and installation (AWWA M11) 3rd edition

Design for sleeve couplings Harness plate thickness, bolt tensile stress,

spacing around pipe, tightening procedure etc.



Type P (smaller diameters)

12



Type RR (larger diameters)

Installation guidelinesThrust blocks:

The sides and bottoms of excavations against which thrust blocks are cast shall be sound and undisturbed and all loose material shall be removed

Excess excavations shall be filled with concrete simultaneously with the concrete of the thrust block

All joints should be leaved accessible If it is a steel pipeline which is flanged or welded, no thrust

block is usually required Valves and plugs at stop-end pipes

must also be adequately anchored Compare pressure forces with

momentum forces Balance upward forces through the

mass of the block

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ExamplesAbove ground installation

Examples

0.05430040040080075

0.0753005005001000100

0.4265013005001000150

0.7780016006001200200

1.50110021506501300250

2.55130027007001400300

Vol(m)

Y (mm)

X (mm)

Z (mm)

D (mm)

DimentionsNominal diameter

(mm)

T-piece underground

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UP Thrust - Thrust block design program (http://www.up.ac.za/academic/civil/divisions/water/software.html)

AISI Steel Water Pipe Design Software (http://www.strucsoft.com)

Software options

Hand calculationWater flows through a reducing 180o bend. The bend is shown in plan. Determine the magnitude of the force exerted on the bend in the x-direction. Assume energy losses to be negligible.

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THRUST BLOCK THRUST BLOCK DESIGNDESIGN

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Thust Block Design