Anchorage Design

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Anchorage zone design


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Anchorage

Determination of anchoragezone stresses in post-tensionedbeams by Magnels method,Guyons method and IS1! code" design of anchorage zonereinforcement " #hec$ for

transfer bond length in pre-tensioned beams


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Methods and Materials

wedgesPT strand

anchor plate

Figure 3Anchorage componentsSource: DSI

Figure 4Cross-section o anchorageSource: !illiams Form "ngineering


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Magnels System

In this method, the end bloc$ is consideredas a deep beam sub%ected to concentrated loads due to anchorages on one side

and

to normal and tangential distributed loads fromthe linear direct stress and shear stressdistribution from the other side&

'he forces acting on the end bloc$ and the stresses

acting on any point on the horizontal a(is parallel tothe beam are sho)n in *igure


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Magnel s#stem


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Guyons method

Guyon has de+eloped design tables for thecomputation of bursting tension in end bloc$s)hich are based on mathematical in+estigationsconcerning the distribution of stresses in end

bloc$s sub%ected to concentrated loads& 'he concept of symmetrical or eui+alent prism

for eccentric cables, and the method ofpartitioning for analysis of stresses de+elopeddue to multiple cables ha+e been introduced by

Guyon& 'he distribution of forces at the ends are treated

under the categories of force e+enly distributedand forces not e+enly distributed


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$u#on%s s#stem


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Introduction

In post tensioned beam prestress istransferred to the concrete either

by bearing of the anchorage componentsagainst the ends of the beam as inMagnels system or

y anchoring the tendons to the beams)ith the aid of anchoring units )hich maybe cast in to the concrete as in th*reyssinet system of prestressing


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.arge prestressing forces concentrated

o+er relati+ely small area causetrans+erse and shear stresses close tothe ends of the member

'he zone bet)een the ends of the beam

and the section )here only longitudinalstress +arying linearly across the depthis set up is called the anchorage zone orend bloc$ or transmission zone

.ength of end bloc$ not less than thedepth of the section


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/roblem

A rectangular beam of crosssection 0mm ( 2mm issub%ected to an e3ecti+e

prestressing force of 4$5acting at the centroid of thesection& 'a$e for ties and

e(pected loss of prestress as46& 'he cables pass through asteel plate symmetrically in an

area of 4mm ( mm& Design

0


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4 (mm

40mm

7mm

0mm

!7mm

earingplate

0mm

2mm

290000360250 mmApun ==

2

161000460350 mmAbr ==


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Design of anchor plate

.et the allo)able bearing stress be

#ontact area reuired 8

Selecting the )idth of the plate 8 40mm

Depth of the plate 8

Select a breath of 40mm and depth of

7mmso that the size of the plate is greater than

4mm ( mm

MPafck 75.223565.065.0 ==

2

3

8791275.22

102000mm=

mm6.351250

87912=


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/unching area

earing area

/age 479clause 1:&7&4&1/ermissible bearing stress 8

or

;hiche+er is smaller


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mm

mm


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As the actual bearing stress is less than the allo)able +alue, theselected size of

the plate is adeuate& 'he thic$ness of the plate is based on thebending

moment acting on the plate due to the cantile+er pro%ection of theplate

beyond cable cones and the bearing pressure&

readth)ise, cantile+er span of the plate 8

Depth)ise, cantile+er span of the plate 8

Ma(imum bending moment,

If >t is the thic$ness of the plate, then

IS 2-4= /age 092&4&1&4

say 4mm

mm252

200250=

mm302

300360

=

mmNmmM /99992

3022.22 2

=

=

99991.1

6

1250

2

0 =

==

tZf

Mm

pzy

dz

mmt 25.16=


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Adopt a thic$ness of 4mm&


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'. / ( 0 (mm to (mm' Pro*ide the same amounto

stirrups in the same 1one 2eeping the legs hori1ontal'

?einforcement for spalling tensile force

$u#on gi*e the ollowing e/pression or thespalling

tensile orce

Area o (mm diameter ,ars re)uired

&o' o (mm diameter ,ars re)uired

( )

( )

3

'

'

2.004.0

+

+=

aa

aaPPFsp

( )( )

{ } kNaa

aaPPFsp 100020002.0200004.02.004.0

3

'

'

=+=

+

+=

227741587.0

1000100mm=

5.5

4

8

277

2 =


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/ro+ide one si( legged stirrup in the +ertical

direction and onesi( legged stirrups in the horizontal direction

%ust behind the

anchorage plate&


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(ampleB 4

'he solid end bloc$ of a post tensionedprestressed beam of

40m span, )ith three cables, each of =-10mmstrands,

tensioned to 14$5 is sho)n in Cgure& 'heanchorage plates

are suare )ith a side length )ith a side of12mm& Design the

end bloc$ for bursting forces and s$etch thedetails of

reinforcement according to the pro+isions ofthe IS 1!-414


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'he end bloc$ has been di+ided into threeeual areas, )ith

one anchorage located appro(imately at thecentroid of each

area& 'he sections of the eui+alent prismscorresponding to

each anchorage force are sho)n in Cgure& 'hebursting tension

may no) be calculated for each prism asfollo)sB


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/age 4=9clause 1:&7&4&4

'he area of steel reuired to resist this

bursting force is

&o' o .mm diameter legged stirrups re)uired

Pro*ide &os o .mm diameter legged stirrups

0

0

0

3.032.0y

y

P

F pbst =

kNy

yPF p

bst 276600

1803.032.012003.032.0

0

0

0 =

=

=

2

3

44.76441587.0

10276 mm=

94.1

54

10

44.764

2=

=


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Anchorage Design

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