Download - 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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