End Block

14/5/2013

MAE 1183 Design of Prestre Pre Prestressed  stressed ssed Concrete Concret Concrete e

End Block Design

2

1

14/5/2013

Introduction •

•

•

•

In post-tensioned construction, the prestressing force is transferred to the concrete through relatiel! s"all anchorage plates #ehind the anchorage #! #earing$ %his results in a er! high concrete #earing stress #ehind the anchorage plate$ In post-tensioned #ea"s, the ca#les are anchored at the ends of  the #ea"s$ Een in flanged #ea"s, the end part of the #ea" &here the ca#les are anchored are often thickened to create a rectangular section 'nchorage (ones failures al&a!s due to uncontrolled cracking or splitting of the concrete fro" insufficient transerse reinforce"ent$ Bearing failures i""ediatel! #ehind the anchorage plate are also co""on, therefore the reinforce"ent in the for" of links or spirals are re)uired at the face of end #lock 3

End Block - *ailure

4

2

14/5/2013

End Block +Di"ension

5

End Block



3

14/5/2013

End Block



4

14/5/2013

5

14/5/2013

.tress Distri#ution 

%he prestressing force in a tendon is applied through the anchorages as a concentrated force



B! .t enants principle, the stress distri#ution in a "e"#er is reasona#l! unifor" a&a! fro" the anchorage, #ut in the region of the anchorage itself D+egion the stress distri#ution &ithin the concrete is co"ple



%he "ost significant effect for design is that the tensile stresses are set up transerse to the ais of the "e"#er, tending to split this concrete apart efer to figures on the follo&ing slides



einforce"ent "ust #e proided to contain these tensile stresses 11

.tress Distri#ution

2

2

2

12



14/5/2013

.tress Distri#ution 0$2!o

2!o

*#st + Bursting *orce 6i

13

.tress Distri#ution 

Eurocode 2 suggests that a strut-tie "odel can #e used to calculate the #ursting force$



%he force 6 can #e assu"ed to disperse at an angle of 2β &here tan  β 7 2/3 7 33$°$ %he force * in the concrete strut 7 0$5 6 sec β 7 0$6 and the tie force &ill #e * sin β 7 0$336

14



14/5/2013

Design for Bursting *orce 

If the di"ensions of the anchor plate are '  ', the di"ensions 9  9 of the associate rectangle are approi"atel! 1$25 ti"es '  '



It is i"portant that the associated rectangle should re"ain inside the concrete of the #ea"$



:here an end #lock contains seeral anchorages, it should #e diided into a series of s!""etricall! loaded pris"s and then each pris" treated as a separate end #lock$ 'dditional reinforce"ent should #e proided around the &hole group of anchorages to "aintain oerall e)uili#riu"

15

Ea"ple 2 Design the end #lock reinforce"ent for the follo&ing #onded posttensioned #ea"$ ' prestressing force of 183; k< is applied #! a single tendon$

1

8

14/5/2013

.olution to e$g$ 2 %r! the si(e of #earing plate 7 280 ""  280 "" %he first check is a#out the #earing stress$ 6"a / 'rea of associated rectangle ≤ 0$f ckt 0$f ckt 7 concrete c!linder strength at the ti"e of stressing 6"a 7 cross sectional are of ca#les  =I< >0$8f pk 0$;f p 0$1k ? f pk 7 182
1

.olution to e$g$ 2 %he "ini"u" a"ounts of reinforce"ent to preent #ursting is@ 's ≥ 0$15 6"a / f !d 7 0$15  23000 / 21$4 7 181; ""2 'ssu"ing 12 "" #ar as links, area for t&o legs 7 22 "" 2 'pproi"atel! ; links are enough +distri#uted oer a length of 1$2  350 7 420 "" einforce"ent to preent spalling@ 's ≥ 0$03 6"a / f !d 7 0$03  23000 / 21$4 7 34 ""2 'ssu"ing 8 "" #ar as links, area for t&o legs 7 100$4 "" 2 'pproi"atel! 4 links are enough +distri#uted oer a length of 350

18

;