Figure H4.15: Effectof(a)timeand(b) temperatureon therelaxation of5mmdiameter wire at various levels ofstress
4.11 End blocks in post-tensioned members
4.12.4 Spacing of prestressing tendons .1 General
-where
f~..
is the prestress in the concrete at the extreme tension fibre at a distancev
from the centroid of the section of second moment of area1.
4.12.3 Cover to prestressing tendons
The recommendations on cover in relation to durability and fire resistance requirements are similar to those for reinforced concrete in 3.3.4 to 3.3.6: in practice. it will often be requirements of fire resistance rather than durability that will control the cover to be provided.
4.12.3.1 Bonded tendons 4.12.3.1.1 General
4.12.3.1.2 Cover against corrosion. The exposure conditions defined in Table 3.2 are used as the basis for Table 4.8. which is essentially Table3.4with the minimum cement content taken not less than 300kg/in3. This limit reflects the importance of the cement content in protecting the steel, and the somewhat greater sensitivity of prestressing tendons to the effects of corrosion, due to their generally small cross-section and high stress level. See also the commentary on 6.2.
4.12.3.1.3 Cover as fire protection. Table 4.9 gives nominal covers to all steel to meet specified periods of fire resistance. The format is similar to Table 3.5 for reinforced concrete and the covers relate specifically to the minimum widths and thicknesses given in Figure 3.2. See also the commentary on 3.3.6 regarding covers and anti-spalling measures.
4.12.3.2 Tendons in ducts 4.12.3.3 External tendons
It should be noted that the cover added to external tendons will not in fact be put into compression by prestress. Some compression may be induced later owing to creep and shrinkage but this may not always be enough to offset the tensile strains due to imposed loading. It is essential therefore that cover provided in this context be thoroughly compacted and that this concrete be anchored to the prestressed member (preferably by reinforcement). The positioning of the tendons and the shape of the cross-section should be so arranged that the influence of any transverse cracking or longitudinal splitting is kept to a minimum.
4.12.3.4 Curved tendons
4.12.4 Spacing of prestressing tendons 4.12.4.1 General
The layout of prestressing tendons should be such that the concrete can be easily placed
I(15
Handbook to BS8IIO:1985
[71
due to prestress atthe end of the transmission length
F
alternative
r
II .,~type of splitting if tendons are
Figure H4. 17: Splitting at ends of p re-tensioned beams.
and thoroughly compacted. No general rules can be formulated because the layout will depend very much on the type of section and on the amount of transverse reinforcement provided: it will also depend to some extent on the method used for vibrating the concrete and on the type of tendon and anchorage system used.
4.12.4.2 Bonded tendons
Where straight tendons are grouped some distance apart in pre-tensioned members.
tension may develop at the end of the beams between the groups of tendons as the pre-compression spreads out from being a series of point loads on the end of the beam to give a linear stress distribution across the section at the end of the transmission zone.
Figure H4.17 shows the areas where splitting is possible, the most likely spot being at any change of cross-section in the depth of the member. Under these circumstances.
stirrups or helices should be usedto contain the tendons at the end of the beam and to prevent splitting from developing. This reinforcement should be designed in accordance with the specialist literaturet4~and4.49)and provided over a distance along the beam at least equal to the total depth of the beam.
4.12.4.3 Tendons in ducts 4.12.4.4 Curved tendons
4.12.5 Curved tendons 4.12.5.1 General
The type of action visualized in this clause is illustrated in Figure H4.18(a). There may also be a risk of the side cover spalling in very narrow webs and of the bottom cover spalling off where tendons run close and approximately parallel to the soffit of slabs.
The manufacturers of most post-tensioning systems specify cover and spacing requirements for their tendons and ducts and these should be regarded as minima.
In general. where a number of prestressing tendons in the same plane are curved in that plane. the innermost tendon should be stressed and grouted first. Where this is not possible. such as in statically indeterminate structures. then it may be necessary to anchor the tendon back into the compression zone (as shown in Figure H4.l8(b)) for highly curved tendons. Consideration could also be given to providing helical reinforcement to carry tensile stresses between the ducts.
The recommendations in4.12.5.2 to 4.12.5.4 are taken from reference 4.50. Further research data and suggested design rules are given in reference 4.51.
A
K K .%•K -K... K~K~K K
.4 .— K
K K K .-.. K... K~— . .4 K — .~K — — —
--“4- K. ~KKK.K..KKKK1-.- KKK-K-KKE
-
--- --.
~K K KK KK K — KK-~K K K K~ — . .K - — KK K — K
‘-4---.-. K-K.
-Part I: Section 4
A
bursting crack
SEC flON A-A
Figure H4. 18: Bursting stresses from tendons with high curvature.
4.12.5.2 Cover
4.12.5.3 Spacing
4.12.5.4 Special measures to reduce spacing of ducts
4.12.6 Longitudinal reinforcement in prestressed concrete beams
4.12.7 Links in prestressed concrete beams
This clause catalogues the various situations in prestressed concrete design where
transverse reinforcement is required. The design and detailing~ of links for shear
considerations is governed by 4.3.8.7 to 4.3.8.10. If a pre-tensioned member is supported
near its ends, such that a considerable proportion of the transmission length (as
determined from 4.10.3) is within the span, the transmission length could be designed
as a reinforced concrete section in accordance with 3.4.5 as a conservative alternative
to the approach given in 4.3.8.4. The requirement for links to resist longitudinal splitting
forces at the ends of pre-tensioned members is dealt with in the commentary to 4.12.4.2.
4.12.8 Shock loading
As in reinforced concrete. the provision of transverse steel in major structural members
isconsidered to be good practice. irrespective of shear requirements and this is especially
so if the member has to resist shock loading. In general, for this situation, minimum
reinforcement requirements should be in accordance with 3.12.5.3. In post-tensioned
members, the ducts should be grouted.
REFERENCES
4.1 ANDERSON. A.R Lateral stability of lone prestressed concrete beams. Journal of the Prestressed
Concrete Institute. Vol.16. No.3. May-June 1971. pp-7-9. See also discussion by SWANNK R.A
Vol.16, No.6. November-December 1971. pp.85-87.
4.2 SWANN. RA. The lateral buckling of concrete beams lifted by cables. The Structural Engineer.
Vol 44, No.1. January 1966. pp-21-33.
4.3 BATES. s.c.c. Some experimental data relating to the design of prestressed concrete. Parts 1.
2 & 3. Civil Engineering and Public Works Review. Vol. 53. No627. September 1958.
pp.lOlO-lO12. Vol.53. No.628. October 1958. pp.1958-1961 and VoL53, No.629. November
1958. pp.1280-1284.
4.4 ABELEs. p.w Partial prestressing and its suitability for limit state design - The Structural Engineer.
Vol.49. No.2. February 1971. pp..67-86.
4.5 BEEBY. AW.. KEYDER. E.and TAYLOR. H.P.J. Cracking and deformation in partially prestressed concrete beams. London. Cement and Concrete Association. January 1972. 26pp. Publication
42.465.
4.6 PANNELL. F.N The ultimate moment of resistance of unbonded prestressed concrete beams.
Magazine of Concrete Research. Vol.21. No.66. March 1969. pp-43-54.
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reinforced concrete beams. Magazine of Concrete Research. Vol.28. No.97. December 1976.
pp 203-208.
4.8 LEONHAROT. F. Abininderung der Tragfahi~keit des Betons infolge stabforiniger. rechtwinklig
zur Drucknchtung angerdraehte Einlagen. pp7l-78. KNtTTEL. G and KUrFER. H eds.
Stahlbetonbau: Berichte aus Forschung und Praxis. Berlin. Wilhelm Ernst & Sohn. 1969.
Festschrift Ruesch.
lal
A
IbI
107
Handbook to BS8IIQ.-i~S5 [~K K
4.9 REY\OLDS.oc. revised byCLARKE. .L.andTAYLOR.H~ Shear provisions forprestressedconcrete [K
in the Unified Code CPI 10 1972. London. Cement and Concrete Association. Octobcr 1974. l6pp. Publication 42.500.
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F
4.11 HAWKtN5. N.M.The shear provision of AS CA35—SAA Code for prestressed concrete. Institution of Engineers Australia. Civil Engineering Transactions. VoLCE6. No.2. September 1964.
pp. 103-116. and University of Sydney. Department of Civil Engineering 1964. 46pp. LS6681
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F
Committee 326. Proceedings of the American Concrete Institute. VoLS9. No.9. Septeinher 1962. pp1341-1S47.
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December 1960. pp649-678.
r
4.14 tHE CONCRETE5octEt4~ Flat slabs inpost-tensioned concrete with particular regard to the use of unbonded tendons—design recommendations. Concrete Society Technical Report No.17.
1979. l6pp.
r..
4.15 THE CONCRETEReport No25. 1984. 44pp.soct~-r~. Post-tensioned tiat-slab design Handbook. Concrete Society Technical 4.16 REGAN.~E.The punching resistanceof prestressed concrete slabs. Proceedings of the Institution
of Civil Engineers. Part 2. Vol.79. December 1985. pp657-680.
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F
and Constructional Engineering. Vol.61. No.8. August 1966. pp.267-274.
4.18 zt~. ~ andMOREADITH. F L.Ultimate load capacity of prestressed concrete columns. Proceedings of the American Concrete Institute. Vol.63. No.7. July 1966. pp767-788.
4.19 BROWN. Ki. The ultimate load-carrying capacity of prestressed concrete columns under direct
[
and eccentric loading. Civil Engineering and Public Works Review. Vol.60. No705 Aprtl1965. pp539-541. Vol.60. No.706.May 1965.pp683-687. Vol60. No.71)7. June 1965.
pp.84l-4.20 HR. andHALL. As. Tests on slender prestressed concrete columns. Detroit. American
F
Concrete Institute. 1965. pp.192.204. SP-l3.
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4.22 CEDERwALL. K.. ELFGREN. L. andLOSBERO. A. Prestressed concrete columns under short-time and long-time loading. Goteborg. Chalmers University of Technology. 1970. l6pp. Publication 70:3.
4.23 KIRRBRIDE.T.w, Review of accelerated curing procedures. Precast Concretc. Vol.2. No.2.
February 1971. pp.93-106.
4.24 FEDERATIONINrERNA-rIONALEDELAPRECONThAINTE.Acceleration of concrete hardening by thermal curing. FTP Guide to Good Practice. 1982. I6pp.
4.25 BANNISTER. IL. Steel reinforcement and tendons for structural concrete. Part 2: tendons for prestressed concrete. Concrete. Vol.2. No.8. pp.333-342. August 1968.
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Vol.197. No.5111.3. April 1964. pp.492.495. Also Building Research Station Current Paper.
Engineering series 12. 1964. 6pp.
4.27 BATE. s.c.c. and CORSON. RH. Effect of temperature on prestressing wires. Conference on prestressed concrete pressure vessels. London. March 1967. London. Institution of Civil
Engineers. 1968. pp.237-24~. Paper No.21.
4.28 CAHILL.r. andBRANCH. GD.Long-term relaxationbehaviour of stabilized prestressing wires and strands. Conference on prestressed concrete pressure vessels. London. March 19fi7. London.
Institutionof Civil Engineers. 1968. pp.219-228. Paper No.19.
4.29 ~BRAM5. ~ts. andCRLZ. C.R. The behaviour at high temperatureof steel strand for prestressed concrete. Journal of the PCA Research and Development Laboratories. Vol.3. No:3.
September 1961. pp.8-19.
L
4.30 BANNISTER. IL.Engineer. Vol.35. No.2. February 1971. pp.81)-90.Steel reinforcement and tendons for structural concrete. The Consulting 4.31 NEVILLEAM. Creep of concrete: plain, reinforced and prestressed. Amsterdam. North-Holland
Publishing Company. 1970. 622pp.
U
4.32 E\ANS. R.H.andKONG. F K.Estimation of creep of concrete in reinforced concrete and prestressed concrete design. Civil Engineering and Public Works Review. Vol61. No.7)8. May 191,6 pp593-596.
4.33 THE CONCRETE SOCIETh
-
The creep of structural concrete. Concrete Society Tcchnical Paper.L
No.101. 1973. 47pp.
4.34 COOLEY. E.H. Friction in post-tensioned prestressing systems. London. Cement and Concrete Association. 1953. S7pp. Publication 41.001.
4.35 ‘vv~i-r. K.J. Measurement of friction in corrugated curved prestressing ducts. SvdnL~.
L
Commonwealth Experimental Building Station. 1964. l7pp. Technical Record 52:75:322
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4.38 BASE. GD. An in’estigation of the transmission length in pre-tensioned concrete. London.
L
K K — — K K K K
K.. - K
I
~ -. -
-K K~.KK
-
- -- - . - K K-Parr I: Section-4 Cement and Concrete Association. 195S. 29pp. Publication 41.005.
4.39 BASE. G.D. An investigation of the use of strand in pre-tensioned prestressed concrete beams.
London. Cement and Concrete Association. 1961. l2pp. Publication4L01 1.
4.40 MAYFIELD.a..DAVIES. 0.andKONG.F.K.Some testson the transmission length and ultimate strength of pre-tensioned concrete beams incorporatine Dvform strand. Magazine of Concrete Research. Vol.22. No.73. December 1970. pp.219-226.
4.41 ZIELINSKI.I. and ROWE. RE. An investigation of the stress distribution in the anchorage zones of post-tensioned concrete members. London. Cement and Concrete Association. 1960. 32pp.
Publication 41.009.
4.42 ZIELIN5Ki.i. and ROWE.R.E. The stress distribution associated with groups of anchorages in post-tensioned concrete members. London. Cement and Concrete Association. 1962. 39pp.
Publication 41.013.
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74 lpp.
4.44 LENsCHOW. RI.andSOZEN. M.A Practical analysis of the anchorage zone problem in prestressed beams. Journal of the American Concrete Institute. Vol62. No.11. November 1965. pp. 1421-1439.
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4.46 YEfl~RAM. A.L. andROBBINS. K. Anchorage zone stressesin post-tensioned uniformmemberswith eccentric and multiple anchorages. Magazine of Concrete Research. Vol.22. No.73. December 1970. pp.209-218.
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4A8 ARTHUR. PD. and GANGULI.s. Tests on end-zone stresses in pre-tensioned concrete I beams.
Magazine of Concrete Research. Vol.17. No.51. June 1965. pp.85-96.
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pre-tensioned concrete members at transfer. Indian Concrete Journal. VoL47. September and October 1973. pp.34-6-351 and 379-385.
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of the Environment. August 1969. 3pp. Interim Memorandum (Bridges) 1M2.
4.51 MCLEISH. A. Bursting stresses due to prestressingtendons in curved ducts. Proceedings of the
Institution of Civil Engineers. Part.2. Vol.79. September 1985. pp.605-615.
I 1)9