road finisher. The wires are released the second day after casting, and the sleepers are removed by overhead crane to storage or for delivery, This plaut is very compact and the floor space of
41.
When the prestressing members of concrete structures are post-tensioned and there is no bond between steel and concrete, the whole prestressing force is induced by anchorage hlaska
order. Diagram, Fig. 24 is a typical ex- ample of such an investigation and it shows at which loading stages the highest stresses, and at which stages the lowest
ERRATA
On Page 48, "Photograph Fig. 22 shows the first of three factories in Eugland producing railway sleepers" should read "Three factories in England produce railway sleepers."'
On Page 50, Fig. 28'' should read “Fig. 25.**
ночны
ACTING WOMEN KONUYLA
AGEN
to wear the whole stretching force, while the concrete is poured, com- pacted, cured and while it matures,
TOP FIBRE
CONCRETE
BOTTOM PIBRE
CONCRETE
PRELIMINARY
STRESS
DEAD WRICHT
LO
LIVE LOND
CREEP & SHRINKAGE
LIL
TIME CF VILANSFER
EET APPARCAYAR THE APPLICAT
TING
L
TOTAL OF A-D
TENSILE. MEMB
BOMULD
STEAL
TIMEL
CASA & CAR C LASE
Fig. 24
The moulds are re-used every 24 hours which is the time required by one pro duction cycle.
In some of the long-line processes with wires up to 1/5 in. diameter, there are no individual moulds provided for each unit, but the moulds are con- tinuous and the concrete is cast in one over the full length of the stret ching bed. In that case, the required length of unit may be cut from the continuous production by a carborun- dum saw, as for the case of Schafer slabs. The bond and the automatic cone anchorages of the wires formed at the places of cutting ensure the effec- tive transfer of the preliminary forces into the couerete of each unit.
bined with an uneconomical use of steel. The elastic elongations are relatively small and require a fine adjustment in the stretching device. In contrast, a high initial steel stress produces a high and reliable concrete compression, obtained with B small amount of steel. The steel elongations are comparatively large and therefore easier to adjust and maintain. High initial steel stresses are therefore more effective(and more economical) thau low initial steel stresses.
The upper limits of the initial ten- sion should be governed by the plastic flow of the steel and by the crack coefficient. This coefficient is the ratio of the load under which capillary cracks appear to the ultimate load under which failure occurs. It should not be lower than 0.5 to ensuro freedom from cracks under all design loads, but should not be higher than 0.8, so that timely warning is given of the approaching destruction by the appearance of cracks. Because of the plastic flow of the steel the initial pre-tension should not exceed 70 per- cent of the ultimate strength, or 85 percent of the 0.1 percent proof stress.
Stages of Loading
In prestressed struc. tures the stresses do not portion as the external change in the same pro- forces, because the value of the prestresses is not dependent on the exter- nal loads. In addition the prestresses diminish slowly from the moment of release until shrinkage and creep have taken place. Consequent- ly, in prestressed units the various load Com- binations must be ex. amined in chronological
40
full
3
TOTAL LOAD IN TON
50
5.
30
2
10
:
CRACKS
Lue prestressed structure is cast in situ; or
(d) the combination of the balanced prestresses and the stresses due to handling, transport and erection of the unit where the prestressed unit is pre- cast;
(e) the stage when loads exceeding the specified values are applied to the prestressed structure, and hair cracks appear;
(f) The loading stage structure fails.
when
the
the
The co-operation of steel and con- crete may be secured by bond or by end anchorages on the prestressing members or by a combination of both. For steel up to 1/5 in. diameter effect of bond is sufficient to ensure the transfer and the maintenance of the preliminary stresses. For heavier bars anchorage blocks should be pro- vided in addition to the bond effect.
For a given type of wire, the anchor- age force obtained from the bond aud from the automatic cone effect at the ends of the wires increases with the degree of pre-tension of the wires and the crushing strength of the concrete at release, and inversely with the dia- meter of the wire. Owing to the creep
PRE-TENSIÓK
78 TON PER SQIN.
t
S2 TON PER SAM
BELOMING VIRALS
DEAD WEIGHT
모
26 TON PER MULIN,
GSTON PER SÅ IN
20
50
40
50
DEFLECTION
OF
PAILURE.
„HOMOGENEOUS | SECTION M-7
(CRACKED SECTION M-15
60
70
BO
30 100 HO
120
PRESTRESSED BEang in fyths op AN INCH
Fig. 25