No_1_May_and_June__1951 — Page 51

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