road finisher. The wires are released the second day after casting, and the sleepers are removed by overheal crane to storage or for delivery. This plant is very compact and the floor space of the factory exceedingly well used.
it
There are a number of factories in which bonded structures are produced in single units. The long-line method is a typical mass production process which requires large floor areas, big capital investment and relatively little labour. The single-unit method, on the other hand, can be worked in small factories with restricted floor space and very much less capital, but requires a greater amount of labour per unit of production. An example of this type is the small joist factory at Orleans, France, Fig. 22, which pro duces floor joists to the desigu of Freyssinet. The steel wires are stret- ched against the steel mould which has to hear the whole stretching force, while the concrete is poured, com- pacted, cured and while it matures.
TOP FISKA
ГОНС АКТЕ
BOTTOM FIBRE
O PONDE
Thui
PRELIMINARY
STHESS
CONCRETE
"Zrime or trANSFER
(NO PORTION
I-TOTAL
DEAD WRIGHT
LIVE LOAD
CAFIP & SHRINKAGE
IT APPLICATION THEO APPLICATUM
JBURIED
NO
TENSILE MEMBE
OTANDO
STEEL
TIME
T
When the prestressing members of couerete structures are post-tensioned and there is no bond between steel and concrete, the whole prestressing force is induced by anchorage blocks at both ends of the structure, Each stretching process can therefore deal only with one single structure, Photo- graph, Fig. 23, shews a concentration of wedge anchorages at one end face of a bridge structure This type of so-called sandwich plates has been pro- posed by Magnel and they are used in all his designs.
PART 11
DESIGN
Principles for the Design and
Construction
are
The designer should aim at a high initial pre-tension of the steel. A low initial steel stress produces a low and, owing to unavoidable losses, rather uncertain concrete compression, com- bined with an uneconomical use of steel. The elastie elongations 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 a 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- CASE cent of the ultimate strength, or 85 Fig. 24
percent of the 0.1 percent proof stress. The moulds are re-used every 24 hours
Stages of Loading which is the time required by one pro duction cycle.
I+4L
сходит
my d
CALE
b
TIME
TOTAL OF A-D
TINE
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 concreto of each unit.
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 amined in chronological
49
full
TOTAL LOAD IN TON
50
40
50.
20
10
CHACKS
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 stresses may be expexted m the steel and in the concrete.
The preliminary design of prestress. ed concrete structures shall be made for not less than two stages:
(a) the stage during
stage during production when the prestresses of the steel have been transferred to the concrete and both are being balanced; and.
(b) the stage when the specified loads are applied to the prestressed
structure.
It is recommended that in the final design a number of additional stages should be investigated such as:
(c) the combination of the balanced prestresses and the stresses due to the dead weight of the structure where the 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 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 the 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 and 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-TENSIÓN
78 TON PER SQIN/
RECOMING VISIBLE
DEAD WEIGHT
10
52 TOX PER SOUN
26 TON PER SQ IN
20
DEFLECTION
50
40
65 TOK PER SA IN
FAILURE.
„HOMOORNEOUS | SECTION m = 7
60
„CRACKED 2ECTION M-15
50
70 60
OF PRESTRESSED BEAMS IN
110
30 100
THS OF AN INCH
Fig. 25
120