CONCRETE CAST 2 STU
PRECAST "UNITS
VanG-FLOORS
BROT KO 320 Shepup
Rhove CONCRETE WES
SARJA Emasigur
IN COVERETE NÓS.
SUPPORTS
CROSSES.
BRIGES
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PELZADAS Že verboer down
KTM CONCRETE FAST A SAU
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Fig. 5.
Fig. 8.
Fig. 6. →
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Fig. 7.
precast units are assembled into one member by prestressing cables which apply the required compression force to the precast units, again by means of end anchorage blocks.
In all these cases, the steel is post-tensioned against the hardened concrete and there is no bond existing at the time of the prestressing process. It is however nowadays quite usual to embed the steel cables in cement grout or mortar, after all prestresses have been adjusted to the specified values. In this way, bondless structures are transformed into bonded structures, and thenceforth behave as such under the action of the external loads.
The losses in the prestresses of bondless structures are considerably smaller than those in the bonded alternatives.
Uniform Prestressing
When beam structures are designed to resist bending moments of given sign, such as roof girders or bridges, it is normal practice to place the prestressing steel eccentrically so as to cancel the bulk of the stresses caused by the dead weight of the structure.
Uniform precompression is suitably applied to structural members which may be subjected to positive and negative bending, such as precast units during transport, handling and erection. It should also be applied to structures which are exposed to loads from all sides, such as towers, poles, piles, pipes, or structures which are exposed to alternating or vibrat- ing loads.
I have produced a number of prestressed beams of various cross sections, such as those shown in Fig. 6, which are uniformly precompressed by a great number of compressor wires. These beams are designed to replace steel joists and their bending resistance is the same for both positive and negative moments. The steel required is only 12-15 percent of that in an equivalent steel joist. The structural depth is some 30 percent higher and its dead weight about 50-80 percent heavier than that of the equivalent steel joist, for average span and loading conditions.
Combined Structures
To make prestressing economical it becomes often essen- tial to combine precast prestressed units with site concrete, and balance carefully the amount of work expended on each, see Fig. 7.
The prestressed units form the tensile zone of the structure and preferably they also form the moulds of the in-situ concrete. The units are usually designed to carry the total dead weight of the structure and such incidental loads as may occur during construction so as to avoid or reduce the need for scaffolding and shuttering on the site.
The in-situ concrete forms the compression zone of the structure and provides the joints which transform the precast units into a monolithic structure. The in-situ concrete is not prestressed and should be designed to provide the additional strength required for carrying the specified live loads. This system has been applied to the construction of floors as well as of bridges.
One of these floors, Fig. 8, is that used for the recon- struction of housing in Orleans, France. The joists are spaced at 1'4" centres and very lightweight filler blocks are dropped in between and seated on their bottom flanges. The concrete slab cast in-situ over joists and blocks completes the floor structure. The ceiling is suspended from the Aller blocks. The wires of the prestressed joists are left protruding for about 6 in. and these ends are embedded into the joints of the brick walls or in a reinforced concrete string course.
The method of combining prestressed precast units with ordinary concrete cast in-situ was used on a large scale in Germany during the war. There it was mainly applied to the construction of bridges and roofs of shelters. The bow- string girders. Fig. 9, which have heavily pre-tensioned tie members formed the lower part of bomb-proof roofs for U-boat
10
Fig. 9
shelters along the Atlantic coast. Their span is 96 ft. They were placed side by side to form the soffit. Heavy web reinforcement of mild steel quality was placed between the precast units and a mass of concrete up to 30 ft. thickness was cast on top to complete the roof. During erection, the prestressed bow-string girders carried the considerable dead weight of the thick mass concrete, while in the finished structure, they formed the tensile zone of. and co-operated with, the slab roof as a compound structure.
Girders of large span
Structures of large span are nowadays built exclusively by the method of post-tensioning, l.e., without bond between steel and concrete. The reason for this is that the prestress- ing forces in big structures become so large that the only abutment which can possibly resist these forces is the con- crete of the structure itself. The roof girders over a hangar at Brussels' airport, Fig. 10, designed by Magnel, are typical examples. Their span is 165 ft. and each such girder carries a portion of the roof 33 ft. wide. The prestressing force amounted to about 3000 ton each, which were exerted by about 840 wires of 7.0 mm. (0.273 in.) dia. each. The wires were post-tensioned in groups of two and held in stretched position by end anchorages in the form of wedges. In this structure the complete roof girder was cast in one unit on the floor of the site, prestressed by the non-bonded cables, and then lifted into position.
The process adopted for the reconstruction of a number of road bridges over the Marne in France was also based on the post-tensioning of steel cables against the hardened concrete, but in this case cach girder consisted of a great number of concrete blocks of from 2 to 4 ton in weight each. The blocks for all bridges were precast in one yard on the river side and later assembled by non-bonded cables and floated into position by barges. All these bridges have a span of 243 ft. and they have to carry main road loadings.
Та eliminate diagonal tension entirely from these exceedingly slender structures, prestresses were employed in three directions. To start with, the two flanges of each I-shaped block were precast with the web reinforcement of high tensile steel embedded in each flange. Fig. 11. After the steel mould for the web had been inserted, the two flanges were pushed apart by a battery of small jacks, the concrete of the web was cast and well compacted by vibration and pressure, and cured by steam. When it had hardened sufficiently. the jacks were released and the web pre- compressed in vertical direction. The second prestress was applied by the longitudinal main cables, during erection. the central zone it was effected by cables in the lower flange and over the rest of the arch by cables in the upper dange which descend through the tie of the springing triangle to be anchored just below the articulation. The third prestress was applied by transverse cables which pressed all six main girders of each bridge against each other and thus ensured their co-operation under concentrated loads. And there is still a fourth type of artificial compression kept in reserve by the insertion of flat jacks in the abutments; these jacks could be inflated at a later date, if the bridge showed any excessive defections due to plastic deformations of the concrete.
Hydraulic Structures
In
The advantages of prestressed concrete become especially obvious in structures which are subjected to pure tensile stresses, such as high-pressure pipes, pressure tunnels and circular tanks for water and oil storage.
In a factory producing pressure pipes according to a process specified by Freyssinet, the pipes are of 32 in. and 48 in. diameter, and 20 ft. long. The underlying principles of this process are to stretch hooped high-grade wires embedded in concrete. by radial expansion of the fresh concrete which is tightly enclosed on all sides; and to speed