ft. (in public buildings) over a clear span of 30'0'` and with cantilevers of 7'6'' at each end to carry the outer walls the depth of the building being 45 ft. The prestressed beams are spaced at 18" e/c. In this picture the joist spans over 15 ft., the depth- span ratio being 1:45,
The same beam placed sideways, spanned 35 ft. between the supports and cantilevering 5 ft. each side. The depth in this position is only 6 in., and the depth-span ratio 1:70, The beam could be easily handled and thrown
Fig. 30
from a lorry without being damaged high prestresses, it is essential to keep the precompression uniform over the whole cross section, Hollow Floor Units
All the deformations seen here were elastic
Fig. 29
·
Another group of prototypes produced were hollow floor units 21 ft. long, Fig. 30. Their eross section was 5" x 12" with a central cavity 31⁄2 X 10", the slabs top and bottom were 3/4" thick, each containing 10 compressor wires single SWG, 16. Parts of the webs were omitted. The dead weight of the floor was 24 lb. per sq. ft., the weight of the steel 2,0 lb. per sq. yd, of floor, These floor beams are designed to span 20 ft. from the front wall to the back wall of standard houses. The spine wall may be positioned anywhere from 7 ft. to 13 ft. distance from the front wall. The load bearing spine wall in the ground floor may be erected after the prestressed units forming the first floor have been laid. To take the live loads the prestressed units work as a continuous floor over, three supports, Planks
A third group of prototypes dealt with planks. Photo Fig. 31 shews a Differential shear stresses, During prestressed board 14 in, thick, 12 in. the production of one of these units wide, 12'9" long, and illustrates its 2 wires positioned in the web slipped elasticity. The concrete of this board when all wires were stretched together, was precompressed to 400 lb. per sq. The two wires were not re-stretched but in. only. Other planks only 3/4 in. embedded without prestress. After thick were precompressed to one ton release, a longitudinal crack appeared per sq. in., and their elasticity was in the beam between the two slipped correspondingly much greater. These wires, see Fig. 29, 3/4 in. wide at planks
have been used, in limited
the end, and 6'9" long, progressing quantities, for roofs with supports at towards midspan. Conclusion: with so 3 ft. c/c
of
To illustrate the elasticity prestressed concrete similar planks, 2 in. thick, have been designed to be used as diving boards in a swimming pool.
The elasticity of prestressed concrete is also well illustrated by this concrete bar 14 in. square and 12'9" long, which is carried in the middle, Fig. 32. The bar was obtained by sawing off a narrow strip from the plank shewn in the last picture. This bar is precom- pressed by one wire positioned exactly in the centre of the square cross sec- tion. It would be quite inconceivablo to try and lift an ordinary concrete bar of similar dimensions. It would crack and break at once. Compound Reinforcements
Compound reinforcements are pre- stressed concrete bars which are in- tended for usc as reinforcements in ordinary concrete structures and re- place mild steel bars. A compound reinforcement, see Fig. 33, consists of a balanced combination of high-grade steel wires and high-strength precast concrete units in which the steel is permanently tensioned and the concrete units are permanently compressed. The wires provides the tensile strength of the compound reinforcement while the co-axially concrete units provide the body in which the greater part of the strength of the steel is stored as pre- compression the great elongations of the steel at high stresses are reduced by the prestressing process to value compatible with ordinary concrete con. struction. To produce a compound re- inforcement precast pipes are laid in one line, and the wires are threaded through. The wires are then stretched
- Fir. 31
Fig. 32→
19