No_2_July_and_August_and_September__1951 — Page 50

PRESTRESSED CONCRETE

(Conclusion)

A Lecture given at the China Fleet Club under the auspices of the Public Works Department, Hong Kong, on December 11th, 1950, by Dr. Kurt Billig, A.M. Inst. C.E., M. Am. Soc. C.E., Professor of Civil Engineering. Hongkong University.

Anchorage of Wires

In the first part of this lecture I tried to give you a general picture on prestressed concrete, the materials used, the various types of prestressing, typi- cal structures, the plant employed, and the various production processes.

The initial investigations dealt with the determination of the embedding length required to develop the full tensile strength of the wire strands in the concrete. Fig. 26 shews one of In the second part I dealt shortly the simple rigs used for testing bond with aspects concerning the design, in anchorage blocks. Result: the such as the principles and assumptions anchorage length of twin strands, at involved, the various loading stages their ultimate tensile strength, was and corresponding stresses, the de formations under load, the ultimate load and the factor of safety.

In the third and last part I intend to give you now a short summary on the

Besearch work and test produe tion of prestressed concrete carried out at the Field Test Unit, Min- istry of Works, London, during

1947/8.

Anticipating the increasing difficulties in the supply of timber and steel, 1946, the Chief Scientific Adviser to the Ministry of Works gave instructions to investigate the use of prestressed concrete as a substitute for timber and steel for transmission poles, tele- graph poles, structural beams, floor units etc. Being the Consultant to the Ministry on prestressed concrete I submitted the program and super. vised its execution.

In the summer months of 1947 we erected an experimental plant and in the Fall we started the production of various prototypes of prestressed units. The central part of the plant is a 55 ft, long prestressing bed of the surface type, where the reaction of the stretching force is taken by heavy *teel girders and brackets. Tho stretching equipment consisted of two hydraulic jacks, 100 ton cach, exten- sion 6 inch, Apart from the mixing plant there was a steam-operated kiln for the curing of smaller units, an overhead gantry for the removal of the finished production and all the usual equipment for the production of high-grade concrete.

The specification of the materials employed in all experiments and test production were:—

Steel: 140-150 ton tensile; 2 percent proof stress 110-120 ton per sq. in. The compressor wires were to be twin- twisted strands of SWG 12 and 11. There should be no creep under fatigue test between 75 and 85 tons per sq. in. Wire delivered in coils should be self-straightening when uncoiled. The concrete was to have a cube crushing

found to amount to less than 120 dia. of the single wires.

Similar investigations were carried out on the bond of wires in castings of rope capping metal for the purpose of gripping and stretching. The com position of the metal was 80% lead, 15% antimony, 5% tin. Preliminary tinuing of the wires was employed to improve the bond. Result: the anchor- age length of twin strands was found to amount to approx. 60 dia, of the single wires when they developed their full tensile strength.

Fig. 27

A third set of investigations dealt with the effectiveness of mechanical devices to grip compressor wires, It was found that a type suitable to

Fig. 26

was

anchor single wires consisted of taper- ed pins fitting into tapered holes of steel plates with grooves to receive the wires; pins and bushings of holes were of hardened steel, A type suit- able

twin to anchor

strands similar, but the pins were flattened and cross riffled on one side, or they consisted of split wedges with central thread gripping as shewn in Fig. 27. All these grips were successful in tensile tests, that is, the steel members fixed in the grip broke before the grip started slipping.

Test Production of I-Joists

The first group of prototypes to be produced were symmetrical, I-joists, 45 ft. long, Fig. 28. They were 12 in. deep. 6 in, wide; their flanges 2 in. and the web 14 in, thick; dead weight 35 lb. per lin, ft. The compressor wires were 28 twin-twisted SWG. 12, the weight of the steel 1.62 lb, per lin, ft. They were evenly distributed over the whole cross section, ten in each flange, eight in the web. Initial pre-tension was 90 ton per sq. in., effective 75 ton per sq. in. The effective uniform precompression of the concrete was one ton per sq. in. There is no preliminary bending in these units. The units have equal resistance to positive and negati- ve bending, positive or negative shear, left and right hand torsion, Bigger units of this type are intended to replace R.S.J.'s in structural engineer- ing. The 12 in, deep profile is designed to carry a floor load of 1 cwt per sq.

Fig. 28

strength, at the age of 7 days of 6000 lb. per sq. in, and at the age of 28 days of 9000 lb. per sq. in.

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