Fig. 38
und. The working table is of rein ced concrete, 1'-6" high and 3' 6" le. The bed is 80 ft. long aud is d for the production of relatively g units, such as transmission poies, > of which can be produced in one
7.
Fig. 39
of SWG. 11. The wires are initially pre-tensioned to 90 ton per sq. in. Their effective pre-tension is 75 ton per sq. in.
To control the stresses set up the in length were following alterations measured in each specimen.
(a) The extension of the steel during
stretching:
(b)
(e)
The contraction of the ram with the piston running back during release;
the contraction of the concrete pole
during relcase;
(d) the contraction of the concerete pole
after release, while it was stored. These readings were compared with the corresponding values calculated in advance. This allowed to draw im- mediate conclusions on the quality of the concrete actually obtained.
The movable stretching bed of Fig. is only 12 ft. long and is used for experimental production of sample tions of new units. Before full totypes are produced, the various es of moulds are tried out on short nples. If necessary, the cross sec- n of the proposed units are changed incorporate the experiences gained a few trial castings. This lilliput etching bed consists of two inde-
Ten poles, 5 of the Vierendeel type, ident latticed stanchions and two and 5 of the full-web type were sub- ss heads, and is easily transportable jected to exhausting loading tests; 3 one light van. It has been sent of each type were loaded up to com- to manufacturers on more than plete failure, see Fig. 42 The most e occasion so that they could make remarkable result of these tests is the lew small trial castings of proposed small divergence of the characteristic units in their yard before going load values in all ten poles. The first any further expense.
cracks appeared under loads between st Production of Transmission Poles 1200 and 1400 lb. Failure occurred One of the main task put to the under loads between 2200 and 2450 lb. perimental Station immediately Deflation at the top of the pole, just er its formation was the production before failure, reached up to 40 in; 97 prestressed transmission poles. The percent of which were elastic. ecification chosen for the prototypes The specification required that the s-overall length 36 ft: planting bending resistance of the pole in the pth 6 ft.; ultimate load applied direction of the transmission line was It. from top, 1750 lb. in a direction 25 percent of the maximum resistance
nsverse to the transmission line. I proposed two designs: one of a erendeel frame, and the other of a 1-web 1-section. Fig. 40 shews the .rendeel pole on the stretching bed
consists of two legs 2" thick and The second type of pole which had wide cach, tapering towards the an I-shaped cross section was very ). The connecting diaphragms are similar to the I-joists discussed pre- aced from 4'3" to 3'0' centres.
viously. It had a slight taper with Fig. 41 shews two Vierendeel poles the effect that the precompression of eased and one of them completed the concrete increased towards the tha 2" web in the planting depth. top. Under loading tests it behaved ither this web nor the cross dia- much stiffer than the Vierendeel mast, ragms are prestressed. They are but the ultimate loads were of exactly inforced by high tensile steel, The the same range.
perpendicular to it. Fig. 43 shews the application of loads to test this parti- cular requirement of the specification. The results were as satisfactory as the main tests.
o legs are uniformly precompressed Fig. 44 shews the two poles erected. slightly more than 1 ton per sq. in., The Vierendeel was about 10 percent ch by 10 twin-twisted wire strands lighter: but due to its more compli
51
Figs. 40, 41 above and Fig. 44 below
cated shape and costlier moulds, its cost is estimated to be 20 percent higher than that of the full-web type. The aesthetics however appear to be in favour of the Vierendeel.
In any case, this Vierendeel has been selected for the first transmission line with
which prestressed poles
has been erected in England early last year.
The full scale loading tests with the 10 prestressed poles has yielded a great amount of valuable information in highly prestressed concrete where
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