Issue 9

Athough the Road and Rail Traffic Act is not a recognised scheme for the relief of unemployment it will certainly have a far reaching effect in this direction. In accordance with the Act, thousands of bridges are to be measured and their carrying capacity calculated. The results obtained will probably entail the reconstruction of a considerable proportion. Included in these bridges are large numbers of masonry arches, many of which were built about a century ago during the development of our roads and canal systems. Owing to the urgency of the work and the large quantity, simple calculations are essential even at the expense of accuracy, providing the error is small. No difficulty will be experienced in finding the axle and vehicle loads for ordinary girder bridges, but with arches it is difficult to get a simple and quick method without making more assumptions besides those used in the mathematical analysis. These new assumptions will unfortunately have to be based on engineering sense which is always open to argument. The majority of these arches will conform to the segmental or parabolic type and be built of common brick in various thicknesses in lime mortar with an average span of about 15 ft. for those spanning canals. One thing to be remembered is that these arches were built with the idea that the line of thrust would come within the middle third, i.e. there is no tension and in the present calculations this has been considered. It will be soon found that no rule of thumb method or emperical formula can be used to give reasonable results, and that either the elastic theory or the theory of least work will have to be used. The first method is considered the simplest, and can be found in most text books, also an example of a segmental arch is worked out by Mr. A.A. Fordham in a paper on Masonry and Concrete Arches read before the Institution of Structural Engineers on the 25th April, 1929. Most work arises in finding the horizontal thrust and the reactions and it is necessary to make our first assumption, i.e., in finding the horizontal thrust (H) and the reactions (RL and RR) the arch may be considered a parabola using the elastic theory. (See diagram below.) P.H. Johnson

In presenting the paper, Mr. Snow added that for the 21 flats in the block at Berners Street, Stepney, there were 300 applications; and for the 320 flats at Amhurst Road, Hackney, there were 1,200 applications.

In the design of reinforced concrete structures certain assumptions are made which are known to be unfulfilled in practice. It is usual when designing for specified working loads to use formulae based on a linear relation between stress and strain or a constant modulus of elasticity for concrete. This assumption, though true for steel within the limit of proportionality, is known to be incorrect for concrete and other building materials and has resulted in much controversy. Furthermore, while the value of the modulus remains sensibly constant for all steels, that for concrete varies considerably with the richness of the mix, quality of the aggregates, water-cement ratio, age at testing, methods of curing and of loading, range of stress and with the interval of time during which the concrete is under stress. It is therefore desirable to examine the effect of a change in the assumed value of the modular ratio on the calculated stresses and deflections of reinforced concrete members. R.H. Evans

THE work of demolishing Waterloo Bridge is now reaching the final stages and the nine arches will soon have completely disappeared. Owing to the tremendous amount of work involved and the necessary care which had to be taken, the task has been somewhat difficult, and it has taken some months to arrive at the present stage. It was necessary to erect steel overhead trusses on, and to suspend steel centering beneath, all but the centre arch, which is supported by piles.