The Author has said that the live load variation for a continuous bridge with equal spans was the same as that for simply supported spans and therefore there was no obvious advantage to be gained by the continuity. The writer has always thought that the solution lay in the use of alternate long and short spans. A full analysis of this problem has been made by Guyon1. There are also other methods of achieving continuity economically in prestressed concrete bridges by altering the geometry of the bridge at the supports.
The Author has given a most interesting resume of recent innovations in the design and construction of water retaining structures, and in connection with two aspects of his paper, certain points involving technique are worthy of mention. In the case of Henley Dam, holes up to 4 in. diameter and 150 ft deep had to be drilled to tolerances of I in. in 10 ft for verticality and in. in 10 ft for straightness which are extremely close tolerances in normal percussion drilling practice.
The Author's concise resume has been of great interest to the writer. The first part of the paper covers a field well known to all practising structural engineers and probably there will be a number of valuable contributions submitted referring to this part. With reference to silos, however, only a comparatively small number of South African engineers have had any experience either in their design or construction. The writer, therefore, confines his brief contribution to the subject of silos.
Although the Author has indicated that nearly all steel structures erected in South Africa at present are designed to B.S.S. 449-1948, this may only be so in areas where Local Authority by-laws are not in force. The full acceptance by local authorities of the B.S.S. or Bureau of Standards regulations is regrettably slow, and this is a great hindrance to progressive engineers employing structural steel. This fact has resulted in higher costs to all concerned and reflects most forcibly on the national economy. Model by-laws formulated by the Bureau of Standards serve as a guide to all local authorities throughout the Union and a national need exists for a uniform adoption of these standards.
The Author has shown that South Africa, in spite of its comparatively short industrial history, is not lagging behind insofar as development in the field of engineering materials is concerned. The opportunities presently offered to engineers by the vast undeveloped hinterland of this continent provide a challenge which we cannot afford to disregard. History is in the making and never was a country so well placed as South Africa to influence the destiny of a continent.
The type of specification for the concrete used on the construction of the Auckland Harbour bridge and approaches illustrates the trends of mix design becoming usual in Great Britain. Various classes of concrete are used in different parts of the work and these are specified by their minimum 28-day strength on work's tests made on 12 in. by 6 in. diameter cylinders. The use of 12 in. by 6 in. diameter cylinders is standard in New Zealand as opposed to the 6 in. cubes used in Great Britain. These strengths vary from 4,500 lb/sq.in. for reinforced concrete to 2,000 lb/sq.in. for the haunchil1g of roadway kerbs.
It is noted that the magnesia content of the slag is much higher than permitted in a normal portland cement. Would the Author express an opinion as to the possible influence this may have on early deterioration of concrete containing slag? The Author states that better workability is obtained in concrete containing slag. Would this be attributed to a better grading resulting from a blending of slag with portland cement?
The Author states that cement is the most commonly used stabilizing agent. This is true in some overseas countries, but it is not essential that South Africa should follow overseas practice. Our circumstances are different from those of other countries. Lime, suitable for soil stabilization, is available in South Africa at little more than one-third of the cost of cement. In some States of the U.S.A. cement is actually cheaper than lime, per mile of road stabilized.
The building industry is being confronted to-day with an. ever-growing flood of new materials and techniques of construction, some representing a real advance on those at present in use. The Author focuses attention on the methods being employed to evaluate such materials, which are particularly valuable inasmuch as they provide a quick check list of those techniques available to laboratories for this purpose.
The writer is in full agreement with the general corollary that a heavier rail section should be used on curves than on straights and considers that this approach deserves investigation, since it would seem to offer some economic advantages. To-day with improved methods of measurement of rail stresses there is a tendency to become too theoretical. The proposed 111 lb rail section is, in the writer's opinion, a good compromise between theoretical knowledge, experience and practical considerations.
The Author's review of tunnelling work on the South African Railways since 1877 is of great interest. The introduction of mechanization in recent years had made it possible for tunnels to be built at a faster rate, and it deserves mention. Early tunnels were built almost entirely by hand, with a little mechanization in the form of wheelbarrows and, later, cocopans. Drilling was done by hand jumper and muck was loaded by pick and shovel. Mechanical aids were introduced gradually up to 1950, after which tunnelling became almost completely mechanized, due mainly to the Author's efforts.
About 19 miles of completed deviation on the Union-Volksrust line which are laid with long-welded rails (5/519 ft and 6/519 ft) have been opened to traffic during the last 12 months. About 13 miles are laid on wood sleepers and six miles on steel sleepers. No concrete sleepered sections have as yet been opened to traffic. It is an interesting fact that although eight pairs of long-welded rails on steel sleepers are without standard splice expansion joints, still having the 32 ft closures fitted with ordinary fishplated joints, no trouble has been experienced in maintaining this section of track.
The basic principles of planning permanent works as descriptionbed on page 278 stress the need for considering future expansion and avoiding features which may preclude subsequent developments. The Author rightly states that although deficiencies may be apparent their exact nature and extent must be established factually. Yet further on the statement is made '... an impartial study and not the acceptance of local opinion may be necessary...' The writer considers that in all cases an impartial study is essential and local opinion should never be relied upon although it should always be consulted. It is only by close observation on the spot coupled with the actual measurement of delays that efficient improvement can be planned.
In spite of modern techniques developed in the design and construction of reinforced concrete structures, corrosion of the reinforcement at the coast is still a source of much trouble. Great care is taken in choosing a properly designed concrete, the emphasis being on correct proportioning, the correct water-cement ratio, and adequate and thorough vibration. Yet difficulties in this regard arise and the saline atmosphere gets to the reinforcement and causes failure of the concrete.
Dit is werklik bemoedigend om weer 'n referaat te sien, wat die onderwerp van metodestudie en van werksmeting behandel. Daar word in die laaste jaar baie gepraat oor hierdie onderwerp maar van voorbeelde in die praktyk hoor 'n mens maar min. Waar moet die rede daarvoor gesoek word. Die vernaamlike rede is weI seker dat, hoewel baie ingenieurs in aanraking met die teorie gekom het, daar maar min is wat die onderwerp grondig bestudeer het, laat staan praktiese ondervinding in werkstudie gehad het. Dit is veral aanmoedigend dat die Suid-Afrikaanse Spoorwe, as die vernaamste werkgewer op ingenieursgebied, begin om gebruik te maak van hierdie metodes op 'n wetenskaplike manier.
The writer considers that there is need to pause and reflect where soil mechanics, an essentially civil engineering discipline, is heading in South Africa. Leaving out compacted soil problems, in the first instance, structural soil mechanics in South Africa may be divided into five main groups, namely: bearing capacity, slope stability, settlement-including collapse of structure, heave, and miscellaneous problems of earth pressures, permeability, etc. The common difficulty with all unsolved problems in these groupings is one of partial saturation. Two problems have been uniquely isolated, namely heave and collapse, and it was in these fields that world attention had been attracted.
The problem of heaving soils appears to have much in common with that of shrinking aggregates ; in both cases the materials contain small particles and exhibit considerable dimensional change with change in moisture content. The writer suggests that the two problems may be different manifestations of the same basic phenomenon and would ask the Author if any research work has been conducted in an attempt to correlate the two problems.
The writer would like to express the view that the collapsing soil phenomenon is perhaps not such an uncommon one as some engineers may think, and he would like to give brief details of three instances in which such soil deposits have recently been encountered in the vicinity of the Witwatersrand. The first was at a site close to the railway college at Kaalfontein, where it was proposed to construct an overhead road bridge over future double track railway lines. The soil profile comprised a loose, red, silty sand with a dry density of 88 lb/cu.ft. to a depth of 24 ft. 6 in. below ground level, followed by a compact broken rock layer of 18 in. thickness constituting the pebble marker, and partly decomposed rock immediately below.
Previous attempts at obtaining a rational solution for the determination of the foundation thickness of a road have been based on the elastic theory with the use of a secant modulus. This modulus incorporates both elastic and certain plastic deformation. These attempts have resulted in designs which are over-conservative, probably because the modulus has too low a value.
The Author has presented an informative paper outlining the numerous problems of soil mechanics in road engineering in South Africa. He states that all the problems stress the need for more detailed study of the soils, that it is apparent that climatic regions are a useful indicator to the formation of a soil, and that research may produce simple chemical and petrographical tests to supplement the standard physical tests now used.