HEAVY VEHICLES FROM THE BRIDGE-MAKER'S STANDPOINT.
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Users of heavy motor vehicles will find some points of interest in the report of a Joint Committee, consisting of representatives of the Concrete Institute, the Institution of Municipal and County Engineers, and the Institution of Municipal Engineers, which was formed some four years ago for the purpose of investigating the design of highway bridges, having regard to the possible loads which may be expected upon them in view of the developments that are now taking place in motor traction.
Small Objections to Present Tire Dimensir ns.
A draft report has just been presented ; it was discussed at a meeting of the Concrete Institution on the 10th inst. For the moment this report is confidential, but we are permitted to refer to it in general terms. The original draft included a discussion of the question of width of wheel tires and diameter of wheel for various loads, but as the committee decided that this matter was outside the scope of the inquiry, it was relegated to an appendix. This appendix consists largely of a recapitulation of the present knowledge on the question and the reproduction of the various 0-2ders affecting locomotives on highways. It is pointed out that by the Heavy Motor Car Order of 1904, a 3 ft. diameter wheel with an axle load of eight tons must have a tire of 10-1 ins, wide, but the proposed report would for this load make the wheel, say, 3 ft 4 ins, diameter and the tires 8 ins. wide.
Loads at Speed.
The transport of loads at the present day, it is pointed out, requires a much higher allowance than eight tons for the maximum load, and when the Local Government Board Regulations are revised the hope is expressed that provision will be made for a maximum axle load of 30 tons. As to the effect of speed in increasing the virtual loads-5 reference is made to a statement at a conference Of local authorities of Surrey and Middlesex, held at Richmond, in September, 1912, that "Engineers' calculations showed that the weight on each back wheel when the (motor) 0mni
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bus was loaded and standing was tons. At. a speed of 10 miles the force equalled 4 torts, and at 15 miles it equalled 7 tons." A consideration of these values, however, on extending them to 25 miles an hour, demonstrates, in the view of the committee, the improbability of their accuracy. At 25 miles an hour the 2.'.2 tons dead load would have become about 20 tons.
Railway Inertia.
Mr. C. S. Meik stated that there was no doubt a disinclination on the part of railway companies to go into the question of bridges and loads, due to the fact that the question of liability is still outstanding. Before it was finally settled it would no doubt have to go to the House of Lords to determine the liability of railway companies as regards these over-bridges. At the same time it would make the value of the report very much greater if the Joint Committee could induce railway companies to go into the conference in some shape or form, so that their approval to the loads now being suggested could be secured.Mr. T. C. Dawson, however, said there was one point in this suggestion which would make railway companies rather chary of having anything to do with the investigation. If they admitted the correctness of the loads put forward by the Joint Committee, it would_ involve the rebuilding of the whole of their old bridges, and he concluded "I do not think you will get the mouse to go into that trap."
A Final Report to Come.
The whole of the details in the report are to undergo further revision and consideration before being issued in a final form. Most of the standards arrived at as to the strength of bridges were settled upon before the war. The enormous development in motor traffic since then, however, and especially military motor traffic, had brought home to the committee the need for consulting the War Office before finally adopting them. This may very well considerably modify, at any rate, some parts of the report. It will convey the final views.