The Road Conference at Olympia.
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Abstracts front. Two of the Papers.
SOME POINTS OF THE, ROAD-BRIDGE PROBLEM. By HOWARD HUMPHREYS, 141.1.Mech.E., Assoc.M.Inst.C.E.
Until the year 1899 public attention was not forcibly directed to the case of highway bridges, but in tnat year the incidence of the 1898 Locomotives Act began to be felt, and local authoin ties awoke to the fact that the legislature had placed in thea hands a new weapon for the chastisement of that unhappy Ianmael of the highways—the traction engine. For a time the scheduling of bridges went merrily on, and the Local Government Board had a busy time with locomotive enquiries, but very soon an .astringent appeared in the shape of a younger brother of the traction engine, whose power of reproduction soon convinced local authorities that the bridge problem must be faced squarely; and, generally speaking, the country now appears to be settling down to the idea that weak bridges must be strengthened, ano that upon important roads new bridges when required must he designed to carry all classes of self-propelled traffic.
The public have hitherto regarded traction-engine traffic as exceptional (and sometimes i•extraordinary "), and have generally refused to make substantial concessions on its behalf. Their attitude with regard to the commercial motor is somewhat different, as the potentialities of that vehicle are becoming rapidly and widely recognised; concessions, therefore, which would have been unanimously refused to the pioneer locomotive are being conceded to the supplementary vehicle, for, in addition to its general utility, its use for municipal purposes has been proved beyond a shadow of doubt. Under the Heavy Motorcar Order of 1904 this vehicle may have a weight approximating to that of a traction engine (12 tons loaded, vice 14 tons unloaded), and may draw one trailer, the axle-weights of which are not very different from those of the ordinary traction-engine wagon. So far, then, as bridges up to a span of about 35 feet are concerned (that is of the average length of a heavy motor and trailer), the bending moments produced by a heavy motorcar will be very little less than those produced by a traction engine, and so much of its train as can be got upon the span of the bridge. At 60-foot span, the ratio of bending moment will be, roughly, as .75 is to 1 in favour of the motor wagon, and, of course, at larger spans the case is still more favourable to the wagon, but, as the majority ot highway bridges are of considerably less span than 50 feet, I have said enough for you to infer that the difference of cost of material, etc., in small-span bridges for the lighter vehicle is very little less than would be necessary for carrying the older type of machine.
Railway and Other Privately-Owned Bridges.
While local authorities have more or less come into line upon the point of bridge maintenance and design, the real difficulty at present to he faced is that of the privately-owned bridges, which have been constructed and maintained by lords of the manor, railway, water and canal companies. Many of these structures were erected long before steam haulage became common, and it is difficult—in fact, almost impossible—to ascertain the precise legal liability which attaches to the several companies. I have never said that bridges constructed by private companies before locomotive traffic became an accomplished fact should be strengthened solely at the cost of those companies, but what I have said, and what I adhere to most strongly, is that some power should be given to county councils to deal with privately-owned bridges which, through the neglect of their owners, have fallen into a state of disrepair. I suggested, when giving evidence before the Royal Commission cm Motorcar Traffic, that a county council shall have the power to serve a notice upon a private company calling upon it to repair " ruinous " bridges within a reasonable time, and that, if such notice was disregarded, the county councils might themselves take the work in hand and recover the cost in precisely the same manner as a local authority may do iii the case of people who neglect to look after their sanitary appliances to the detriment or possible detriment of their neighbours. I can see nothing unreasonable or revolutionary in such a proposal, and while, of course, it would be strenuously opposed, it seems to me that the opposers would be rather giving their own case away. If such summary power existed, I am convinced that there wculd be very little trouble apart from a few cast-iron and wrought-iron bridges.
• It is true that the Railway Clauses, Consolidation Act, Sec. 65, provides a way of dealing with railway companies who may have been neglecting their duties in bridge maintenance, but this clause by itself is not entirely satisractory, for railway companies only count one of the several private companies which have to maintain, highway bridges, ano, further, they are not, in my opinion, the worst offenders : canal companies with a far shorter mileage own a greater number of derelict structures : the lengths are, respectively, 4,000 miles, as against 16,005 miles, in England and Wales.
New Private Bridges.
I must say a word with regard to bridges to be constructed. by private companies in the Suture. When the standardisation committees of the Institute at Civil Engineers were chosen a few years ago, I hoped that a high Way brine committee might pernaps, be appointed, but this has not been done, nor does any uniform system as to loading seem to be followed by the Board of Trade with regard to over bridges.
Among other things, it is absolutely fallacious to take the uniform floor loads per square foot usually given in text-books, gir such a course in the case of small bridges will result in both cross girders and main girders being designed too weak, whereas, in large-span bridges, I have often found that the cross girders have been too weak, but that an enormous waste of material existed in the main girders As bridge design to carry self-propelled traffic is occupying more and more attention, I will make one or two suggestions, especially with regard to arches. In the first place, and generally, it is a mistake to use iron or steel girders where it is possible (even at a slight excess of cost) to use good brick, stone, or properly-designed armoured concrete.
With regard to arch bridges. In this class of bridge little attention has been given in the past to the ratio of rise to span. I have had to determine the direction of the thrust line in very many bridges of this class, and, speaking broadly, the conclusion I have come to is that the strongest form is that in which the ratio of rise to span is from one-quarter or one-third to one, as then he arch realises its best theoretical form under ordinary loading—namely, that of an inverted catenary. In the construction of the arch itself the fact is often overlooked that when loaded such arch may be considered as a special condition of a "bent column" ; no one would think of building a vertical column of brickwork to carry a load without most carefully bonding the courses together ; but moving loads acting upon a bent column have far more tendency to separate the rings of brickwork than dead or moving loads have to cause severancebet Ween the courses of an unbonded vertical pillar : it would seem, therefore, as though there was overwhelming reason for bonding arches. I am aware, of course, that a difficulty-will at once present itself to your minds—namely, that in Small-radius bridges the joints at the extrados of the arch will be exceedingly wide if my suggestion is followed, but Where the radius exceeds 14 feet this is not the case; where the radius is less it is possible to build partly in rings, but with bonders through here and there. My attention was originally drawn to this point by Mr. Richardson, the well-known engineer of the Severn Tunnel, who held, I think, that an arch built in vertical bond was about one-third to one-quarter stronger than one built in rings, and I see no theoretical . or practical reason for doubting this conclusion. Arches should invariably be set in Portland cement mortar, not only on account of the greater power of adhesion, but because it prevents permeation of water through the brickwork, and consequent erosion of joints.
Weakness sometimes shows itself in brick and stone bridges through the spandril walls being too light, and either bulging or sliding bodily outwards. There is a natural dislike to putting tie-rods with unsightly washers in new bridges, but it is quite possible to put in such tie-rods and to conceal the washers under the facing course of the spandril wall. I know of no better preventative for bulging or sliding than properly-placed tie-rods, as the bonding of spandrils to the extrados of an arch is difficult and unsatisfactory.
In concluding, may I urge that some definite and immediate policy shall be adopted with regard to neglectful private companies. In support of my contention for fresh legislation herein I quote the action of the County Councils of Buckinghamshire and 'Worcestershire who, in effect, said to railway companies who desired their bridges closed: "You have cut through our highways for the purpose of earning dividends, we are not going to he parties to limiting the public use of our roads for the benefit of private interests."
WHAT COULD BE DONE TO REDUCE SKIDDING ON CITY ROAD SURFACES.
By E. B. B. NEWTON, Borough Engineer
of Paddingion, A Select Committee in 1906 termed skidding or side-slip one of the greatest dangers in the use of all self-propelled vehicles with rubber tires, and the. causes mainly a greasy or slippery road surface, and the mechanical action of the differential gear in propelling driving wheels unequally, under certain circumstances, aided by the needlessly-high camber of many roads, and a bad and improperly-kept road surface. Bad driving is condemned.
Of the police record of 2,448 motor omnibus accidents ia 1906, 367 are due to skidding, and of 418 causing personal injuries, 66 are due to skidding. In Paddington the annual cost of repairing and number of accidents to public lamps by vehicles has risen from £48 and 33 to £138 and 128. Road authorities dislike skidding, for doubtless it damages a road. London street surfaces are seldom so greasy as those of the large manufacturing towns, but, in the latter, the traffic is slower and smaller, and skidding may be less noticeable. It has been stated that " side-slip amounts to this, that one cannot rapidly apply the brakes on greasy wood, oolite, macadam or stone blocks." Scorchers may be comforted to know that according to Galton's experiments in 1878 the co-efficient of adhesion is independent of the velocity.
A witness told the Select Committee in 1906 that he knew no really effective and practicable non-slip device, and that the remedy was to keep the roads clean, and in snore uniform pavements ; but another witness said to the Royal Commission on Motorcars that a non-skid was not necessary, and that he found that if he used flat-tread tires and drove carefully there was little chance of skidding. The author has to deal with roads, however, and so leaves these questions of improvements in the gear, tires, brakes, weight, load, construction and driving of motorcars. One may say, none the less, that the use of studs, spikes, sharp-edged tires, etc., is to be deprecated.
Design and Camber.
Where there are tramlines, a width of at least 17 feet on each side of the track is desirable.
The following remarks refer to level roads. For roads with a longitudinal fall, from a mathematical point of view only the camber should be greater than for a level road; practical considerations require less. Camber of itself does not make a road more or less slippery, but may intensify the effect. Nothing will justify a road being unsound ; such a one will be greasy often, but a sound one occasionally. Therefore, as sufficient camber is essential at present if a road is to be sound, the lesser evil, viz., a proper, but not an excessive, camber, is obligatory.
The by-laws for new streets require a camber of between a inch and I inch for each 1 foot in width. For granite macadam and flint gravel carriage-ways at a point midway between the summit of a channel and the galley, the author favours a camber of inch, For granite set roads about I inch of camber ; for wood roads about a inch, and for asphalt roads about
inch. These, properly set out, though low, are adequate and assist the vehicular traffic. As usually set out, the haunches are too steep and the centre too flat. A cross-section formed of two straight lines is not practicable. The usual circular arc causes the faults previously mentioned.
Mr. Deacon recommended an approximate hyperbola. Dividing the half cross-section into four equal parts, the rise to be given at one-quarter of the width from the channel is .35, halfway .65, threequarter way .81, and at the centre 1. The authors rule is a mean between a straight line and a parabola, i.e., .33, .63, .85 and 1 respectively. The figures for the usual circular arc are, respectively, .44, .75, .94 and 1, which shows its defects. If the cross-section of a level road has a small camber, the longitudinal fall in the gutters causes the summit of the gutter to be nearly level with the centre of the road. To avoid this, asphalt is best laid sometimes with an undulating surface longitudinally.
Construction.
A good foundation is imperative, and for the surface coating the choice of a material depends on local requirements. Wood holds the field perhaps at present. Cork may prove useful. Asphalt must be kept quite clean to be practicable. Tar paving is still uncertain for streets with more than the lightest traffic. Flint gravel has little beyond cheapness and noiselessness to commend it, but it is perhaps the least greasy of all roads. Sets of igneous rocks have numerous points in their favour, but are too noisy as a rule. Some special sets, and perhaps Kleinpflaster, etc., are not quite so noisy. Sets of stratified rocks are little used in cities. Of the igneous rocks, for macadam choice must be made of a material which will wear "gritty" and not into, a very sticky or clay-like compound with water. As to blinding, its quantity is best miniraised, but in the majority of steam-rolled roads it is indispensable, and, even for patching small holes in cities, various circumstances render its use almost essential. Creosoting wood blocks gives a greasy surface, but its many advantages make it desirable even in the case of hardwood blocks.
Maintenance and Scavenging.
An imperfect scheme well executed may he better than a good one badly administered. Only "money will make the mud to go." When a road is dry, there is little danger of sideslip. Absolute cleanliness of road surfaces removes the danger of slip, but it cannot prevail with excreta, dust, mud and sludge from broken stone roads, and soot, etc., in the air. To be clean, the roads must be non-friable and well shingled.
The proper use of water in sufficiently large quantities will keep the roads non-slippery, but the London County Council will not let us keep clean our streets by means of unlimited quantities of water, and flushing direct into the sewers, and the washing of wood roads at night is not always advantageous, especially in the winter time, quite apart from the danger of frost. A gleaming wet road does not inspire confidence in a nervous driver. Though a frozen road just thawing has a bad surface, one may remember how motor vehicles have revelled in the recent snow and slush. When a road has upon it sticky 'mud, the best plan is to liquefy the mud so that it can be swept or squeegeed into the gutters for quick removal. Dust is unbearable, and watering or washing is essential, but, in busy thoroughfares, with impervious surfaces, only the channel's of the road, perhaps with advantages, may be watered, and the rest of the road be kept free from dust by numerous orderly men. Without referring to any proprietary article, the use of creosite oil on wood roads as a dust preventer results in the production of slipperiness, and the immediate requirement of quantities of shingle. In cities, a proper system of scavenging by orderlies is essential, with frequent receptacles into which the mud, etc., may be swept during the day and cleared away at night. It is obvious that in busy thoroughfares such mud collection during the day is out of the question. Judicious shingling will always keep a road non-slippery, but, as fast as the shingle is ground to a paste with moisture, the sticky mud must be removed and broken up by the addition of more shingle. This may create an 'unpleasant state of affairs occasionally,
but oftentimes it is the only practical remedy. Regular shingling of creosoted wood roads is necessary. Shingle preserves its grittiness far longer than igneous rock chinnings, which soon grind down into a pasty mud. All should pass through a 1-inch or g-inch square mesh sieve. One. objection to flint shingle is that it cuts the rubber of tires, but the tire makers cannot grumble at this : owners are saved faorn sideslin, and the few people who prefer igneous rock chippings are not likely to be faced with a deficit in their favourite grit, an nearly everybody will have some comforting reflection if we pia
our faith to water and the material which has proved so useful since pre-historic times. (Other papers will appear in subsequent issues.)