SECT. I.-RULES OF THE GOVERNMENT OF INDIA FOR THE DESIGN AND INSPECTION OF GIRDER-BRIDGES.

The first five articles lay down the scope of the rules generally, and define the duties of the Government Inspector and the procedure to be adopted by him and the Railway Administration where a lower standard is considered sufficient to meet the case.

Maximum Permissible Stress.

6. For any member of a railway bridge of wrought iron or steel the total working load is to be taken as the 'moving load,' increased by an appropriate allowance for impact,' and added to the actual 'fixed load.'

7. For the purposes of this rule fixed load' is to be taken to mean the weight of the structure itself, with the roadway, flooring, ballast, permanent-way, etc., complete. (See also Wind Pressure.')

'Moving load' is to be taken as a train-load on each line of rails as specified in Rule 15; and if there be a road or footway that can be occupied at the same time as the railway track, an additional moving load as specified in Rules 17 and 18.

8. The increment for impact to be allowed in the case of railway load is to be calculated by the formula

where S is the stress due to the train load specified, considered as at rest in the position which gives the maximum stress in the member under consideration, and L is the length in feet of that portion of the span which the train has had to traverse to reach that position from the point where it first began to produce stress in that member and I is the I amount to be added to S to allow for impact.' Values of the ratio S for various values of L are appended as Table I, Appendix I.

Note (1) For bending moments caused by an assumed equivalent moving load, L may be taken as equal to the span of the girder. For shears it will usually be the distance of the point under consideration from the further support. For cross-girder concentrations it will usually be twice the interval between the cross girders.

9. The increment for impact to be allowed in the case of loads moving on the roadway or footways of a combined road and railway bridge shall be half the railway increment specified in Rule 8.

10. The intensity per square inch of different kinds of stress due to the 'total working load' thus calculated is not to exceed the following:

11. But for occasional loads, such as those due to wind pressure, or exceptional loadings of a separate roadway simultaneously with the absolute maximum train-load, stresses 25 per cent. in excess of the above may be permitted.

[N.B. This implies that the effect of wind may be neglected when the stresses caused by it are not more than 25 per cent. of those caused by the total working load.']

12. For members in tension the working stress per square inch given above is to be taken on the net available area at the weakest part of the member of the structure to which it is applied, after deducting all holes for rivets, pins, bolts, etc.

13. For members in direct compression well-filled rivet holes need not be deducted, but the working stress given above is subject to such reduction as may be necessary according to a suitable column formula. In calculations submitted to Government, the formula given in Table II, Appendix I, should preferably be used. No main compression member should have a greater free length than 100 times its least radius of gyration, but in subsidiary members, such as lateral struts, the ratio may be 120. For plate girders it will suffice if the compression flange has the same sectional area as the tension flange.

14. Members and connections subject to alternating stresses are to be proportioned for tension and compression separately, and half the smaller area is to be added to the larger area to give the total section.

Standard of Permissible Train-load.

15. The maximum train-load referred to in Rules 7 and 8 is the heaviest train of two engines, followed by as many fully loaded wagons as can get on the bridge, which will not cause in the girders greater bending moments, shears and cross-girder concentrations than those laid down in Tables III, IV and V, Appendix I.

Road or Footway Loads.

16. For those members of a combined road and railway bridge which carry the roadway only, the moving load on the road or footway shall be taken as 90 lbs. per square foot (4 tons per 100 square feet) of effective surface, or the heaviest elephant, cannon, traction-engine, vehicle or train of vehicles which is likely to be allowed on the same, whichever will give the greatest stress in the members.

17. But for those members which carry both loads, only 20 lbs. per square foot of effective surface need be taken, except in special cases where this moderate load would seem to be inadequate, such as a bridge at a large city, the roadway of which is likely to be constantly crowded. Such cases should be treated on their merits. (See also Rule 9.)

Wind Pressure.

19. Every structure must be capable of bearing a wind pressure of 2:5 tons per 100 square feet (56 lbs. per square foot) when unloaded, or 1.5 ton per 100 square feet (33 6 lbs. per square foot) when loaded with

the maximum moving load, without exceeding the stresses laid down for occasional loads in paragraph 11 above.

22. No allowance for impact' is required for wind load, and the excess vertical load on the leeward rail due to a horizontal wind pressure tending to cant the train need not be taken into account. (See also Note to Rule 11.)

The remaining rules deal with the applicability of the foregoing rules to existing structures as against new or proposed girders.

The foregoing are the bridge rules which have been in force since 1903. Of the Tables referred to (see Appendix I), Tables II, III, IV and V incorporate the revised standards of 1908. Until 1893 the rules in force were those of the Board of Trade.

1893 Rules. In 1892 the Government of India framed rules designed to supply the deficiencies of the Board-of-Trade rules by ensuring uniformity of design, and circulated the draft rules amongst a large number of representative railway engineers in India for criticism. The only items of these rules which it is necessary to touch upon now are:

(a) The coefficient of impact.

(b) The standard equivalent distributed uniform load to be allowed for moving load.

(c) The working-stress.

For item (a) the rules were:

·

"For any member of a railway bridge of wrought iron or steel the total working load is to be taken as the greatest moving load' multiplied by a coefficient and added to the actual fixed load.' The coefficient to be used for this purpose is 2.0 in all cases, except for the upper and lower booms of triangulated girders, for which a coefficient of 1.5 may be used."

Item (b) was deduced from the maximum bending-moments obtained at any point of a span from two coupled tank engines of imaginary type placed in such position in the span, and in such position with regard to the train, as would produce the greatest effect.

Item (c) allowed a working-stress (for total working load) of 7 tons per square inch for wrought iron and 9 tons per square inch for steel on the net area, the stress in compression being, of course, subject to modification by column formula, the particular formula being unspecified.

Fifty-five of the engineers to whom the draft rules were submitted either approved of them as they stood, or proposed amendments which dealt principally with the coefficient of impact

for the moving load. Some advocated wider limits of the coefficient, but the dissent was only in favour of a refinement of its application, and all agreed that the use of a coefficient of 2 to 1.5 to multiply the moving load and add to the fixed load to arrive at an equivalent total working load would give a closer approximation to the actual effects than that attained by the Board-ofTrade rule, and that the use of the same amount of material would result in a stronger bridge, owing to the disposal of the material to better advantage.

reasons.

The rules as drafted therefore came into force in 1893. 1903 Rules.-In 1903 the rules were revised again, for several One of these was that the standard equivalent uniform load for bending-moments resulted-for spans up to 100 feet-in the design of girders which were very largely in excess of the strength required to carry the heaviest engines in use or recently proposed for use. Accordingly a number of these engines were selected, and their equivalent loads for bending at a point one-sixth of the span, and also for shear loads, were worked out, with the result that a new and intermediate standard was adopted which gave a moderate margin for bending and shears for all spans.

At the same time the working-stresses of 7 tons and 9 tons per square inch for wrought iron and steel respectively were reduced, on the advice of the consulting engineers, to 6 tons and 8 tons, and the Pencoyd formula for impact replaced the original coefficients applied to the moving load.

The 1903 draft rules were submitted to the railway managers, bridge engineers, consulting engineers and others, of whom about sixty furnished comments. Of these the greater number accepted the standard loads and the Pencoyd formula for impact. Some dissented from the latter as being more suited to American practice, in which the girders are comparatively light, and advocated a less 200 150 severe formula for Indian use, such as 200+ L' 150+ L

while a considerable number were of opinion that a formula depending rather upon the ratio of the fixed to the moving load than upon span should be adopted. Eventually, after reference to a committee of four consulting engineers, the Pencoyd formula was adopted.

Between 1903 and 1908 it became evident that the standard of equivalent loads no longer gave a sufficient margin to provide for the increasing weight of the rolling stock, and accordingly the 1908 rules specified an increase of 25 per cent. en bloc in the loads for which new or strengthened structures were to be designed.

SECT. II.-STANDARD MOVING LOADS.

In a rule like that of the Board of Trade, which prescribes no definite allowance for the dynamic effect of a moving load upon the structure, and is intended to provide with safety for the strains in that structure from all causes whatsoever, it is evident that there is infinite latitude for the individual designer in considering what proportion of the working-stress per square inch he shall assign to the effect of the load considered as static, and what proportion he shall assign to its dynamic effect.

The Indian rules, on the other hand, by means of the Table of standard equivalent uniformly-distributed loads, specify exactly the proportion of the load which is to be considered as static.

This question of the standard load, as well as the rule for the dynamic increment, has latterly been the subject of discussion, and the Indian railways were invited by the Railway Board to give their views, with the result-the Author understands-that the majority of the railways have concluded that the loads prescribed are not too far in advance of present and probable future requirements. On the question of impact there is a greater diversity of opinion, not only between the various railway-companies, but among their individual engineers. In the meantime articles in the technical press have dealt with the matter in a manner which somewhat blends the two issues of loading and impact. The Author feels that the latter subject cannot be satisfactorily reviewed from an engineering point of view unless it is first dissociated from the former, and with this object he has prepared diagrams (Figs. 1, 2, 3, Plate 3) comparing with the rules the equivalent distributed load for a few typical engines and trains that are in common use, or are likely to be used in the near future.

Typical Moving Loads.-The 2-8-0 goods and the De Glehn engines are as used on the Bengal-Nagpur Railway. The 2-6-2 tank engine is one recently passed by the Standards Committee for use on the Great Indian Peninsula Railway. The Mallet and 2-8-0 type goods-engines are possible developments in the near future. As used on the Bengal-Nagpur Railway the latter has 15.15 tons as a maximum axle-load, but the standard for use on several railways is 15.9 tons, and with 90-lb. rails in the track it would be possible to increase the axle-load to 18 tons. It will be seen that, speaking generally, this 18-ton-axle engine leaves but little margin below the Government-of-India 1908 standard loads, either for bending-moments or for shears. The excess of the Government