_Excess of Deflection and straining Action of a moving Load over that due to a resting Load._
Dead load in per cent | | | | | | | | of total load .... | 10 | 20 | 30 | 40 | 50 | 70 | 90 | Live load in per cent | | | | | | | | of total load .... | 90 | 80 | 70 | 60 | 50 | 30 | 10 | Ratio of live to dead | | | | | | | | load ............. | 9 | 4 |2.3 |1.5 |1.0 |0.43|0.10| Excess of deflection | | | | | | | | and stress due to | | | | | | | | moving load | | | | | | | | per cent ......... | 23 | 13 | 8 |5.5 |4.0 |1.6 |0.3 |
These results are for the centre deflections of main girders, but Stone infers that the augmentation of stress for any member, due to causes included in impact allowance, will be the same percentage for the same ratios of live to dead load stresses. Valuable measurements of the deformations of girders and tension members due to moving trains have been made by S.W. Robinson (_Trans. Am. Soc. C.E._ xvi.) and by F.E. Turneaure (_Trans. Am. Soc. C.E._ xli.). The latter used a recording deflectometer and two recording extensometers. The observations are difficult, and the inertia of the instrument is liable to cause error, but much care was taken. The most striking conclusions from the results are that the locomotive balance weights have a large effect in causing vibration, and next, that in certain cases the vibrations are cumulative, reaching a value greater than that due to any single impact action. Generally: (1) At speeds less than 25 m. an hour there is not much vibration. (2) The increase of deflection due to impact at 40 or 50 m. an hour is likely to reach 40 to 50% for girder spans of less than 50 ft. (3) This percentage decreases rapidly for longer spans, becoming about 25% for 75-ft. spans. (4) The increase per cent of boom stresses due to impact is about the same as that of deflection; that in web bracing bars is rather greater. (5) Speed of train produces no effect on the mean deflection, but only on the magnitude of the vibrations.
A purely empirical allowance for impact stresses has been proposed, amounting to 20% of the live load stresses for floor stringers; 15% for floor cross girders; and for main girders, 10% for 40-ft. spans, and 5% for 100-ft. spans. These percentages are added to the live load stresses.