What is dead weight in a bridge

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Load assumptions for bridges

[128]Load assumptions for bridges. A bridge construction is stressed by the following forces (external or acting forces): 1. By its own weight, which represents a permanent, static and perpendicular load. 2. by the traffic load. This is changeable and temporary; it is also connected with dynamic effects as a result of its movement. The traffic loads also stress the bridge structure mainly through their vertical gravitational forces, but horizontal forces can also be caused by the moving loads. (Side pressures of the vehicles, centrifugal forces in curves, braking forces.) 3. By the wind pressure, which is taken into account as a horizontal force acting perpendicular to the longitudinal axis of the bridge. 4. In certain (statically indeterminate) structural systems, changes in temperature generally also produce forces and stresses.

1. The dead weight must be assumed in advance for the purposes of the static calculation of a bridge structure. The weights provide clues for this. well constructed bridges and the weight formulas developed from them. However, these only provide average figures that apply to certain bridge systems and normal designs. The following information can be used for this.

Wooden bridges. Incidental total weight (including road surface) in kg the mTrack:

Main linesstandard gauge
Branch lines
Toothed or
pegged beams 900 + 75 l 780 + 65 l
Truss (Howe-)
bridges 1100 + 39l 950 + 32l
1 – 0∙006 l 1 – 0∙006 l

Iron bridges. Total weight, including the roadway (rails, sleepers, planking or ballast bedding) in kg for the mTrack (see table on p. 129).

For double-track railway truss bridges with the carriageway below, the total weight is in kg for the m Bridge length about

for spans of20–40 m2860 + 65 l
for spans of40–80 m2860 + 74 l

of this, about 1360 are accounted for by rails, sleepers and flooring kg for the m.

Here the use of the usual construction material (fluoro iron from 3600 to 4300 kg/cm2 Tensile strength).

Iron road bridges. The dead weight in kg for the m2 can be set (according to Engesser):

for road bridges with ballast: 125 + 2 ∙ 8 l + 0∙025 l2; there are around 400 kg Gravel and 65 kg Zoresis iron;

for city road bridges with ballast: 170 + 3 ∙ 2 l + 0∙028 l2; about 480 more kg Gravel and 80 kg Zoresis iron;

for city road bridges with paving: 180 + 3 ∙ 7 l + 0∙029 l2; about 700 more kg Paving and 80 kg Zoresis iron.

For main railways according to the current loading regulations. (After Schaper, Eiserne Brücken. Berlin 1911.)

2. The traffic load on the railway bridges consists of the means of transport, including the heaviest locomotives and trains that could drive on the bridge. For the static [128] calculation, an ideal load train is assumed, the wheelbases and axle loads of which are to be dimensioned in such a way that it stresses the individual links of a bridge at least as much as any of the actually running truck trains. In earlier years, however, in identifying these ideal freight trains, too little consideration was given to the increase in the weight of the trains running on the main lines, a circumstance that resulted in the later reinforcement or replacement of these older bridges. In 1900 the general assembly of the VDEV. A provision has been included in the technical agreements, according to which all new bridges to be built are to be set up with a loading train consisting of two locomotives including tenders and an unlimited number of goods wagons attached to one side. The five-axle locomotives should have the first axle with at least 14 t, the other four axes at 1 ∙ 4 mWheelbase with at least 16 t have burdened. The three-axle tenders have 13 t, the two-axle freight wagons with 2 ∙ 5 mWheelbase have 9 t Axle load.

As a result, the association administrations were prompted to either take over the prescribed haulage or to choose a new haulage with similar axle loads on their own. In 1901, the Prussian State Railways introduced the freight train shown in Fig. 11, which had only two different axle pressures of 17 t and 13 t and in which all wheelbases are equal to 1 ∙ 5 m or equal to a multiple of this value. Due to the German railway construction and operating regulations of November 4, 1904, this freight train was transferred to almost all German railways, while the other administrations of the association prescribed slightly different freight trains, which, however, do not differ too much in their effect from the Prussian freight train. For small spans in which there is not enough space for all five locomotive axles, if four axle loads are taken into account, these are to be 18 t, with three axle loads on 19 t and for one or two axes to 20 t to increase.

In view of the steady increase in traffic, which is pushing for a greater load on trains and for increasing travel speeds [129], the Prussian state railways have taken action to rule out any need to strengthen and renovate structures for the foreseeable future to prescribe a heavier freight train for those iron bridges to be rebuilt for which the new heavy rails will also be used. The general arrangement of this new, heavier billing truck corresponds to that shown in Fig. 11, only the axle weights of the locomotive drive wheels are 17 t on 20 t, all other axle weights from 13 t on 15 t increased (decree of March 25, 1911).

The Bavarian State Railways reckon with a freight train consisting of two locomotives and an unlimited number of freight wagons Fig. 12. The axle loads are all 16 t, the wheelbase 1 ∙ 4 m or a multiple of this value. The locomotives are to be switched between the freight cars if this has a less favorable effect. For constructions, for which more than one machine can not be used, an axle load of 18 t to increase.

In Austria, the regulation of the Ministry of Railways of August 29, 1904 is decisive with regard to B. for railway bridges.

This prescribes the following loads:

a) For full-track main lines, two five-axle locomotives with 16 t Axle load, 1 ∙ 4 mWheelbase, including three-axle tenders with 13 t Axle load, 1 ∙ 5 mWheelbase and freight wagons with axle loads of 11 t in 3 m Distance Fig. 13. For the calculation of smaller girders on which there are fewer than 5 locomotive axles, the pressure of a locomotive axle is 20 t to increase.

b) For full-track branch lines either two three-axle locomotives with 14 t Axial pressure, 1 ∙ 2 mWheelbase, including three-axle tenders with 10 t Axle pressure, 1 ∙ 5 mWheelbase Fig. 14 or two tank locomotives according to Fig. 15 with one-sided wagons of the main railways.

c) For narrow-gauge railways (760 mm Gauge) two tank locomotives as shown in Fig. 16 a and wagons lined up on one side, and if there is no roller-bearing traffic on the railway line, the vehicle as shown in Fig. 16 b or, if roller-bearing traffic takes place, the vehicle as shown in Fig. 16 c

According to the ordinance of the Federal Council of August 19, 1892, the following burden applies to Swiss railways:

For the main lines a train consisting of three locomotives Fig. 17 a and an unlimited number of cars Fig. 17 b.

For beams with one span l < 15 m are the loads by 2 (15 - l)% to enlarge. For secondary lines, the loads can be reduced by 25% be accepted in a reduced manner

In France, the Ministerial Circulaire of August 29, 1891 stipulates a loading train consisting of two four-axle locomotives of 14 t Axle pressure, two-axle tenders of 12 t Axle pressure and carriage with 8 t Axle loads at equal intervals of 3 m Fig. 18. For small spans, an axle load of 20 t to increase.

In the report on the reinforcement of the tracks and bridges, which was submitted to the International Railway Congress Association in Bern in 1910, the government and building councilor Labes also discussed the question of how economically it seems to be for the B. for new ones to be built To secure railway bridges against [130] the future growth of rolling loads. The heaviest locomotive currently used by the Prussian State Railways (superheated steam locomotive) has 17 ∙ 82 m Length a total weight of 104 ∙ 55 t or 5 ∙ 86 t for the m. In contrast, the locomotive of the Prussian load train at 18 m Length 124 t Weight or 6 ∙ 9 t for the m, the locomotive of the Austrian freight train at 16 ∙ 6 m 119 t or 7 ∙ 17 t for the m. The bridges calculated in accordance with the German or Austrian regulations currently in force can therefore withstand a further increase in loads until the permissible loads are reached. Moreover, the latter are chosen so moderately that in most administrations one can exceed them by up to about 20% before the reinforcement of a bridge structure is found necessary. The German railway administrations are therefore of the opinion that the current load scheme should be sufficient for a long time. A permanent safeguard against the increase of the rolling loads will not be possible at all, at least not without very considerable economic disadvantages; Because according to the view of the American railway technicians, the weight limit of the running equipment for the standard gauge would only be reached with one type of locomotive, the four drive axles with 32 each t, a total weight of 220 t or around 12 t for the m owns, as well as the heaviest wagons of 10 m Length and 100 t Weight on four axles, these are loads that are more than twice as high as the weights of the heaviest equipment currently in use.

The traffic load on road bridges is a crowd of people, as well as loads from wagons and road rollers. Depending on the frequency of the bridge, the crowd will have an evenly distributed load of 300 to 460 kg for the m2 set. For the footpaths of urban bridges, the load becomes even higher, up to 560 kg/m2 accepted. For the road sections, the load on the wagon or the load from road rollers is less favorable than the load from the crowd. The wagon load also affects the main girders with spans of up to 30–40 m usually less favorable. Individual countries, districts or cities have stipulated the B. through special regulations.

In Austria, the ordinance of the Ministry of the Interior of 1905 and the identical ordinance of the Railway Ministry of 1904 come into consideration. This prescribes the following B.

For 1st class bridges a) Four-wheeled trucks of 12 each t Total weight at 7 ∙ 8 m Length (without drawbar), 2 ∙ 5 m Width, 3 ∙ 8 mWheelbase and 1 ∙ 6 m Track with a covering of 4 horses with a total weight of 3 t to 7 ∙ 2 m Length; b) a human load of 460 kg on 1 m2; c) a steam road roller from 18 t Total weight, of which 8 t on the front roller and 5 each t on each of the two back rolls, 6 ∙ 1 m Length, 2 ∙ 5 m Overall width, 3 ∙ 5 m Center distance, 1 ∙ 3 m clear track width of the rear rollers, 1 ∙ 4 m Width of the front roller and 0 ∙ 5 m Width of each back roll.

For bridges 2nd class: a) Four-wheeled trucks of 8 each t Total weight at 5 ∙ ​​4 m Length (without drawbar), 2 ∙ 4 m Width, 2 ∙ 8 mWheelbase, 1 ∙ 5 m Gauge, with a covering of 2 horses with a total weight of 1 ∙ 5 t to 3 ∙ 6 m Length; b) a human load of 400 kg on 1 m2; c) a steam road roller from 14 t Total weight, of which 6 t on the front roller and 4 each t on each of the two back rolls, 5 ∙ 3 m Length, 2 ∙ 4 m Total width, 3 ∙ 0 m Center distance, 1 ∙ 1 m clear track width of the rear rollers, 1 ∙ 2 m Width of the front and 0 ∙ 4 m Width of each back roll.

For 3rd class bridges: a) Four-wheeled trucks of 3 each t Total weight at 4 ∙ 8 m Length (without drawbar), 2 ∙ 3 m Width, 2 ∙ 4 mWheelbase, 1 ∙ 4 m Gauge, with a covering of 2 horses with a total weight of 1 t to 3 ∙ 2 m Length; b) a human load of 340 kg on 1 m2.

Similar assumptions also apply to the road bridges in Germany.

The Swiss regulations stipulate: An evenly distributed load of 450 for the main roads kg for the m2 or car from 20 t on two axles, carriage length 8 m,Wheelbase 4 m, Car width 2 ∙ 5 m, Gauge 1 ∙ 6 m; for the 1st class country roads an evenly distributed load of 350 kg/m2 or car from 12 t on two axles, carriage length 6 m,Wheelbase 3 m, Carriage width 2 m, Gauge 1 ∙ 6 m; for 2nd class country roads an evenly distributed load of 250 kg on 1 m2 or car from 6 t Weight on two axles, carriage length 4 ∙ 6 m,Wheelbase 2 ∙ 0 m, Car width 2 ∙ 0 m, Gauge 1 ∙ 4 m.

For the state road bridges in France, B. is required: 400 kg f. d. m2 on the roadway and footpaths or a load from wagon trains, which either consist of single-axle wagons with 6 t Axle load, 3 m Car length (without drawbar), 2 ∙ 25 m Width, 1 ∙ 7 m Track width and stringing with two [131] horses each stretched one behind the other, each of 0 ∙ 7 t Weight and 2 ∙ 5 each m Length or two-axle wagons with 8 t Axle load, 3 mWheelbase, 6 m Car length, 2 ∙ 25 m Width, 1 ∙ 7 m Gauge and covering with 4 pairs of horses of 1 ∙ 4 each t Weight and 2 ∙ 5 each m Length.

The side effects of traffic loads, especially in the case of railway bridges, are: The horizontal side pressures of the vehicles, the centrifugal forces in the bends of the track, and also the braking forces.

The former forces are in accordance with the Austrian Bridge Ordinance 1/20 the axle loads of the locomotives, elsewhere also with 4 t attacking the first axis, taken into account. The centrifugal forces are according to the formula

to calculate in what G the axle load, r the radius of curvature and v denotes the train speed. The latter is for main orbits depending on the radius of curvature (200–700 m) at 15–30 m to accept. The braking forces are taken along for bridges with small and medium spans 1/7 of the train weight, for large bridges about with 1/10 of the total train weight. They are only to be taken into account for bridges in railway lines with more than 10 Inclination as well as those that are located in stations or subsequent braking sections.

3. The wind pressure is taken into account as a horizontal lateral force. The Austrian and French regulations set 270 for this kg on 1 m2 Wind impact area of ​​the unloaded and 170 kg on 1 m2 the loaded bridge. The regulations of the Prussian State Railways determine these digits as 250 kg, or 150 kg, the Swiss regulations only with 150 kg, or 100 kg. The visible area of ​​a supporting wall is to be taken into account as the wind impact area, but with perforated supporting walls it has to be enlarged by a part of the area of ​​the second supporting wall, among other things.this enlargement (according to the Austrian regulation), if the ratio of the open mesh areas of the first supporting wall to its total outline area is 0 ∙ 4, 0 ∙ 6, 0 ∙ 8, is 0 ∙ 2, 0 ∙ 4, 1 ∙ 0 times the area the second supporting wall. According to French and Swiss regulations, a fraction is calculated from the second supporting wall, which corresponds directly to the ratio of the open mesh areas to the total outline area. In the case of loaded railway bridges, the tension area exposed to the wind is a 3 m high, progressive rectangle in 0 ∙ 5 m Assume height above the rail. Of course, the part of the supporting wall surface covered by this is to be deducted. For road bridges, the wagon train is supported by a 2nd m high rectangle replaced.

4. Thermal effects. The changes in volume (changes in length) of its rod-shaped parts produced by temperature changes in a structure cause stresses if the system arrangement or the mounting of the structure is such that these changes in length cannot take place unhindered. In the so-called statically indeterminate systems, this is often the case with an even temperature change of all parts, but always with an uneven change in heat.

When determining the stresses caused by the temperature change in statically indeterminate constructions, one normally assumes for our climatic conditions that heat fluctuations within - 25 ° C and + 45 ° C occur in iron constructions, that is, an average installation temperature of + 10 ° C provided that the temperature fluctuation in iron bridges is ± 35 ° C. The Swiss regulations limit this assumption to ± 25 °. If parts of an iron supporting structure are exposed to direct sunlight while others are protected from it, it is advisable to expect a heat difference in these parts of 10 to 15 ° C.