All structures shall be designed for the following lateral wind forces. These forces shall be considered to act horizontally and in such a direction that the resultant stresses in the member under consideration are the maximum.
The wind force on a structure shall be assumed as a horizontal force of the intensity specified in the following table and acting on an area calculated as follows:
(a) For a deck structure. The area of the structure as seen in elevation including the floor system and railing, less area of perforations in the hand railing or parapet walls. (While calculating the wind pressure as on live load, the clear distance between the trailers of a train of vehicles shall not be omitted.)
(b) For a through or half through structure the windward truss as specified at- (a) above plus half the area of elevation above the deck level of all other trusses or girders.
ADVERTISEMENTS:
The pressures given in the following table shall be doubled for bridges situated in areas such as the Kathiawar Peninsula and the Bengal and Orissa coasts.
where, H = The average height in metres of the exposed surface above the mean retarding surface (ground or bed level or water level)
V = Horizontal velocity of wind in kilometres per hour at height H.
ADVERTISEMENTS:
P = Horizontal wind pressure in kg per sq. m. at height H.
The lateral wind force against any exposed moving live load shall be considered as acting at 1.5 m above the roadway and shall be assumed to have the following values:
Highway Bridges, ordinary … 300 kg/linear m.
Highway Bridges carrying tramway … 450 kg/linear m.
ADVERTISEMENTS:
While calculating the wind pressure of live load, the clear distance between the trailers of a train of vehicles shall not be omitted.
The bridges shall not be considered to be carrying any live load when the wind velocity at deck level exceeds 130 km. per hour.
The total assumed wind force as calculated according to the above clauses shall, however, not be less than 450 kg per linear metre in the plane of the loaded chord and 225 kg. per linear metre in the plane of the unloaded chord on through or half through truss, latticed or other similar spans and not less than 450 kg per linear metre on deck spans.
A wind pressure of 240 kg/metre2 on the unloaded structure shall be used if it produces greater stresses than those produced by the combined wind forces as per above clause.
ADVERTISEMENTS:
Railway and Foot Bridges:
For railway and foot bridges the wind pressure shall be computed from the appropriate basic wind pressure given by IS 875 or the table already given above on the exposed area as given below:
(a) For unloaded spand and trestles the exposed area shall be considered as one and half times the horizontal projected area of the span or the trestle, except for plate girders for which the area of the leeward girder shall be multiplied by the factors shown below and added to the area of the windward girder-
(i) When the spacing of the leeward girder does not exceed half its depth – 0
ADVERTISEMENTS:
(ii) For spacing exceeding half depth and upto full depth – 0.25
(iii) For spacing exceeding full depth and upto one and half times depth – 0.50
(iv) For spacing exceeding one and half times depth and upto twice its depth or more – 1.00
(b) For loaded spans the net exposed area shall be computed as the sum of (i) and (ii) below.
(i) One and half times that portion of the horizontal projected area of the span not covered by the moving load except for plate girders for which the area of the leeward girder not covered by the moving load shall be multiplied by the factors shown under (a) above and added to the area of the windward girder above or below the moving load, and
(ii) The horizontal projected area of the moving load.
Note:
In the case of railway bridges, the area of the moving load shall be taken as from 600 mm above rail level to the top of the highest stock for which the bridge is designed. In the case of foot bridges the height of the moving load is to be taken as 2 metres throughout the length of the span.
The wind pressure effect is considered as a horizontal force acting in such a direction that resultant stresses in the member under consideration are the maximum.
The effect of wind pressure to be considered are as follows:
(a) Lateral effect on the top chords and wind bracing considered as a horizontal girder.
(b) The same effect on the lower chords.
(c) The vertical loads on the main girders due to the overturning effect of the wind on the span and on the live load.
(d) Bending and direct stresses on the members transmitting the wind load from the top to the bottom chords or vice versa.
Note:
The members of the main girder should be designed for entire wind load on the top chord being transmitted through the portals. Their sections, however, shall not be less than that required to take the additional vertical load on the leeward girder derived form an overturning moment equal to the total wind load on the fixed structure and train multiplied by the height of the centre of pressure above the plane of the top lateral bracings in the case of deck type spans and the bottom lateral bracings in the case of through type spans.
Racking Forces:
Lateral bracings of the loaded deck of railway spans shall be designed to resist in addition to the wind and centrifugal loads, specified above, a lateral load due to racking forces of 60 kg/m treated as a moving load. This lateral load need not be taken into account when calculating stresses in chords or flanges of main girders.
In the case of effective spans upto 20 m it is not necessary to calculate wind stresses but in railway bridges lateral bracings shall be provided designed for a lateral load due to wind and racking forces of 900 kg/m treated as moving load in addition to the centrifugal load, if any.
Forces on Parapets:
Railings or parapets shall have a minimum height above the adjacent roadway or footway surface of one metre less one half the horizontal width of the top rail or top of the parapet. They shall be designed to resist a lateral horizontal force and a vertical force each of 150 kg/m applied simultaneously at the top of the railing or parapet.
Longitudinal Forces:
Road Bridges:
In all road bridges, provision shall be made for longitudinal forces arising from any one or more of the following causes:
(a) Tractive effort caused through acceleration of the driving wheels;
(b) Braking effect resulting from the application of the brakes to braked wheels; and
(c) Frictional resistance offered to the movement of free bearings due to change of temperature or any other cause.
Note:
Braking effect is invariably greater than the tractive effect.
The braking effect on a simply supported span or a continuous unit of spans or any other type of bridge unit shall be assumed to have the following value:
(a) In the Case of a Single Lane or a Two Lane Bridge:
20 percent of the first train load plus 10 percent of the loads of the succeeding trains or part thereof, the train loads in one lane only being considered for the purposes of this sub-clause. Where the entire first train is not on the full span, the braking force shall be taken as equal to twenty percent of the loads actually on the span.
(b) In the Case of Bridges having more than Two Lanes:
As in (a) above for the first two lanes + five percent of the loads on the lanes in excess of two.
Note:
The loads in this clause shall not be increased on account of impact.
The force due to braking effect shall be assumed to act along a line parallel to the roadway and 1.2 metre above it. While transferring the force to the bearings, the change in the vertical reaction at the bearings should be taken into account.
The longitudinal force at any free bearing shall be limited to the sum of dead and live load reactions at the bearing multiplied by this appropriate co-efficient of friction.
The co-efficient of friction at the bearing shall be assumed to have the following values:
The longitudinal force at the fixed bearing shall be taken as the algebraic sum of the longitudinal forces at the free bearings in the bridge unit under consideration and the force due to the braking effect on the wheels as mentioned above.
The effects of braking force on bridge structures without bearings such as arches, rigid frames etc., shall be calculated in accordance with approved methods of analysis of in determine structures.
The effects of longitudinal forces and all other horizontal forces should be calculated up to a level where the resultant passive earth resistance of the soil below the deepest scour level (flow level in case of a bridge having pucca floor) balances these forces.
Railway Bridges:
Where a structure carries a railway track, provision as under shall be made for the longitudinal loads arising from any one or more of the following cases:
(a) The tractive effort of the driving wheels of the locomotives.
(b) The braking force resulting from the application of the brakes to all braked wheels.
(c) Resistance to the movement of the bearings due to change of temperature.
No increase for impact shall be made for longitudinal loads.
These loads shall be considered as adding horizontally through the knuckle pins, or through the girder seats where the girders have sliding bearing.
For spans supported on sliding bearings the horizontal loads shall be considered as being divided equally between the two ends; for spans which have roller bearings at one end the whole of the horizontal load shall be considered to act through the fixed end.
For Railway bridges the value of the longitudinal load due to either the tractive effort or the braking force for the loaded length L shall be obtained from tables given below.
The loaded length L shall be taken as equal to:
(a) The length of one span when considering the effect of the longitudinal loads:
(i) On the girders,
(ii) On the stability of the abutments.
(iii) On the stability of piers under the condition of one span loaded or.
(iv) On the stability of piers carrying one fixed and one roller bearing.
(b) The length of two spans when considering under the condition of both spans loaded, the stability of piers carrying fixed or sliding bearings.
In this case, the total longitudinal load shall be divided between the two spans in proportion to their lengths.
For determining the value of tractive effort L shall not be taken to exceed the following:
(a) For Broad Gauge bridges – 30 m for M.L. and B.L. loadings.
(b) For Metre Gauge bridges – 24 m for all loadings.
Where the structure carries more than one track, longitudinal loads shall be considered to act simultaneously on all tracks. The maximum effect on any girder with two tracks so occupied shall be allowed for, but where there are more than two tracks, a suitable reduction may be made on these loads for the additional tracks.
Seismic Force (Railway Bridges):
Seismic load need only be taken into account if local conditions so require and the allowance for horizontal acceleration shall depend on such local conditions; but shall not exceed 0.12 of gravity.
Road Bridges:
If a bridge is situated in a region subject to earthquakes, allowance shall be made in the design for the seismic force and earthquake-resistant features shall be embodied in the structural details of design.
The seismic force shall be taken as a horizontal force equal to the appropriate fractions specified below, of the weight of the dead load and live loads acting above the section under consideration. (Parts of the structure embedded in soil shall not be considered to produce any horizontal forces).
For bridges situated in epicentral tracts where large devastations have occurred in the past due to earthquakes, the percentage shall be fixed by the engineer responsible for the design with due regard to the local conditions regarding the intensity of earthquakes generally experienced in these regions.
These horizontal forces due to seismic force shall be taken to act through the centre of gravity of all the loads under consideration. The direction of this force should be such that the resultant stresses in the member under consideration are the maximum.
Seismic and wind forces shall not be considered to act simultaneously.
Temperature Effects – Road Bridges:
Provision shall be made for stresses or movements from variations in temperature.
The rise and fall of temperature shall be fixed for the locality in which the structure is to be constructed and shall be figured from an assumed temperature at the time of erection.
Due consideration shall be given to the lag between air temperature and interior temperature of massive concrete members or structure.
Except where stated otherwise, the following range of temperature shall generally be assumed in the design.
But in both the cases (a) and (b) intermediate values can be allowed at the discretion of the engineer responsible for the design.
The co-efficient of expansion per degree centigrade shall be taken as 0.000017 for steel and reinforced concrete structures and 0.0000108 for plain concrete structures.
Railway Bridges:
Where any position of structure is not free to expand or contract under variation of temperature, allowance shall be made for the stresses resulting from this condition. The temperature limit shall be specified by the Engineer.
Erection Stresses:
The weight of all permanent and temporary material, together with all other forces and effects which can operate on any part of the structure during erection, shall be taken into account.
Allowance shall be made in the design of stresses set up in any member during erection – such stresses may be different from those which the member will be subjected to during actual working.
Deformation Stresses:
A deformation stress is defined as the bending stress in any member of an open web-girder caused by the vertical deflection of the girder combined with the rigidity of the joints. No other stresses are included in this definition.
All steel bridges shall be designed, manufactured and erected in a manner such that the deformation stresses are reduced to a minimum. In the absence of calculations, deformation stresses shall be assumed to be not less than 16 percent of the dead and live load stresses.
Secondary Stresses:
Secondary stresses are additional stresses brought into play due to the eccentricity of connection, floor beam loads applied at intermediate points in a panel, cross girders being connected away from panel points, lateral wind loads on the end posts of through girders etc. and stresses due to the movement of supports.
All bridges shall be designed and constructed in a manner such that the secondary stresses are reduced to a minimum, and they shall be allowed for in the design.