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TYPES OF FORCES ON CONCRETE BRIDGES

Updated: Aug 9, 2022


BOX GIRDER SKETCH
Evaluating the Loads on bridge is amongst the first steps in designing bridges. All loads upon a bridge have to travel via the bridge pier to be transferred to the foundation below. Hence, the integrity of the pier is of remarkable importance. As a pier structure is loaded with a multitude of forces natural and imposed, a definitive understanding of the cases are essential in assessing the suitability of design and detailing.

TYPES OF LOADS


1. SELF WEIGHT



The shape and section of RCC piers primarily determines the magnitude of self-weight from the pier. The material of construction, mainly concrete, is multiplied to the volume calculated considering all the geometrical variations. While analyzing the structure on software packages, this load is automatically calculated by self weight function/command. Usually, a self-load multiplier of 1.1 is used in the analysis.

When the pier section is hollow, appropriate deductions must be made from the total weight.

Self-weight consideration is made for the pier member, piercap flaring, bearing pedestals, seismic arrestors, crash parapet, etc. Self-weight is used only as a point load incident upon the footing excluding a moment emanating from it.


2. DEAD LOAD


Unequal Spans

Dead loads are a permanent category of loads that bridges have to endure for the entirety of their service/ design life.

The arrangement of spans on either sides of a pier dictates the magnitude of the dead load. The dead load incident upon the pier is generated by the spans on either sides of the pier. Hence, piers might require supporting un-symmetric spans on either sides, as a result, experience varying dead load values on either sides of the bearing groups respectively as shown in figure.

Prior to applying the forces on the deck, the designer must compute the DL of all the loads of different structural and non-structural elements upon the deck. This includes the mechanical equipment that will remain upon the deck in the service life of, cable racks, railway tracks, dividers, wearing course, light posts, other fixtures, etc.

Structurally, the main factors of consideration are the length of the span, the section of the girders, crash parapets, longitudinal rail loads (in case of a railway bridge), etc.

Dead load completely exerts itself in magnitude in the construction stage, after which it remains constant in value.



3. SUPERIMPOSED DEAD LOAD

Superimposed dead loads are gravity loads that consist of other permanent non-structural or architectural parts of bridge such as parapets and road surfacing, etc. These items are ‘long term’ loads the nature of which might change or remain unchanged for most part of a structures design life. The densities of each of such components might be different.

The most notable item of superimposed dead load is the road pavement or surfacing. It is not uncommon for road pavements to get progressively thicker over a number of years as each new surfacing is simply laid on top of the one before it. Non-structural entities like steel rails, cables, steel barriers, road-medians, etc. are considered for this category.

4. LIVE LOAD


Track Live Load Combination

There are different types of live loads that should be considered in attaining the most critical load case scenario for design. Standard multiple axle vehicles with their standard dimension, load tonnage per axle, distance between the axles, minimum distance between two successive vehicles, etc. are specified as per the design code referred to. Bogie loads for railway and metro bridges are defined based on the number of tracks on a span. To find out more about the number of combinations of train load cases, read this article.


Some important consideration for live loads are mentioned further.


Critical Vehicle Position

As different vehicles travel over the bridge viaduct, a plethora of possibilities arise which produce different critical load cases generating varying magnitudes of stresses on the superstructure.

In order to reach the most critical load case for the design, different variations in the position of design vehicle must be analysed and a load envelope be created. Comparing this load envelope, we can reach a conclusion as to the maximum axial load, moment, transverse moment, torsion, etc upon the structure.

Using this matrix of maximum, perhaps critical, load values, the structures may be designed individually. The factors of consideration include the width and length of superstructure, number of lanes, etc.

Congestion Factor

Traffic congestion on vehicular bridges must be addressed in the axle load as a factor that is multiplied to the loads. This factor mainly depends upon the number of lanes for traffic. Thus the loads are increased by 20-40% and the resulting factor that is multiplied to the loads is called as the congestion factor, based on the data.


Vehicle traction is the generated friction between a drive wheel and the road surface and is caused by the vehicle's acceleration and the subsequent movement over the bridge carriageway. These loads assume the nature of horizontal loads applied to the superstructure.

Watch this video to find out about the load cases on dual track railway/metro bridges :




5. WIND LOAD


Regional Wind Velocity Diagram

Wind force acts as an area load or pressure. However, to compute the moment magnitude emanating from it, it can be considered incident upon the centroid of exposed area/surface.

The magnitude of wind applicable depends on the are geography, location terrain, environmental obstructions in the surrounding area, orientation of the structure against the upwind direction, topography, height of the bridge above clear ground, aerodynamics of the structure (cross-section), etc.

When the viaduct comprises of box girders, the gradual curve of the web provides aerodynamic shape to the superstructure minimising the impact of the wind.

Wind load is considered on the pier as well as the superstructure. An additional load case for wind on the live load (train bogie) is considered for high wind speed locations or if required.


6. TEMPERATURE LOADS

The changes in the temperature are Uniform and differential which incite effects on the structural member appropriately.


Temperature Stresses

There are two thermal effects which potentially induce stresses in bridges, these are :


1. Uniform temperature:

It is the change in the bridge out of uniform temperature exposure which results in an axial expansion or contraction. If restrained, such as in an arch or a integral bridge, this can generate significant axial force, bending moment and shear.


2. Differential temperature:

Differential temperature exposure on the structure indicates that different temperature intensities are experienced by different components of a member. This is due to differential changes in temperature. For example, the bottom flange of a box girder might be exposed to lower temperature compared to the top flange. If top of a beam heats up relative to the bottom, it tends to expand; if it is restrained, bending moment and shear force are generated.



7. EARTHQUAKE LOADS

Earthquake damage to bridge structures may occur in the superstructure, the substructure or the approach slabs. Connection failures are the most common type of damage and these may take several forms. They include-the failure of bearings and expansion joints which connect the superstructure to the substructure, shear or flexural failures that occur within the substructure, etc. Connection failure is classified as the principal reason for bridges to fail.

Since earthquake loads are predominantly horizontal in-plane loads, and because bridge superstructures are inherently very strong in-their-own-plane, earthquake related structural damage to a bridge superstructure is very rare.

On the other hand, loss of support due to gross horizontal movement of one or more segments of a superstructure is quite commonplace and may cause the partial or total collapse of one or more spans of the bridge.


8. WATER PRESSURE

The intensity of the water pressure pounding the pier structure


Water pressure Forces

For waterfront structures or bridges spanning water bodies, the pressure exerted by the water is applied and considered in design. This consideration is not valid for urban designing where by the threat of the pier being submerged is absent.

The intensity of the water pressure parallel to the water current is computed as below:



where,

V is the velocity of the water current at the location, and k is a constant depending on the pier's shape.

the k value ranges from 0.6 to 1.5 for the piers except , based on its cross section.

The mean velocity of the water is usually around 3m/s and the maximum velocity is taken as times √2 the mean velocity.

The magnitude if the pressure varies with the height of the highest water level.

In case of the skewed flow direction, velocity is divided into parallel and normal components.

Scour level is usually the top of the rock level. The highest point to which the water can rise on a pier is known as the highest flood level or HFL.


The pier section geometry plays an important role in the formation of vortexes, viz. horseshoe vortexes, in the flowing water. Hence, pier geometry must enable appropriate vortex shedding at the opposite end to the face of the pier interface.

Consideration while designing the footing to safeguard against scouring potential of the waves at higher flow velocity will prevent the scour destabilizing the pier in future.

In many cases, a water debris loading might be considered in design.

9. CENTRIFUGAL FORCES AT CURVES

Centrifugal forces are the direct result of a span with a curvature.


Banking for CF Forces

The movement of live loads on the curved span produces an outward pull on the vehicle. This pull exerted by the axles on the superstructure have a horizontal component of force along with a vertical one.


Curved Span

Since the pulling force varies as per the dynamics of the live load in consideration, this force is applied at the CG of the vehicle, which is 1.2m above the carriageway.


Girder Tilt

For this reason, the cross section of the superstructure require to be banked.

The magnitude of this force depends upon the severity of the curve, the cross slope, axle loads, friction, etc.


The centrifugal force is calculated as:

ADDITIONAL FORCES TO CONSIDER


· BUOYANCY EFFECT ON THE PIERS

Buoyancy is an uplift force on the foundations because of the pores below the pier zone compressed by air and water.


Vehicles moving around the pier might hit the pier structure and cause damage to the base of the pier. Hence, the pier must be braced or designed for this type of an impact arising out of unfortunate accidents. For this, a horizontal force, normal to the plane of the pier substructure is applied at a vertical distance equal to the Cg of the vehicle. act parallel to the carriageway.

· BRAKING LOAD

· ICE LOAD

· SHIP COLLISION LOAD

· CREEP & SHRINKAGE EFFECTS

· EFFECT OF PRESTRESS

· EFFECT OF BRIDGE VIBRATION

· DIFFERENTIAL SETTLEMENT

· PEDESTRIAN LIVE LOAD

· WATER DEBRIS LOAD


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