Wind Load on Structure

The wind angle is defined as the angle between the normal to the longitudinal direction of the bridge and wind direction, positive if anti-clockwise, measured from the bridge normal to the wind direction. The wind pressure on the superstructure depends on the wind angle and superstructure dimensions. By default, predefined values for wind pressure, in accordance with IRC specifications, are used. However, you can overwrite the default wind pressure and input custom values. The superstructure parameters must be defined before Substructure can automatically generate the wind load on structure.

This generation internally accounts for the pier view direction and then generates loads as per the view direction.

The wind load is calculated in two steps:

Calculate the wind load acting on superstructure
Calculate the wind load acting on substructure

Wind on Superstructure

To calculate the wind load acting on superstructure, you must first calculate the superstructure area exposed to the wind load. This area is equal to the product of the total superstructure height and the average span length. Next, the wind pressures in the bridge longitudinal and transverse directions are multiplied with area to obtain the total forces, which are decomposed to the global X and Z directions if the bridge has a skew angle. Then, the total forces are averaged over the bearing points. The wind load also produces a moment due to the arm between the total forces and bearing points. This moment is assumed to be balanced by a force couple (in global Y-direction) acting on the external bearing points at left and right end of pier.

Wind on Substructure

To calculate the wind load acting directly on the substructure, the projected area of pier cap and column are calculated first, depending on the wind direction and the bridge skew angle. Taking a column as an example, the calculation of its projected area is illustrated as follows:

A₁ = b_zcos(θ_w - θ_s)×h

A₂ = b_xcos(θ_w - θ_s)×h

Projected area, A = |A₁| + |A₂|

where

b_z	=	column thickness, in the global Z-direction
b_x	=	column width, in the global X-direction
h	=	column height exposed to wind pressure
θ_w	=	wind angle
θ_s	=	bridge skew angle

The projected area is multiplied by the wind pressure to obtain the total wind force.

F = P_w×A

where

P_w	=	wind pressure
A	=	projected area

The total wind force is subsequently resolved into global X- and Z-directions, depending on the wind and bridge skew angles, as follows:

X-direction component, F_x = Fcos(θ_w - θ_s)
Z-direction component, F_z = Fsin(θ_w - θ_s)

The force acting on the cap in Z-direction is represented as a uniformly distributed load over the length of the cap, while the force acting in X-direction is applied as a concentrated load. The length of the column subject to the wind load depends on the elevation above which the wind is applied (value you enter on the Auto Load Generation: Wind on Structure screen).