Week 5 Challenge

Aim: To calculate the wind loads and do the structural analysis of the bridge by applying wind load.

Given data:

• The height of the bridge above ground level is 9 m.
• It is located in open terrain with well-scattered obstructions of height less than 10m. (terrain category-2)
• The pier size is 1.5 x 1.5 m.

Assumption:

Bridge deck width = 10 m & thickness 0.2 m.

The total length of the bridge is considered as 20 m with intermediate pier support at 10 m in length.

3 nos of the pier at each end.

The depth of the pier below GL =3 m.

The main girder beam section is considered as 0.5 m x 1 m.

The Cross girder beam section is considered as 0.4 m x 0.8 m.

The cross girders are given at an interval of 5 m length of the bridge.

Fixed support condition is considered for the pier end.

Solution:

Let's consider the location is Chennai,

so basic wind speed (Vb) is 50 m/s

k1(Risk coefficient) = 1.08 (from table 1 of IS 875 part 3)

k2(Height factor) = 1 ( from table 2 of IS 875 part 3)

k3 ( Topography factor)=1

k4 ( Importance factor for the cyclonic region) =1.3 ( As Chennai is a coastal area)

Design wind speed (Vz) = Vb x k1 x k2 x k3 x k4 = 50 x 1.08 x 1 x 1 x 1.3 = 70.3 m/s

wind pressure (Pz) = 0.6 * Vz^2 = 2.97 kN/m2

For design wind pressure (Pd) calculation,

Wind directionality factor (kd) = 1

For area averaging calculation (ka),

Tributary area = pier height above GL x minimum c/c distance of pier in either direction

=9 x 5 =45 m2

Ka=0.9 + ((0.8-0.9)/(100-25))*(45-25) = 0.873

Kc (Combination factor) =1

Now Pd = Pz x Ka x Kd x Kc =2.59 kN/m2.

Structural Analysis in Staad Pro.

Considering the above details of the bridge , A bridge model was prepared in staad pro.

(3D model of the bridge)

Then in the wind definition custom pressure was given for a particular height as shown below.

Then 4 wind load cases were generated i.e wind load in +ve X and -ve X direction, wind load in +ve Z and -ve Z direction.

It can be noted that the Pier starts from -3 m up to 9 m in Y-direction.

The 4 load cases generated are shown below.

WL(+X)

WL(-X)

WL(+Z)

WL(-Z)

ANALYSIS AND RESULTS

The result of the analysis due to the application of wind load is summarised below for each component of the bridge.

PIER:

(Bending Moment diagram (Mz) for load case WL(+X))

(Bending Moment diagram (My) for load case WL(+Z))

Maximum Bending Moment Observed (My) = 168.488 kN.m for load case (WL(+Z) & WL( -Z))

Maximum Bending Moment Observed (Mz) = 345.740 kN.m for load case (WL(+X) & WL(-X))

Maximum Axial Force observed = 31.083 for load case (WL(+X))

MAIN GIRDER:

(Bending Moment diagram (Mz) for load case WL(+Z))

Maximum bending moment observed (Mz) = 37.762 kN.m (Span moment) & 36.819 kN.m (Support Moment)

Maximum shear force observed = 7.893 kN

CROSS GIRDER:

(Bending Moment diagram (Mz) for load case WL(+X))

Maximum bending moment observed (Mz) = 61.428 kN.m (Span moment) & 60.7 kN.m (Support Moment)

Maximum shear force observed = 24.538 kN

BRIDGE DECK:

(Bending Moment (Mx) Contour for load case WL(+X))

(Bending Moment (My) Contour for load case WL(+Z))

Maximum Bending moment observed (Mx) = 2.198 kN.m

Maximum Bending moment observed (My) = 2.198 kN.m