Week 7 Challenge
Aim: To determine the water current loads acting on bridge and analyze accordingly in Staad.Pro.
Introduction:
The objective is to calculate water current loads from the given data manually and attempt the same in Staad.Pro for the bridge and analyse the results and effects on the structure.
The Illustrated bridge is 90 m in length over a water body having the Highest Flood Level as 2 m from the foundation level. Mean velocity of water body is 3 m/s.
Given Data (for calculation):
Highest Flood Level = 2 m above foundation level
Span of Bridge = 45 m
Mean Velocity = 3 m/s
Assumption:
Size of Pier = 1.5 m Diameter
Depth of foundation = 2.0 m
By the consideration of all the dimension of abutment given in figure, Depth of abutment is 3.965m and accordingly depth of foundation is 3.96-(1.83+0.915) = 1.22m
And Total depth of footing is 1.22 + 0.765 = 1.985m, Since according to question HFL is 2.0m from the foundation level. Then water current act on abutment is 15mm. So ignore the water current load on abutments.
Thickness of abutment = 0.9 m
Width of bridge = 10.0 m
Size of Cross beam = 0.9 m x 1.2 m
Size of Girder beam = 1.2 m x 2.0 m
Thickness of Deck slab = 300 mm
Procedure:
All the loads are calculated in such a manner as mentioned below,
Height of piers & Abutments subjected to water current = 2 m
Vmean = 3 m/s
Maximum Velocity = V = √2 x Vmean = √2 x 3 = 4.243 m/s
Water current pressure = P = 0.52 x k x V2
(As Piers are circular in shape)
So, value of k = 0.66
P = 0.52 x 0.66 x (4.243)2 = 617.865 KN/m2
Calculation of Scour Depth: -
Mean Discharge = 3 m/s
Design Discharge = Db = [(30/100) x 3] + 3 =3.9 m/s
(Considering catchment area below 3000 km2 ) (From IRC 78, 703.11)
Considering Bed of soil as medium sand
Silt Factor of Coarse Sand will be = 1.25 from 703.2.2.1, IRC 78
Mean Scour Depth = dsm = 1.34 x (Db/Ksf) 1/3 = 1.34 x (3.9/1.25) 1/3 = 1.95 m
Scour Depth = 2 x dsm = 2 x 1.95 = 3.9 m (For Pier)
= 1.27 x dsm = 2.48 m (For Abutment)
Computer Proficiency:
- Create a model by putting the necessary inputs as file location, units etc.
- Create three nodes (x,y,z) represents as (0,0,0) & (0,2,0) & (0, 7.36,0)).Because the height of pier is assumed to be 5.36m & depth of foundation is assumed as 2.0 m.
- Since the span of bridge is 45m each, make two nodes adjacent each other and connect them with add beam option.
- Replicate the end nodes at the distance of 3.965m below in order to make abutments as per question dimension. And connect them accordingly.
- Go to support tab, and create fixed support and assigned to the piers and abutment ends.
- Go to Properties tab, and define size of pier as mentioned above and assign accordingly.
- Define the size of beam girder as discussed above and assign accordingly.
- After assigning all the necessary size of different section select the whole assembly and replicate it at spacing of 10m in z direction.
- Connect the necessary beam by making cross beam at ends and middle piers.
- Select the create mesh option, and connect the all four nodes of abutment and select quadrilateral meshing.
- Provide the division in order to make 0.5m x 0.5m mesh. Refer image below.
- Replicate the abutment plate to the other end as well. And provide the size of plate as discussed above.
- Define the size of cross beams as discussed above. And assign accordingly.
- Replicate the cross beam in both direction at spacing of 9m from the transitional repeat option.
- Select the generate mesh option in order to make deck slab mesh of 0.5m x 0.5m size with the same process discussed in step 11. And replicate to each deck panel.
- After that go to loading option, and create three load cases as Dead, Water current load +Z and Water current -Z.
- In dead load, provide the self-weight with factor -1. And For water current load use member load at trapezoidal option and provide the value as discussed above by multiplying the cross section of pier according to the direction of load case. Refer images below.
- Go to Analysis tab, and select analysis commands and click add.
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Analytical Results:
Bridge has been analyzed and check for different load combination and different displacement for different scenarios has been record.
Maximum node displacements is recorded as 103 mm.
Maximum Base reactions are recorded axial force as 5661.68 KN and moment as 555.538 KN-m.
Maximum shear force recorded in beam in Fy direction is 2436.74 KN.
Maximum Bending Moment recorded in beam in Mz direction is 21700.984 KN-m.
Maximum Shear Force in SQX (local) direction for deck slab is recorded as 1941.1 KN/m2.
Maximum Shear Force in SQY (local) direction for deck slab is recorded as 4904.35 KN/m2.
Maximum Moment in MX (local) direction for deck slab is recorded as 792.742 KN-m/m.
Maximum Moment in MY (local) direction for deck slab is recorded as 6196 KN-m/m.
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Conclusion:
Different results have been found under the water current loads,
Maximum node displacements is recorded as 103 mm.
Maximum Base reactions are recorded axial force as 5661.68 KN and moment as 555.538 KN-m.
Maximum shear force recorded in beam in Fy direction is 2436.74 KN.
Maximum Bending Moment recorded in beam in Mz direction is 21700.984 KN-m.
Maximum Shear Force in SQX (local) direction for deck slab is recorded as 1941.1 KN/m2.
Maximum Shear Force in SQY (local) direction for deck slab is recorded as 4904.35 KN/m2.
Maximum Moment in MX (local) direction for deck slab is recorded as 792.742 KN-m/m.
Maximum Moment in MY (local) direction for deck slab is recorded as 6196 KN-m/m.