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The centrifugal force (Horizontal Moving Load) has two components, the radial force and the overturning force. The radial component of the centrifugal force is assumed to be transmitted from the deck through the end cross frames or diaphragms and to the bearings and to the substructure. The overturning component of centrifugal force occurs because the radial force is applied at a distance above the top of the deck. The overturning component causes the exterior wheel line to apply more than half the weight of the truck and the interior wheel line to apply less than half the weight of the truck by the same amount. Thus, the outside of the bridge is more heavily loaded with live load. The overturning force is computed by taking the sum of the moments about the inside wheel and setting the sum equal to zero. The result is that the outermost girder will receive slightly higher load and the innermost girder will receive slightly lower load. Thus, it is also necessary to compute the condition with no centrifugal force, i.e., a stationary vehicle, and select the worst case.
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Fig. Vehicular Centrifugal Force Wheel-Load Reactions
Once the wheel-load reactions are manually calculated, the overturning effects of centrifugal forces can be taken into account during moving load analysis in midas Civil
(new enhancement in Civil 2021 (v1.1)). In the previous versions, it is also possible to automatically assign the centrifugal force for AASHTO live load code using static load cases converted from the moving load tracer. The steps to be followed are as below:
Using this feature, the overtunring effect on wheel loads due to centrifugal loading can be taken into account for particular elements selected by the user.
Here is the procedure. For example, the maximum positive moment of the j-end of element no. 444 is 1749.1 k-ft and we want to find the corresponding centrifugal loads.
Step 1.
Perform moving load analysis. Maximum positive moment due to live load = 1749.1 k-ft.
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Step 2.
Go to Moving Load Tracer and find the vehicle position which gives maximum positive moment for the element no. 444.
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Step 3.
Hit the 'Write Min/Max Load to File' button and
check on the 'Centrifugal Forces' option in the dialog below. Click OK.
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The MCT file will be generated.
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Step 4.
Copy and paste the text in the MCT file into MCT Command Shell. Click Run.
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Both vertical live loads and corresponding centrifugal loads
will be generated as a static load.
%20Load/10.bmp)
Step 5.
Check the Centrifugal Load (Overturning component).
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Step 6.
Add the effect of the centrifugal load (overturning effect)
to the effect of the moving load analysis.
1749.1 k-ft + 198 k-ft = 1947.1k-ft
If you want to consider the centrifugal load (radial component) on the substructure design, the horizontal component of the centrifugal load can be used.
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Note :
- Centrifugal force will only be available for the vehicle containing concentrated load.
- Direction of Centrifugal Forces with reference to Vehicle Direction: Select the direction of centrifugal forces on transverse direction of lane based on the longitudinal direction of lane. For more details, refer to the online manual.
Right-to-Left Direction / Left-to-Right Direction
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