4 Lane - Reinforced Concrete Bridge verification IRC design

[fiery heart]

Greetings,

Currently, i am checking the viability of 4 lane of RCC T beam bridge. The features of bridge are:
Span= 25m
No. of longitudinal girder =6
Depth of longitudinal girder = 2.5m

I have modeled in sap and done manual calculation.
The live load distribution among the longitudinal girder was done by Little and morrice method as well as K.S. rakshit method to check the authenticity of the manual calculation.

The distribution coefficient for outer most longitudinal girder (spacing 3.28m) was found to be 1.64 by little and morrice method.
The distribution coefficient for outer most longitudinal girder (spacing 3.28m) was found to be 1.6 by K.S. Rakshit method.

The bending moment due to 2 number of 70R (W) assuming a single beam elemnet was found to be 9256.4 KN-m.
Here the softwatre and manual calculation result matches.
However, upon checking the software for outer longitudinal girder, the bending moment value is 4208 KN-m
BUt manual calculation shows, the value of bending moment of 2783 KN-m.

 

[BridgeSmith]

The software may be considering what those of us who design by the AASHTO LRFD Bridge design spec call the rigid method, which considers the superstructure as being rigid body in the transverse direction. It assumes intermediate diaphragms are stiff and strong enough to make the girders react as a group, rotating the entire superstructure, rather than the girders deflecting independently. This very often produces the controlling load case on the exterior girders. If that is the case, you will have to make a determination of whether the girders are interconnected sufficiently for that to be a valid loading case.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[fiery heart]

The basis of little and morrice method, K.S. rakshit method is that the stiffness of cross girder are taken into account ( by parameter alpha and theta) but not infinitely rigid.

 

[BridgeSmith]

The AASHTO LRFD also specifies the use of the lever rule to calculate the distribution factor for exterior girders with one lane loaded.

Doesn't your software report what distribution factor it's using?

Just so I'm clear on the terminology, is the "distribution coefficient" the number of wheel lines (wheel fraction used in the AASHTO Standard Specification) or the number of axle lines (distribution factor used in the AASHTO LRFD spec) applied to the girder of interest?

Rod Smith, P.E., The artist formerly known as HotRod10