Shear Calculations for Vo for AS5100.5

[justin.conrad]

Hi All,

I have found a contradiction in the way the Structural Bridge Design software calculates Vo in relation to clause 8.2.7.2.

For background. I have created a standard prestressed 1500mm deep Super T girder with reinforcement under the following applied moments: (Primary bending) Mx = 1000kNm, My = 100kNm, and Vy = 500kN.

Clause 8.2.7.2 states:

"Vo is equal to the shear force which would occur at the section under consideration when the bending moment at that section was equal to the decompression moment (Mo).

For simply supported conditions - 

Vo = Mo/(M*/V*), where M* and V* are the design bending moment and design shear force respectively, at the section under consideration, due to the same design load.

M* and V* shall be calculated from - 

1. the design maximum moment and corresponding shear; and
2. the design maximum shear and corresponding moment."

What I have found is that in the section module of SBD, the M* value used refers to the un-factored moment capacity of the section and not the applied moment of 1000kNm, resulting in a severe under estimation of the Vuc.f value. It should be noted that the beam module of SBD uses the applied moment and not the moment capacity for M*. Are you able to provide clarification or advice on why the section module would be using the un-factored moment as M*?

Regards,

Justin

Photos below for clarification:

[david.geeves]

Hi Justin,

In your description it sounds like you have set the "Capacity "option to "X Moment Positive" rather than leaving this  to "*not used*".  It is important to not set the capacity option or the shear calculations will then  be done at the capacity value.  If you set it to *not used* I think you will find that the calculations will be ok and the correct moment used.

Please mark this reply as a solution if this has solved your problem so that others can find it more easily.  Thanks

Kind regards

[justin.conrad]

Hi David,

Thanks for your reply.

I've tried setting the capacity with the aforementioned load case and the section analysis fails to converge despite different analysis parameters.

I've found that setting the capacity to 'Y Positive Moment' yields the correct applied moment value.

Regards,

Justin

[david.geeves]

Hi Justin,

I think you may have misunderstood me.  It is important not to have the capacity set when doing the shear calculations.

If you like to attach your data file to your post I will take a better look at your section and may be able to see what the problem is.

Kind regards

[justin.conrad]

Hi David,

To clarify from my earlier post. I have tried setting the capacity to 'not used' under my given load case and the analysis does not converge. The analysis for shear will perform if I define no loads and set the capacity to 'not used',

Regards,

Justin

[david.geeves]

Hi Justin.

I really need to see your data file to make any judgment on this.  There could be many reasons for the problem so it is quite difficult to narrow down.  For example,  you may have fixed the neutral axis to be horizontal, and if the applied moment is not big enough to counteract the moment caused by the prestress  (if there is any prestress tendon force applied) then it will be trying to turn the NA  by 180 degrees and will thus fail.  There are many more scenarios that I would need to rule out  before being able to comment.

It is quite easy to attach the data file to the post so I hope you are able to do this.

Kind regards

[justin.conrad]

Hi David,

I've attached the model file in this post.

Regards,

Justin

[david.geeves · Accepted solution]

Hi Justin,

Thanks for attaching your file.  I get exactly the same as you when I run it in ASBD 2014 but I get a solution in ASBD 2016 and ASBD 2017.  There were a few refinements done on the iterative algorithms for the strain compatibility analysis between 2014 and 2016 so I suspect this will have ironed out the problem you are experiencing.  I don't seem to be able to get a solution in 2014 with the load case as supplied.

One thing I was interested in is that you have an Mx moment and an My Moments - providing a rotation in the NA when the moments are resolved.  The applied shear. which is in the Fy direction, will be resolved into a direction perpendiculars to the NA so will be a reduced value.  Is this what is intended?  For biaxial bending it is common to split the calculation into 2 looking first at Mx with the appropriate shear and then MY with the shear perpendicular to this.  This will ensure the max shear value is catered for.  In this respect, if I set the My value to zero in you data file and set the neutral axis angle to 180  then I get a solution that should be valid for the shear direction specified.

I hope this helps and I'm sorry I can't be of more help.

Kind regards

[justin.conrad]

Hi David,

Thanks for your investigation. I will have to keep this mind for the future.

As for the combined moments, I was testing the file with the three concurrent loads to examine the results. This would explain why fixing the neutral axis provided a result.

You are correct, in a realistic analysis we would not look at the 3 of these loads concurrently for maximum shear.

Regards,

Justin