topic Re: Shear design - uls persistent / transient graph in Structural Bridge Design Forum

Shear design - uls persistent / transient graph

Jan-999 — Fri, 27 Jan 2023 17:08:25 GMT

This is the pre-tensioned beam analysis carried out in ASBD2020. I have got a weird-looking shear graph with a peak in the middle of the beam.

I had a look into the calculations and it seems that this is caused by a cracked/uncracked section. However, this appears to be odd. I would expect to receive something similar to this:

(Ref. EU Examples Manual)

I think that this might be caused by some sort of error in the software.

Re: Shear design - uls persistent / transient graph

dave_geeves — Sun, 29 Jan 2023 17:06:04 GMT

Hi,

It is difficult for me to comment on this as although I can see the design shear in your diagram I would need to know the associated design moment, which would govern whether the section was cracked or not. Could you please "zip up" your data file and attach it to a reply to this post and I will take a look and try to explain what is happening. Thanks

Kind regards

Dave Geeves

Re: Shear design - uls persistent / transient graph

Jan-999 — Mon, 30 Jan 2023 09:15:35 GMT

Hi,

Thank you for your reply. Please find attached the model.

The beam with this diagram is MGI2.

Re: Shear design - uls persistent / transient graph

dave_geeves — Mon, 30 Jan 2023 15:14:20 GMT

Hi, thanks for attaching your model file.

I examined the shear and associated moment diagrams from the beam load tables and it can be seen that the associated moment for max shear reduces at mid span

For the purpose of the shear calculation the equation to use depends on whether the section is cracked or uncracked. As you noticed in the results, the reduced moment means that the section is uncracked for a certain length near mid span and towards the supports but is cracked in lengths either side of the central uncracked region - thus reducing the shear capacity.

To get a better understanding of the length involved I added a number of "points of interest" in the mid span region as the calculations are only done at these POI. You can see below that the uncracked region is not just at a single point.

the additional points of interest can be added to the table using the "+" button or you can simply click on the graphics screen where you want them.

So, in summary , I believe the program does not have a defect here but is calculating the shear capacity in accordance with the design code.

I hope this helps and had answered your question.

Kind regards

Dave Geeves

Re: Shear design - uls persistent / transient graph

Jan-999 — Mon, 30 Jan 2023 15:18:11 GMT

Thank you, Dave. I have noticed the change between cracked/uncracked sections. Thank you for the clarification and for showing me how to add POIs.