Design Beams Module-RC BEAM

[maj.rentung]

Hi,

Im working on different models for bridge pier. 3D model has been carried out externally to analyze the load effect on cantilever crosshead and the FINAL TOTAL ULS & SLS results then used in separated 3D model for design purposed using Design Beams Module for RC Beam design. Usually I use Design Sections module for section design, but for this project assignment Im figuring to try out using Design Beams Module. I manage to run the bending check for total ULS load combination (Persistent/Transient) under "Construction Stage" features but fail to run the shear design check. I also create another file for SLS Quasi-Permanent which include the total SLS (Live Load+Permanent Load) and Permanent Load only for crack width check. But the program also cannot run the crack width check. Attached the ASBD file for your reference and your assistance is much appreciated.

[dave_geeves]

Hi,

It is great to see different uses of the software and yes this should be perfectly applicable. It is not necessary to have two models for ULS and SLS as each design beam loading table has columns for both ULS and SLS for each loading type.

The problem you are having is that for ULS shear beam analysis you need to have a live load case as well as construction stages.  The same applies for SLS crack width calculations (ie you need a live load table with SLS values).  To get round this you need to create a dummy live load case (eg Traffic gr1b-gr5) with almost zero load effects.  To do this I created the table for this case and used the generate button to define a UDL of 0.001kN/m.

I can then analyse for this case and the construction load table values will also be added to give the total design effect value.

Give this a try and let me know how you get on.

Kind regards

Dave Geeves

[maj.rentung]

Hi Dave,

I manage to give a try as per your recommendations. However the results from the Design Beams module seems doesn't make sense  as it stated inadequate for design shear and bigger crack for crack width check. There is also no option to pick the outermost bar for the crack width check. I also noted that the live load will not be added to the total combination although the design values for the live load included.

I then converted the Design Beam Module to Design Sections in the same file, it still stated the same for shear design and the crack width now is much lesser because there is option to pick the outermost bar near to to the surface.

I also reproduce a separate Design Sections file for the similar section for comparison, and now the shear is adequate.

Im wondering why there is significant different in shear results for similar section as compared above.

Attached both files for your review and your assistance is much appreciated.

[dave_geeves]

Hi,

There seems to be a defect in the shear calculation of  Vrd,max as it is finding that it is zero.  That is why it is failing

There is not a lot I can recommend for this and hopefully the developers will see this post and comment accordingly.  I have tried changing various parameters but it makes no difference.

Sorry I can't be of more help

Kind regards

Dave Geeves

[dave_geeves]

Hi,

The problem of the max shear resistance  being zero was worrying me so I have spent quite a bit of time investigating this.  I first built my own design beam in a different file from scratch using your dimensions, materials and reinforcement locations.  When I did this I didn't get the same problem and I got a max shear resistance of 16992kN.  This of course is still failing but at least it wasn't zero.

After a lot of changing of design parameters I eventually found the cause of the zero value of VRd_max was due to a nationally determined parameter being changed in your file such that Cmax,shear was set to 0.0 rather than the default of 50.  I'm not sure how this can be as it is impossible to actually set this to zero in the form.  If I changed this to the default then the value of VRc_max was correctly calculated.

I then checked out the difference between the design beam calculations and the Design section calculations to see why it was failing in the beam but not in the section.

I found that the difference was in the value of "z" being used where there is a note marked at the bottom of the calculations.  In the section calculations it stipulates that it is calculated using the design moment and dividing by the tensile force whereas in the beam module it is calculated using the ultimate resistance moment and dividing by the tensile force.  I don't know why the difference but it says that the beam is the more conservative approach.

This can be checked in the section design by setting the capacity field in the analysis form to "My bending" rather than it not being used and the value of  VRd_max is than the same as that in the design beam.

I'm not sure what is the correct method for you but you do have the option to use either for the shear calculations.

With regard to the crack width calculations,  I agree that the ability to select bars ( or at least to set the surfaces checked should be available here to get more realistic values for heavily reinforced sections.  I should point out that for these calculations that the pseudu permanent load case should be used here.  Unfortunately your construction loading includes the live loads so the values you will get will not necessarily be correct in these calculations.

I hope this has helped.

Kind regards

Dave Geeves