VBA functions for determining (and graphing of) moment, shear, and deflection for a simply supported beams +/- cantilevers (Part 3)

Well after saying here that there was limited practical use for point moments for a simply supported beam, so I wasn’t going to (didn’t see a need to) include them. I’ve eaten those words now and added the ability to consider point moments.

Thanks to some work I’ve been doing recently at work on steel design using the alternative design provisions within NZS3404 for combined actions checks, I found a practical use for having applied moments considered in working out some of the parameters used. Yum words.

More on this use case in a later post I guess, but the deflection within a design segment subject to transverse loads is required to evaluate the βM factor using CL8. of NZS3404, which directs you to make βM = βT from CL4. if transverse loads are present, and this requires you to consider the deflection with end moments applied. All very convoluted!

Excerpt from CL of NZS3404

Well to do this properly you obviously need to have the moments at the end of the segment applied like noted above…. Like I said earlier… Yum words…

A formulation for adding point moments anywhere on the main span and cantilever have been added to the beam formulae code module. Download the latest version here on GitHub.

The inputs for the moments follow the exact same logic as for the point loads, entered in a n rows by 2 column range or array for both the main span and the cantilever span, see part 2 for further info on this.

I’ve updated the BEAM_analysis function to have two further inputs (in bold): –

=BEAM_analysis(length_step, L_main, L_cant, E_beam, I_beam, Main_UDL, Main_PT, Main_PTM, Cant_UDL, Cant_PT, Cant_PTM, close_diagram, moment_shear_only)
length_stepThe max distance step along the beam to derive the points at which results are to be returned
L_mainMain span beam length
L_cantCantilever span length
E_beamYoungs modulus for the beam
I_beamMoment of inertia of the beam
Main_UDLCell Range with UDL data for the main span
Main_PTCell Range with point load data for the main span
Main_PTMCell Range with point moment data for the main span
Cant_UDLCell Range with UDL data for the cantilever
Cant_PTCell Range with point load data for the cantilever
Cant_PTMCell Range with point moment data for the cantilever
close_diagramOptional Boolean value to add extra data points to close off moment and shear diagrams to allow for plotting.
moment_shear_onlyOptional Boolean value to select only returning Moment and Shear

Examples with moments

anticlockwise 10kNm moment applied at centre span location
anticlockwise 10kNm moment applied at tip of cantilever

So now you can literally consider any type of loading possible, constant UDL’s, triangular UDL’s, trapezoidal UDL’s, point loads, point moments!

Fixed a small error for cantilever formulation..

I also noticed when doing earlier verification that while the answers for the cantilevers were always slightly different than the computer model used to verify the answers, they were still awfully close. Like noticing the second decimal place was out by some small margin. I put this down to some numerical precision creeping in and thought nothing more of it. But I have now discovered the error of my ways!

The issue was I was working using very small loads in the verification cases, so the deflections were very small. So while they appeared correct to several decimal places, they were in fact out by this very very small margin and it was impossible to notice any deviations between the correct answer and results obtained from the code.

Now scale this up to looking at a larger deflection case and the differences became more obvious in a recent practical case where I was using the spreadsheet. Looking into the code the culprit was I had enclosed some of the rotations in a TAN() function. No idea what I was thinking at the time in doing this, but the end result is this was incorrect and has now been corrected!

So, if you downloaded the previous version, make sure you update your code from the repository, or manually make the same correction noted in this commit.

Leave a Reply