Solved: Re: Beam bending using finite differences

SM_6465178 · ‎Jun 29, 2023

Hi All,

We have been using this worksheet for a while which in theory allow us to look at different restrain configurations. Its always been a bit flakey, but converting to Prime seems to have finally killed it.

I just wonder if anyone has some advice on how to get it to work?

Werner_E · ‎Jun 29, 2023

I stopped at the first error thrown

The error about unit mismatch is thrown because you can't add a dimensionless quantity (C) and a quantity with unit N*m/s^2 (P).

Prime isn't using SUC (static unit check) anymore and so its necessary to apply a unit even if the value is zero!

But even if we fix this, we get a similar error, This time because Load is a vector in N, but add(P) results in a vector in N*m/s^2.

Haven't look further as this sure has to be fixed first.

It may have worked in Mathcad because all involved values seem to be zero anyway. But you sure can't and shouldn't add quantities of different dimensions!

View solution in original post

Werner_E · ‎Jun 29, 2023

I stopped at the first error thrown

The error about unit mismatch is thrown because you can't add a dimensionless quantity (C) and a quantity with unit N*m/s^2 (P).

Prime isn't using SUC (static unit check) anymore and so its necessary to apply a unit even if the value is zero!

But even if we fix this, we get a similar error, This time because Load is a vector in N, but add(P) results in a vector in N*m/s^2.

Haven't look further as this sure has to be fixed first.

It may have worked in Mathcad because all involved values seem to be zero anyway. But you sure can't and shouldn't add quantities of different dimensions!

terryhendicott · ‎Jun 29, 2023

Hi,

Here is a fully working sheet with all errors gone.

SM_6465178 · ‎Jun 29, 2023

Thank you guys. It was doing my head in that it was working in MathCAD 15 last week, but now I want to use it in Prime it wasn't,

What a fantastic community this is!

BillWadsworth · ‎Jul 12, 2023

From the snippet of code I recognised this as mine. (I'm sorry it has proved to be 'flaky', it works OK for me in 'proper' mathcad.)

Of course the Load vector add(P) vector need to be in the same units, which are kN.

Thought you might like the latest version, I discovered that it only took some small additions

to allow the distributed loads and suppots to be trapezoidal, not just uniformly distributed.

ttokoro · ‎Jul 12, 2023

Add 0 right units.

SM_6465178 · ‎Jul 12, 2023

Hi Bill,

It has proved to be a worthwhile thing to have, and its has been used a few times when we have had multiple reactions with reasonable success. Its been around for a while, and I imagine there are a number of people at work who have "bashed" it around to suit what they wanted, and unfortunately its always the bashed version that continues on and becomes more and more unreliable (in terms of getting it to calculate a result) as more and more people introduce junk.

But thank you so much for your original work, and for the update.