Sign convention for the internal forces and reaction forces

[thisisapple]

First of all, thank you for the contributor to make such a nice library for structural analysis. I spent a few days on it and enjoy it.

Below is a common sign convention from Page 134, Structural Analysis (8th Edition) by R. C. Hibbeler. You can find the pdf by googling.

I hope there is an API that will allow the user to make adjustments to the signs. The sign convention in the current version of anaStruct may be confusing to understand. Let me use a simply supported beam to illustrate my point. structure-anastruct-simply-supported-beam.zip

Model

Reaction forces

They should be positive. The direction is correct.

Shear diagram

It should be positive on the left side.

Moment diagram

It should be all positive.

[thisisapple]

Correct or preferred shear diagram

Correct or preferred moment diagram

[ritchie46]

There is no universal "correct" diagram. These differ per country. AnaStruct was build using the convention of the Netherlands and (likely other countries). A feature that does localization might be a good addition, but note that there is no good or wrong.

[thisisapple]

Hi, Ritchie Thanks for the clarification.

Reaction forces

I think their signs should be consistent with global coordinate system. It is just very odd that they are NEGATIVE in this simple case.

Internal forces

I would suggest two predefined preferences for the sign conventions. User just picks the one they prefer.

Thanks again for your efforts.

[smith120bh]

Thanks for your thoughts and feedback here, @thisisapple ! As Ritchie noted, this is all conventions that differ between different countries (and sometimes even between different textbooks in the same country). I wish it were more standardised. Your suggestion of adding options to abide by different conventions is a good one. I'm happy to mark this as a feature request.

To your last comment, I did want to specifically note that it is a very common convention for reaction forces to have opposite sign conventions to the loads. That is, so that a positive gravity load equates to a positive reaction. You're right that that switches the global coordinate sign, but it is a common convention that I've seen in the US and Australia, where I've worked as a structural engineer.