Truss behaviour, load-deformation curve

[SofiaB]

TL;DR Summary

Hello everyone! I am just trying to verify if the results I obtained for my truss behaviour analysis are correct

Hello everyone!
I am analysing an 18 m per 1.2 m truss, simply supported, with 140x5 chords and 90x8 braces. I then loaded the superior nodes with 500 KN. The top nodes were also laterally constrained to prevent out-of-plane displacements.
After imputing the structure in Abaqus (FEA software), I obtained the load-displacement curves in the top nodes where the concentrated loads were applied. I should note that I am doing a nonlinear analysis with a nonlinear material(S355).
What I found weird was the abrupt drop in the curve after yielding, as well as the fact that the value of the yielding strength seems very small. Can someone help me understand if this behaviour is correct? I will leave some pictures of the truss, the material input and the load-displacement curves for each node for better understatement.

Should also note that ultimately what I am studying is the joints behaviour (in a further analysis I will increase their rigidity) and thus I want to explore the behaviour above the yield limit.

Thanks in advance!

 

[jrmichler]

I like to look at a deformed geometry plot on all of my FEA analyses. Possible failure modes of a truss include plastic buckling of a compression member, elastic (Euler) buckling of a compression member, plastic yielding of a tension member, lateral buckling of the top member (which you restrained), and joint failure. The deformed geometry plot shows this, and is also useful as a check on your restraints and loads. And on how well you model joint rigidity, when you get that far.

It's also a good idea to do a hand calculation for the truss element that yielded or buckled.

 

[SofiaB]

I like to look at a deformed geometry plot on all of my FEA analyses. Possible failure modes of a truss include plastic buckling of a compression member, elastic (Euler) buckling of a compression member, plastic yielding of a tension member, lateral buckling of the top member (which you restrained), and joint failure. The deformed geometry plot shows this, and is also useful as a check on your restraints and loads. And on how well you model joint rigidity, when you get that far.

It's also a good idea to do a hand calculation for the truss element that yielded or buckled.

There were no signs of buckling as well as joint failure which I also found a little weird. I will leave some pictures of the deformed configuration and von mises stress values.

 

[jrmichler]

The deformed geometry plot needs to be scaled such that the peak deformation is larger and more visible. Typical FEA defaults make the peak deformation about 10% of the truss depth. You might need to put a scale factor in there.

Are the end supports simple supports, or rigid? It makes a difference. One end support should restrain in both horizontal and vertical directions, the other should restrain in the vertical direction only. Both should restrain in the out of page direction.

This is a simple truss to analyze by hand. What is the hand calculated stress in the top and bottom members near the middle of the truss and away from the joints? How does that compare to the FEA values?