Inconsistent Reaction Values

[Anonymous]

Hi everyone,

I've attached a simple 2D truss with applied loads to it. I calculated the results and obtained the reaction loads for it. However, when I change (for example) all the bottom chord members of the truss to another section, for some reason the horizontal reaction values seem to vary quite significantly (depending on the member size). This is a simple statics problem that should not output different reaction loads for varying member sizes. Can anyone tell me why this is happening? See images below:

[Rafacascudo]

Structure is not isostatic.
If self weight is being generated, then it is correct to get different reactions due to different sections

[Anonymous]

I tried to find out a suggestion but the results on my computer came as you see , for this, ask master Rafal and share us his answer

[Artur.Kosakowski]

If you have a wire between two walls and you hang 10 kg and then 100 kg on it, do you think that the anchors you mounted the wire to these walls will be under the same force which 'wants' to pull them out of the wall ? 🙂

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Rafacascudo,

This is still apparent for the other load cases. For example, if I select the live load case (load case number 3) and run the analysis using the same procedure as in my first post, you'll see different reaction values as shown below. 

The live load case only should not affect the reaction loads in any way even if the member changes. I mean, the horizontal reactions cancel each other out but why the increase in loads when the bottom chord members change? This could cause problems if, say for example, one of the end supports had a roller instead of a pin connection.

[Rafacascudo · Accepted solution]

  As I said before , the structure is not isostatic , so the stiffness of the members  matters.

Your assumption is correct if one of the supports was a roller. But it is not.

In an isostatic truss ( pin-roller), if you change the botton chord to a section that has less area (axial stiffness) , then the horizontal displacement on the roller node will be greater compared to the model with the larger section.

But , your model has 2 pinned supports . So that horizontal displacement is restrained. And if your support has to restrain a larger displacement than the one of your 1st model , than you should expect larger horizontal reactions for this 2nd model with "weaker " botton chord sections.

Hope it is clear.

[Anonymous]

That would seem to make sense to me. Thanks!