Top 5 Reasons Why Your Structural Model is Not Solving

Everyday, SkyCiv receives emails from users requesting assistance solving their structural models. With this information, we've compiled a list of the top reasons why your structural model is not solving, based on the most common problems experienced by engineers like you.

1. The Units are Incorrect

We recently had a structure sent to us that was solving but showed huge deflection; the deflection results were somewhere in the vicinity of 100,000 mm (100m). Clearly something had gone horribly wrong! It wasn't until we opened the project in the 3D renderer that we discovered the issue was that the section was extremely small in comparison to the member length.

How to Check: Open the structure in your 3D Renderer - if you see the members are incredibly thin, then it is probably a unit or section dimension issue.
How to Repair: Check the dimensions of your section in Section Builder and the dimensions/lengths of your members.

2. There are Disconnected Nodes/Members

This one is a little harder to detect, but just as common as an issue with the units. It occurs when a node splits a member without actually being connected. Just remember: a node is only connected if it is at the start or end of a member!

hinge members
How to Check: Visual inspection - make sure all your nodes are either at the start or the end of a member.
How to Repair: Run 'Repair Model' - this will detect any nodes that are not properly connected to your model and repair them for you.

3. The Structure is Not Properly Restrained

This is another difficult one. Basically, in order for your structure to be static, it needs to be adequately restrained, i.e. it can't move or rotate. If you receive an error such as "X translation is not constrained", then chances are this is your issue. Note: if you are having issues understanding what these support codes mean, read our article on fixities.

How to Check: Try solving the structure
How to Repair: Try changing one of your fixed supports to 'FFFFFF' and see if it solves. If it does, you know that you need to increase the restraints of your supports.

4. The Connections are Not Constrained Enough

This is similar to the above in the sense that there are too many degrees of freedom at a node. Often when we are trying to edit a connection (such as modeling a hinge), we adjust the fixities at the ends of the members. This often leads to the joint becoming too released, stopping the support of the loads on the structure. If they become too 'loose', the forces push them to translate or rotate to infinity.

How to Check: Try solving the structure - this will often result in an error. Alternatively, try changing all your connections to 'FFFFFF' and see if the structure solves.
How to Repair: Inspect each joint and ensure they make sense. A member is able to translate/rotate as much as it needs, provided the other side of the member and the rest of the structure is significantly connected and supported.

5. No Section Has Been Added

Surprisingly, this is one of the most common reasons for a failed solve. Every model needs section properties in order to solve. A section gives the member a cross section: necessary information to accurately analyze the structure. Since our software is able to perform hand calculations (without including sections) to analyze our structures, users often believe including every section is unnecessary. Unfortunately this is not the case, as the solvers use complex analysis methods that require section properties.

How to Check: You will automatically be alerted when you attempt to solve.
How to Repair: Simply add a section by clicking "Section" -> Section Builder.

Sam Carigliano, PEng CEO and Co-Founder of SkyCiv
Sam Carigliano, PEng
CEO and Co-Founder of SkyCiv
BEng (Civil), BCom
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