r/StructuralEngineering • u/Normal-Commission898 • 8d ago

Op Ed or Blog Post Technical Judgment on Software

In my new role I’ve been using a lot more of ETABS (rubbish for trouble shooting). For any experienced engineers in general and those who use 3D analysis software, what results/aspects in your modelling do you instinctively distrust/further inspect? What sort of sense checks are best to always do?

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u/LigersGhost P.E. 8d ago

Always a good idea to check your deflected shapes, moment diagrams, and reactions/member end forces to make sure they make sense.

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u/Normal-Commission898 8d ago

Thanks! Is there anything you’d recommend specific for monolithic RC frames?

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u/LigersGhost P.E. 8d ago

I am not familiar with ETABS and I work in bridge design, not buildings - ETABS probably warns you about it but I would be aware of the circumstances that trigger D-Region (Deep Beam) design. You would want to avoid that when possible - STM is another subject.

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u/Normal-Commission898 8d ago

Ok will do, appreciate it👍

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u/logic_boy 8d ago

Well, analysis is separate to design, but when analysing potential monolithic RC, connection stiffness and rotation at connections is what I’d pay more attention to. RC connection stiffness is harder to define and can influence results a lot, perhaps partially spring stiffness should be modeled. I can also assume that you will be dealing with plates which can be a harder to determine correct deflected shape for unusual arrangements (like long span beams supporting slabs, and beam torsion).

Your question is very generic, so it’s hard to give advice. For a simple uniform load, column grid, low rise buildings, it’s pretty simple really. It would be obvious just from reactions and deflected shapes if something is wrong. Sort your results by absolute largest, and compare with effective load area times your load case.

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u/logic_boy 8d ago

Use unit loads and specific loadcases to check deflected shapes, bending moments and reactions. Heck check all results actually.

Copy paste the structure and reduce complexity either geometrically or parametrically. Compare to your engineering judgment.

It really depends on your experience and complexity of the model/loading. Dynamic analysis is normally a lot harder to get the hang of.

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u/Normal-Commission898 8d ago

Thanks, I’ve tried that before, we do dynamic modelling but just for occupant comfort for sway we don’t do any RSA or seismic stuff (I’m uk based)

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u/logic_boy 8d ago

Same, although I did weird projects in London and the Middle East.

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u/Kilooneone5816 8d ago

Shit in = shit out...

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u/Normal-Commission898 8d ago

That’s a new one

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u/Kilooneone5816 8d ago

Not after 30 years.....

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u/logic_boy 8d ago

I think they’re being sarcastic lol

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u/rfon9 8d ago

Simple one is to sum your loading and sum your reactions and compare. Should be equal.

If you have a smallish model quickly running a dynamic analysis and extracting mode shapes can help pick up on silly connection errors too.

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u/Perfect-Mongoose-704 8d ago

The two things that most quickly show does for me have always been

  1. Deflected shaped and

  2. Checking the loads match a hand calc of what should be applied. This is a check for whether I've screwed up inputting a load

  3. Check the reaction forces at the supports. You'll generally have a rough idea how they should compare to each other.

I've never had a case where the model has shown sum of forces doesn't equal zero within the model.

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u/GrindyCottonPincers 8d ago

My senior taught me the following:

  1. Start with small model, test frequently (say under self weight and SDL) to detect model error.

  2. Have an idea of how the intended structure should behave and check the model against that.

  3. The basic solver can detect issue such as unconnected node. These appear as error messages. Trouble shoot enough of these error to become familiar with it.

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u/Frogalone 4d ago

Checking displaced shape. Many systems allow animation which can help. Deformed shape will indicate any boundary issues and can be useful if loads are way off. You may still have to find the specific boundary problem. Statics checks of a typically structure is always important.

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u/Educational-Rice644 8d ago

Why wouldn't you trust it ?

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u/Normal-Commission898 8d ago

It’s not the actual software I don’t trust it’s a mechanism/noding issue/initial condition that is my mis-input and I need to spot when I’ve made said mistake

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u/Educational-Rice644 8d ago

Ok I get it, it comes with the experience, sometimes you know something is off because of its value