Idea Statica Connection Design Review 3

[bookowski]

I'm looking for practical advice on reviewing connection designs performed in statica.

I have a lot of steel connections that I need to review and sign off on as the EoR that are being performed in statica. This is not a typical delegated design situation where the designer is providing signed/sealed calcs, I am ultimately responsible for these (and I can't change this so not looking for alternative workflows on this one). The connections are mostly not something that you could pull out of an aisc table, but they are mostly ones that could be designed by traditional aisc methods (a lot of bracing connections, some end plate M conns, a lot axial drag forces, etc). I know the guys designing these and I'm not worried about complete incompetence they're quite sharp in general. I also can't review the fem results for hundreds of these so looking for some reasonable way to cover this. I plan on having them do aisc checks for a handful as sort of a benchmark to make sure we understand how statica works and compares to traditional methods, but beyond that I need to review the rest with some confidence but also not make it a phd project.

For anyone that is using Statica are there any particular things that I should look out for - any common mistakes that are made, settings that need to be checked, any conneciton types that tend to differ substantially in results vs aisc, etc?

 

[CANPRO]

I'm a semi-regular user of Idea Statica.

My first suggestion is to review some of their verification documentation. They have a lot of worked examples comparing their results to traditional hand calcs.

Major thing to look for is how they have defined the eccentricities of the various members being supported. Much like doing hand calculations for connection design, if you neglect or improperly define the eccentricity, your design could be significantly deficient. Member forces can be applied "at the node", "at the bolts", or a "user-defined" position. In the simplest example, if you're designing a simple shear connection and the load is applied "at the node" the bolts in the connection resist rotation but if the load is applied "at the bolts" the bolts see direct shear only and the supporting member resists the eccentricity.

Ensure equilibrium is achieved in the load combinations. If the load inputs don't balance the program will do it for you and it may not do it in a way that reflects reality. There are (2) options in the program for load input, one that displays unbalanced loads and one that does not. If you do not require equilibrium, I believe the program provides equilibrium by additional forces applied to the supporting member. I make a point of always using the "equilibrium" option...that reflects reality and it provides a good opportunity for me to double check my load inputs make sense. This is particularly important for bracing, truss nodes, and other multi-member nodes.

If the program is being used to check buckling there is some engineering judgement required. An acceptable buckling factor is different depending on the element under consideration. If the buckled element impacts member stability a higher buckling factor is required. Idea Statica has some documentation on this as well.

My first step in verifying the accuracy of the results is taking a close look at the deflected shape. Improperly defined loads/eccentricities or modeling errors will often reveal themselves in the exaggerated deflected shape. It is fairly easy to add images to the reports. If the reports you're reviewing don't include deflected shapes of the connections it would be reasonable to ask for the report to be re-submitted with additional images of the deflected shapes. It would be fairly easy to comply with this.

In the reporting options, there are different levels of detail. The basic summary doesn't provide much. The most detailed version will provide code equations for checks on the welds and bolts. If you don't see that in the report you have, ask for the detailed version.

I believe they still have a fully functioning demo version. If you have a good working relationship with connection designers maybe you can ask for a copy of a couple of their models so you can review yourself and a get a feel for what was done.

 

[human909]

I've picked up Ideastatica in the last couple of months. I cannot speak more highly of it. It does it's job amazingly well. Though like any piece of software it is rubbish in rubbish out and can give you false confidence if you put rubbish in.

For anyone that is using Statica are there any particular things that I should look out for - any common mistakes that are made, settings that need to be checked, any conneciton types that tend to differ substantially in results vs aisc, etc?

CANPRO has already nailed the importance of the location of the load. But here is a video showing that and more:

IDEA StatiCa The most common mistakes in connection design and how to avoid them

In addition to deflections that CANPRO suggested I'd always look at the the bending moment diagram is explained in the video. It is another way to see the location of the applied load and the behaviour.

One more thing. Watch out for eccentric compression struts if you do those sort of connections. This is something I tested out before and found that IDEA Statica was unconservative. I believe this was more my error rather than IDEA Statica's. I'm talking about these:

https://scnz.org/wp-content/uploads/2020/12/CON1302.pdf

 

[bookowski]

Thanks to both of you. They will have no problem sending me the idea files so I'll get a free trial and see what I can figure out.

@human909 - Can you clarify what you mean about it being unconservative for compression bracing with the eccentric end connection?

One issue/question that came up was a check for an hss to hss welded moment connection. I asked them to do an aisc check on that one and it did not work per chapter k. In that case it didn't seem like anything incorrect in statica, rather that it was a more realistic analysis of the particulars for that connection. I can foresee this happening, getting better results and having to decide if they're acceptable.

 

[human909]

@human909 - Can you clarify what you mean about it being unconservative for compression bracing with the eccentric end connection?

As described in the pdf linked. You really need to read the first couple of pages of that or THIS to understand the background and the issue.

I use these connections extensively and am aware of the eccentricity issue so testing this was part of me running IDEASTATICA through its paces. It didn't pass, but I'm currently putting that down to user error[until I have the chance for further testing. The software seems robust and reliable, but it is only as good as the operator.

To model it properly you probably need to remove lateral restraint from the compression member and manually change the location of the load application. That is the good thing about guides, they hopefully should capture ALL failure modes. Modelling software like IDEASTATICA can readily not capture some failure modes unless you specifically go hunting for them. EG try to get this compression failure mode happening IDEA:

One issue/question that came up was a check for an hss to hss welded moment connection. I asked them to do an aisc check on that one and it did not work per chapter k. In that case it didn't seem like anything incorrect in statica, rather that it was a more realistic analysis of the particulars for that connection. I can foresee this happening, getting better results and having to decide if they're acceptable.

Yes, in many cases that is to be expected. Predefined checks often have conservative assumptions all through them in order for them to be easier to perform and widely applicable. That is the beauty of IDEASTATIC.

I've been doing some extensive iterations on hss welded and bolted moment connections. While I haven't compared it directly to other hand calc guides it seems to perform exactly as I'd expect and I regularly get failures being flagged exactly where I'd expect. You do need to be sensible though. IDEA statica can readily put welds in places that are impossible or challenging. Eg Fillet welds inside HSS, or welds on large radiuses.

 

[human909]

My first suggestion is to review some of their verification documentation.

Thanks CANPRO for that fantastic resource. I've read some of the verifications, but never browsed the big set.

And it turns out that my 'test' earlier has already been verified by IDEASTATIC. And no surprise IDEASTATIC works, like I expected user error. Me. But it does highlight, like most software tools, that you need to be very diligent in the your configurations, assumptions and boundary conditions.

https://assets-us-01.kc-usercontent...te_connections_in_IDEA_StatiCa_Connection.pdf

Though in this case of the verification approach they used the eccentricity force has to be added manually which is not at all intuitive for the user of the software. I'd have expected that the IDEASTATICA would have incorporated that. Evidently it doesn't regarding bolt/plate interface eccentricity.

Eccentric gusset plates are the norm around here. Yet IDEASTATICA suggests "Eccentric gusset plate connection should be
designed only if completely necessary due to erection." I'd argue that alternatives are generally cost prohibitive in many cases. The eccentricities can be accommodated with thick plates.

 

[271828]

But it does highlight, like most software tools, that you need to be very diligent in the your configurations, assumptions and boundary conditions.

Bravo for figuring out how to make the program do what you want it to do.

There's no stopping progress, but IDEASTATICA is the scariest program I've seen, perhaps ever. The marketing "looks like" the program automatically generates and designs practically any connection. There are already a lot of unqualified engineer-like people punching buttons out there. Now it seems like they could nintendo design any connection and generate the best eyewash work products; that's bound to be attractive to that crowd.

I guess this isn't new. Frame analysis has been like that for this entire century.

 

[human909]

The same comments have been said about excel spreadsheets or even calculators.

 

[271828]

Vaguely perhaps. Neither of those is a black box that will design the connection automatically.

I should've kept quiet considering I haven't used IDEASTATICA. I'm just going by the marketing material.

 

[human909]

Being able to see deflections, stresses and strains make is FAR less of a black box than most design guide formulas.

I'd say that many design guides for connections and even formulas out of structural codes are often black boxes. Sure if you dig hard enough you can might be able to find a fair bit about the theoretical and empirical under-pinning. But most engineers don't do that for every single formula.

When I design monorail beams this is the formula I use from the code based extensively off empirical testing apparently:

Oh and it doesn't really design the connection for you. You the user needs to do that. It does CHECK the design against the loads you input.

For standard connections it is slower than other approaches whether you have a ready made spreadsheet for a moment end plate or a structural software that designs and checks the connection to a design guide.

 

[Sparweb]

human909,
Re: your posts from March 2.
I am struggling to imagine a more inefficient connection. The ligament connects structures constrained in multiple degrees of freedom with a member that is constrained in only one. Why bother? The frame joint may lack stiffness but the member supports it in only one plane of rotation, and in only in tension, not compression, at that. Even in tension the ligament seems barely any stiffer than the beam's joint to the column.

Is it safe to say that this works because there's a mirror image on the other wall? So that even if these ligaments are ineffective in compression, for any shear +X or -X direction there's at least one ligament in tension to provide some support?

Why isn't a gusset used for stabilizing this roof-to-wall joint? No welding, just bolting, and easier to make much stiffer. Heavier, but I don't think you steelwork guys care much about that! Actually maybe not heavier because a gusset is effective in both opening and closing the frame joint, meaning both gussets on two corners are equally effective, beating out the ligaments that are effective only one direction at a time. It might even waste less space in the enclosure.

(bookowski, let me know if you feel like this hijacks your thread.)

 

[human909]

human909,
Re: your posts from March 2.
I am struggling to imagine a more inefficient connection.

I agree with all of what you are saying but the picture was supposed to demonstrate eccentric HSS cleat connections.

I wasn't meaning to have any comment on portal frame knee braces. Which I have never used, have rarely seen use (except in a negligent engineering project I'm currently reviewing as a third party)**. I have often used them on small platforms that have minimal lateral loads and need some bracing, but I'd normally choose angle for that.

**A 20m span portal frame industrial shed with C30024 columns and beams. Those are cold formed members that weigh 10kg/m (6.7pound/foot). It is an impressive piece of engineering if you live in a place where wind and snow doesn't exist.

 

[Sparweb]

Like this one?

 

[Sparweb]

OK I get your point. Sorry for the distraction!

 

[rowingengineer]

In cold formed portal frames knee braces are very common. The connection design is made easier with knee braces turning it into tension, compression and shear design, as moment connections are more difficult in cold formed.