Connection Design: Using Software Tools and Human Imagination to Build Better
Peer Perspective: Jason McCool, P.E., Robbins Engineering Consultants in Little Rock, Arkansas
We spoke with Jason McCool, project engineer at Robbins Engineering Consultants. Read on for his take on connection design, thoughtful use of software programs, and the importance of taking a global perspective to preserve the integrity of creative problem solving in the world of engineering.
Q: Can you tell us a little bit about your current role?
A: I’m a project engineer at Robbins Engineering Consultants in Little Rock, Arkansas, and I’ve worked here for almost 11 years now. I do a mix of delegated connection design for fabricators, often working with my colleagues here at the office on various in-house projects, and sometimes working as an EOR. I’ve also served on the editorial board of Structure Magazine for about three years now.
Q: We’d love to hear about the types of steel projects you typically work on.
A: I mostly do delegated steel connection design or in-house connection design. My projects range from schools to hospitals, industrial facilities, aircraft hangars and office buildings to even a few high-end residential projects. Typically, my work involves hollow structural sections (HSS) or wide-flange columns, usually with wide-flange beams and HSS braces.
The delegated work with different fabricators has led to doing delegated stair design for fabricators, so we often use HSS for the stringers and posts. One of the more exotic stair projects had glass treads, risers and landings. It was bearing on a cantilevered switchback stair of some fairly large HSS sections to handle the deflection requirements for the glass. Another one was an offset mono stringer stair with cantilevering HSS treads. Since you only have one stringer off to the side, the torsional advantages of HSS were absolutely essential to that project working.
Q: Do you have a go-to shear connection that you employ for wide-flange beam to HSS columns?
A: Definitely the shear tab. It’s simple to fabricate and doesn’t require cutting the beams or columns. I can put short-slotted holes in there for beam tolerances and it has a good capacity for gravity connections. It can be skewed if I need it to be, and it still has a decent capacity for beam axial loads if the HSS column walls are sized accordingly. There’s no minimum width for the flat portion of the HSS column like for clip angles and prying action isn’t a concern. The shear tab is contained within the beam web, unlike bearing seat connections, and it’s generally compatible with typical moment connections. It really just seems to be all around the most fabricator- and erector-friendly.
Q: What are some of the challenges you’ve encountered in designing connections?
A: The biggest challenges are probably bolting access with closed sections like HSS, the potential for needing CJP welds, and the problem with single-sided, full-penetration welds on closed sections that require backing, unless the welders have additional certifications for open-root welding. The last one is mainly an issue at splices, but I’ve found the use of a 1” thick plate cut to the interior dimensions of the HSS to be the simplest and most preferable for the fabricators I work with. That lets them get a single-sided full-pen weld with backing instead of difficult and expensive open-root welding.
One of the greatest strengths of HSS is also the cause for one of the biggest challenges of using it. You get very sleek, streamlined structures with HSS. It looks really good, until you get to a connection with a big, obtrusive end plate sticking out on the sides. The sleek, streamlined look of HSS tends to push you toward more minimized, hidden connections or flush connections, which complicates the connection design.
Q: What steel connection design software programs do you personally prefer to use?
A: Most of my connection designs now are more focused on nontypical connections. A lot of times, if fabricators have a very simple job, they don’t contact me about it. They contact me about the ones that nobody else wants to take on. So, I pretty much use finite element analyses on most of my connection designs just because of the atypical joints I’m dealing with.
For the last four years now, I’ve been using Idea Statica Connection out of the Czech Republic. I would say that’s the one I’ve been happiest with, both because of their interface and because they’ve done a really good job of implementing AISC and ACI provisions for column base plates and things like that. They allow some integration with Revit and a lot of common analysis programs. They have some template functionality that allows you to set up your own library of what’s common for you as opposed to a preestablished one. So, if I do a what-if scenario and find a configuration that works really well for something I see a lot of, then I can set that up as a library of common connections and run with that.
Q: Could you share any design tips that you would like to impart on entry-level engineers?
A: First, your global structural model will most likely be trying to minimize member weight without regard to connections. But parameters like HSS wall thickness and ratio of connection width versus HSS width can really govern the member size. So, HSS connections are important to consider early on — not as an afterthought. I see that come up on delegated connection design jobs even from experienced engineers just because it gets delegated and is somebody else’s problem now, and you don’t really see the effect from those choices as readily. For entry-level engineers, I would say just be aware of that. Your structural analysis software might tell you that an HSS 5x5x1/8 is the most efficient column size, and it may be from a global standpoint, but it’s going to cause problems at the HSS connections. So plan member sizes accordingly. I would say to look at the HSS formulas like in AISC Chapter K and see how many times HSS wall thickness gets squared in those different formulas. That really has a big impact. Also, note when different limits start or stop applying.
A second thing would be to be able to do by hand whatever your software is automating so you can develop good judgment of when the software output is correct or when it’s suspicious. The problem in connection design specifically is that connection checks are often complex formulas and complex branching logic. The building codes are constantly changing, and these formulas are being updated. It’s not hard to have a correct analysis and design provision get lost in translation between the development engineers and the programmers who are actually coding the software.
Lastly, I would caution against a black box mindset. That’s an issue for engineers in general, but there is just this idea of, “I put in my problem, and the program magically spits out a solution I can’t explain.” Every program has to make simplifying assumptions at some point, but it’s our name and our “stamp” going on the project, not the programmer’s. So it comes back to us. And connections are a load path bottleneck with a lot of different limit states being checked in a very constrained space compared to the typical framing members. I would say take the time to learn what every input does and what the program assumptions are.
Q: Whether designing steel connections involving HSS or verifying the output, what are some of the resources you lean on?
A: I do have Steel Tube Institute’s full set of HSS design manuals on my shelf as well as electronic copies of the CIDECT’s many volumes, and that’s a European assembly of volumes on HSS design connections and member design, fire design, everything. I do try to think kind of outside the U.S. just because we come up with certain solutions to problems we see. In other countries, a lot of times, they have different mindsets or different availability of parts, so they come up with different solutions. I look at conference proceedings from international conferences and typical construction and fabrication details used in other countries. This helps me think outside the box on my connections. And I found if you’re stumped by a particular problem, a lot of times it helps to redefine the problem. If you keep beating your head against a wall, then you step to the side and avoid the wall entirely.
Q: What updates do you hope to see in the coming years, and how can these help expand the possibilities?
A: Yeah, besides the hopefully improved sharing of design information, it seems like building complexity is only increasing and the associated connections are only getting more difficult, more nontypical, more complex. So, the ability of finite element modeling techniques to accurately represent real-life tests is encouraging, and it’s a powerful tool. But with any powerful tool, it requires care in use. As long as the limitations are understood by the engineer using it, that can open the door for some really creative solutions to new design challenges. Finite element connection design helps get us back to that role of developing creative solutions to problems and not getting stuck in a rut based on what our software tools allow us to do.
To learn more, read STI’s article.
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