Introduction to HSS Connections (Hollow Structural Sections)
Hi, I’m Brad Fletcher. I’m structural engineer with Atlas Tube, and today I’m continuing on with our video series on designing with hallow structural sections. And today we’re going to talk about connections. But first let’s kind of review and talk about what hollow structural settings are.
Now, tube-like structures are a naturally occurring size and shape. Between bones of animals, or stems of flowers, or bamboo, you can see that nature has come up with really a perfect size and shape for carrying structural loads.
Now, what we’re talking about today, is we’re talking about the steel sections, a cold-form welded tube, which I use for bolted and welding construction. And these come in square, round, and rectangular sizes. Now, why are HSS used? Well the main reason is for aesthetics. They’re nice to look at. As you can see there are many nice structures that have been built out of tubes and hollow structural sections, and that’s the main reason.
But there are other reasons, such as they’re economical. They are efficient at resisting torsion. They have great compressive strength. They have less surface area and can have less weight in certain applications. They also come in a wide range of sizes. The Atlas tube size range is from one inch up to 22 inch, and we can go up to 7/8-inch wall. They’re also, as I mentioned, they come in square, rectangular, and round sizes, but they do even also come in elliptical sections.
Now, other advantages that HSS have is that they’re easy to handle. A lot of fabricators like to have them in their shop because they lay down flat, they’re easy to jig. They also have lower finishing costs because of the lower surface area, fireproofing, and painting cost can be lower. They also have excellent torsion resistance. The torsional constant of a closed section is 200 times greater than that of an open section, which gives its torsional strength.
The RX and RY factor: significantly higher than an open section, and [therefore] gives you a great unbraced length. Also, robust structures and structures that require some blast resistance. If you use square and round HSS, because of their ability to resist loads in any direction, those non- directional loads such seismic or blast, are easy to resist.
Now, where HSS are used are mainly in columns or bracing. We also see em’ in trusses. But there’s horizontal applications such as curved beams where there’s a lot of torsion. Exposed structures as well as robust structures.
So now, let’s talk a little bit about connections. Well, the challenges we face with connections when it comes to hollow, structural sections is, you know, people say, “Tthey’re too hard. They’re too expensive, maybe there’s not enough resources out there, I don’t know where to go to to learn about hollow structural sections connections.” And a lot of time the engineer record doesn’t actually do the design for the connection. So therefore they don’t give much thought to the connections during the design process.
Talking about the different types of connection that we’re going to be talking about in this video series, tension and compression, these are your bracing connections for diagonal bracing. For braced frames. Or your splices when you’re splicing two hollow sections together.
The next type are connections that involve line-loads or concentrated loads. These are your sure connections or your connections that take in account for the line-loads, directly connected to hollow structure columns.
Mobile connections? These are probably some of the more challenging connections. Here we’re talking about wide flames to HSS columns. And there’s different types of connections that can be used for this, and we’re going to explore those in this video series as well.
Trust connections, or HSS truss connections. These are generally when, when you design the truss to have tension and compression in your web members? This is the next type of welded connections that we’re going to be talking about.
And then there’s HSS to moment connections. These are areas where there isn’t a lot of research done but that is changing: there is research that’s being done directly welded HSS to HSS connections as well as mechanically fastened connections. Now, let’s talk a little bit about the resources. Up until about 10, 15 years ago, there really weren’t a lot of resources for us to dig into as engineers.
But in 2005 and 2010, AIC-360, the Chapter K came into effect. So Chapter K Now, is a great resource for engineers when it comes to HSS connections. Design guide 24 from AIC is based on Chapter K. It’s an excellent resource in understanding how Chapter K works and the requirements behind it. The CIC, the Canadian Institute of Steel Construction, published a design guide in 1997, which was written by Professor Jeff Packer.
And that’s an excellent resource in HSS connections as well. It’s a little out dated as far as some of the specifications out there right Now, but still is an excellent engineering guide for connections. Now, the Side Deck Design Guides, these green books here which are available off of AIC website, they’re an excellent resource, but they’re very Eurocentric. They’re based on the Eurocodes in Europe.
And but they’re still a good engineering resource, sometimes you have to dig through the differences between the Eurcode and the American and the Canadian codes, but they’re based on some excellent information. And then of course there’s the Atlas website. I encourage you to visit our website on a regular basis as I continue to update it and have it become a good resource for you and a connection in a hollow, structural section of the world.
So let’s talk a little bit about where these resources get their information. Well, there’s the international institute of welding, the IIW, which has been providing recommendations for the design of static strength of tubular joints for a long time. All the way back to 1981 was when they put their first edition out. And they’ve since put out their editions in 89′ and 2009.
Now, those recommendations were taken on by Sydeck, which is an international committee for the development and study of tubular structures. It’s a collection of manufacturers of HSS and Pipe, mainly based in Europe, but Atlas Tube is the North American representative on that committee. And they have, as I said, those green books we talked about. They are a great resource for connection design and the first set of design guides was based on the requirements of 1989.
They are now going through the process of updating it for their second edition for the requirements that came out in 2009.
Now, why this is important, if you look at all the design codes out there that most engineers are going to run into, whether it be the Eurocode, or the AIC code, or the CISC code, as well as the static design guides. They’re all based on the recommendation of the IIW 1989. And while there’s always going to be differences between Eurocode and American codes and Canadian codes based on regional influences or format issues.
At the end of the day, the designs you get from these different codes are all going to be very comparable. So in some rate, why understanding HSS connections is important is because the strength of HSS, the local strength of HSS has a very integral part into the connection design. And what I mean by local strength is your side wall yielding, your chord-wall classification, the flexibility of the face of the HSS when it comes to connections.
Those are all limit states that you need to understand during your member selection. And what I mean by that is the engineer of record needs to be thinking about these things when they’re sizing their members, for their main members before they even get to their connections. Because if they don’t think about these things and they always choose the most efficient section, whether it be the lightest, least-weight sections, you may run into problems with your connections down the road.
Your fabricator, who’s doing the design for the connections, may have to end up reinforcing connections, and reinforcing is generally not an acceptable option, whether it be the fact that they’re difficult and expensive, but also aesthetically, your architect doesn’t want a bunch of reinforcing plates all over his tube.
So now, even though EOR may not be doing the final connection, it’s important that they do some checks throughout the process so that they choose the right member designs. And then once again, it’s important to pass along good information for your connections to the party who is responsible for the connections, whether it be your fabricator or your third party engineer.
Good load information such as the proper loads, the combinations that have to be used as well as transfer forces, these are all really important issues. So that’s a quick overview on what we’re going to be talking about during our video series on connections. Appreciate you watching. If you have any questions, please feel free to contact me.