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HSS Connections: The Top Things You Should Know Part 1
Part I of the top things you should know about HSS connections.
Now we’re going to talk about HSS connections. We’re going to focus on ten things you should know about HSS connections. We’re going to talk about a couple different subjects, more than ten, less than ten, however you look at it; but we’re basically going to be talking about some things that I think are good food for thought that I think you should know about HSS connections as we move forward.
When it comes to connections it’s a bit of mystery for HSS. A lot of times we don’t know where the resources are. We don’t know what’s available to us. It really wasn’t until 2005 that there was really something in the code that specifically addressed HSS connections, but even with that out there, there are a lot of questions. So what I decided to do today was to kind of talk about, what I say, I’m saying “ten things you should know about HSS connections.” This is not how to do connections kind of talk, but I just want to highlight some things that maybe you hadn’t thought about before, maybe you’re not aware of before in some of the more common type connections.
So we’re going to start off with something that probably doesn’t sound too much like connections, but I’m going to ask the question, why .9-3? 9-3 has to do with the factor that is defined in AISC as what you need to apply to the nominal wall thickness to come up with what we call the design wall thickness.
If you think about the design tables that are in current AISC specification, or the current manual I should say, you’ll see there’s a column for T and a column for T-des, T design; and you’ll see for a half inch wall T-des is .465 and that ratio is .93. So a lot of people know that. They know that the properties associated with HSS are reduced and they’re all there in tabular form and no one has to think too hard about it, but let’s kind of investigate about where this came from and why.
So if we look at section B4-2 of AISC 360 the current wording, and this wording has changed in each one of the manuals over the past few years, so the current wording is, “design wall thickness T shall be used in calculations involving the wall thickness of [inaudible 02:06] wall sections.” So that tells us we need to use T-des.
“The design wall thickness T shall be taken equal .93 times the nominal wall thickness for electric resistance wall that HSS and equal to the nominal thickness for submerged arc welded HSS,” and that’s basically everything that’s made to A500 to A53.
Now SAW HSS is probably, maybe, a new term for some of you. That is typically sizes that are outside the range of A500. We talked early about A500 having a max size of 64 inch, historically, but it has been expanded to 88 inch. With that expansion of 88 inch we’ve kind of slopped over little bit, the size range from between 64 and 88 inches there are ERW HSS but there’s also SAW HSS in that size range as well.
Historically, there has not been a specification behind SAW HSS, but back about maybe five, six years ago they did produce a specification called 10- 65 which now governs what we call SAW HSS. So why is this in the code?
Well, it has to do with tolerances associated with the specifications. So 8500 has tolerances of wall thickness of plus or minus ten percent. On a couple of projects that we’re using, both a Canadian product and a US product, they found that the same size had different properties because the specifications were actually different.
CSA spec actually has tighter tolerance than 8500, and so people kind of freaked out when they realized that you’re [inaudible 03:45] 8500 product, you’re actually getting less than what you realize. That’s when this .93 factor started to creep into the specification.
o that’s kind of where this all came from and kind of why it exists this way. What does this have to do with connections? Good question.
If you go Chapter K, the new Chapter K within the AISC 360, and 1.1 they define the T that’s used in Chapter K as design wall thickness. If you’re not using the T-des, the reduced number, you’re actually overestimating your connection capacity. You’re being unconservative. That’s something just to be aware of that, I bring this up because if you’re using 8500 product and you’re designing connections you got to make sure you use T- des, T design, make sure that .93 gets in there. All right, next subject.
Welded plate versus slotted through plate. Talked about fabricators, talked about engineers, everyone has different opinions about this subject. Fabricators tend to not to like the slotted through plates, and really, this is not a sheer connection; I want to make sure we’re clear.
This is not a slotted plate for sheer this is more for having to transfer axial loads. If you’re talking about a branch member for a truss, you’re using a slotted plate type connection, or if you’ve got a column that’s got a diagonal brace coming to it those are the types of connections we’re talking about here in this case.
So a lot of fabricators say, “Why can’t we just weld a plate to the face of a HSS?” and a lot of engineers that I’ve talked with say, “Well I don’t like to do that because you have to worry about [classification] of the face of the HSS. I can’t ever get my connection capacity to reach where I want it to go.” So there are some things around that.
What I want to highlight here are the two equations K1-12 and K1-13. K1-12 is for if you have a plate that you’re welding to the HSS and the bottom one there is if you have a slotted. You can see that the equations are almost identical except that there’s a factor of two in the slotted plate connection. The slotted plate connection has twice the capacity, so you need twice the capacity because you’re going to pay for that.
Now the question that also gets posed . . . so the equations are pretty obvious for square sections, now if you go to the round sections, the formula I see there, K1-2, that’s actually for a plate welded to a round HSS. So not the picture I have associated there but it’s actually for a round, but there is no equation for the picture you see there. That picture doesn’t exist in Chapter K.
The questions is, what do I do in that situation? I know that’s got to have more capacity than what this equation gives me but is it twice. Well the answer is yes, it is twice the capacity. Now there’s been research that has shown two things – number one, that the actually ratio between this formula, or between the slotted HSS and a plate welded to a round HSS, it factors more like 1.6.
So it’s not quite two. The equation we see here actually underestimates the limit stay for classification so it’s a conservative. Once again, nowhere is it written in AISC that you can double this, but there is research out there and there’s papers written that you can get your hands on, that does talk about this. If you have this situation with a round HSS you can actually look at doubling the capacity by using a slotted HSS.
What side of the fence you fit on this, it’s just something to be aware of, you know, that I can’t get this situation to work, my only option if I can’t get this to work, is to double the capacity by doing a slotted connection, but if I only need ten percent more capacity to get that connection to work, then why not make it little more cost effective and upsize your two.
Slightly similar but different subject, slotted HSS gusset plate connections, this would be your connections that we see a lot of us for bracing applications. You can see the picture there. It’s pretty common. I just want to highlight the things you need to think about for this.
Clearly, when you’re doing this type of connection the things you need to look at, you need to look at the limit states with HSS. Those limit states are tensile yielding on the gross area, it’s pretty obvious, does the brace member yield? But then you also have to look at the tensile rupture on the net section.
What we’re talking about is that section A there, normally that slot is continued longer than the plate so you actually have a net section there. If you think about this area there’s a slot that comes past that and that’s usually open. Current practice is to not fill that in with any weld metal so that’s open.
That’s the net section you need to look at, but not only do you need to look at net section, but you also need to look at the effect of [sheer lag], and that sheer lag will have a pretty good effect. It’s going to, your U factor that’s in the tables. There’s a U factor that you have to calculate for your sheer lag. So those things are, that’s the limit state that can control a lot of times and a lot of people don’t realize where you need to check that section.
Obviously, you also need to check the base metal sheer on the weld as well, I’m sorry, the base metal sheer and the HSS and the gusset plate as well as the sheer in the weld metal. Those are pretty standard and those are pretty well described in the codes, and then of course, you have to check all the typical gusset plate, limit states, bearing of bolts, bolt fracture and stuff like that.
Now the key thing to realize here is because of the sheer lag issue you really can’t develop the full yield strength, I’m talking about the tensile yield strength, the tensile yield strength of the bracing member. A lot of times we size these braces for yielding, but when you get down to your connections, and this is true of just about any HSS connections, you really have to pay attention to the force transfer that you’re going through, that a lot of times you can’t develop that full capacity of your HSS.
But you do need to make sure that you’re using the right factors and that you’re checking that sheer lag issue, and D3-1 in 360, I’m going to show you that table in just a second. There is an exception to that rule and that’s case five for round HSS. If you actually use a length of weld equal to 1.3 times diameter you can, your yield factor actually becomes one so the sheer lag doesn’t affect the strength of your connection, but that’s the only case where you actually can get a U of one.
Now one of things we talked about the length of weld. It’s implied, it’s not actually stated anywhere, but it’s implied by the tables that if you use a weld length, your weld length should be equal to the dimensions of your tube, okay. As a minimum. There’s nothing in there that says that you can use less, but there’s nothing that says, there’s really nothing to tell you what the strength of that connections is.
It’s implied by these U factors, and here’s the table I was talking about. It’s an abbreviated version of the table. It’s showing the HSS versions. If you look at the sheer lag factor, U over there, first of all I’ll identify the first one there as L greater than 1.3D, U is equal to one.
So that’s what we’re talking about and what, the reason why it’s 1.3D is that’s actually, research has shown that the distance between the weld is actually the distance around the round part of the HSS. It’s not the actual, even though it’s referring to D there, they’re amplifying that D by 30 percent because the distance that’s actually effective is actually the round the circumference of the tube, whereas for the square sections, it’s actually the distance between the two weld lines which is H.
But the implication that’s here from these formulas is that you can’t go, your L can’t be anything less than D or anything less than H or else you’re not dealing, you’re not in these parameters, you can’t for the sheer lag. The research has supported that, but I just wanted to highlight that because nowhere does it say the weld length should be greater than the tube dimension, but it’s implied here.
So next subject–cast connections. Most of the time when we think of cast connections we think of nodes. I’ve got some examples here and you can see that they can get pretty large, but it’s a very unique and a very aesthetic way of handling the HSS connection problem, if you will.
We’ve got a lot of maybe HSS members coming into one place and use a cast connection and these are usually very highly aesthetic driven structures that are expensive structures, if you will, and you can kind of see how we go from a casting to a construction of a node and then you see the final pre-painted version of an HSS node and it’s pretty elegant solution.
While these are really elegant, great solutions for HSS, I think probably 90 percent of people in this room are never going to deal with this type of situation, but it’s a really good, competitive, cost-competitive solution. Especially if you’re looking at especially concentric brace frames and seismic range. These have been fully tested in full scale laboratories as well as going to the ICC-ES process in California.
While there is not pre-qualified connection for bracing as far as AISC’s pre-qualified connections it’s really focused on [moment] connections. There are entities out there, especially in California, between [inaudible 13:11] and ICC-ES that do the pre-qualification, if you will, of these types of connections.
If you’re going to do an energy dissipating bracing system that, when you do that you’re looking to create your hinges both in the [mid-lane] of the brace but also in the gusset plate. These connections have been proven to show that you can develop those hinges.
These are really a great solution and as I said they’re making them cost effective by making them more off the shelf and when I say that, they’re really geared toward round HSS so they make, as you can see, a wide variety of sizes that are associated with certain sizes of diameters, and it’s independent of thickness.
Whether you’re an eight inch nominal pipe or schedule 40 or schedule 80 or if you’re at an eight and 5/8ths inch and quarter wall, it will all, the same casting will fit independent of the wall thickness. It’s based on diameter alone. They come pre-beveled so you can actually get that complete or partial-joint penetration weld there. So it’s a really elegant solution to your everyday problem of diagonal bracing.