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Steel Truss

Steel Truss

Steel Truss

(OP)
I’m looking at doing steel girder roof trusses to frame a 70’x200’ structure. The trusses will be spanning the 70’ dimension, probably 5’ or 6’ to 7’ in depth, and will be spaced at about 20’ on center. The configuration resembles a typical bar joist but with 2 HSS 12x6 top and bottom chords sandwhiching 5” or 6” HSS web members. There are ~10’ top chord cantilever extensions, and with a 1:12 roof slope it makes the truss a little under 100’ in total length. The trusses will be exposed so the plan is to have no bottom chord bracing. I am planning on putting enough concrete on the roof to negate any uplift forces. My 2 concerns are:

1) Are there any concerns with running a truss this long without any bottom chord bracing, even if the bottom chord never goes into compression?


2) Should I anticipate having to provide a splice detail due to shipping limitations? If so, should making the cantilevers separate pieces to limit the total truss length to about 76’ be considered?

RE: Steel Truss

(OP)
Another question I have is if cambering the truss for the dead weight of the concrete would be appropriate in this situation.

RE: Steel Truss

1) I would prefer to brace the bottom chord with a maximum L/r of 300 even if there is no compression in the bottom chord. Otherwise the truss is too limber.

2) Splicing is a good idea. It makes handling much easier. Splicing at midspan is an idea I have used in the past with a similar project. In your case, there would be two sections each 45' long. Top and bottom chords may be spliced using end plates welded to the HSS and bolted together. Bolts in the top chord splice would be nominal as the chord force is always acting in compression. The bottom chord splice will carry the full chord tension so the end plates will be quite thick. Bolts penetrate both end plates and act in tension. Large diameter A490 bolts are needed for this purpose.

3) Cambering for dead weight is normal practice but with a central splice, both sections can be built without camber, then connected to provide any slope you wish. My preference would be to provide 1/4" per foot of slope after the dead weight is in place...provides good drainage for the roof.

BA

RE: Steel Truss

Is your roof a gable roof or a monoslope roof?

Depending on your project and wind loads, you might want to re-consider the concrete on the roof. At that span and trib area, you can use MWFRS wind loads for your truss uplift.

If your trusses are spaced at 20' on center, are you planning on having infill framing in between? If so, are you using regular wide flange beams spanning the 20' or using bar joists that are perpendicular to the roof slope? You will need to consider the roof slope when designing beams perpendicular to the roof slope.

Below is a document I have found useful.
http://rmsca.org/wp-content/uploads/2017/03/Sloped...

RE: Steel Truss

The Importance of Tension Chord Bracing

It doesn't mean that you absolutely must have bottom chord bracing. But, if you don't have it, you'll want to justify that decision carefully with self weight etc. In this regard, extra weight on the roof actually makes the problem worse.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Very interesting. Does anyone have any accounts of truss failures from gravity loads due to this issue?

RE: Steel Truss

I, for one, do not. But, then:

- The overwhelming majority of trusses out there do have some tension chord bracing.
- When I've tinkered with the numbers, it doesn't take much to get this done with self weight and other tough to quantify "stuff".

When you look at bar joists anecdotally, you almost always see at least two lines of bridging, uplift or no. To my chagrin, there are some at my local fitness center that have only one line of bridging at midspan. That baffles me, for either uplift or tension chord buckling.

You can create an interesting example on your desk:

- set a pen up vertically on your desk.

- drape a tensed, broken rubber band over the top of the pen and hold it down at the level of your desk with the drawing end at the bottom. Like an upside down truss. Stable, if marginally so.

- Now put an eraser with a decent thickness under the pen and do the same thing. Should be armageddon unless you're picking up some unintended restraint. I consider this to be a "slightly encouraged" form tension chord buckling.

Another interesting example is just a wide flange beam. There's no such thing as a free lunch so, if this is a "thing" for trusses then I'm sure that it's also a thing for beams under the right combination of load and support conditions. I don't know of anyone actually considering that of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

My bet in the wide flange beam scenario, is for the most part the web is stiff enough to brace the tension flange appropriately. In a truss however, it's just a piddly little connection between the web and chord that never really had lateral loading taken into account when designed.

RE: Steel Truss

The opposite I think. In both cases it's the web that wants to buckle and the flange/chord doing the restraining. From time to time, I'll wind up with a wide flange beam that ends up needing to be heavily coped to match joist seat depth. In those situations, I always wonder if one ought to have bracing/bridging near the ends for this reason. I've not done it yet though. I'm not all that worried about it for a few reasons and, mostly, I'm too lazy to create a new detail.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

I concede you are correct now that I think about it more. Where the difference between beam and truss lies is the strength of the connection between web and flange. There's significantly more capacity there to prevent the armageddon type buckling in your real world small scale test scenario.

RE: Steel Truss

I actually disagree with that too although I'd be lying if I pretended that I had this all sorted out with great confidence. Both the truss and beam need to be restrained from what is, fundamentally, a rigid body twisting. As such, I don't feel that connection quality would come into play that much. My guess is that the difference is this with beams:

1) The ratio of torsional stiffness to flexural stiffness is much higher than with your average truss.

2) When a beam is top loaded, there's usually a relatively wide loaded surface (flange) over which the point of load delivery can shift to rectify a twisting tendency. This, so long as the member delivering the loads extends past the supporting beam center line.

I suppose that could argue that #2 is helped by a stiff-ish web/flange connection. That said, even though they're non-solid, most truss webs possess more lateral stiffness than beam webs I suspect. You're comparing HSS/WF/LL discretely versus a thin web continuously.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

You're talking about rigid body twisting. However typically the top flange is restrained from twisting significantly by whatever is framing onto/into it even steel deck has the ability to restrain this twisting at the magnitude of load we are likely discussing. Then the only thing left it the remaining web and tension flange buckling/twisting. The connection from flange to web definitely comes into play in my eyes.

RE: Steel Truss

Sort of tying back to BA's point, I feel that it's generally good practice to brace all truss chords generously. Fundamentally, that's part and parcel with translating these idealized 2D things into 3D realities. If aesthetic concerns mean that's not possible, then it's time to bust out the calculators and sharp pencils. And to gauge one's own tolerance for risk. And I don't mean that as a deterrent. Merely as a nod towards sage business practices. You can't be an effective structural engineer without some tolerance/understanding of risk.

Often here, folks will post about large trusses where the bottom chords are tied to the columns. The issue dujour will be whether the trusses should be modeled as pinned... blabbity blab. My supsicion is that, often, these trusses were detailed this way so that the bottom chords could span laterally between columns like girts and thus brace the bottoms of the compression webs. Surely no numbers on that, just considered detailing. You can get the same thing done, theoretically, with an axial slip connection. They didn't seem to fuss around with that froufrou stuff back in the day though. They just tied things together with a vengeance and dealt with the implications accordingly. Or turned a blind eye to the implications. Seems to go both ways.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Google "Sidesway Web Buckling of Steel Beams" and some papers come up that discuss this.

RE: Steel Truss

Quote (jayrod)

Then the only thing left it the remaining web and tension flange buckling/twisting

Yup, I see what you're getting at. If top flange rotational restraint is given as a prerequisite to the problem then, certainly, the stiffness of the web and it's flange connections would come into play. Effectively, the restrained top flange reaches down via the web to laterally restrain the bottom flange which then restrains the bottoms of the compression webs.

This same mechanism can work in trusses too. Sometimes you'll have beams or secondary trusses tying into the girder truss webs with nominally capable moment connectins that will brace the girder bottom flange the same way. Basically AISC style rotational bracing. Of course, with big trusses, the relative efficacy of torsional restraint offered by the framing tying in, and the chords themselves, tends to be much less than is the case with beams. Things are just of a different scale and character. And that takes us back to the need for discrete bracing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Keen observation XR. Did a little of that Googling you suggested: http://files.engineering.com/download.aspx?folder=...[1].pdf. I'd do a proper hyperlink but the brackets screw it up. Clips from the intro below. At the least, this is a close cousin to my concern. I was really thinking of a case with minimal rotational support at the top flange, minimal rotational support at the supports, and a tendency towards a more true rigid body rotation as is the case with trusses I believe. Admittedly, that's a pretty contrived situation in normal structural steel.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

12x6HSS chords could pose a small problem of wall stress where joined by 5" or 6" HSS compression web members but this will not be known until an analysis is conducted. It may be better to reduce the width of the chords.

BA

RE: Steel Truss

(OP)
Thanks for the replies. I’ve attached a schematic of the truss, it’s spanning over a large fire truck bay. We are still early on in the project, so I’m not married to anything yet. I’m assuming the preference architecturally will be to have the secondary framing concealed by bearing it on the top chord of the truss, I believe they are trying to go with a wood soffit. I think I’m left with a few options to pitch to the architect:

1) Run HSS members between the top chords (flush) of the trusses, maybe at quarter points, to provide axial bracing for the top chord and, ideally, rotational bracing for the entire truss

2) Use diagonal bracing (probably cables) for the bottom chord, maybe at midspan, to brace the whole truss. In this case I think I would size the top chords as a composite section unbraced for its entire length between columns.

3) If I cannot justify the first 2 options after running some calculations I will try to use a combination of the above options.

I’ll need to read through the posted literature and get educated but I think the options I described above are viable. I’ll look into the wall stresses at the web connections but if it’s an issue I think I can get around it by knifing a gusset through the top chords.

https://res.cloudinary.com/engineering-com/image/upload/v1513387954/tips/Truss_Screenshots_fjnxz9.pdf

RE: Steel Truss

(OP)
After giving it some thought, I think the cable bracing (option 2) is the best solution that satisfies both architectural and structural needs. The plan is to brace at the midspan splice location and where the bottom chord meets the end diagonals if needed while keeping all of the secondary framing concealed. Hopefully I can justify the out of plane strength aand stiffness of the chord/web connection at that location to omit diagonal bracing there but I will determine that after doing some calculations. I’ll follow up with with some connecton details but I think I have some in mind that will satisfy all required load paths and ideally brace the top and bottom chord for compression and eliminate the concrete on the roof. The dead weight of the concrete is a lot more “real” than wind uplift and I have seen deflection issues with it on long span trusses in the past due to bad construction and misfabrication so it would be preferable to avoid. Thanks for all the responses.

Kootk- Nice link on the tension chord bracing, I will be using that for sure.

RE: Steel Truss

At the risk of being a Debbie Downer, I don't love cables as bracing for a major truss like this. Consider:

1) Brace stiffness is just as important as brace strength. More so probably.

2) All of your cables need to be tension capable, all of the time, for this to work.

3) Cables need to be pre-tensioned to keep them tension capable after creep stretch etc. It can be tough to get/verify that robustly in the initial install.

4) If one truss deflects more than its neighbors for any reason, two of the cables coming off of that truss may loose their tension.

In my heat of hearts, I feel that cables probably would work. A little buckling movement would surely draw out any slack and get things taugh again. For me though, the scale's just off with this. I'm happy to cable brace some little curtain wall support truss etc. Not a large primary girder though. My preferences would be:

a) Justify no bracing.

b) Horizontal truss the bottom chords at the end bays and run two strut lines in between. I know... you've got an architect. All this stuff could be done in sexy timber though. And it would tend to look more "right" architecturally in my opinion. Just gotta sell it. If the strut lines could hit solid wall or columns at the ends of the truss runs, you could eliminate the trussing too.

c) Run you bottom chords out to supporting columns if they exist.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Yes, especially when cable sag is taken into consideration... I once calculated the effective stiffness of an inclined cable including the behavior of pulling sag out under load. It's very, very low.

On that topic.. how much "buckling movement" before a brace kicks in is too much (without getting into FEA analysis)? At some point you'll run into obviously problematic second-order effects... but is that the only limit?

----
The name is a long story -- just call me Lo.

RE: Steel Truss

The 2nd order effects were all that I had in mind. Once you draw out any slack and start reengaging the full stiffness of the cable, I suspect that stiffness gets in line in a hurry. I'm not sure that I know how to calculate that without making a thesis of it though. And the stakes are pretty high.

Interestingly, camber actually makes this system less stable as it raises the load above the potential point of LTB-ish rotation. Another way to see it is that it reduces the lever arm available to the bottom chord to use its self weight to stabilize.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

(OP)
Hmm I see your point. I would say that items 1 & 4 could be resolved with proper analysis and detailing, assuming the numbers work out, while 2 & 3 rely on proper and diligent erection of the trusses. It’s design/bid/build and I know the types of contractors that get the specific job I am working on and I probably shouldn’t rely on them carrying out “advanced” construction techniques for the stability of the structure. That said, assuming I could extend the bottom chord to the columns and strength and stiffness calculations were satisfied, I feel like I could just as well design a cable bracing system that should be much stiffer and stronger than the bottom chords spanning, although I could be completely wrong, still need to verify with calculations. If elongation and some construction tolerance is considered and adequate pre-tension is verified before and after the dead load is applied is there really an issue if the components are adequately sized and detailed correctly for all loading conditions? My only question to you is would be if you think the same issues would arise with using another form of tension-only bracing such as rods or other slender rolled members.

Obviously, I need to quit assuming what’s architecturally acceltable and just sit down with the architect and workshop some ideas. I’m really just trying to get as many options as I can into my back pocket for when I do. When I’m starting from a blank slate like this I tend to get too caught up in what I think the strucuture should look like instead pushing what’s best structurally and letting the architect do their job.

Really appreciate the time and effort you put into your responses Koot. I can’t tell you how many of these threads pop up on google searches and you’re in the replies with some good insight and literature. Maybe you have to play the role of devils advocate/Debbie downer sometimes but it’s worth it if it saves some poor engineer in the future getting the call asking to clean up someone else’s mess.

RE: Steel Truss

(OP)
Sorry, last 2 replies came in while I was drafting mine, and I see your point that it might not be possible to justify cable bracing with analysis. While the initial stiffness can be calculated and designed per AISC guidelines, when you start considering 2nd order effects it gets hazy and would require some assumptions and the right software and quite a bit of time resesrching how to accurately model and analyze the system. I’ll plan on staying away from the cables for now but I’m still curious on anyone’s opinions with other forms of tension-only bracing.

RE: Steel Truss

Other means of tension-only bracing (or at least the only other I can think of, rods) are fine. #1 and #3 of Koot's comments still apply, and #2 and #4 as well, just to a lesser extent.

Going back to my cable sag exercise once more, I don't recall the exact deflections I got before cable stiffness began to kick in (besides, absolute numbers wouldn't mean much without more context), but it was roughly an order of magnitude above the 1/8" or so that I could justify by inspection. In my case, the 2nd order effects quickly ruled that sort of movement out.

----
The name is a long story -- just call me Lo.

RE: Steel Truss

Thanks for the kind words dnlv. For what it's worth, I suspect that many engineers would be fine with the cable bracing. I might even get there myself were it my project and my client for whom I was striving to provide value. I really think that your path here may well be justifying the no-bracing option. Based on the direction that you seem to be taking with member profiles, I think that you'll have two things working in your favor here:

1) It sounds as though your top chord will have a high torsional stiffness. That helps as your failure mode is effectively LTB about the top chord.

2) It sounds as though your bottom chord will have a high lateral stiffness. That probably means that you can span unbraced between supporting columns or, at worst, bridging/bracing lines at the the far ends of the truss.

For the no bracing option, you're essentially counting on bottom chord weight to brace against LTB. A little off the wall but, if it doesn't check out, I wonder if one might concrete fill the bottom chords to improve matters. That's right where the weight would be most effective.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Quote (KootK)

For the no bracing option, you're essentially counting on bottom chord weight to brace against LTB

Koot,

Wouldn't you have to get a fair amount of truss rotation before the weight of the bottom chord would provide meaningful restraint?

RE: Steel Truss

I like to think of these things in energy terms as I find it easier to make good decisions that way. Buckling in an LTB-ish mode doesn't happen unless:

1) Truss rotation sets up a condition where truss weak axis bending is involved in downwards deflection.

2) #1 results in the applied loads moving closer to the earth than they otherwise would, thus reducing potential energy (mathematical instability).

Quote (XR250)

Wouldn't you have to get a fair amount of truss rotation before the weight of the bottom chord would provide meaningful restraint?

You would. But, by the same token, you would also need a fair bit of rotation before the the applied loads would start moving closer to the earth in a meaningful way. So I think that you have balance in that regard. By the time that you need the resistance, you probably have it. It's certainly worthy of some numerical exploration of course, starting from an appropriate, presumed out of plumbness.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Quote (KootK)

You would. But, by the same token, you would also need a fair bit of rotation before the the applied loads would start moving closer to the earth in a meaningful way. So I think that you have balance in that regard. By the time that you need the resistance, you probably have it. It's certainly worthy of some numerical exploration of course, starting from an appropriate, presumed out of plumbness.

Makes sense. So based on this discussion, should we assume a no-heel gable (triangular) truss with is always stable under gravity loads as long as the top chord is sufficiently braced?

RE: Steel Truss

Quote (XR250)

So based on this discussion, should we assume a no-heel gable (triangular) truss with is always stable under gravity loads as long as the top chord is sufficiently braced

That makes for an interesting example. And the answer isn't at all obvious to me. My take:

1) In theoretical terms, I don't believe that a gable truss would be perfectly stable in this respect by default. The compression webs still need to be kept from kicking out at the bottom chord.

2) You've got a bottom chord that extends to the bearings which is great for restraining the compression webs from kicking out.

3) You've got a situation where kicking out the compression webs and bowing the bottom chord would actually raise the peaks of the truss and thus move load further from the earth initially. This is obviously a pretty great situation from a stability perspective. It might be that the stability graph here would be one that has multiple peaks and valleys. While you're initially raising the load, you may eventually crest and descend into an unstable region beyond. I'd think that this would take pretty extreme amount of movement though.

4) My conception of this actually differs a bit from the presentation given in paper that I referenced. They deal pretty much exclusively with the kicking out of the bottom chord. I see that phenomenon as really a symptom of a form of truss LTB which the paper doesn't really speak to. Obviously, a gable truss with the top chord restrained is in pretty good shape with regard to anything that resembles LTB. By comparison, the classic "bad truss" in these discussions is always an upside down king post truss (it is in the paper). That's effectively your gable truss flipped upside down which reverses all the good stability stuff listed above. And I do recognize that, in my disagreeing with the paper, I'm putting myself up against Fisher, Yura, and Galambos. I acknowledge that my odds of being "righter" than that gang are pretty low. That said, this is democratic space and even dummies are allowed to express their wayward opinions.

5) For practical purposes, I do consider the top chord restrained gable truss to be inherently stable to the point that I normally wouldn't waste real design time looking into it. I know from experience that the wood truss guys certainly don't.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Steel Truss

Quote (KootK)

For practical purposes, I do consider the top chord restrained gable truss to be inherently stable to the point that I normally wouldn't waste real design time looking into it. I know from experience that the wood truss guys certainly don't.

I figured as much which is why i asked you in particular. Thanks for the comprehensive insight.

RE: Steel Truss

Don't have a copy right here with me, but I'm pretty sure the truss section of CSA S16 (Canadian steel code) has a specific requirement for tension chord bracing to keep it below a certain slenderness limit.

RE: Steel Truss

They do but it isn't mandatory. Their suggested limit is 300 for KL/r of a tension member.

BA

RE: Steel Truss

There's a separate clause specifically for trusses. Just checked it:

Quote (S16)

15.2.7 Maximum slenderness ratio of tension chords
The maximum slenderness ratio shall be limited to 240, except when other means are provided to control flexibility, sag, vibration and slack in a manner commensurate with the service conditions of the structures.

RE: Steel Truss

Okay, that is good to know. I wasn't aware of that.

BA

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