CAST YOUR VOTE! The Great DIY Steel Joist Detailing Competition of 2022 - Win $250 USD 30

[KootK]

Things have been a little slow around here lately for my liking. In an attempt to liven things up, I'm attempting something experimental: an engineering contest with a cash prize of $150 USD. This is your chance to:

A) Earn yourself some KootBucks and;

B) Establish yourself as a GSEL (Goddam Structural Engineering Legend).

THE CHALLENGE

In the spirit of times, develop a scheme for the fabrication of a steel joist that would be fabricated on site rather than by a conventional joist supplier (Vulcraft, Canam, etc).

THE RULES

1) Include sketches or be forever disparaged.

2) Describe the benefits of your proposal as you see them.

3) Submissions will be accepted until midnight on June 5th, Pacific Time.

4) On June 6th, voting will commence.

5) Voting will close at midnight on June 12th, Pacific Time

5) Votes will be cast by way of members visiting the thread and writing a quick note to indicate their favorite scheme.

6) Votes will NOT be cast by way of giving out little purple stars. Give out all the little purple stars that you wish but none will be recorded as a formal vote.

7) Anyone may enter the contest and win the prize. However, you need to have received at least 9 little purple stars on this forum in the past in order to qualify as a judge and cast a vote that will be counted. I'm setting this restriction only as an attempt to prevent this thing from spiraling into some weird, spammy, cheating affair. Basically, if you're to be in charge of awarding the KootBucks, you need a reputation.

8) The prize will be awarded in the form of an Amazon eGift card. The winner will need to send their burner email address to my burner address or whatever. We're smart kids, we'll figure it out.

9) No prize will be awarded if there are not at least five entries to choose from, including my own.

10) If JAE shows up to judge, he can cast my vote along with his own.

May the best engineer win!

 

[KootK]

(Since a bunch of people are listing multiple votes, I'm assuming this is going to be a ranked choice/instant runoff type process?)

We're not doing ranked choice voting or anything fancy like that. I only listed my 2nd and 3rd choices for fun.

I'm only game if someone else tends to the tallying. You up for it?

 

[phamENG]

Ha. I can read...I promise...

If we end up with a tie I'll jump in and do the ranked choice.

 

[phamENG]

bhiggins - neat idea, but I'd be worried about the bolted connections at the HSS. How are you going to get enough pretension to avoid slip at a hundred bolt holes and the accompanying, additional deflections without crushing the walls?

 

[skeletron]

My vote is for @canwesteng's Unistrut joist. Although the technical aspects of the analysis might be a bit of a stretch compared to other joist configurations, I think the Unistrut solution quite obviously is a field built solution. This was my view of the most important criteria for the contest.

In a similar realm of thought, my second pick was @Celt83's rod-and-channel joist.

All the other hot-rolled steel solutions are third place. Very creative and some good detailing thought, but I can't pick out one that stands out like the other two ideas.

 

[KootK]

Ha. I can read...I promise...

No worries. We're well into TLDR territory here. I've got a laundry list of contributions that I've been intending to make and am now at the point where I can no longer even remember what I've tackled and what I haven't without rereading all of my stuff again.

I've actually found the administration of the contest to be more difficult than I imagined it would be at the outset:.

1) I should have defined the range of applications better and maybe just targeted K-series replacement as the goal. I didn't anticipate long span stuff creeping into the mix.

2) It was probably a mistake to use the acronym DIY. What I meant was really DIY as executed by a commercial contractor an for commercial application. But then some folks clearly think homeowner stuff when they hear "DIY".

3) I'm not looking forward to having to scan through this to tally the votes. That sounds tedious. Should we do to the ranked choice voting? Is it fair to switch to that at the 11th hour? What if there is a tie?

4) The time limit on when an OP can modify the opening post has gotten shorter than I remember it. I had to reach out to management in order to change the thread title in order to kick off voting and update the purse.

5) Is it kosher to continue critiquing ideas once we've entered the voting phase? Or is that tacky?Everyone seems to be providing "pros" to justify their selections so I would think that ongoing critique would be appropriate as well.

I need to just keep my Type A tendencies in check I think. Relax and enjoy the ride. I don't care who wins the contest really. The goal was simply to have an interesting discussion with lots of creative sketch work. And that's been achieved in spades in my opinion.

Digressions and varying interpretations of the setup have only enhanced the value of this in my opinion.

 

[KootK]

Double angles & channel web { joist design} by (Kootk)

Thank goodness. Without your vote it seems as though I'm not on track to receive any votes other than my own. There's got to be some kind of conspiracy to separate me from my money or something. Not even a 3rd place mention so far? Jeez...

 

[KootK]

Things like using unqualified welds, not removing slag, etc. Pretty sure that would be a problem for any and all of these ideas.

I still harbor concern over the rod welding although, admittedly, my confidence in my understanding of this stuff is poor:

1) I've seen numerous problems with flare bevel welds at HSS corners. I've no idea if similar issues apply to rods / rebar. I do this all the time in my precast work but, there, the welds are being done under plant conditions by folks that know there craft fairly well.

2) I'd certainly trust the rod welding were it to be done in a plant environment and, preferably, using an automated process subject to a QC program and some validation testing. To me, this does not befit a situation where a contractor is is just slapping these things together on site with a general purpose welder doing the work.

3) Below, I've sketched up a to scale version of this using a C3 which I assume would be the default choice. It appears to me that the weld access condition wouldn't be great which would tend to exacerbate any potential QC problems.

4) Load eccentricity will create a twist tendency that will tend to pry the weld apart. This same condition exists (successfully) when bent rods are used with hat channel chords. it doe not exist when bent rods are used with double angle chords as with the Robb joists which, clearly, had their own problems.

5) As a visceral, gut feel thing, I very much feel that it would be easier for to do a high quality fillet weld between two overlapping plate elements than it would be to do a reliable flare bevel rod weld. Am I really alone in that? Obviously, there would be a massive difference in expectations for joists assembled in a steel fabricators shop relative to joists fabricated on site or at a contractors yard.

 

[KootK]

I have a particular interest in Celt's truss because, in many ways, it is similar to a scheme that is very common in my area and has led to a lot of reinforcement head scratching on my part. The systems that I see tend to have some common features that are not present in Celt's scheme:

1) The shear stiff / strong part of the chord is located right over top of the rod webs.

2) End diagonal detailing often delivers the truss vertical shear right to the support, bypassing the chords.

My intuition leads me to wonder if these choices are deliberate and taken to ensure that the vertical component of the web shear spends as little time within the chord as possible. And that, naturally, leads me to want to carefully study Celt's option to understand what the implications of not doing these things might be.

The issues that I've raised below, in the contexts of Celt's truss, are almost certainly resolvable by simply designing for them via YLM or modified detailing where required. I'm not saying otherwise. This is just me taking the opportunity to discuss some features of these kinds of trusses that I find interesting.

 

[Celt83]

I can't much speak for the general quality of the field welding but I'm just working off the assumption that a qualified welder could do it.

Load eccentricity would be a real problem in my setup that would need special detailing.

I recognized dumping the end web shear right at the support with some of the rando existing trusses we get down here in the district, and was aiming for an elongated bend on the first web bar to minimize that eccentricity but it really should get into that seat area thinking about it more.

The vertical web shear hitting the "weak" channel web would need some more thought for sure, the magnitude of the load would also dictate what needs to happen here. I suspect at just some typical roof loading a YLM may check out.

Could get a bit more crazy with it and do the webbing in a helix such that the web bars would land inside the fillet between the flange and web of the channel.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[canwesteng]

I'm going to vote for sticks and triangles. Mostly because it is a modular solution with no field welding (also because I'm not voting for myself). I just can't see anything with field welding being a great repeatable field solution - seems like most of the above designs would all be better built in a shop and shipped to site (just not a truss fabricators shop). Maybe one off in a pinch you could build them on site but I don't see the advantage.

 

[KootK]

@Celt: what do you think of these speculations on my part:

1) Bent rod systems work on the analytical model that I showed in my last post with the web joints eccentric from the chord centroid.

2) Part of the secret sauce of bent rod systems is that, as result of #1, the primary truss shear never spends any time in the chord except at the last diagonal shear dump if the diagonal doesn't go right to the support instead.

An interesting feature of your setup relative to the other versions that I'm familiar with is that your would halve pretty much the least amount of eccentricity betwen the web and chord centerlines of any of them. That is, of course, unless I'm completely out to lunch on the analytical model and it should really be like this. In that case, the webs would meet outside of your truss. Not that there's anything terribly wrong with that.

In all of my travels, I can't say that I've ever seen any guidance on how a bent rod webbing system ought to be treated analytically.

 

[Celt83]

The rigid offset model was my quick assumption, it really probably is more like a curved bar node of a strut tie model. Need a sweet Brad805 model .

"...pretty much the least amount of eccentricity between the web and chord centerlines of any of them..." this was a specific aim of my sketch.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Celt83]

Excel Sheet

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Enable]

1st Place Vote: Kipfoot version 2.0. His WT with sheared angle ends wins all day long for me. It can be fabricated from a single side only, and the welds are easy as pie fillets, and only minor eccentricity is involved. Given that it's a traditional (probably misc) fabricator doing this and not a joist supplier (they'd just do their own thing) I cannot stress how important it would be to do something in the fabricator's wheelhouse from QC perspective. Kipfoot's design has that in spades + even if the fabricator can't do the WTs they'll at least be available. Perhaps not the most economical choice but if we cared that much about economy we'd just wait for the regular suppliers.

2nd Place Vote: Bobby Flay version 2.0 or what I'm calling 2.0. The version where KootK's welds are from one side only on his original design. Like it for the same reasons: can be done from one side + welds are typical for the people likely to be doing this.

3rd Place Vote: canwesteng for a very cool / original contribution. Slick and makes for easy assembly for those that don't do joists at all. I'd be weary of all the bolts for anything other than the lightest loads (and non-onerous deflection requirements due to the slip as others mentioned), and if we are that light I would probably prefer CFS truss system with screws.

 

[KootK]

..this was a specific aim of my sketch.

Speaking of specific aims, I didn't understand what you were doing with the coupler until yesterday when you said this...

...and was aiming for an elongated bend on the first web bar to minimize that eccentricity but it really should get into that seat area thinking about it more.

Originally, I thought that you were just using the couplers to splice together lengths of bent bar that were not the full length of the truss. I found that odd and clunky in terms of fitup. Now is see the magic. You did that so that you could customize the length of the last truss panel and bring the shear dump in close to the support.

Consider the alternative below having the following benefits:

1) Probably a simpler web splice.

2) The last TC panel length doesn't grow.

3) You eliminate the issue of shear dump into the channel web at the end.

4) You've doubled your web capacity at the last diagonal where the web angle will be shallower than normal and, thus, the demand higher than normal.

 

[KootK]

My vote would be for Milkshakelake's CFS option. Sure, maybe the strength is lacking in comparison to the other options on the table, but there's no welding and easily manipulatable materials.

milkshake acknowledged that the strength would likely be lacking when he posted his sketch. I agree and I struggle to know how to handle that given that his solution is clearly a frontrunner for the win.

I think that it's to be expected that lighter duty, easier to fabricate trusses will be competitive for short spans that are lightly loaded. At the same time, if such options are considered to compete on an equivalent basis with solutions that would more appropriate for typical commercial work, I feel that unfairly disadvantages the other, heavier duty solutions.

Long story short, I feel that the CFM option needs some qualification of it's likely capacity, whatever that is. Below, I've run some demand numbers on what I consider to be a marginally acceptable, commercial joist situation. Questions that need answering, in my opinion, include:

1) Can these track chords handle 14,000 lbs compression when evaluated for weak axis buckling over an unbraced panel length of 38? That's 71% of the cross section yield strength at 50 ksi.

2) Given the geometries involved, I don't see each web to chord connection containing more than 8 screws (4 screws each side). Can such a connection handle 4800 lb? Or should such a connection be assumed to be welded?

To be fair, I considered that the depth of a CFM truss might be permitted to be 50% greater than would typically be the case for a hot rolled steel joist.

Another path to take would be to halve the joist spacing in order to lower the stresses to more manageable levels. But, then, one would also need to weight the penalties associated to fabricate, ship, erect, and fasten deck and bridging to twice as many joists.

Certainly, I respect the creativity and DIY merits of the CFM truss option as you and others clearly do. I'm just doing my best to try to run a fair competition. Taken to the extreme, one might propose that the best solution here would be to:

1) Use prefabricated wood trusses and;

2) Do whatever is required in terms of intermediate support and joist spacing to make that work.

Obviously, that would not be in the spirit of what we're doing here. Yet again, this a contest management issue that I did not foresee at the outset.

 

[Celt83]

“ I found that odd and clunky in terms of fitup. Now is see the magic. You did that so that you could customize the length of the last truss panel and bring the shear dump in close to the support.”

Wish I thought of that in the original run. The couplers where an attempt to allow this thing to be broken up for field fab and shipping.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[KootK]

Damn it... presumed too much.

 

[KootK]

I feel as though you could get away with a web splice like this too. Yes, it does introduce some eccentricity. And no, I don't know how to prove that it's okay.

 

[KootK]

dold's deck as web was my other one I feel if those were all intended to be bottom chord bearing, then it would be no different from I-joists in theory.

I agree, the root concept is mechanically sound. The corrugated web business already has history in the realm of steel bridge girders where it offers the following benefits:

1) With continuously welded boundaries, it's basically a pre-stiffened web as far as shear buckling etc go.

2) With flange connections on both sides of the web, you engage gobs of web flexural stiffness to torsionally tie the flanges to the twist of the global cross section.

My concerns with the CFM deck solution are centered around how that is different from #1 & #2 and what the associated implications of those differences may be.