Built-up column Conundrum

Les Engineer

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Nov 5, 2013
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Huntington
I'm planning on building some very tall (and medium tall) columns to hold platforms. Having poured over the literature for several weeks, I've still not found information on built-up columns (i.e., mechanically laminated two-bys) that include a variation on the theme of "hog-troughing," a term I've seen on this site from time to time. Does anyone out there know of any literature on built-up columns that include the attachment of members to the built-up studs laterally, that is, as if one were building a sort of modified "I-beam." I have no structural standards to point to, but it seems to me that adding studs to the edges of a built-up column would give that column greater resistance to buckling along the weak axis. Is this just my imagination? I would love to hear the voices of experience wax eloquent on this theme before I commit!
 

bobgaggle

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Nov 19, 2007
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Philadelphia, PA
I think you're going to need to provide more detail. I'm a little confused by what you're trying to do. Generally platforms are legged with 2x or 4x, and cross braced underneath for stability. I imagine you're thinking of columns in wall construction in homes. where 2,3,4... 2x4 studs are nailed together face to face.

like this


This is generally done around windows or doorways to bear the load being transferred around the window/door. You won't find info on hog troughing them because the practice doesn't exist in home building. There isn't a need as bracing is provided by the perpendicular walls and floor/ceiling.

It would help to know what your application is, how tall your platforms are.

More importantly, why is your instinct to go with built up columns? Is there a reason you followed that thought train?
 

Les Engineer

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Nov 5, 2013
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Huntington
I think you're going to need to provide more detail. I'm a little confused by what you're trying to do. Generally platforms are legged with 2x or 4x, and cross braced underneath for stability. I imagine you're thinking of columns in wall construction in homes. where 2,3,4... 2x4 studs are nailed together face to face.

Aaron, sorry, I was a little cryptic wasn't I? The 16' tall columns I'm building will have limited-to-no-bracing in the middle sections. And only light bracing at the bottom. At least one face, the downstage side of the rectangle, will have no cross-bracing along it's length. To make this happen safely, the column must be strong enough to support the load, live, dead, and in-between, with a comfortable safety factor. Built-up columns have several advantages over solid timbers. Properly configured, constructed, and implemented, they approach timber strength - can actually surpass timber strength, depending on the wood grade. However, it's the deflection along the weak axis that concerns me. I'm a pipeline engineer, not a structural engineer, and the thing I can't get past is that in my line of work everything is tested before we give it the thumbs up. With structural I'm, frankly, working with averages: formulas with variables, NDS fact sheets, experiential opinion, and industry bulletins. I don't mind overkill so long as I know the minimum I'm "overkilling." I should also say that when I design my columns, I design them UNBRACED. Design-wise they have to "stand on their own" without bracing. Then, in actual construction, I can add such bracing as I desire, knowing that it's all additional safety stuff, but not a part of the original math.

OK, so the "hog-troughing" I've referred to, which is often used in scene-craft, must be born of experience and logic, right? Two pieces of lumber, one for x-axis strength and one for y-axis strength. You're right Aaron, in that for the example above, the "wall" becomes the trough! I'm looking to do the same but using 1x or 2x studs in place of the "wall." I guess I could make my own modified hog-trough out of built-up pieces and treat the opposing pieces as separate supports, but I suspect that the strength of the whole is greater than the strength of the individual members, I just don't think there is any empirical data I can use to determine how much stronger. SO... I guess I was hoping someone had experience with this; someone had made one of these up and what they thought of it in the end.
 

Les Engineer

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Nov 5, 2013
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Huntington
Also, I'm not starting construction for 3 months - plenty of time to worry this to death. Also, unlike here at work, where I have a bottomless budget, skilled labor, and the best equipment money can buy, at the theater I have no money, ma and pa kettle putting this thing together (and taking it apart), and zero heavy equipment. Zero. So part of this is a quixotic attempt to make the impossible not only possible, but safe. Impossible, but that's not stopped me yet!
 

kicknargel

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Aug 10, 2009
Location
Denver, CO
You're an ideal candidate for Structural Design for the Stage by Alys Holden. It contains the math you're looking for, including a whole chapter on column design.

I think the relevant factor you're looking for is the slenderness ratio. I don't have my book here, so can't look it up, but it gives a height to thickness ratio for different materials used in a column.
 

Les Engineer

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Nov 5, 2013
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Huntington
You're an ideal candidate for Structural Design for the Stage by Alys Holden. It contains the math you're looking for, including a whole chapter on column design.

I think the relevant factor you're looking for is the slenderness ratio. I don't have my book here, so can't look it up, but it gives a height to thickness ratio for different materials used in a column.
It's now on my Christmas List!
 

robartsd

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Apr 18, 2011
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Sacramento, CA
Yes, you're looking at the slenderness ratio. The slenderness ratio is all about buckling. The good news is that the force required to brace against buckling for a straight, axially loaded member is very low; the bad news is that the force required can go up rapidly if the load is off axis or the member is not straight. So as long as your geometry is straight and you've worked out how to load the column properly, you shouldn't need to worry much about the strength of the connection between the members of your built-up column (I'm guessing that this is the part that you're having trouble finding data for). If I were buliding up a column from 2x members I'd use glue and screws to marry them together and not have a second thought about the strenght of the joint. However, the shear strength of the joint between members of a built-up beam is VITAL - worthy of careful testing in the absense of reliable engineering data.
 

Les Engineer

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Nov 5, 2013
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Huntington
If I were buliding up a column from 2x members I'd use glue and screws to marry them together and not have a second thought about the strenght of the joint. However, the shear strength of the joint between members of a built-up beam is VITAL - worthy of careful testing in the absense of reliable engineering data.
Thank you. Absolutely. I'll probably use carriage bolts, Simpson Strong-Drive SDW Truss-Ply screws or the equivalent, although I know that we theater folks normally like to use whatever's in the big bucket under the fake sink, as well as epoxy and good clamping techniques. This should minimize slipping and shear at the joints considerably. My question, however, is still this: in the absence of "cross-bracing" how will the book-end-hog-troughing (for lack of a better term) of built-ups impact the natural deflection of the studs across the weak axis? I haven't purchased the book mentioned above, yet, so I'm off to the local University library later this week. [I'm way overthinking this, and starting to feel guilty about bothering anyone with it. My understanding is quickly evolving!]
 

robartsd

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Apr 18, 2011
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Sacramento, CA
If the members are perpendicular to each other and joined so that they deflect together, then any force that would contribute to the weak axis deflection of one member will be transferred to the strong axis of the other member. In composite sections, strength always follows stiffness - so the strength and deflection of the weak axis can safely be considered negliable and, provided the built up section is loaded axially only, the strength of the joint is not very critical either. If the built up section is primarily subjected to bending instead of axial loads, the shear strength of the joint between members could easily be the most critcical part.

I agree that you might be overthinking this, but don't think you should feel guilty about "bothering" anyone. This forum is intended as a place to learn for novices through experienced hands.

On the other hand, if your concern about these columns is if they provide enough lateral support for your platform; you're not over thinking this at all and my "axially loaded" quailfier reveals that my interpretation is overly simplifying the issue you are trying to deal with.
 

Les Engineer

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Nov 5, 2013
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Huntington
Can we get a follow up on this? What did you end up building? Photos?
Mr. Hess, I'm just beginning the construction phase this week. The design stage was difficult, in that an unforeseen difficulty forced me to abandon 3 months of planning, a thick book of blueprints, and cut sheets for the builders: everything was measured to the 16th of an inch, and not on centers, but to the edges! I was so depressed when I realized my gaff that I took a couple of weeks away just to drink and drown and my sorrows. My welder took me aside yesterday to remind me, as he likes to do, that "we ain't building a piano," and to take it all in stride.
It looks like I'll be going with something different than I had originally planned for the built-up columns, although the same design and construction issues remain. I promise I'll report back with the results as soon as I have something concrete and helpful to report, good or bad. Thanks for checking up!
 

Les Engineer

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Nov 5, 2013
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Project completed successfully. Actually, it was far better than I hoped it would be. No issues with columns, beams. joists, or overall structure. If anything, it was too sturdy: strike took four days. Wish I had photos to share (I have them, but not on this computer.) Huge set, 28 feet tall, 24 deep and 42 across built for around a grand. Learned a few things: 1) I worry too much; 2) Nothing wrong with erring on the side of safety; 3) Actors appreciate a well-made, beautiful set; 4) Audience, too 5) Quality is really, really hard work 6) There is a fine line between "too much" and not enough; that line is almost impossible to walk, so the builder is often faced with either over-doing it or falling short. Better the former than disaster; 7) even the strongest unsupported columns might "walk" over time. Mom went to see the show and came back to report that the DS column two from SR was slightly off-center. I went back that night and checked. She was right. I had to kick it two inches to the right. Mothers - sheesh!
 

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