engineering calculation needed

Chris,


By its very complexity, what are describing is far more trouble than a conventional pinned frame, which BTW is not difficult to analyze. In fact, it can probably be done in less time than it would take to correctly draw your proposed system.
In the words of Professor Hoover, "take it to pieces". If I understand you correctly, the beam is not horizonally supported at the top, and is therefore not in equilibrium. Add another horizontal support to fix that, and it is no longer statically determinate.
Bill
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Bill Schwab wrote:

It is a conventional pinned frame. Furthermore the idealisation I proposed simplifies analysis.

I believe it was a correctly drawn idealisation. The two members and support all connect to the pin, not to each other.

It is connected to the roller support via the pin. Please see the diagram.
Goodness me! You're making me feel like I'm in court here. You make it your business to automatically disagree with everything I say. Fortunately I did think through what I did here, and can support all bar one of my decisions, although the odds are you'll disagree with that statement, too.
I really will give this a break now.
Chris
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Christopher Tidy wrote:

You mean C:\docs\davit_calc2.jpg? If so, isn't that the pin you said doesn't act on the beam? If that's the case, then we're back to the missing horizontal force.
Otherwise, you should draw the diagrams, write the equations, and either count the unknowns and equations, or check the moment balance on the beam.
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Bill Schwab wrote:
<snip>

The pin does not transmit the horizontal component of the tension to the top end of the column, if you accept the idealised joint I proposed. This does not mean that the pin is unable to transmit any horizontal force to the column. It does provide a horizontal reaction which opposes the compression in the horizontal beam. This is a subtlety of the idealised joint, which you may like or dislike, but the FBD is consistent with the idealisation.
I call it quits.
Chris
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Chris,

What you describe is almost certainly either unsupported, or statically indeterminate. Even if you have designed something that is determinate, it is by no means a simplification relative to the what was presented to you in your mechanics classes. If you think the pinned frame is complicated, you didn't learn the material.
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Either help the OP or give facts, a diagram, or a formula that presents your worries and concerns. Continually repeating that someone is wrong solves nothing, helps nothing, changes nothing.
Perhaps you did not notice that OP intends to try his original concept - win, lose, or draw. He will be watching for empirical evidence that he is underdesigned.
I would point out that I just loaded on my trailer two pieces of 2x2x1/4 wall twenty foot sticks that had been unloaded with a single choker at mid point. They both have a permanent 2-3" crown in them. Too good to toss, too bent to use without modification/adaptation. Could not be used on this job for their original intent.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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DanG wrote:

Perhaps. If you have an engineering background, please review Chris' 12-17-05 12:39 pm post; he says everything is pinned. In that case the horizontal component of tension is indeed transmitted to the beam via the pin, and must be included in the diagram and equations, as I explained early on in this process.
Then go to 12-17-05 2:17 pm and note that he now says the force goes around the column/beam. The problem with the latter becomes clear when you look at an FBD of the beam; external moments to do balance. The horizontal member pushes the beam to the right; per Chris' design, the only other horizontal force is at the base, and it clearly needs to push to the left or the whole structure goes flying. Note that the beam is left wanting to rotate clockwise, which is a contradiction unless there is some other horizontal force at the top of the beam [*]. We are looking at two rigid bodies and the pin supporting the load, which gives seven equilibrium equations. Add that extra support and the system becomes underdetermined.
[*] the support reactions change, but hopefully you get the idea.
Even if there is a way to build a gizmo that can do all of this and be statically determinate, it will be far more complex than the pinned frame, and if built, would be more likely to fail than the frame.

I did notice; that's not the point. What if I were telling people hold endmills in a tapered chuck and than crank up the feed rate?
> He will be watching for empirical

I'm sure he will.
Bill
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Sure they can be used with a bit of a bend in them! Put the long vertical leg with the crown toward the davit arm, and when you load up the davit that crown will magically disappear. If not appear on the other side.
I've been watching this College Calculus & Engineering gab-fest wondering why nobody has suggested a KISS solution - hang a small chunk of I-beam off the porch beams and rafters, sticking two or three feet out over the loading area. Possibly with a couple of added support legs to concrete footings for insurance, or upgrade the area by adding another post and footing that matches the existing porch construction. And you can make a hidden gate in the railing so the loads don't have to go as high.
Use the same hoist for the load but mounted on a roller beam trolley. Convert the overhanging and swinging loads of that davit arm system to purely vertical loads of making sure the hoist beam stays up.
And don't forget a couple of rubber bump stops at the ends for trolley travel stops, impact loads can cause failure.
A proper use of a davit arm crane is outside where there isn't any other structure available, and they normally aren't designed for a support at the top of the post - you transfer all the loads down through a healthy vertical member to a large footing. You have a very handy house sitting there - use it.
If you take down the trolley and hoist, all that's visible is the beam. Ultra Slickness would extend the hoist beam across the porch to the house wall, and you make a convenient little closet that the hoist rolls into, with room below for storing the slings straps and spreader bars. You close a little door and "Presto!" it all disappears.
KISS!
--<< Bruce >>--
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
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My 19 year old son is a freshman Mech. Eng. student at the Univ of Texas at Austin. He had his Statics exam on Thursday, and I took him out to dinner last night, where I presented him with this problem. We discussed it for the whole meal, drawing with crayons on the back of the kid's menu.
After looking at his results, we agreed that we would redesign. I like Bruce's I-beam idea, myself, but what we decided was that the post should be fixed, and the davit pieces should swivel around the post with a collar, hinge, or other means. We also brought the winch in, right next to the post, and made it pull up a platform with a wheeled carriage riding on the post, something like a dumbwaiter. We didn't see the need to hang the winch so far out from the post.
We also came up with the gate idea so that the platform only would come up to the level of the deck rather than trying to go over the railing - this would make it a lot easier to unload as well. Of course, my son immediately saw the possibilities of making this platform a people lift, which I told him had already been vetoed by the professionals here...
Anyway, it was interesting to him, as the previously mentioned ME undergrad...
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Emmo,

Great test question for statics - not the deflection part of course, that's the next course. Your son might enjoy checking out the 4th edition of Hibbler's Engineering Mechanics: Statics, example 6-15. The "triangle" is curved, points downward instead of upward, and the dimensions are different, but it is otherwise the same problem, pinned of course. It's a one pager, including a lot of explanation.
Good luck to your son!
Bill
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Not 100% sure the roof joists could handle it, and wife would never allow me to hang anything that ugly over the porch swing. Only reason I get this is because it is way down to the side and doesn't interfere with the view.
myself, but what we decided was that the post should be

Thought of that, too, but I think for the purposes of swinging load into the deck, an axis that goes through the center of the vertical is best. Anything else increases the leverage and flex. Right?
We also brought the winch in, right next to the

Now, you're getting complicated.
We didn't see the need to hang the winch

Beat you to it, just didn't post it in the original post. I will probably do wrought iron on that end, probably the Indital "vineyard" stuff with metal leaves and nice decorations to make it look like foliage. Then airbrush in details.
Of course, my son immediately

That may still happen. Of course, I would never admit it. However, I do weigh #195, and that would be a good test load. (I have ridden a headache ball down from the top of a drilling derrick, but that was when the safety man was in the doghouse.)

Thanks for bouncing it off a fresh brain.
Steve
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When looking at the situation, this is what I came up with. The rafters are there making the porch covering. It is just that they are notched on their ends where they sit on the beam, and at the wall where they tie in. So, they are equivalent to 2x4s. The intended site is about the strongest thing around to hook on to. Plus, I didn't want an industrial looking I beam thingus hanging over my porch swing.
Had the thought of cutting the railing, and will do that, but will am considering replacing current railing with mortise and tenon rough cut railings. The current is all square cut lumber. I may even do a fancy Indital panel on that end of the porch complete with swinging section. I would certainly like to lessen the height that I lift anything to, and that would cut it down by about three feet.
Believe me, if that beam was supported better, I would have just hitched onto it, and not had to go to the ground. Did a few lifts of things with the old rope and pulley trick, but it was hard to get in over the railing with the lifting point located where it is.
Other thing is that OTHER PEOPLE will be operating this from time to time. Were it me only, I could come up with something a little simpler. I don't want anyone who lacks experience from doing a header off the deck rassling with a load.
I thank all for the continued input. I am being able to really consider this before final fab and install.
We will be going up there for New Year's Eve if there isn't too much snow, and we can get in there with 4wd and chains. So, I will have a final look, beings as we probably can't get in until March without snowshoes. I'm too old and lazy for that.
Steve
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Christopher Tidy wrote:

Responsibility w/o compensation. Nice try, but I'm not going on the record with a solution.

It is not an insult; it is a professional opinion.

The identity of the member would be clear to an engineer; it would be clear to just about anyone since I used the unique name _you_ gave it. The h/v thing was a typo, no doubt the result of my distraction with trying to find a nice way to put this.

I think you will find the level of knowledge of the "three guys" varies just a bit. Hopefully the OP has figured out that your advice is not to be taken at face value.
Toward taking my leave of this thread, I will address some of your other comments here.
> As has been demonstrated, one of the problems with your proposed > structure is that it isn't as simple as it looks. Its behaviour > is more > complicated and less easy to predict than I thought, and probably than > Ned and Bill thought, too.
Speak for yourself. You haven't come close to the complex part (searching for the weak link in the chain so to speak). The deformation analysis is at the level of an undergrad homework problem.
> I would attack the > problem by making some changes to the design (you should still be able > to use the materials you've bought) rather than trying to find someone > who's a professional structural engineer.
More bad advice.
To the group at large, I offer an apology for the nature of this discussion, but I have learned to respect you. To not point out the flaws would be unfair to you, and potentially dangerous to the OP. I trust you will do the same should anyone give me bad advice about clamping or feed rates.
Bill
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Bill,
I feel I must enter this thread one more time.

Amid your stream of criticism and confusion between horizontal and vertical, I gave the answer to a different question: the question "Why is there no vertical force shown on the left-hand FBD acting at the junction of the horizontal and vertical members?" I stand by the answer I gave to this question. The answer to your question is very simple. I believe the free body diagram on the left to be complete and correct. No horizontal force is shown acting on the top end of the beam because the upper support provides a reaction against this (see the roller support in my diagram). If you disagree with this, please elaborate.

I agree with your description of the tension member. I don't see what you're complaining about here.
<snip>

If it's so simple, I still don't understand why you don't go ahead and solve it. From my recollection of undergraduate structures questions, this might have been a difficult second year "bonus" question. The kind which looks simple but isn't. We are also presented with the problem of having to model a vague structure from scratch; we haven't been given an idealised structure on which to perform the mathematics.

Why? Steve is unlikely to hire a structural engineer. Some structures are easier to analyse than others. Some structures are designed so that their behaviour is easier to predict. I've seen older bridges which are built with a roller support at one end. Why? So that the structure is statically determinate and easier to model. Changing a design so that its behaviour is more predictable is a sensible thing to do. If you disagree with this, please explain why.

You seem devoted to accusing me of offering bad advice without explaining why my advice is bad, or attempting to offer better advice of your own. Your refusal to solve what you claim is an easy problem is, to me, suspicious. It's all too convenient. The risk involved in offering Steve a solution is not great. You're not a practising structural engineer, so you needn't worry about your reputation, and if you say that your advice is offered without warranty I don't believe you have any legal worries. The risk doesn't concern me, and it doesn't appear to concern Ned either.
I tried to shed some light on Steve's problem in good faith. Perhaps the model I proposed was somewhat flawed, but you just comdemned it as "nonsense". At no point did you attempt to explain why it was nonsense, or how I might refine the model to give a better answer. You never explain why my advice is "bad". Furthermore, some of your questions and comments barely make sense.
I'm of the opinion that you're attempting to make me look a fool, and I don't appreciate it. An engineer contacted me by e-mail this afternoon and offered some suggestions about how to refine my model. Why didn't you take this approach?
I am also intrigued to learn that you work in an anaesthesiology lab: http://needle.anest.ufl.edu/anest4/bills /
Chris
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Chris,

I don't know how to be any more plain about it: you are wrong. The horizontal reaction is one applicable force, but that does not change the behavior of two-force members. Your FBD is incorrect. It's not "less accurate" - it's wrong.
Bill
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Construction and installation of this major project will take place in Spring of 2006. I will keep all appraised. Photos will be available as fabrication progresses, at installation, testing, and finish.
What I intend to do is fully mount everything, and leave the middle standoff out. I will then make a 200# load and measure deflection. I will then install the standoff as a CYA feature. I am going to make a public guess on record that the deflection in the middle of 15' and the lifting point 2'6" off the vertical will be less than one inch. We'll see if I am even close.
Thanks for all the help, input, and insights into areas I had never thought of.
Steve
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says...

I don't doubt you can find *some* case where your model is predictive, but a model that only works in one specific instance isn't very useful.

In your original sketch you made the assumption that the triangle was a rigid body. Considering that the triangle is only 1 foot tall, and the rest of the tube is 14 feet, that seems like a reasonable simplification. In other words, I'm confident that the deflection I calculated is quite accurate, erring on the conservative side, as a result of my using the full 15 feet as the beam's length. In hindsight, it may have been more accurate to treat the beam as 15' long, but figure the slope at 14'.
Of course there is also a vertical compression load in the vertical tube equal in magnitude to the suspended load, but its contribution to deflection will be negligible, so I ignored it.

I think you could solve the problem this way as well, and the result would be within a few percent of the way I figured it. You would still have to calculate the slope of the vertical tube at the attachment point of the horizontal.

Unless we've completely misinterpreted Steve's description, the only thing I can think of that would have a signficant effect on the results of the deflection calculations would be a change in the way the structure is supported, e.g., adding another support near the horizontal member, as you've suggested. In that case the structure would be statically indeterminate, and the problem would become one appropriate for the end of a first sememster structures course instead of one introduced near the beginning.
Ned Simmons
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Chris,

That might end up being true, but I do not think you can hand-wave it that way. With its max deflection somewhere in the middle of the beam, it is more likely that it will function as two half(more or less)-length cantelevers, which puts quite a hurt on the cubic term in the deflection.

My gut tells me that the deflection will be quadratic in length, vs. cubic as for the cantelever. My gut has been known to be wrong.

I think it will be a factor only in the stress calculations anyway, but thought I'd mention it. If your deflection is correct, the thing will tear itself apart anyway =:0
Regards,
Bill
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Bill Schwab wrote:

I take your point here. The root of the cantilever may rotate a bit so that the maximum deflection is somewhere in the middle of the beam. But I believe that the cantilever model will give a useful result for a 10-minute, one-side-of-paper calculation. Whether it is more like a single cantilever or two half-length cantilevers will depend on Steve's joints and anchor points. When we know so little, there will inevitably be some hand waving. I still think it's a fair model based on the information we have. It is only intended to give an order of magnitude result.
Chris
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Steve,
Thinking about your project a little more, I would choose to alter the design rather than trying to add a stand-off. There are simpler and stronger designs possible, such as this for example:
http://www.hi-techedu.com/products/hfc14.htm
And here are a couple of older cranes which might give you ideas:
http://www.tclayton.demon.co.uk/pics/canal/npb5.html
http://www.stroudwater.co.uk/cpsn/dudbridge%20crane.jpg
Note that both of these have a compression member which is at an angle to the vertical. This is a nice, simple design. You just need to avoid applying large moments to the beam as in your original proposal. In pure compression 2" x 2" x 0.25" tubing should be fine.
The most critical part of the crane is likely to be the point at which the top is attached to the wall. If you choose to use expanding bolts, look up their load rating and use a good margin of safety (a factor of 10, say). Use the right size drill and make sure the wall isn't crumbly. Should it fail, a joint like this will not fail in a nice way.
All the best,
Chris
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