Knee Braces
Knee Braces
(OP)
I've got a scenario right now; one that I have had to analyze many times, with a crane runway with knee braces.
First of all, from my experience with runways knee braces = BAD as they cause the runway connections to fatigue and break.
My question is, did the original designers count on these knee braces for bending capacity of the runway or simply for stability?
My suspicion in review design loads on old drawings is that the knee-bracing was strictly intended for stability.
First of all, from my experience with runways knee braces = BAD as they cause the runway connections to fatigue and break.
My question is, did the original designers count on these knee braces for bending capacity of the runway or simply for stability?
My suspicion in review design loads on old drawings is that the knee-bracing was strictly intended for stability.






RE: Knee Braces
With this we turn our sight to one of the inner sanctums of steel design, quite inexplored, or better, not stated clearly and comprehensively enough: the subjects around overall instability, what is the behaviour when the connections usually calculated in a plane are not, and proper and foremost complete design of adequate bracing; how practiced in the past, and how better is and should be practiced today. I would say that my collection of papers on the matter is one of the most erratical in content, lacking systematic layout, and leaving so very wide expanses of unknowns or doubts. I am certain that a book of the extension of the Galambo's just on these matters would be very welcome by the structural designers.
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RE: Knee Braces
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RE: Knee Braces
I am not trying to separate. I realize that it is what it is.
For the most part, having the knee braces reduces moments in the girders. In looking at the original design moments on the drawings, the knee braces were not taken into account in the design of the runways. The moments were determined as simple spans.
RE: Knee Braces
Seems you and I have locked horns on computin power and prowess a time or two before. And, with the software you have today you should be able to see and get a pretty good handle on what is going on with that area of a crane runway girder. And, possibly even get some insight into what the original designer had in mind the day he did that design.
The structure doesn't give a damn what you and I think, how we think it should act, or how we model it; it will act the way we forced it to act by the way we detailed it, irrespective of how we analyzed it. Without the benefit of the old calcs., maybe studying the exact way it is detailed will reveal something, and give you some insight. I suspect this knee brace detail was treated different ways by different engineers, on different days. I suspect the knew braces were primarily considered for stability; but if the designer could make the next smaller size beam, or a size he had in stock check, he certainly might allow that the knee brace offered some fixity and reduced the center line moment, or some such. Whichever way he was thinking at the moment, what probably wasn't done, was to truly consider the amount of fixity or secondary forces which the knee brace would (could) induce into the joints and connections, thus your fatigue and joint parts failures.
This is where you should shine, with all your FEA and computin power, you can analyze and see things that we could only dream of, or make educated guesses at. Now go play with your computer and get something constructive done, instead of just social networking. Remember, model carefully, because that structure doesn't care what you think; maybe fairly simple, clean, frame analysis around that joint until you really understand what causes what to change in the way of moments and forces around the joint; at this stage we don't care what von Mises thinks either. What you want to see is the upward force on the col./bm. connection, or some such, which nobody assumed existed, or forgot about after girder was sized with the knee's help, and then didn't detail for, that's your fatigue or joint parts failures problem.
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re-read the OP.
I never asked single question about what the knee braces actually do. I know what they do. I can analyze these by hand and by the "computin prowess".
I simply stated the girder moments on the original drawings do not account for the knee-braces.
I honestly don't know what you are getting at. So I'll ignore your posts from now on.
"Now go play with your computer and get something constructive done, instead of just social networking"
RE: Knee Braces
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I googled this and didn't see this crane you guys are talking about, but I finally know where my meniscus is located.
knee brace + structure + picture
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htt
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I do like the sketch on pg 58 on the LLC, 300k??? Funny stuff. That is how I feel doing calcs sometimes. I wish I could just stop there with some question marks and let someone else wrap it up, like a "closer" structural engineer that comes in about 530pm and works till 10pm to wrap up your project. Hell, I'd like a middle relief guy around lunch too.
RE: Knee Braces
I run into this pretty often in my line of work and it is never fun.
I try to post questions on here in plain English so all who read will understand. I try not to go over the heads of those who might be in a slightly different area of work.
If I come off as a dummy, well, that's just fine with me.
Other times when I run into something and figure it out, I will post the question anyway, so that maybe someone else will benefit.
As for others, well, they are omniscient and only chime in with answers.
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I have seen the type failure described in the article you posted. When I worked for a primary aluminum producer, we once took out all the knee braces in an old mill building.
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Hokie- We have done the same on a few occasions. It sometimes can lead to other problems. One way or the other, they always seem to cause problems.
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Just like hokie66, I've done a complete re-build of a bridge crane line (350 ft. long x 70 ft.) where there were knee braces.
In most cases there were fatigue cracks (some not visible to the naked eye) running horizontally from the top bolt at the column-girder connection - back towards the span. Very similar to what your attached article shows in its Figure 3.
I think your main question - what did the designers intend for the use of the knees - most probably - if the beams are old - were to provide for lateral stability of the crane beam system.
If very old, they most likely wouldn't have considered the frame action of the knee braces, bending in the columns, etc.
I get the feeling you will be analyzing these as they are anyway - but the sense I got with my craneway was that they were there simply as a mechanism to resist longitudinal crane forces.
RE: Knee Braces
RE: Knee Braces
Yes, Usually what I try to do is analyze the runway for the seemingly unintended frame action of the knee braces (using modeling software in order to get relative stiffness of the runway girder and knee brace correct) and also for the simple span condition and compare the two. In the case of am upgrade, using the knee braces can result in some extra capacity.
However, I am not all that comfortable with using that reserve capacity of a knee brace, hence my original post.
I am usually not afforded drawings that have original design shears and moments for the runways as I have been given in this case. I was simply reviewing the old drawings in order to surmise the original intent of the knee braces
Fatigue cracks in runways with knee braces and in the webs where web splice plates are located is very common. I see them all the time. These problems are not usually very easy to fix as most runways are littered with utilities and of course, hot rails.
RE: Knee Braces
One other issue that cropped up with the runway I did was that those horizontal cracks at the top bolts were also probably caused by LATERAL deflections in the top flange with respect to the webs.
As the crane shifted sideways with a moving hoist, the web portion between the top bolt and top flange bent over (all the lateral shear was directed out of the system through a clip plate from web to column).
This caused repetative bending in the web. The better detail, of course, is to connect the top flange to the column and take the load out directly, rather than through the web.
I think the moments you were provided probably answers your question here better than we could.
Hope you do some dye testing on the webs. Good luck.
RE: Knee Braces
Yea, those web plates you are talking about are what I would call Web Tie-Backs or Web Diaphragm Tie-Backs.
They are a bad detail. You're right, side thrust forces roll the top flange over, back and forth and a crack propagates from the nearest hole or stress riser.
I do a lot of dye and mag particle testing and repair procedures for those all too common types of cracks.
If possible and feasible, I usually recommend a new top flange tie back.
Those types of cracks are less common with a thrust plate (walk plate) and a back-up truss or girder as the thrust plate helps to mitigate the top flange rolling.
Sometimes the cracks in the webs at the seats are a combination of the phenomena you describe and the web splice plates creating a bit of a continuous girder situation.
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Michael.
Timing has a lot to do with the outcome of a rain dance.
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http://ww
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I can see your line of thinking, but with a rail that can float longitudinally, I'm not sure that would be an issue. I guess it could be an issue rather instantaneously depending on wheel locations.
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"The thought was that the connection had enough slip in it to accommodate the beam deflection"
This was the type of input I was looking for as I wondered how these fellows ignored the brace in the beam design.
Thanks.
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It was the same reference, but the one linked in the earlier thread was the second edition.
thread507-217733: Bearing stiffeners on crane-runway beams
Supposed to cost $40, which is a bargain, but it freely downloaded for me. Hope we are not doing the wrong thing.
RE: Knee Braces
You know that the beams were designed as simple spans. What would the defection be for this simple span? If simple span deflection is greater than (approximately) L/600, then the knee braces (intentionally) contribute to deflection control.
If assumed simple span deflection is less than L/600, then the knee braces have little or nothing to do with deflection control.
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RE: Knee Braces
The deflection of a properly designed girder, close to the support is tiny. We assumed that there was enough slippage in the connection and if not, a small amount of yielding in the plates, we did make efforts to see that they were the place where yielding would occur.
You know, back when many of these were being designed, there was little knowledge of fatigue. research only started in earnest after a de Havilland Comet crashed in 1954 and then it took time to percolate into the type of work we do.
Michael.
Timing has a lot to do with the outcome of a rain dance.
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I certainly hope I did not insult you. You're input was exactly what I as looking for.
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I haven't retrofitted any old cranes, I would be interested to know if you found similar fatigue in girders that did not have knee braces. As I said, the knowledge of fatigue has developed over the past fifty years.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Knee Braces
RE: Knee Braces
I have found cracks in many girder webs without knee braces and usually they are the result of web splice plates, or, web daphragm tie backs that double as web splice plates.
This to me is the result of poor details (as Hokie says) and, in the case of web tie-backs, unintended fixity of a joint thought to be a simple support.
Hokie- I guess fatigue gets tossed in with knee braces because they cause negative moments --> stress reversal
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There is no way you are old enough to have designed the building I am analyzing; circa 1930.
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Given enough stress cycles at high levels, this type of connection could definitely experience fatigue. Given a 50 year service life and 10 cycles per day (which may be conservative), the connection could see over 150,000 stress cycles. This would be within the range 2 loading condition per AISC which requires a significant reduction in allowable stress.
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Michael.
Timing has a lot to do with the outcome of a rain dance.
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Seems like a lot of your career was spent industrial settings.
As usual, I appreciate the input.
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The statement by ToadJones was that the indicated moments didn't correlate with the presence of the knees. Thus, the engineer (not you - the actual design engineer on this particular project) didn't account for them in the analysis. These are just factual observations of what TJ described.
RE: Knee Braces
My point is; that to say they limit deflection, while at the same time not affecting the bending moment would be unprofessional, unless it were done out of ignorance.
Now, I'm done with this subject.
Michael.
Timing has a lot to do with the outcome of a rain dance.
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That is what I think a lot of crane beam rail designers used to do. (I don't do that at all but I've seen it done)
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and
I don't see anything said, or implied, about your comments or professional opinions on this thread.
To the contrary, if the calculations that I suggested in my second post are performed the results will likely make my first comment look foolish. That's ok with me. As far as I'm concerned, Engineering-Tips is all about good engineering practice and accurate results - NOT "insults" or "annoyance".
There's more that I could say... but that's enough.
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