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Knee Braces

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.  

RE: Knee Braces

To say what was the intent in every case is adventurous, and only a detailed review may or might with some likelihood to assert the probable intent.

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.

RE: Knee Braces

TJ...I'm not sure you can separate the two, without regard to the intent.  Knee braces have the same effect as a rigid connection, except that the connection doesn't take the rotation, yet it is transferred to the member.

RE: Knee Braces

Quote (ToadJones):

...did the original designers count on these knee braces for bending capacity of the runway or simply for stability?
In my opinion, neither. Probably for a third reason: reduce beam deflection to the recommended L/600 to L/1000 range.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Knee Braces

(OP)
Ron-
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

Toad:
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.
 

RE: Knee Braces

Since the knee braces exist, they will tend to pick up the end shear reaction of the girder and depending on the geometry of your setup and wheel spacing, this could cause uplift at the actual girder/col. connection that it may not have been designed for.

RE: Knee Braces

TJ...gotcha.  In that case, SlideRuleEra has a point, but stability is more likely the intent since the beams were designed as simple spans.  Were the columns designed for bending?   

RE: Knee Braces

(OP)
dhengr-

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

(OP)
SAIL, Ron, SlideRule, Ishvaag...thanks for hearing me out. I appreciate the honest input.

 

RE: Knee Braces

(OP)
I had no clue the fatigue problems had to do with stress reversals caused by knee braces or that the web splice plates break and crack the girder webs / prevent beam end rotation or that the knee braces could cause and uplift at the girder seat. I really had no idea. Fascinating stuff. My computer output didn't explicitly state that so I really had no clue.  

RE: Knee Braces

What's a knee brace? I have only ever needed ankle braces. Stupid basketball, reversing the stress in my ankle joint...

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

RE: Knee Braces

Thanks Toad, very interesting. So it was a method left over from the prescriptive days, where something looks good at first site but has unintended consequences. I would not have thought about that effect at first glance, I'd have to do the statics. See, not being that smart or cocky, I have to do some FBDs before I can really get it, and hopefully I would catch that :)

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

(OP)
a2mfk-
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.  

RE: Knee Braces

TJ...go have a beer...! Have one for a2mfk and me while you're at it!

RE: Knee Braces

TJ,
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.

RE: Knee Braces

(OP)
Ron- you're right, I should. Been a super busy/hectic few months.

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.

RE: Knee Braces

ToadJones,

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

I think SlideRuleEra nailed it. I remember my mentor (we're going back a long way here) telling me that, in the old days, knee braces were only considered for deflection considerations.  I believe the deflection was calculated for a simple span of length between the knee braces.

RE: Knee Braces

(OP)
JAE-
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

ToadJones,

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

(OP)
JAE-
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.  

RE: Knee Braces

The knee braces are there for stability, pure and simple. If they had been included to reduce deflection they would most certainly have been used to reduce bending stress. The thought was that the connection had enough slip in it to accommodate the beam deflection, I am not convinced that this is untrue. In my mind, the main contributor to making beams behave as continuous over the support is the continuity of the rail through the joint and the friction between rail ans girder when under crane load.

 

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Knee Braces

Someone posted a very good reference here some time ago, Canadian I think, about things to avoid in crane girder design.  I will try to find it.  Probably not yet seen by some.

RE: Knee Braces

(OP)
Ricker also wrote a paper called "Tips for Avoiding Crane Runway Problems" for the AISC engineering journal some years ago. I don't believe it is available for free.  

RE: Knee Braces

(OP)
Paddington-
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.  

RE: Knee Braces

That may be the one.  Looks familiar, but I will search some more.

RE: Knee Braces

(OP)
Pad-
"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.  

RE: Knee Braces

TJ -

Quote (ToadJones):

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.
Why not make an easy calc to see if the knee braces were used for defection control, or not?
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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Knee Braces

I'm beginning to feel insulted, we were neither ignorant of structural analysis nor unprofessional. If we used the knee braces for deflection control, we would have been forced to include them in the bending moment, knee brace and connection calculations. We would also have had to take account of the axial load induced into the girder.

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.

RE: Knee Braces

(OP)
Michael-
I certainly hope I did not insult you. You're input was exactly what I as looking for.
 

RE: Knee Braces

@Toad, no not you. It's the repeated suggestion, by others, that we deliberately ignored important structural consequences when it was convenient; that we would use knee braces to reduce deflection without considering the the consequential effects.

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

I wouldn't classify the knee brace initiated failures as fatigue.  It is just poor detailing, resulting in local overstress.  Probably done with the best of intentions and before it became apparent that the detail was problematic.  The failures at web braces to the building column fall in the same category.

RE: Knee Braces

(OP)
Pad-
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

RE: Knee Braces

(OP)
Pad-
There is no way you are old enough to have designed the building I am analyzing; circa 1930.  

RE: Knee Braces


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.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Fatigue.htm

RE: Knee Braces

Toad, you're right, but my mentors did design in the thirties.  

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Knee Braces

(OP)
Pad-
Seems like a lot of your career was spent industrial settings.
As usual, I appreciate the input.  

RE: Knee Braces

Pad - one of my mentors started his career in 1927.  Two others in 1947 and one in 1949.  I don't feel insulted by anything here and neither should you.  

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

JAE, It wasn't Toad that I was annoyed with, Toad and I agree on more than we disagree. I think SlideRuleEra and someone else said, more than once, that the designer might have used the braces to limit deflection even though Toad said that they were not in the bending moment design.

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.

RE: Knee Braces

Pad - thanks for the reply - I just took those comments to be more for LATERAL deflections rather than vertical deflections.  A lot of "us" older engineers learned from "even older" engineers the concept of designing a straight-up gravity frame and then adding knee braces, bottom chord joist extensions, etc. to limit wind deflections and add, in a seat-of-the-pants way, the lateral drift of a structure.

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)

 

RE: Knee Braces

Paddingtongreen - You say that I, SlideRuleEra, have both insulted and annoyed you. For the record, here, in total, is what I said on this thread:

Quote (SlideRuleEra):

In my opinion, neither. Probably for a third reason: reduce beam deflection to the recommended L/600 to L/1000 range.
and

Quote (SlideRuleEra):

Why not make an easy calc to see if the knee braces were used for defection control, or not?
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.

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.

SlideRuleEra

 

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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