Knee Braces

[ToadJones]

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.

 

[ishvaaag]

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.

 

[Ron]

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.

 

[SlideRuleEra]

...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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[ToadJones]

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.

 

[dhengr]

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.

 

[SAIL3]

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.

 

[Ron]

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?

 

[ToadJones]

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"

 

[ToadJones]

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

 

[ToadJones]

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.

 

[a2mfk]

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

 

[ToadJones]

a2mfk- see link below

http://www.middough.com/Newsroom-Docs/Iron-and-Steel_Knee-Braced-Crane-Girders_Ray-Milma.aspx

 

[a2mfk]

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.

 

[ToadJones]

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.

 

[Ron]

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

 

[hokie66]

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.

 

[ToadJones]

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.

 

[JAE]

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.

 

[miecz]

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.