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.

 

[ToadJones]

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.

 

[JAE]

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.

 

[ToadJones]

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.

 

[paddingtongreen]

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.

 

[hokie66]

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.

 

[ToadJones]

there is this one ...basically a design guide...some good stuff in it.

http://www.sigi.ca/engineering/documents/cisc_crane_support_steel.pdf

 

[ToadJones]

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.

 

[ToadJones]

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.

 

[hokie66]

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

 

[ToadJones]

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.

 

[hokie66]

Toad,
It was the same reference, but the one linked in the earlier thread was the second edition.
thread507-217733
Supposed to cost $40, which is a bargain, but it freely downloaded for me. Hope we are not doing the wrong thing.

 

[SlideRuleEra]

TJ -

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[paddingtongreen]

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.

 

[ToadJones]

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

 

[paddingtongreen]

@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.

 

[hokie66]

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.

 

[ToadJones]

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

 

[ToadJones]

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

 

[cvg]

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

 

[paddingtongreen]

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.