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Shear web with unstiffened holes

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trainguy

Structural
Joined
Apr 26, 2002
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706
Location
CA
Hi all.

Can anyone suggest a design guideline for a shear panel that has holes without stiffening? I am actually evaluating a shear plate with certain cracks, and I figure that replacing cracks with holes for one time proof loads is a decent approximation for the order of magnitude of capacity lost due to cracking. This is for resisting buff loads on a passenger railcar.

Thanks!
 
cracks are cracks and holes are holes ... i don't think i'd use one to represent the other.

and buffet loads implies out of plane loading ?
 
I agree with rb1957, I don’t think I’d try to represent cracks in a plate with holes either. They are, and act, very differently under stress. And, with so little info. on the real situation who knows what’s really going on. You gotta give us some clues or you are probably not going to get much real good help. What do you think the stress levels and orientations are? What is the rolling direction of the plate. What are the loads, moment, shears and the number of cycles? Are the loads due to normal train action loading, high coupling loads, impact loads? Show a sketch with sufficient detail so we can see what’s going on. Where do the cracks start and stop? Is this one-of-a-kind or one of many? Many times you can stop cracks from growing by drilling small holes at the tips of the cracks. You can also gouge them out, weld them up with a slightly softer filler metal, and you’re good for another thousand miles. What about your detail is causing this to crack? Can you do something slightly different to absorb this energy, stretch, not crack?
 
The shear plate in question connects a short center sill with the 2 side sills on a passenger railcar, between the body bolster and the car's end sills. The loading is a one time crash load of 800 k applied by the coupler. The shear plate only carries in plane shear and pretty much zero fatigue loading. In service loads are approx 1/40 of the proof load mentioned above. My client wants to know if they can avoid repairing cracks in these plates.

I certainly can provide fea of various cracked arrangements, but it's not clear to me what allowables I could use given buckling.

I'm not a big fan of FE eigenvalue buckling analyses because it's been shown they tend to overestimate buckling loads.


Any suggestions would be appreciated.

Tg
 
I take it these are existing passenger cars which are starting to show cracks from normal operating conditions, tension and compression loads from the draft gear, and now you are being asked if they still meet the 800k buff loading criteria without catastrophic failure. It seems to me that you do have a fatigue problem or you wouldn’t have the cracking, although at a much lower stress level than the 800k buff load would cause. I think you have to look at where the cracks are forming and why, and how these cracks are oriented w.r.t. the loads and stress fields if this shear plate now sees the 800k buff loading. Do the cracks settle down once they form, or do they keep growing over time? What details are causing this cracking? I would at least drill small holes at the tips of the cracks. Without seeing the details and the end arrangement, outboard of (but including) the body bolster, I would imagine (expect, by design) that the shear plate would initially buckle under the compression loading, and it would them become two diagonal tension fields from the inner end of the stub sill, at the body bolster, to the outer ends of the two side sills. The body bolster and the end sill and any other cross car framing (cross ties, etc.) provide a cross car force system to hold the side sills and center sill apart during this tension field action. They also act in horiz. bending in concert with the shear pl. The shear pl. acts to do this over the stub sill/draft pocket. But, all of the welded joinery is acting to cause the existing cracks and may cause them to grow during this impact loading. If the cracks are oriented in the direction of the tension field stress lines, they are less of a problem on impact. If the cracks are diagonal to or cross the tension field stress lines, they are almost certainly an early failure mechanism. The weld details are all important to prevent the cracking from starting.

I’d look at these shear plates like buckled webs on a plate girder. They first start acting in shear, then buckle out of plane, and then start acting as a tension field of some depth, from the stub sill web to the side sill. And, I would certainly use FEA to study this problem, just not expect exact (certain, absolute) answers from it. I’d do the FEA model to represent the as built end frame, on two supports at the side sills, and loaded at the rear draft lugs, and run that through the normal draft and buff loads, and the 800k impact load. Then introduce the cracks one at a time (a linear discontinuity 1" long, for example), and only one at a time, to see what these did to the stress picture After the impact a slightly crumpled car end outboard of the body bolster is o.k. as long as the rest of the car is intact. Are these transit cars or over the road passenger cars, how old? I don’t think there is an exact answer to your problem, the next crack will be slightly different, starting at a slightly different defect. All you can do is offer some good engineering judgement about a range of failure loads and the nature of the failure. You might see that some cracks and the details that caused them should be welded up and cleaned up, while others are of little consequence, but I’d still drill the tips. I’d like to see an end arrangement drawing and some details.
 
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