Filler plates vs shim plates slotted in direction of shear - bolt strength reduction due to bending?

[icebloom]

For finger shims used in the erection of steelwork, often these have slots to allow them to either be installed horizontally or vertically as required. For the case of an endplate splice or some variant of a plate to plate connection, you may have the scenario when you have a slotted shim separating the endplates, with slots orientated in the direction of the shear force as shown below. For standard filler plates, codes generally have some form of bolt strength reduction factor similar to the equation shown below (taken from Australian standard) which is based on the thickness of the filler plate. Would anyone know if this equation is still valid in the case that the shims are slotted? I..e is this a generic "gap between plates" strength reduction equation, or specifically for the scenario in which you have a "full" filler plate as in fig 2 below. I can find no reference to this in codes or design guides, yet these kind of slotted shims seems to be common practice in steel erection. My thinking is that for the full filler plate (2 in image below) if any bending of the bolt occurs due to the presence of the filler plate, there is some kind of clamping or jamming action occurring which may be captured in the bolt strength reduction equation (which from my understanding are generally empirically based) whereas with the slotted shim this would not occur and you'd get more of a pure bending of the bolt, and hence less capacity.

 

[dauwerda]

I don't have it in front of me right now so I can't offer too much detail, but I know section 9 of the AISC steel construction manual address this (as does section J5 of the specification, but the manual part goes into more detail I believe).
My interpretation would be that the reduction is due to the bending that can occur due to the filler plate, so the slot is being accounted for.
AISC allows as an alternative to bolt strength reduction, welding the filler plate to one member. This does require the filler plate to also be a bearing member so I believe that wouldn't work with the slotted holes in the same direction as the load.

 

[human909]

I'm digging this up from a few weeks ago because I have the same challenge myself. Not specifically finger plates but filler plates in general do affect strength. A current design I'm working on will have ~6mm filler plates with 24mm bolts. There is plenty of spare capacity in the bolts though.

Here is a good bit of research on the topic:

https://ntrs.nasa.gov/api/citations/20205000526/downloads/20205000526_TM_1.1.pdf

 

[icebloom]

Thanks for the reference @human909, I will have a read. I've looked through the literature myself and found the common papers and equations for reduction in strength due to filler plates. Finger shims seem quite common and yet intuitively I'd say there is less capacity than with filler plates, due to the slots.

 

[KootK]

Finger shims seem quite common and yet intuitively I'd say there is less capacity than with filler plates, due to the slots.

I disagree. My understanding is that you develop a clamping force as shown below. As with most thig looked at at a small scale, a strut & tie kind of setup. Viewed in this way, it is entirely intuitive to me that the presence of reasonable slots would not impair the mechanism relied upon.

I see the shim as little more than a stiff spacer.

While some shear may well traverse the shim via inter-plate friction, I don't believe that we rely on that overtly.

 

[icebloom]

I disagree. My understanding is that you develop a clamping force as shown below. As with most thig looked at at a small scale, a strut & tie kind of setup. Viewed in this way, it is entirely intuitive to me that the presence of reasonable slots would not impair the mechanism relied upon.

I see the shim as little more than a stiff spacer.

While some shear may well traverse the shim via inter-plate friction, I don't believe that we rely on that overtly.

View attachment 2581

With a slotted finger shim I'd say there is inherently less clamping than if you had a full filler plate as your endplates need to span horizontally across the gap. I guess it all depends on context, what is the force in the bolts, what is the gap, and what is the thickness of the plates. We wouldn't rely on the clamping for any shear resistance, however from what I can find the reduction factors in codes are generally empirically derived from experiments with full filler plates rather than slotted shim plates. My worry (maybe I'm overthinking it) is that for slots in the direction of shear force the bending in the bolts may be greater than that of a full filler plate as the clamping mechanism cannot develop directly below the bolt, and as such the empirical reduction factor may be unconservative. I guess just don't have the bolts working hard and have reasonable plate thicknesses is the way to go. Alternatively I guess there is the "bolts in bending" equation that is used in standards for bolt bending in shear when anchoring to concrete with a lever arm - if worried about bolts in pure bending this could be an avenue to check it conservatively(if say the clamping mechanism is reduced and the empirical reduction factor is not applicable).

 

[KootK]

With a slotted finger shim I'd say there is inherently less clamping than if you had a full filler plate as your endplates need to span horizontally across the gap.

Relative to a non-fingered plate (sounds gross), I would say that the only difference in the clamping is the incredibly small amount of bending that you would get in the outer plate because it would only be able to span 3 ways rather than four. So no practical difference at all so long as the width of the fingers is similar to the oversize in a common bolt hole-ish.

Alternatively I guess there is the "bolts in bending" equation that is used in standards for bolt bending in shear when anchoring to concrete with a lever arm - if worried about bolts in pure bending this could be an avenue to check it conservatively(if say the clamping mechanism is reduced and the empirical reduction factor is not applicable).

I feel that would be excessively punitive and not reflective of the way that we should expect this system to behave. The bolt rotation in my sketch allows there to be a horizontal component of the tension force in the bolt. It's that we should rely on for shear resistance across the shim, not bolt bending which is highly inefficient and flexible relative to bolt axial strain.

I guess just don't have the bolts working hard and have reasonable plate thicknesses is the way to go.

If that's what it takes to get you back in your comfort zone, so be it. I personally see no need to treat the fingered shim any different from a conventional shim.

"Conventional" shim may not be the best terminology. In my experience, fingered shims are altogether common and, if they exhibited a special weakness, I would have expected to hear of it long ago.

 

[human909]

This is an interesting discussion and I've been following along. I've had my thoughts and I've sought of agreed with both points of view and I've been uncertain so I haven't jumped in yet.

I feel that would be excessively punitive and not reflective of the way that we should expect this system to behave. The bolt rotation in my sketch allows there to be a horizontal component of the tension force in the bolt. It's that we should rely on for shear resistance across the shim, not bolt bending which is highly inefficient and flexible relative to bolt axial strain.

My understanding is that bolt bending is what is happening. It is also mostly exhibited in the failures in the paper linked. However the behaviour you have outlines is also present and the more ductile bolts do fail like that in the paper. Both can occur and are not mutually exclusive.

As the paper concludes:

The NASA criterion adequately bounded the test data with the simple assumption that the induced bending moment is equal to the shear load times the shim thickness. The single equation nature of the criterion does not capture the actual varied loading or capability in the bolt or the difference caused by joint members of different strength. This makes the criterion very conservative for bolts and joints limited by shear failure of the shank. A criterion that can account for these differences would potentially be more accurate. The McCombs criterion is a simple example. Yet, the McCombs criterion also proved to be conservative compared to the test results .This criterion considers shear failure at the shank and moment failure at the head/nut.