Filler in Bolted Connections

[iv63]

Section J5.2 of the 2010 AISC Specification addresses filler plates in bolted connections. When a bolt that carries load passes through filler plates that are greater than 1/4" thick, the shear strength of the bolts shall be multiplied by factor 1-0.4(t-0.25) but not less than 0.85 which limits t to 5/8". How are bolts checked in the connection with filler plate greater than 5/8”?
Regards,
iv

 

[cliff234]

For connections with bearing bolts, the 0.85 factor is the maximum permitted reduction factor. The value gets smaller as the filler plate gets thicker. There is no upper limit on the filler plate thickness. For fillers less than 5/8", the reduction factor is 0.85. For fillers thicker than 5/8", the reduction factor will be a value less than 0.85. (example: for a 1" filler plate, the reduction factor is 0.70.) See AISC 360-16, Section J5.2.

There is a different procedure for calculating shear strength of slip-critical bolted connections with shims and filler plates. See AISC 360-16, Section J3.8

 

[Lomarandil]

Tangentially -- If your filler plate gets to be thicker than the bolt diameter (and is not connected to adjacent material on either side), you should probably also start considering bending stresses in the bolt.

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just call me Lo.

 

[BridgeSmith]

Lo, the commentary in the AASHTO code indicates that purpose the reduction factor for bolt shear capacity was to account for bending in the bolt. Are you saying that bending in the bolt sometimes needs to be considered separately, or in addition to, the reduction of the shear capacity?

Rod Smith, P.E.

 

[Lomarandil]

HotRod -- my (imperfect) understanding is that the reduction factor accounts for bolt bending in most common cases (when the gap/filler is not unusually thick). However, in the transmission line industry, we often considered bending effects separately for very large gaps or fillers.

(Or, more practically, design a "structural fill plate" which can be welded to one faying surface or the other.)

----
just call me Lo.

 

[iv63]

Thank you all for your contributions. I have interpreted “but not less than 0.85” that reduction factor cannot be less than 0.85.

 

[dauwerda]

I agree with your interpretation iv63. However, I don't believe the intent is to limit the thickness of the filler, the intent is that the multiplier need not be taken less than 0.85 no matter how thick the filler is. If you go to the commentary (15th ed) it states, "Starting with 2010 AISC Specification, bearing connections with fillers over 3/4" thick were no longer required to be developed provided the bolts were designed by multiplying the shear strength by a 0.85 factor."

 

[iv63]

So is 3" filler allowed?
iv

 

[dauwerda]

Here is the paper that the 0.85 factor comes from:

http://www1.coe.neu.edu/~jfhajjar/h...ers -- Report No. NSEL-017 -- August 2009.pdf

From the paper (page 87):"The shear strength of a connection is influenced by the introduction of filler plates. The filler plate separates the splice plate and column shear planes and introduces bending into the bolt. The interaction of shear and bending reduces the shear strength of the bolt. The bolt bending increases with the thickness of the filler. Additional clamping force is also induced by the reaction of the bolt onto the filler, proportional to the filler thickness, which tends to increase the bolt shear strength of the connection. These two mechanisms offset each other and the shear strength is initially reduced with increasing thickness in relatively thin fillers, and then increases in strength for thicker fillers."

The tests described in the paper used fillers up to 3-3/4" thick.

Based on the above, I would say a 3" filler is allowed

 

[iv63]

Thank you dauwerda. Your paper deals with fillers in slip-critical connections. Are fillers 3" thick allowed in bearing-type connections?
iv

 

[dauwerda]

That is the paper referenced in the commentary for section J5 (and J3.8) - this leads me to believe that it is the intent of AISC to say that it is applicable for both cases.

 

[BridgeSmith]

In the AASHTO spec, there is no minimum for the reduction factor. It's based on the ratio of the filler plate area to that of the smaller of the flange or splice plates. For fillers the same area as the flange or splice plates, the reduction is 0.667; at fillers twice the area, it's 0.60; at fillers half the area, it's 0.75.

Rod Smith, P.E.

P.S. AASHTO doesn't have any reduction for slip resistance.