Anchor bolt diameter? 1

[300redbeard]

Public Works Issue-21' tall abutment with 26' span

I have an issue regarding anchor bolt diameters. AISC table J3.3 states for 1/2" dia. holes: std-9/16", oversize-5/8" and long slot- 9/16"x1 1/4" respectively.

I have a bearing plate on a pedestrian bridge that the fixed end has 3/4" dia. holes for the A307 1/2" anchor bolts and the sliding bearing end has 3/4"x1" slotted holes with 1/2" anchor bolts.

The engineer of record wants to fix the oversized holes by 'A': use a 1/2" washer over a 3/4" washer. Table 6.1 AISC (RCSC): ok if washers not thicker than 5/16". If both washers are at extreme for thickness tolerance, they will exceed 5/16". Or 'B' use 1/2" thick plate x 1 1/2" x 1 1/2" with 9/16" dia. hole.

My concern is that this criteria is for standard oversized holes, not larger than oversized holes as I have stated for my case. Am I interpreting this correctly? Also the washers or the 1/2" plate will bear next to a 1/4" fillet weld, this will result in the washers or plates taking the lateral force instead of the anchor bolts.

Thanks for any comments!

 

[nutte]

This is why AISC calls them "anchor rods" and not "anchor bolts." The tables you reference aren't intended to apply to your scenario.

 

[steellion]

Table 14-2 in 13th Ed. AISC gives recommended maximum sizes for anchor rod holes in base plates. The EOR is correct to want oversized holes for anchor rods to ease construction tolerances.

 

[300redbeard]

I understand the desire for oversized bolt holes for ease of construction, but where do we draw the line? Isn't that what the tables in the AISC and the RCSC are for? I know on School projects in CA they won't accept anything over 1/16".

 

[DaveAtkins]

As stated above, the 1/16" tolerance is for typical connections, not base plates. AISC states the proper size hole for anchor rods as small as 3/4" is 5/16" oversize. It appears to me a 3/4" hole for a 1/2" anchor rod is not unreasonable.

DaveAtkins

 

[nutte]

Isn't that what the tables in the AISC and the RCSC are for?

Those are for steel-to-steel bolted connections, not base plate connections.

 

[JAE]

(continuing nutte's thought): ...and for high strength bolts not anchor rods.

 

[300redbeard]

Thanks Gentlemen! Advice is appreciated.

 

[sandman21]

The DSA requirement applies to anchor rods if you are trying to transfer shear through the bolt to the concrete. I don't have the section with me, but if need I can look up tomorrow.

 

[weab]

In general direct shear is not the correct method assumed for shear transfer in base plates. A hole can be almost any size you want for anchor bolts. If you need direct shear transfer, you need to weld plate washers to the base plate after column installation.

 

[300redbeard]

"In general direct shear is not the correct method assumed for shear transfer in base plates" - jsdpe25684


What is the determining factor as to whether you should use direct shear for base plates or not?

 

[sandman21]

Correction the section 2204A2.2 applies to any base plate with oversized holes are greater than 1/8". I do not see how direct shear would not be a correct method to transfer shear.

 

[weab]

I was not clear in my response, and maybe I'm saying what you already known. What I meant to say is that anchor bolt design is based on a shear friction assumption, not directly the shear strength of the anchors.

As the anchor bolts make contact, a concrete wedge breaks out and tries to slide up into the base plate. Friction then occurs which is used to resist the lateral force. It should not be assumed that all anchors make contact due to oversized holes. When forces get too large, use shear lugs. See AISC design guides 1 or 7.