Base Plate Prying Action?

[KootK]

When you've got an angle or a tee section supporting a tension load, you have to amplify the bolt forces for prying action.

Is there a similar need for bolt amplificati0n when designing anchor bolts in fixed base plates where the anchor bolts provide a tie down force for the base plate?

Obviously there is SOME prying action due to the 1.5" or so base plate overhang. Perhaps, since concrete is considerably less stiff than steel, this makes the prying effect insignificant?

Thanks,

Kootenay

 

[271828]

With the way people usually determine the plate thickness, there's no need to consider prying action. The usual assumption is that "failure" equals a single yield line forming at the flange adjacent to the tension rods. Prying action will only happen if the plate is made thinner, so a second yield line is required to resist the load--the second one will be at the bolt lines. If I was going to try to really design base plates to the gnat's butt, I'd use the AISC DG16 (or maybe 4) and use their procedure that includes prying action. It would have to be adapted a little for use with base plates, but this would work.

 

[KootK]

281828,

Thanks for your post.

I'm doing something a little unusual with this. I'm elongating my base plate and making it as thin as possible in order to make the connection more flexible and minimize the tension in my achor bolts (long story). This is how I've come to be concerned with prying action.

I've always been curious about yield line design in steel. I'll check out the design guides that you recommended.

Kootenay

 

[rb1957]

"tie down force" implies a tension load on the fitting, no? i would analyze like a Tee fitting under tension.

 

[KootK]

I gave that a whirl Aerospace. It triples the bolt force. Additionally, the AISC procedure doesn't account for the fact that the material pried against is concrete, not steel. Maybe it doesn't matter, I'm not sure.

 

[mudflaps]

K-kid, You mentioned a base plate. Is this a column on a footing. If so they will probably double nut it for erection and mess up your assumptions. Maybe limit the base plate grout to the area between bolts, or just under the column. this could become a strange looking detail.

Old CA SE

 

[KootK]

This is a column on a footing. I thought about the limiting the grout to the area between bolts too. Trouble is, I need to get some serious shear out of the bolts too...

 

[Lion06]

I put this in the wrong thread somehow earlier:

I had to do a yield line analysis for a column cap plate on an existing building once. It was terribly overstressed using elastic analysis (450%), but using yield line analysis knocked it down to around 125%. I was quite impressed with that. I guess that's the extra capacity you get out of 4 yeild lines.

Anyway, the point was that I used the procedure outlined in my concrete text (McGregor/Wight), because I couldn't find any easy to follow references for steel. The concrete example was easy to follow and implement.

 

[Tomfh]

In Australia it is common practice to add ~30% to the bolt tension when prying is determined to be an issue. See here:

http://books.google.com.au/books?id...8qiNCg&sa=X&oi=book_result&ct=result&resnum=1

 

[transmissiontowers]

You might try out the new Hilti Profis 2.0 software available for free on their web site. I believe it allows you to have a baseplate on concrete or on leveling nuts.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[swearingen]

Any thought to a shear key to help out your anchor rods?


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