Design approach for anchor rod plate washers? 2

[structuresguy]

Hi all. This is a question I have had for a while, and I can't find any references to help me out. When designing steel column base plates with large oversize holes, plate washers are required to cover the hole. However, all the references I have say that the thickness needs to be designed for the loads. Does not say how, though.

I am in Florida and frequently design light structures (hangars, etc) that have significant net uplift forces on the footings. Using standard AISC hole sizes, one ends up with a nut/hard washer outside diameter approximately equal, or just slightly smaller, than the base plate hole diameter. With a net uplift, the base plate pushes on the plate washer which then bears on the nut/hard washer.

How would you design the thickness of the plate washer? I usually use 3/8" thick for up to 1" dia rods, and 1/2" thick for larger rods. But this is based on my gut feel. I have never been able to calculate a required thickness anywhere near this usign assumed shear/bearing/bending of the plate washer.

If the hole size was significantly larger than the nut diameter, then the plate would go in bending. But when the two are equal, or nearly equal, no bending should occur. Bearing stress is usually no problem.

This is not something that keeps me up at night, but I would like to see how others approach this problem. Thanks.

 

[jheidt2543]

The way I would look at it is; you know the load on the bolt, you know the allowable bending stress of the washer, treat it as a beam. Calculate the thickness based on bending, similar to what you do to find the thickness of the base plate.

 

[edba]

Recently, a similar type of questions were raised in aisc.org, maybe you can try reading the response there.

You can read also Chapter 9 of RCSC Design Guide (see the link below). It is not exactly the reference but can give you some guidelines.

http://www.boltcouncil.org/2nd Edition Guide.pdf

Thanks

 

[structuresguy]

edba: Wow, thanks for the reference. I was not aware there was a design guide like that. I just downloaded it. Looks great. Have not absorbed it all yet of course.

Thanks.

jheidt2543: Ya, that is bascially the procedure I would use, but I usually get a zero lever arm for bending, so I have not moment. This occurs because I assume the bending to take place over the distance from the unsupported edge at the base plate hole to the bearing of the nut/hard washer. As I mentioned, usually the hard washer is the same outside diameter as the base plate hole, or close to it, so the lever arm distance (dia of washer - dia of hole) is very small. I guess a conservative approach would be the difference from base plate hole dia. to plate washer ID.

I still think this is an interesting discussion, even though I am sure most engineers give it very little attention.

When in doubt, make it stout. Of course, that can lead to other problems. But that's another story.

 

[prex]

If there is now bending, as you say, then it is just a matter of shear: the active area is &[ignore]pi[/ignore];Dt (the meaning of symbols should be evident) and the allowable stress is the stress in shear.
However I'm not pretty sure there is no bending: as plate washers are there to adapt for misalignments between the bolt and the hole, there will be some non axisymmetric bending.
As you don't need to save much material on the plate washers, you could adopt a very safe approach: assuming the bolt is completely out of center, then the maximum offset between bolt CL and hole border would be D-d/2 (d is bolt diameter). Now take a circular plate with a center hole having the diameter of the hard washer, loaded with your load at a radius equal to the latter formula, with outer diameter equal to the outer diameter of the plate washer (or side x 2/&[ignore]radic[/ignore];&[ignore]pi[/ignore]; if it is square) and supported at inner hole.
In the site below there are some forms for circular hollow plate calculations.

prex

http://www.xcalcs.com

Online tools for structural design

 

[dbuzz]

Roark's Formulas for Stress and Strain has a comprehensive table of moments, shears and deformations for annular circular plates for a variety of edge restaints and loadings.

Do you have access to a copy of Roark's?

 

[jheidt2543]

structuresguy,

I see what you are saying, but the lever arm is what it is. If the lever arm is very small, then the bending moment is very small and that's that. However, I would take the lenght from the edge of the wahser hole to the end edge of the washer since the edge of the washer is free to rotate, as you mentioned. By the way, I think we have all seen dished/bent bolt washers, so it is very possible.

prex's comment is valid too, both shear and bending should be checked.

Also, since you select from standard thicknesses of washers, to going to the next thickness up the ladder is no big expense.

 

[Taro]

I submitted the same question to AISC's Steel Solutions Center some time ago. Their response was:

"I have not seem a formal design procedure for this case. However, our Design Guide #10 states:

'As a rule of thumb, it is suggested that the thickness of the washers be a minimum of one-third the diameter of the anchor rod, and that the length and width of the washers equal the base plate hole diameter plus one inch.'

Hope this helps.

Sergio Zoruba, Ph.D.
AISC Solutions Center"

 

[structuresguy]

Ya, I have seen that 1/3 db rule before. I generally try to match it.

And I have also figured on worst case bolt out of place as well. But my take on that is the portion of the plate washer over the open hole does not receive any load, as it doesnt touch the base plate. Hence, no load equal no contribution to the uplift reaction. However, the flip side of this is that the other side of plate washer is in direct bearing contact with both the nut face adn the base plate. So again no bending.

This is an interesting problem. I wish I was back at university thinking up my master's thesis again. I might be interested to run some pull through tests.

Realistically, this problem does not worry me too much. I am much more concerned about nut push thru of the leveling nuts with oversized holes. A little while back I started explicitly specifying plate washers of equal size top and bottom of the base plate when leveling nuts are used. This is after an erector had a large 3 story building up, with concrete on the floors and 15000 lb air handlers on the roof but had not grouted the base plates yet. And, you guessed it, no plate washers or even flat washers on the leveling nuts. I went out to check on things and noticed this problem on several main columns and I was shocked. I immidiately contacted the owner to recommend he force the contractor to stop all work until this problem was fixed. I am still amazed that push thru had not occured. I had columns with 400 kip reactions sitting on only four 1" leveling nuts. It also fortunate the anchor rods did not buckle. To this day I am not sure how the inspector missed that. Makes me really push for frequent site visits, but there usually is not money for as many as I would like.

 

[STEELCHECKER]

I AM DETAILING A COLUMN WITH 5/8" ANCHOR BOLTS. THE ENGINEER WANTS 1/2" OVERSIZE HOLES, WITH PLATE WASHERS. WHAT IS THE MINIMUN EDGE DISTANCE?

 

[14159]

I think the following are good thicknesses as long as you're using typical anchor rod holes as given in the Manual.

bolt = 3/4, washer = 1/4
bolt = 7/8, washer = 5/16
bolt = 1, washer = 3/8
bolt = 1.25, washer = 1/2
bolt = 1.5, washer = 1/2
bolt = 1.75, washer = 5/8
bolt = 2, washer = 3/4
bolt = 2.5, washer = 7/8