Appropriate steel grades for Anchor Rod Plate Washers

[jochav5280]

Good evening:

Does anybody have any design equations or guidance for determining the appropriate plate washer steel grade for anchor rods? AISC's Base Plate Design Guide does not give any guidance, and only indicates that F436 washers should not be specified since they are usually too small to cover the oversized base plate holes recommended by AISC. There's got to be a design equation for verifying that the washer strength is sufficient to prevent pull-through, and I would expect that since they are cut from plate that they would fall within AISC's umbrella of design equations.

Additionally, does anybody know if there's any problem with embedding the threaded portion of anchor rods in concrete?

Many thanks in advance!

Best regards,

jochav5280

 

[KootK]

I've used A36 plate. As for the design, if you search for "washer plate" here, you'll get a wealth of advice.

As for embedding the threaded portion of a bolt, we do it all the time. Even if it wasn't threaded, there would still be significant bond. Supposedly, it can result in localized bond / spalling failures in the concrete where the anchor bolt enters. Using a prestressed anchor rod can help this and, apparently, is the method of choice for situations involving high level of cyclic loading (marchinery, high aspect ratio towers, etc.).

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ibhorus]

ASCE - ANCHORAGE DESIGN FOR PETROCHEMICAL FACILITIES (2013), ISBN 978-0-7844-1258-9; page 89 and 90 Table 3.3 "Recommended Maximum Sizes for Anchor Holes in Base Plates and
Minimum Fabricated Washer Sizes" with Notes, provides similar recommendation for A-36 material as the previous post with additional information for washer size and thickness.

Page 69 of same documemnt provides the following: For tension stretching or
debonding of concrete to the anchor shaft may be achieved by wrapping the shaft
with industrial tape to within 1 in. (25 mm) of the embedded anchor head before
placing concrete (Figure 3.21). Care must be taken not to allow tape to come into
contact with the head of the anchor nut. This is the reason for stopping the tape one inch
from the anchor head. Likewise, grout must not be allowed to bond to the anchor using a
similar method. Sleeved anchors that are to be tensioned should be installed using the
methods mentioned above for debonding the shaft below the sleeve.

 

[jochav5280]

Thank you KootK & ibhorus:

I will check out your references; it's too bad that AISC doesn't just specify in their anchor bolt table the appropriate washer grade. Since structural steel generally has the same modulus of elasticity, I believe the stiffness requirements can be satisfied by any grade of plate washer. However, since strength is dependent upon the steel properties, theoretically, not just any grade of plate washer would satisfy the strength requirements.

Thanks again for your input.

Best regards,

jochav5280

 

[WillisV]

A36 is typical. The thickness is approximately based on the amount that would prevent the washer from pulling through the maximum hole size should the entire washer yield and fold (aka like an umbrella) and is therefore independent of yield strength. This does assuming the yield strength is high enough to prevent a direct shear failure, which any standard grade of steel will be. If you are worried about preventing distress/yielding of the washer under maximum loads, you should look at Roark for applicable circular plate loading equations.

 

[jochav5280]

Hello WillisV:

Thank you for your response.

Your second sentence is a little confusing/conflicting as you are saying that the thickness is based on the entire washer yielding, yet you then say that it is independent of yield strength. Your third sentence then goes onto indicate that the yield strength determines the shear yield strength, yet you then indicate that any grade will provide enough strength. Can you support this claim with some documentation or calculations?

I will look into the Roark equations for further clarification.

Thank you,

jochav5280

 

[WillisV]

I agree my post was confusing - I will try to clarify my thoughts:

There are two possible failure modes for a washer to pull through a hole:

1. Flexural yielding / bending such that the washer folds up in bending and passes through the hole.
2. If the washer is sufficient to prevent 1, then the only other feasible limit state is direct shear through the washer perimeter.

What I was trying to say is that limit state 1 is largely prevented in the AISC provisions not by sizing the thickness of the washer based on a yielding/flexural criteria, but by sizing the washer to ensure that the thickness is greater than the annular space between the bolt and the hole so that even if the washer does completely fold up to 90 degrees, it cannot physically pull through the hole. For example, for a 1" rod, the maximum hole diameter is 1 13/16". If the anchor rod is sufficiently stressed to start yielding the washer it will tend to center the anchor rod in the hole, thus leaving 13/32" of annular space around the sides of the hole. This (0.4") is approximately the minimum washer thickness given in the table (0.375") thus forcing the limit state to direct shear (failure mode 2).

Regarding limit state 2, using the 1" rod above as example and assuming that you would have to shear off a thin ring of the washer directly around the anchor rod hole, the ultimate direct shear strength in A36 steel would equal to Phi*twsher*(Pi*Holedia^2/4)*Fy = 34.8 kips. The ultimate tension strength of a 1" F1554 rod are: GR36 = 26.3kip; GR55 = 34.08; GR105 = 56.8 kips. So, the min. washer thickness in GR36 steel is adequate for a GR36 and GR55 rod, but the thickness or grade needs to be increased for a GR 105 bolt (hence the "minimum" label for thickness in the table).

This is my opinion and is not documented anywhere in AISC. The table itself is a product of committee and has no formal research background that I am aware of.

All of the above is assuming standard building design with transient loading causing uplift of bases. In constant tension situations, I would tend to size the washer slightly thicker using elastic/Roark formulas to prevent any flexural yielding.

 

[WillisV]

Edit to above - shear strength = Phi*twasher*Pi*Holedia*Fy = 76.8 kips so actually the minimum washer size in GR36 is adequate even for a Gr105 rod.

 

[KootK]

@Willis: interesting theory. Two questions regarding shear strength:

1) Shouldn't Fy be something like 0.66 Fy?
2) Wouldn't it be appropriate to use Fu for this?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[WillisV]

@KootK

-I agree - missed the von Mises reduction. The use of rupture criterion can certainly be argued. This would change the design example above to 0.75*0.6*0.375*Pi*1 13/16*58 = 55.7 kips, or approx equivalent to the 105 rod tension strength.

 

[racookpe1978]

I understand the requirement for wrapping anchor bolts that are pre-stressed in place is to prevent the concrete around the bolt from restraining the stretch (axial growth) of the embedded anchor bolt under load, right?

That is, you want the bottom (base and its washer) to be held rigidly in place deep inside the concrete, but the top (the threaded portion sticking up) to be free to move slightly when pre-stressed.