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Base Plate Anchor Rod Hole Size

Base Plate Anchor Rod Hole Size

Sorry I missed Subject in my previous post.

I designed cooling tower structure (per UBC'97, Seizmic Zone III) and provided foundation load w/ base plate detail to the customer who will design foundation. I specified standard oversized 1 3/4" hole for 1 1/4" anchor rod and customer asked why 1/2" oversize - their concern is shear transfer. Please advice.

RE: Base Plate Anchor Rod Hole Size

AISC provides standard hole sizes in Chapter 14 of the AISC Steel Manual.  For 1 1/4" rods, the hole diameter is 2 1/16".  Min. washer dia = 2 7/8".

AISC also states in J1.11 all of the limitations on Bolted Connections - and at the end, states that "In all other cases connections are permitted to be made with A307 bolts or snug-tight high strength bolts."

No commentary in Chapter 14 or in the spec (that I can find) appears to concern itself with the oversized holes.  I have seen, in high seismic areas, where the base plate had a key plate set down in a slot in the concrete between the projecting anchors to take shear.  

RE: Base Plate Anchor Rod Hole Size

The purpose of oversized holes in base plates is to provide tolerance for misaligned anchor rods.  If shear needs to be transferred to the anchor rods, you can use plate washers with standard holes that are field welded to the base plate.  Another option is to use a template for setting the anchor rods and providing standard holes in the base plate.

RE: Base Plate Anchor Rod Hole Size

Oversized holes are provided for erection tolerance to be able to fit the base plate over the cast in place anchor bolts (which may not be placed exactly perfect in the field).

Shear may be transfered thru the anchor bolts if a plate washer (with 1/16" oversized hole)is field welded to the base plate at each anchor bolt. Another way to transfer shear is with a shear lug welded to the bottom of the base plate.

RE: Base Plate Anchor Rod Hole Size

I tell you what i like to do these days is to use adhesive anchors. I use the base plate holes as my drill template. Through the years, sloppy construction some times will tend to misplace the AB locations. A contractor comes back to me and asks if he can slot the base plate holes, which means mutilating the base plate.

If you have normal column, consider using adhesive anchors. You can have the bolthole size 1/16 or 1/8 of an inch larger than the AD diameter.

Good luck.

RE: Base Plate Anchor Rod Hole Size

In one project, I didn't allow drilling holes in the base plate till concrete and bolts were well set. Holes (1.5mm larger than bolt dia) were then drilled to the actual measurements of the bolt locations. With slight reaming of holes, it came alright.

RE: Base Plate Anchor Rod Hole Size

Roark's Formula's

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