anchor rod holes in base plates

[olr]

please help me understand AISC specs for anchor rod hole diameters in base plates

table 14-1 in AISC 16th edition (recommended sizes for washers and anchor rod holes in base plates) specs the maximum hole diameters as long as special washers are used, like square washers or round washers that meet the acceptable width in the same table.

if I want to use non-special washers (like ASTM F844 washers), Table 14-1 has a note at the bottom stating if F844 washers are used, the hole diameters are limited to
D + 5/16 (for D <= 1")
D + 1/2 (for 1" < D <= 2")
D + 1 (for 2" < D)

i've been taught to assumed contractors will use common F884 washers, so I follow the hole spec from the note. during QC I often get comments saying that hole sizes can just be D + 1/4" in all cases form one engineer, another engineer says just do D + 1/2" in all cases.

Does the note below table 14-1 also spec max hole sizes for F884 and engineers can spec anything less than that? for example 1/4" oversize is a more convenient spec than 5/16" oversize for D<= 1"

What is the correct way to size hole diameters for F844 washers?

 

[HDStructural]

The purpose of this rule is due to the difficulties of accurately placing anchor bolts in concrete. I have seen MANY MANY RFI's asking for anchor bolt fixes because they were put in the wrong spot. They can vary from 1/2" out of tolerance to more than a foot out of tolerance. The larger the hole, the less RFI's we will receive and more construction budget we can save for other items/profit.

I will always specify the original hole dimensions from the table (which used to be in AISC DG 1). I sometimes see smaller hole sizes on the shop drawings for my projects, and I comment that they are to be oversized per our project documents. If they choose to use a smaller hole, and there end up being alignment issues in the field, it is not our fault.

 

[RWW0002]

The footnote to the table [D + 5/16 (for D <= 1"), D + 1/2 (for 1" < D <= 2"), D + 1 (for 2" < D)] yields anchor rod holes similar to historic - i.e. "green book" - anchor rod hole sizes. Tons of projects have been successfully completed with the tolerances necessary to meet these hole sizes. I generally allow the use of the smaller holes, however I put a warning to the contractor to ensure that tighter tolerances are communicated with concrete subcontractors.

I agree that the larger bolt holes may reduce significance of field errors - although it is worth noting that extra clearance should be planned for for the larger plate washers+ anchor tolerance. I have seen many base plates where the larger holed and plate washers were used, but there was no extra room left between column and plate washers - there goes your "extra" tolerance"..

Also, be aware that plate washers may need to be welded to base plates for the case of significant shear transfer through anchors.

 

[olr]

are the sizes in the footnote to the table (D + 5/16 (for D <= 1"), D + 1/2 (for 1" < D <= 2"), D + 1 (for 2" < D)) a maximum size and can engineers spec smaller sizes than than that?

I understand it will cause complications in the field. I'm trying to understand why someone would spec +1/4" instead of +5/16" (for D <= 1"). is there a standard size drill bit that is convenient to design to?

 

[HDStructural]

I don't know why they would spec + 1/4" instead of + 5/16". That seems to take on more risk if you ask me for having tolerance issues.

There should not be an issue with drill sizes to cut these holes, and they are often cut using lasers or other methods. Consider that the standard hole size for bolts in steel to steel connections is + 1/16" for bolts smaller than 1" and + 1/8" for bolts greater than 1". For many standard hole sizes in shear tabs, you are having 13/16" or 15/16" holes, so hole sizes to the 16th of an inch is not an issue.

 

[olr]

I don't know why they would spec + 1/4" instead of + 5/16". That seems to take on more risk if you ask me for having tolerance issues.

There should not be an issue with drill sizes to cut these holes, and they are often cut using lasers or other methods. Consider that the standard hole size for bolts in steel to steel connections is + 1/16" for bolts smaller than 1" and + 1/8" for bolts greater than 1". For many standard hole sizes in shear tabs, you are having 13/16" or 15/16" holes, so hole sizes to the 16th of an inch is not an issue.

what would you say is the correct way to spec hole sizes? It sounds like you use the table and not the footnote. i'm trying to find a good answer to this. thanks!

 

[HDStructural]

For steel to steel connections, I say std or ovs holes. For cast in anchor bolts, I usually have a table that gives the oversized hole size per AISC DG1 with the applicable anchor bolt diameters used, or I will just call out the hole size on the base plate detail, and it would be the oversized hole size per AISC DG1.

 

[EngDM]

For steel to steel connections, I say std or ovs holes. For cast in anchor bolts, I usually have a table that gives the oversized hole size per AISC DG1 with the applicable anchor bolt diameters used, or I will just call out the hole size on the base plate detail, and it would be the oversized hole size per AISC DG1.

If you use oversized holes, do you need to specify anchor bolts to be torqued down so that the washer friction actually transfers the shear?

 

[HDStructural]

Most of my projects have had large forces, so we have washers welded to the base plate. If we don't have washers, we would do the ACI breakout and steel checks assuming only one row of bolts resists the forces.

I guess this allows for the columns to slip in the second condition, but typically these have minimal shear and large gravity loads.

What do you typically do in these situations?

 

[EngDM]

Most of my projects have had large forces, so we have washers welded to the base plate. If we don't have washers, we would do the ACI breakout and steel checks assuming only one row of bolts resists the forces.

I guess this allows for the columns to slip in the second condition, but typically these have minimal shear and large gravity loads.

What do you typically do in these situations?

Anything with high ish shear (like from cross bracing in a simple CRU) I try to spec smaller holes. I wonder if there is a slip critical tightening method for anchor bolts, similar to what you'd do for a bolted beam connection.

 

[HDStructural]

Have you had any construction issues with misalignment with those smaller holes?

Another method that I've seen done is to spec that they fill the oversized holes with grout before applying the anchor nut. That would solve the problem.

 

[Brad805]

"another engineer says just do D + 1/2" in all cases" Um, no. Is this still a common problem today? Many of our steel trades send 1/8" thick anchor bolt templates with their steel packages and trades use total stations for layout. They have figured out who pays for these mistakes. Our anchor bolts holes are usually 5/16" over, but we never are working with particularliy large shear forces or anchor bolts larger than 1 1/4"d.

That said, the question friom the detailer seems a tad whiny. Many will be using a detailing component like you see below. Change the variable, done. If they have any skills that can be automated as well based on the diameter of the rod. After that, most will be using CNC equipment to handle the fabrication aspects.

 

[HDStructural]

The anchor bolt template holes should not be oversized, they should have standard holes. We are talking about the holes in the base plate.

I'm not as concerned about the impact of a larger tolerance on the forces in the bolts. A baseline tolerance is accommodated in our design assumptions and safety factors. It is about constructability and ensuring the column base is acting how we intend it to.

Even with new better technology, there have been anchor bolt placement issues on all large projects that I have been a part of. Whether out of tolerance by 1/2", or surveyor screwed up and everything is 6" off, or used the wrong anchor bolt pattern. Some times it is just a few locations on a project, recently I had one where over 40% of the columns had anchors out of tolerance for a variety of different reasons.

 

[Brad805]

"We are talking about the holes in the base plate." I understood that.

There will always be some mistakes. 15years or more ago we did one where 50% of the anchor bolts were misplaced. The fights that ensued led to some changes in attitude. Now we see less than 5-10% except in cases where there is a survey error.