Base plate modifications for misaligned anchor rods

[gte447f]

The anchor rods for a column were supposed to be set in a 1'-4" square pattern, but instead they were set in a 1'-2" square pattern. The column is part of a moment frame. It is a W10x112, the base plate is 2" thick with 2-5/16" diameter holes, and the anchor rods are 1-1/2" diameter with 3-1/2"x3-1/2"x1/2" plate washers. I have checked the plate thickness and anchor rod capacity for the effect of the anchor rods in the in-situ locations and the plate thickness, anchor rod tensile strength, and concrete break out strength are all ok. There appears to be just barely room to fit the specified washers over the in-situ anchor rods and clear the corner of the wide flange section. So, I would like to use the anchor rods in the in-situ locations.

My question is how to go about modifying the base plate to fit on the in-situ anchor rods. I have attached a detail of the original base plate and anchor rod layout with the actual in-situ location of one of the anchor rods also shown (top left anchor rod, but the other 3 are similarly offset toward the center from the specified locations). Is it OK to slot the existing base plate holes, or should I specify that the holes be filled and new holes drilled. There is a recommended procedure in AWS D1.1 5.26.5 and C5.26.5 for filling the holes. One of my concerns with slotting the existing base plate holes would be that I guess the plate washers would have to span the gap over the slotted holes.

 

[SlideRuleEra]

Both slotting the existing base plate or filling the existing holes / redrilling will take time and cost money. Why not just get a new base plate with holes positioned as needed?
Smaller replacement plate washers (if the calcs allow) are probably a good idea, too. Because the existing washers appear to just fit "on paper" does not mean they will fit in the field.

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[MGaMart]

Since you already checked the plate and anchor bolts for the in-situ case, slotting the holes should not be an issue. Since the new holes migrate towards the center of the plate, the critical sections of the base plate haven't been affected too greatly for the load transferring from the column to the base plate to the anchor bolts. The plate washers would need to be oversized to cover the existing holes and the plate washer welded to the base plate to develop any shear the column is transferring to the base plate. The new holes and the existing are likely to close to each other to pass code, resembling closer to a irregular slotted hole, hence the need for welding.

 

[gte447f]

SlideRuleEra,
To clarify, do you mean cutting/gouging off the existing base plate and welding on a new replacement base plate with holes positioned as needed?,... since the column has already been fabricated and delivered to the site. Would this be cheaper and faster than the 2 options I asked about?

New holes, whether drilled in the existing plate after filling the old holes, or in a new replacement base plate would not need to be oversized, and so should allow for smaller plate washers as you suggest.

 

[gte447f]

MGaMart,
I agree with you that slotting the holes shouldn't affect the capacity of the plate since it is not at the critical section for bending. My concern would be the plate washers spanning across the slotted hole. I am not sure how to check this. Honestly, I never check the plate washer thickness for the typical oversized holes in base plates, I just use the recommended minimum sizes and thicknesses from AISC Table 14-2 and AISC Design Guide 1 Table 2.3.

 

[SlideRuleEra]

...cutting/gouging off the existing base plate and welding on a new replacement base plate with holes positioned as needed?

Yes, absolutely.
Modifying a 2" thick plate with a W10 attached to it will be quite a challenge (expensive). Also, a fair amount of accuracy is necessary for everything to fit.

There is going to be both cost and time involved now matter what solution is used. IMHO, will be much better to control costs and quality by replacing the existing plate with a new plate.

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[gte447f]

Thanks SlideRuleEra. I think I agree that your suggestion is probably the best solution. Much appreciated.
Thanks also to MGaMart for sharing your thoughts.

 

[MGaMart]

In my experience, whenever anchor bolts are erroneously placed, erectors will instinctual want to burn slots in order expedite resolving the problem (especially if the plate is 2" THK). Irregular holes (slots, oversized, etc.) eliminate an equal distribution of shear load across the bolts as it's likely they are not all subject to bearing at the same instance in time. Field welded plate washers compensate for this by ensuring that anchor bolt bearing is occurring simultaneously across all bolts. Your plate washer is then subject to the same requirements you would check for shear plates (shear rupture, block shear, tensile rupture, etc.). The weld is designed predominantly for the shear that needs to transfer from the base plate to the plate washer. The main region of concern is the uplift portion of the base plate, which imparts compression on the plate washer (since it is the anchor bolt nut that holds the plate down. If the hole/slot in the base plate is large enough, your plate washer is bridging the gap between the two piece, and bending of the plate washer may result. In this case, you would need to size the plate to resist both shear and flexural stresses.

 

[gte447f]

A quick follow up question to SlideRuleEra's suggestion... If you fabricate a new base plate and field weld it to the column, since you know the in-situ anchor rod layout, is it safe to use standard holes 1/16" larger than the rods, or should you still use something larger but maybe not as large as the base plate holes per AISC Table 14-2? Maybe a regular "oversize" hole per AISC Table J3.3, which would be 1-13/16" (i.e. d+5/16") instead of 2-5/16".

 

[MGaMart]

Anchor bolts can be bumped and bent before the new plate arrives. Never assume the standard hole will work when it comes to anchors, that decision will always come back to haunt you.

 

[gte447f]

MGaMart,
Good advice. I agree. Thanks.

 

[SlideRuleEra]

gte447f - Keep the scope of the problem in mind:

1. The holes in the base plate have to fit over four bolts simultaneously. Just how accurate do you know the position of the bolts in relation to each other?

2. What is projection of the 1 1/2" diameter anchor bolts? Maybe a very few inches? How much can a "short" 1 1/2" diameter bolt be safely bent? Not much.

3. AISC "standard" and "oversized" holes are for steel erection, where the bolts are placed one-at-a-time. Not for a base plate (See my Item 1, above).

4. I'm not familiar with the current AISC manual, but the 9th Edition has recommendations for column base plate hole sizes (pages 4-130 and 4-131). For 1 1/2" diameter bolts, 2" diameter holes are recommended. I would go with that.

While you are doing this, I suggest that the dimensioning of the drawing be improved. This is not theoretical... it makes a measurable difference (i.e. error). The issue is controlling manufacturing tolerances - the most important dimension needs to be controlled - in this case that is the 1' 2" hole spacing. If you want a more detailed explanation, I'll be happy to do so.

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[gte447f]

SlideRuleEra,

All good points. Thank you.

The 13th edition AISC manual recommends 2-5/16" diameter holes for 1-1/2" anchor rods, an increase from 2" holes per your 9th edition. The large hole size is of course to accommodate setting tolerances of the anchor rods. In the case of anchor rods that are already set and can be field verified, surely smaller holes would suffice, but I suppose standard size "bolt" holes (i.e. d+1/16") would be overly optimistic.

I settled on 1-13/16" holes in the new base plate, which happens to be equal to an oversize "bolt" hole for bolts greater than 1-1/8" diameter (i.e. d+5/16"), even thought these are not bolt holes. I feel like this is a good middle ground, and it will allow smaller plate washers, since you rightly pointed out upthread that because the existing washers appear to just barely fit on paper does not mean they will fit in the field.

For base plate washers, AISC recommends the size of the washers equal the base plate hole diameter plus 1" rounded to the nearest 1/2", so 1-13/16" holes instead of 2-5/16" holes results in a 3" washer instead of a 3-1/2" washer. By the same formula, your recommended 2" holes would have also resulted in 3" washers, so maybe I should have gone with 2" holes. Oh well, I am satisfied with it at this point (until the wailing and gnashing of teeth starts coming from the contractor tomorrow morning).

Duly noted on the dimensioning advice. I will do it as you suggest from now on. Thanks.

 

[hokie66]

I agree that cutting off the plate and welding on a new one is the best solution. But I wouldn't want this done on site, unless by the fabricator rather than the erector.

As to the potential clash with the plate washers, just rotate them 45 degrees.

 

[gte447f]

hokie66...lol... good call on rotating the plate washers.. a case of not seeing the forest for the trees.