Anchor Bolt Projection - Field Issue 3

[oengineer]

Anchor bolts at baseplate for a braced column does not have the required projection for seating of the nut. The construction drawings call for a minimum 5" projection and a " 1/2 inch plate washer welded to the baseplate". In the field, the anchor bolts are shown flush with the nut & washer, and plate washer welded to the baseplate. I would like to tell the contractor to match the construction documents's 5" projection, but since it has already been built out in the field I am not sure how to advise them on correcting the projections. Based on the comments & pictures for the contractor, the anchor bolt embedment is not an issue. It's just the projection.

Would using Hex Coupling Nuts be helpful? Based on some reading in the AISC baseplate design guide, it seems as long as the anchor bolt is not projecting below the foundation, this short projection is not an issue. Please confirm. Any suggestions/comments are appreciated.

 

[oengineer]

@winelandv

So, the contractor was asking if it was okay to remove the washer in order to get the anchor flush with the nut. I realize that is not allowed structurally. So, now I am trying to determine if it is okay to have the anchor bolt be 1/4" shy of being flush with the nut, while keeping the washer intact.

Based on Table 3.1 of the AISC Design Guide 1 for the design and allowable strengths for various anchor rods, 1 3/4" anchor bolt has a capacity of 102 kips which is more than enough for my applied tension load of 73 kips. I am using Grade 55 anchor bolt.

Shown in Table 3.2 of the AISC Design Guide 1 are design pullout strengths for anchor rods with heavy hex head nuts. 1 3/4" anchor bolt has a capacity of 93.4 kips which is more than enough for my applied tension load of 73 kips.

If I am interpreting you response correctly, then my 1 3/4" anchor bolt is sufficient for an applied tension load of 73 kips, regardless if the anchor bolt is 1/4" shy of being flush with the nut.

Do you agree with my interpretation of your response?

 

[structSU10]

I dont think you have it all. I am guessing this is an anchor bolt group - the values stated above are for single anchors, and also do not consider concrete breakout, only pull out and steel strength.

concrete breakout is controlled by anchor embedment and edge distance. It is a seperate issue that generally is the weak link in anchor design. See ACI for all the limit states.

the design guide states if you have half thread engagement at a minimum, you can count on a percentage of the steel capacity. The pullout and breakout strengths are not affected by this.

 

[graybeach]

You are most likely ok, based on your numbers. I have learned the hard way in a similar situation to never specify anchor bolt projections on contract drawings.

 

[miningman]

In my experience, field engineers have to develop the backbone necessary to be able to say " That does not quite meet code , but it is adequate " This seems to be one of those circumstances

 

[phamENG]

I would say that you're probably ok. Per the design guide, 2.11.3, "A conservative estimate of the resulting nut strength can be mad based on the percentage of threads engaged, as long as at least half of the threads in the nut are engaged." So if you're 1/4" short, and an 1-1/4" A563 nut is 1-7/64" tall, then by my math you have about 77.4% engagement. .774*102=79kips. That's a 92.4% utilization. You'd better be sure that 1/4" dimension is dead on. Its in the contractor's best interest for it to be more engaged, so he might tend toward the wrong side of the tape measurer's tolerance.

That being said - I'm thoroughly concerned by the fact that you're still talking about pull out strength, which has nothing to do with low projection issues. I'd also be concerned if you used that chart in the design guide to actually design your anchors. It's there as an illustration of potentially controlling limit states. The concrete side of things should have been designed using ACI 318 for the myriad failure modes and influences of edge distance, group spacing, etc..

 

[oengineer]

@structSU10

Table 3.1 of the AISC Design Guide 1 is for the Anchor Rod (Rod Only) Available Strength.

Table 3.2 of the AISC Design Guide 1 is for the Anchor Rod Concrete Pullout Strength.

I went ahead and determined the tension load on each individual anchor bolt for a group of (6)- 1 3/4" anchor bolts. The maximum tension load in a single anchor bolt is 73 kips.

Therefore, I referenced these tables for a single anchor bolt, being subjected to a 73 kip tension load.

 

[WinelandV]

oengineer,

Table 3.2 is semi-useful, but you need to realize that it's checking pullout. Design Guide 1 is mentioning the Heavy Hex nut because that nut is at the bottom of the rod, embedded in the concrete. Note that this is a separate failure state from the concrete breakout strength (See ACI 318 Appendix D, in pre-2014 editions). I'm a little wary that you sized (6) 1-3/4" into the concrete based only on a pullout check.

Putting that aside, yes, it appears you should be able to de-rate the strength of the anchor based on the partial thread engagement. As long as 1/4" short still gives 75% engagement, you should be ok.

Tell your contractor that the next time he's installing high strength rods, he should take 2 hours prior to the pour and make sure those rods are 1) in the correct spot (in plan and elevation, i.e. correct projection) and 2) not going to get moved as they cast the concrete around them. Yes, it's a pain in the rear, but it sure beats having to possibly do a fix, or even rip-out and re-do.

 

[oengineer]

@winelandv

I did not size these anchors. My goal is to verify these designs for this field condition, in order to respond to the field issue.

 

[phamENG]

I did not size these anchors. My goal is to verify these designs for this field condition, in order to respond to the field issue.

That tells me you're not the original designer, and certainly not the EOR. Whatever happens you SHOULD NOT APPROVE this without the buy in from the EOR. A braced frame to foundation connection is a critical structural connection that the stability of entire structure depends on.

 

[oengineer]

@phamENG

I will notify the EOR. I just wanted to have a comprehensive conclusion before I discussed it with the EOR. Thanks for your help!

@everyone

Thank you all!

 

[Aesur]

@oengineer, can you provide us with a detail/sketch showing the baseplate size, anchor locations relative to the edges of baseplate, column size and all forces, concrete strength, concrete footing/caisson size with baseplate located on top of this showing edge distances and thickness? This will get you to your answer the fastest.

 

[phamENG]

You're welcome, oengineer. Good luck getting it resolved.

 

[oengineer]

@winelandv

Thank you for your break-down in the approach to this field issue!

 

[A.B]

What you want to make sure is that the nut is fully engaged with the bolt, if is not then you have an issue.

 

[Lomarandil]

Per the design guide, 2.11.3, "A conservative estimate of the resulting nut strength can be mad based on the percentage of threads engaged, as long as at least half of the threads in the nut are engaged." So if you're 1/4" short, and an 1-1/4" A563 nut is 1-7/64" tall, then by my math you have about 77.4% engagement. .774*102=79kips. That's a 92.4% utilization. You'd better be sure that 1/4" dimension is dead on.

This is the conservative path (and I'd be hard pressed to stick my neck out past this in most cases). But the reality is that bolts transfer most of their load to the nuts in the first few threads.. it's more of an exponential relationship than a linear one.

So I'd feel more confident than 92.4% based on a linear relationship might lead you to initially believe.

----
just call me Lo.

 

[phamENG]

Lo,

Do you have any good references in mind that show the exponential relationship? I'd be interested in reading them.

 

[Lomarandil]

Sorry for the late reply.

I don't know where I've picked that up, sorry. I know it has come up in two topics I was investigating, one being what should happen when a bolt in tension has threads flush with the end of the nut rather than projecting (or even one or two threads short) (short answer -- can be OK in certain cases). The second was a case where constructability/access was leading me to seriously consider tack-welding the projection end of a high strength nut to hold it in place, and wondering what the weld heat would do to the nut material strength (short answer, might be OK, but risky enough that we found another way).

----
just call me Lo.

 

[phamENG]

No worries. If you happen upon it, I'd be grateful if you remember me. In the meantime, I'll put it on my ever growing list of things to look into....someday....

Thanks.