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.

 

[canwesteng]

What do you need the projection for? If you're flush with the nut and have the plate washer installed you're fine. I also call projection as the distance from TOC to top of anchor - a bunch of this is eaten up by grout, base plate, nut, washer, and elevation tolerance.

 

[phamENG]

How critical is the application? By that I mean, did you design these anchors to use 99.9% of the steel strength in tension at design loading? If you didn't, and it's truly flush with the top of the nut, you're likely ok. Usually best to have at least one thread exposed above the nut to be sure you have 100% engagement, but as you said - it's already built. If you don't have full or even sufficient engagement, then we'd need to know more about the anchors. Size? Grade? Chemistry for welding? Etc.

 

[dik]

If you're flush, It's OK... anything more is about as useful as 'runway behind you'.

The purpose of the projection is to address issues like this... imagine if you had a 4" projection.

Dik

 

[Aesur]

Read through this article for solutions:

https://www.aisc.org/globalassets/modern-steel/archives/2016/09/steelwise_1.pdf

 

[Once20036]

As others have said, if the nut is fully threaded onto the bolt, you`re ok.
In instances where that isn't the case, I often look into elecone nuts.

 

[oengineer]

One of the anchor bolts is 1/4" less than the nut. would that be an issue?

 

[jayrod12]

that depends on whether you're utilizing the tension capacity of that anchor bolt.

 

[phamENG]

You said you have access to Design Guide 1 - look a little closer and I think you'll find a section regarding thread engagement and a method for estimating partial capacity based on partial engagement (subject to limitations, of course). I think it was in DG1, anyway...

 

[oengineer]

@jayrod12

my goal is to utilize the tension capacity of that anchor bolt. I have been told that the bolt is less than 1/4" of the bolt.
Based on just concrete pull out strength alone, using the Table 3.2 form the AISC Anchor bolt deign guide, the (6)- 1 3/4" anchor bolts are acceptable. Would this be an acceptable check for such a situation?

 

[oengineer]

What code or document covers calculations for 1/4" less of projection to account for tension capacity of anchor bolts? The anchor bolt is 1/4" less than the top of the washer.

 

[human909]

What code or document covers calculations for 1/4" less of projection to account for tension capacity of anchor bolts? The anchor bolt is 1/4" less than the top of the washer.

Simple shear calculation base on the depth of thread engaged. If you have 6/7ths of the threads engages then you have 6/7ths of the capacity. But throw in an extra margin to be comfortable.

It is a good idea to ask yourself how the bolt capacity is calculated in the first place beyond just reading it off a chart.

 

[oengineer]

I have a shear lug in the base plate, so shear is not a problem. The tension force is governing. The Concrete Pullout Strength is adequate. The issue I am having is with the concrete breakout strength.

 

[oengineer]

The projection being off by 1/4" should not effect the concrete breakout strength or the overall strength of the anchor bolt, correct?

 

[phamENG]

See my response in your other thread about (apparently) the same issue. Though I'm still confused - how are you having concrete breakout strength issues? Was it not designed properly? If you have insufficient project but correct length anchors installed, then you would have more embedment and more breakout strength than you would have initially calculated.

Engagement of the bolt of in the nut ABOVE the baseplate has nothing to do with breakout strength. But that doesn't mean you can ignore the load path from the baseplate, through the washer, to the nut, to the anchor, to the nut at the bottom of the anchor, and into the concrete.

 

[jayrod12]

As long as the nut is fully engaged, i.e. bolt flush with top of nut, then you can account for the full tension resistance of the anchor bolt from a steel capacity. From a concrete breakout capacity, the nut has no effect.

If you are having breakout issues, then your design is inadequate and no amount of projection is going to fix it.

 

[oengineer]

@jayrod12

What if the bolt is a 1/4" short of being flush with the nut? Would that effect the design capacity of the anchor bolt?

 

[jayrod12]

There have already been people above that have given you guidance on this. But if you don't have full engagement of the threads, you cannot enact the full steel tension capacity of the anchor bolt. The question is whether you need the full steel capacity of the anchor bolt or not. If not, maybe it's ok but you need to check for the tension capacity you require. If you do need the full steel tension capacity, then you've got an issue.

 

[oengineer]

@jayrod12

In order to determine if I need the full steel capacity of the anchor bolt or not for my tension loaf, would the calculation just be based on mainly the anchor bolt embedment? Or is there something I am missing?

 

[WinelandV]

oengineer,

You've got a tensile load in your column. It needs to pass that load into the baseplate. (I'm assuming) You sized a weld between the column and baseplate, and sized the thickness of baseplate required (whether controlled by column compression or tension). Now that the load is in the base plate, you need to pass it into the ANCHOR RODS. The diameter of the anchor rods should have been set by the tension demand on them (per your other thread, I believe there is a shear lug taking out the shear). That is a straight AISC Chapter J3 check. The embedment into the concrete doesn't come into play yet. How much steel area do you need to carry the tension load in the bolts? If the area required is LESS THAN the area of the 1-3/4" diameter bolts, then there's a chance that the reduced thread engagement is ok. If you need every bit of area of your bolts, then no, the reduced thread engagement is no good.

Now, based your other thread about washers, I'm curious if your nut engagement is based on the washers being in place, or with no washers. In which scenario are you getting the rod to be 1/4" short of full engagement?


Now then, after the load has made it into the Anchor Rods, NOW you can start worrying about concrete side of the design. But, as mentioned by many others above, the anchorage calcs should have been done prior to casting, and a suitable design should have been communicated.