Anchors Stud Concrete Cover 2

[AAEC]

I need suggestion, opinion or anything you can share on this tank replacement project. I have this existing old tank that needed to be replace with a new tank using the same concrete foundation. However, new tank is slightly different from the old tank. This difference in size will cause the anchor stud to have a concrete cover of 2-1/4". The existing stud has a concrete cover of 3". Another difference is the amount of anchor studs. The old tank have 4 studs and the new tank have 7 studs. Although the studs have only 2-1/4" cover, the 7 studs should compensate on the pull out strength as compare to the 4 studs. Please refer to the attached sketch to give you more detail information about this problem. Should I go ahead and follow what I have sketch out using the 2-1/4" cover? Keep in mind that I have to use the existing concrete foundation. Is there anything I could do to make sure that this new tank is appropriately anchored to the concrete that will meet new wind and seismic requirements. This tank is located in Hawaii.

 

[Ron]

The capacity of a drilled anchor is reduced based on edge proximity. In this case, you need to check the reduction required by Hilti's tables and see if you still have adequate capacity. If not, you could consider adding steel plate to the outside vertical face of the concrete to increase capacity.

 

[coloeng]

Your drawing seems to indicate that the existing rebar has 2" of cover. If your new anchor bolts only have 2-1/4 " of cover, won't they conflict with the existing rebar?

 

[msquared48]

Good point here coloeng. With the placement shoen, you will be cutting the outside horizontal barsw of section A.

Better rethink your design here...

Mike McCann
MMC Engineering

 

[msquared48]

I can see why the existing studs have 3" of cover here considering the 2" cover on the stirrups, and the diameters of the stirrups and horizontal bars.

As the new embeds are 1.5" in diameter, this means that the centerline of the bolt circle needs to be a minimum of 3.75" from the edge of the grade beam.

You will have to rethink your method of attachment here,

Mike McCann
MMC Engineering

 

[JedClampett]

If you really need the hold downs (some times they do more harm than good) I would bite the bullet and replace the foundation. You're only saving a small amount of money by re-using it.

 

[lovethecold]

Couldn't you either have a longer chair welded on, or cut the existing chairs off to weld longer ones on? This would push the bolts further out from the inside edge of the existing foundation.

 

[AAEC]

Thank you all for replying on my post. I have another questions regarding the responses I received. Since replacing the foundation to increase the stud concrete cover is not an option, do you think I can angle the studs inward to avoid hitting the existing stirrups and horizontal rebars? Will this add capacity to the stud anchors?

 

[msquared48]

Not no, but H*&& NO!

You should really consider bolting a stiffened metal plate to the existing foundation with two bolts concentric with the existing holddown location. These bolts could be smaller in diameter, with the 1.5" diameter bolt field welded to the plate. The new anchor bolts could then be set inside the existing reinforcing steel. It would double the number of anchor bolts, but so what? It will work.

Mike McCann
MMC Engineering