Leveling nuts under column base plate and non-shrink grout. 3

[RabitPete]

Erector wants to use leveling nuts instead of shims under the column base plate before filling 1" space with non shrink grout. If that is the case, I assume compression load will be transferred to the anchors instead of a grout and that's not how I designed it.

1. Is that a problem? What is the concrete failure mechanism of the headed anchor under axial compression? Would it be reasonable to assume punching shear originating at the depth of the embedded anchor head?

2. Anyone recommends using soft nylon washers under the base plate, so they will compress first and let the grout and concrete to take over the load?

Thanks!

 

[azcats]

If the comments above about baseplate flexibility haven't assuaged your concerns, you could have the fabricator add another nut and washer set into the concrete just below the top of pedestal.

I do this when my anchor bolts end up near bottom of foundation on telecom towers. But we don't use grout.

 

[dik]

Even putting a 1" piece of neoprene tube beneath the bottom levelling nut would prevent load transfer, if really concerned about it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[RabitPete]

I am with you guys. Thinking more about it, and assuming the worse case scenario with zero bond between anchor and concrete, we have a rather long rod under compression. Yes, steel has higher modulus than grout and concrete, but is it really stiffer? The rod section area is much smaller and very little percentage of the design force will displace the top end down enough for the grout to take over. Displacement under the full load is about 0.030" or 0.75mm, I cant imagine a situation with such a gap between the base and non-shrink grout. And that is not even considering the flexibility and creep under the embedded anchor head. Flexibility of the base plate helps to redistribute the load too.

So it does not seem there is any risk with using steel leveling nuts and steel washers in this example.

 

[StrucPatholgst]

We used leveling plates and jamb nuts on tank farm pedestals for decades, never had a single problem.

 

[WillisV]

It is incredibly common to have leveling nuts under columns in commercial construction - there is no issue here.