Increasing shear capacity of metal building column connection to concrete pier 1

[McSEpllc]

Hi All,

In order to increase shear capacity at the connection of a per-engineered metal building column to a concrete pier, the metal building engineer suggested the attached detail. It uses a steel angle that the anchor bolts extend through within the concrete pier. I guess to engage the concrete.

Have any of you used this detail?
What do you think about it?

It seems it offsets the location of shear resistance down into the concrete, and one could argue that induces bending into the anchor bolts. On the other hand the anchor bolts are within the reinforced concrete pier, so can not deform.

Thanks!

 

[bones206]

If I recall correctly, the effective depth of anchor for shear transfer into the concrete is something like 8 x the anchor diameter. It would be difficult to justify the distribution of load sharing between the anchor shaft and the embedded angle. I'd much prefer using a traditional anchor reinforcing scheme using rebar stirrups or hairpins.

 

[dik]

Can the angle be located just at the underside of the base Plate?

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

-Dik

 

[McSEpllc]

bones206, yes, I am skeptical about how this is supposed to work as well.

dik, the detail was suggested by the metal building engineer. I see your point, which makes a lot of sense.

 

[dik]

comes from years of playing engineer. I've used an angle often with rebar welded to it to take the horizontal load into the slab and slab reinforcing.

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

-Dik

 

[McSEpllc]

@ dik, Maybe I was looking at this wrong: I was looking at horizontal shear being transferred
from column base plate to anchor bolts,
from anchor bolts into concrete pier,
and from concrete pier to the tie rods in the slab.

When I follow the route described by me above, the anchor bolts lack shear capacity. The anchor bolts do not have sufficient spacing to use all four anchor bolts. The two diagonal anchor bolts just meet the spacing limit, but are insufficient in shear.

If I follow you correctly, I would not have to count on the shear capacity of the anchor bolts in the concrete pier, but can transfer the load by the angle directly to the tie rod (the columns are pin-connected, no moment transferred.)
Is that how you see it?

 

[KootK]

I think that the detail would be largely ineffective, in large part because I disagree with this statement:

On the other hand the anchor bolts are within the reinforced concrete pier, so can not deform.

Steel is much stiffer than concrete and, therefore, will not be perfectly restrained by it. Trust your instincts regarding the bending in the anchor bolts. This setup looks like a permutation of the hairpin reinforcing scheme.

With the angle perpendicular to the applied shear, Dick's proposal is basically a permutation of the shear lug method. You can read all about that in AISC Design Guide 01.

 

[dik]

I've used this, not to transfer the load to the pier to the slab. On the other hand it transfers the load to the pier more effectively, too.

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

-Dik

 

[bones206]

My assumption is that the anchor rods themselves are adequate to transfer shear from the base plate to the concrete, since they are typically sized by the PEMB engineer for the loads. So really the capacity that needs to be increased is the concrete breakout capacity, correct? The concrete breakout capacity is basically the same regardless of shear load delivered through the anchor shanks or delivered through that angle. So that's why I was suggesting anchor reinforcement to restrain that breakout cone and achieve a higher overall capacity.

 

[GC_Hopi]

My general approach for PEMB is to add supplementary reinforcement at the anchors. They typically like to use bypass girts and lap the siding down the edge of the foundation which forces you to ~ 1 ft edge distances to your column centerline. CSA published a guide called Design of Anchor Reinforcement in Concrete Pedestals that is helpful, snip below.

 

[jayrod12]

I would never assume the PEMB guy has adequately sized the anchor bolts for anything more than the anchor steel capacity. They don't give two hoots what happens once the load in the anchors gets to top of pier. And in my experience, they assume no bending in the anchor bolts either.

 

[dik]

That's why the restraint has to be at the U/S of the base plate.

What is the anchor rod material? In these environs, I always use ASTM F1554 Gr 55S1.

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

-Dik

 

[bones206]

I just think the PEMB engineer has a fundamental misunderstanding of anchorage if they suggested the angle to increase the anchor capacity. If the concrete breakout capacity is deficient for the shear load, then it doesn't matter what mechanism is delivering the shear load to that concrete breakout cone. The angle doesn't do anything to restrain the breakout cone, so it's pretty useless IMO.

 

[dik]

Here's our building and these are the forces; you tie it down...

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

-Dik

 

[McSEpllc]

Thank you All for your thoughts

 

[dik]

Let us know what you decide... thanks.

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

-Dik

 

[Spanky7]

If you are having problems with the concrete shear breakout and do not want to add supplementary reinforcement at the anchors, try using a shear key under the base plate to give yourself a larger breakout wedge. AISC DG2 has a good example of a shear key design.

 

[dik]

The shear plate is not likely part of the PEMB package, and if you add one, you may void any warranty you might have had.

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

-Dik

 

[bones206]

This is just my opinion, but if the breakout capacity is deficient using the anchor group pattern, but sufficient with a shear plate that is only 3” or so wider than the anchor spacing, then I wouldn’t be comfortable with that safety margin. Using reinforcement to restrain the breakout cone has almost become the standard of care at this point in time, and I think that is the best practice. If you have a concrete pier, the ties are going to be there anyways. It’s just a matter of quantifying their capacity as anchor reinforcement using the method GC_Hopi references. I know I may be a bit of an anchorage snob, but I don’t like to mess around when it comes to PEMBs.

 

[McSEpllc]

Thanks guys.

I am hesitant to modify the PEMB steel. The PEMB engineer is quite communicative, but not forward coming in doing changes on the PEMB side.

So my approach will be to figure reinforcement to restrain concrete breakout.