Concrete Breakout Strength for Shear Force of a Pre-Fabricated Metal Building

[ivanga7]

Hi. I'm working on a project where I have to design the footings and anchorage for a pre-fabricated metal building. The slab rests on top of the footing and they are cast in separate pours leaving a joint between them (see attached photo). The building column anchors go through the slab and into the footing. My question is, when it comes to finding the concrete breakout strength in shear, do I consider the edge of concrete as the edge of the slab or the edge of the footing? The joint between the slab and footing is throwing me off. The slab is only 6" thick and slab edge is only 8" from the closest anchor. The anchor is embedded 24" into the footing.

 

[malikasal]

i would go with the slab, otherwise if the slab cracked your rods are gonna have to resist a considerable value of moment resulting from eccentricity while transferring the load from the plate to the footing.



ôIf you don't build your dream someone will hire you to help build theirs.ö

Tony A. Gaskins Jr.

 

[ivanga7]

Can I treat it as if the joint between the slab and footing was not there and my hef = 24" or is my embedment depth just the thickness of the slab?

 

[bhiggins]

Why not install the baseplate at the top of the footing and provide a diamond blockout for the slab at the columns? I feel like this is more typical.

 

[Ron247]

Are you talking about shear from the horizontal force on the anchor bolt (thrust)? What magnitude of force are you talking about?

 

[Ron247]

Also:
Does anything connect the slab to the foundation other than the anchor bolts?

 

[BAretired]

Is there any confinement reinforcement within the slab which would tend to prevent breakout?

BA

 

[ivanga7]

Ron247: At some points the loads are as high as 30 kips.

BAretired: There is no confinement reinforcement.

For further clarification, we designed the slab and footing to be cast as a monolithic pour, however, the contractor wants to do it in two pours.

 

[Ron247]

Sounds like you have a problem. 30 kips (USD or ASD) is a lot of thrust. With no confinement, the slab can shear off which then causes the eccentricity that Malikasal mentioned. The hardest things I find to design for is the outward thrust under DL+LL and the uplift of the anchor bolts for Wind. I am in a 12 psf LL area with light wind. Other than those 2, most of a foundation is relatively easy to size.


The PEMB supplier does not care how close the anchor bolts are to the edge of slab. They do not care about how tight the bolt pattern is. Their only job is to specify quantity and diameter. The concrete code really penalizes fasteners near the edge of concrete. Even pouring monolithic I would think will require a confinement cage made of dowels and ties.

 

[skeletron]

I would look at the breakout cones independently first to see how much capacity there is in the footing and in the slab. You might be able to justify the load resistance that way, but the slab could be vulnerable in this situation.

 

[BridgeSmith]

Without reinforcing to transfer the shear on the bolts (tension on the slab edge concrete) back into the into the slab, away from the edge, the edge of the slab will break off and you'll have a 15 kip-ft moment on the anchor bolts. Unless those anchor bolts are huge, they're going to bend.