Concrete Pier Lateral Load

[Sweever]

I have a large concrete pier/foundation (2'x3'x5'High) and a large lateral force around 720KN. I have provided stirrups at the top of this pier to take this lateral force, however due to construction realities, I can only get the required stirrups within the top 7-8" of the pier and not the top 5" of the pier that is recommended by various sources and codes. My question is should I be concerned? My thoughts are no. I am not clear where/why this 5" measurement comes from. Because the load is in the long direction of the pier, could you not make the case that the shear load from the anchor bolts would be dispersed downward within the pier concrete and as long as you had the required stirrups within a distance of d from the top of the pier, that load would get pickup by stirrups. I get that you want a decent concentration of the stirrups near the top to help with getting the load from the anchor bolts into the pier but 5" seems very constraining. Thanks for you assistance.

 

[CDLD]

If you can get one stirrup near the top of pedestal (within the 5"), you can do a strut and tie check for the high shear load. Refer to PIP STEO5121.

Alternately, you can check your concrete breakout strength and other anchorage requirements from your code.

 

[Sweever]

I can get several ties within the top 5" of the peir. However, I cant get enough stirrup area within the 5" for the full load. The full amount of steel needed for the S and T method can only fit witin the upper 7-8" of the peir. My understanding that only the steel in the top 5" of the pier could be utilized in the S an T method. This is what was confusing and I did not understand why.

 

[JLNJ]

What mechanism delivers the force? That a big shear. Almost 200 psi on the gross section.

 

[Sweever]

The load transfer is via 10- Anchor bolts

 

[hokie66]

Just a couple of thoughts.

1) Maybe you need to deliver the force to the top of the column in direct bearing down the side of the column rather than by a bunch of bolts.

2) The shear is the same full height, not just at the top.

3) It is best not to mix units. Inches and feet are not compatible with kN.

 

[Sweever]

Thanks Hokie66 for your input. A couple of additional questions

When you say deliver the forces in direct bearing down the side of the column. What exactly do you mean? Steel side plates on the side of concrete pier with bolts in shear into the pier?

Agreed the shear is the same the full height of the pier. That is why I am confused why there are papers out there that say you need to resist the shear load within the top 125mm (5") at the top of the pier.

The shear resistance of the pier is based on the shear of the concrete and the shear of the stirrups over a length much longer/greater than 125mm (5"). I am just trying to understand do I have an actual problem here.

Sorry about the units mix up. I know we are all not in metric.

 

[hokie66]

No, I mean to bear on the face of the column. Your base plate would be 'L' shaped with a leg down the face.

 

[Sweever]

Hokie66- That is a possibility. Any comments on the 125mm mm distance to engage stirrups? Is this a real concept that needs to be followed? It just does not make alot of sense to me.

 

[hokie66]

No, I am not familiar with the 125mm requirement, but know that the force should not be delivered at the very top.