Embeded Steel Plates in Piers

[CowboyBill74]

Hello all -

I have a foundation pier that is supporting a major wind brace in a building. The tension force in the bolt group is around 270 kips and the shear force is approximately 350 kips. Large forces I agree but it is what it is.

I am having a hard time coming up with a achoring system to the pier that can accomidate both forces.

A fellow engineer in the office suggested that I design the anchor bolts to with stand the upward tension force and embed a steel plate with steel studs into the pier and weld a steel plate to both the embeded plate and the column base plate that way I could utilize the concrete pier for resisting the shear forces and extend the anchor bolts through the pier and into the concrete foundation to with stand the tension force.

However I am still having a hard time getting a reasonable size pier, and when I say reasonable I mean not having a pier the size of a foundation, to work.

Any suggestions of finding references to this type of pier connection would be greatly appreciated.

Thanks

 

[jike]

I have used the system that your fellow engineer has suggested but not for those kind of forces.

What size pier do you need to take the shear and moment?

I have a hard time believing the pier will be almost as large as the footing, especially with that amount of uplift and overturning. Do you have a pile foundation with tension piles?

 

[ChipB]

Ugh!
Well, you've got an interaction diagram for concrete. And we all know, concrete works great in tension. ;-) That would put you below the pure moment line, and your vertical steel is going to have to accomadate the tensile forces due to uplift, and the tensile forces due to the moment the shear has created. But, if you are transfering your tension through the pedestal, down into the footing, it should place you on the pure moment line.

Size and space your stirrups to take the shear, size your vertical steel to take the induced moment.

As for the detailing, I don't like it. Put a shear key on the bottom of your base plate and provide a grout pocket in your pier.

 

[CowboyBill74]

jike -

Well right now I have analysized a pier that is 2'-6" wide (perpendicular to the loading) by 4'-9" long (parallel to loading) and it is giving my a side blow out capacity of only around 15k, so with out going any further it seems that to be able to with stand the shear force that I have the pier is going to need to be something around 8'-0" or so.

ChipB -

My pier depths range from 16" to around 5'-0". So if I understand you correctly I can embed my steel studs into the pier and design the pier itself to with stand the shear force? Do I not need to worry about side face and front face blow out of the pier? From what I have read there is a factor that takes into account the reinforcing in the pier to help out but this factor is rather small. Do you have a reference that I can get to look at the procedure you speak of?

 

[jike]

It sounds like you might be using Appendix D for blowout. RD.4.2.1 seems to imply that when you have supplementary reinforcement has to rely on other design theories or procedures.

 

[ChipB]

Nope, you missed it.

The more correct terminology for what I'm discussing is "shear lug". It is a plate, welded vertically to the bottom of your base plate. This in turn, sits inside a grout pocket which is inlaid into your pedestal. With this load, it is likely going to be a big plate. The pocket on your pedestal, is typically 1.5" to 2" past the outside edges of the plate (3" to 4" wider"). You'll likely need confinement steel in the pedestal, around this pocket.

Look at AISC's Design Guide #1 for designing your shear lug.

Interaction diagram, CRSI

With that high of shear, can you tie this pedestal to others via a grade beam, and share the total shear along this column line between the braced and unbraced frames?

Not knowing how you calculated your loads, but you are going to need a chunk of concrete under this column to resist that uplift. There is no way your pedestal size should be getting anywhere close to your footing size.

 

[ChipB]

I found this thread: thread507-116220

 

[CowboyBill74]

Thanks ChipB that really does help and one of the engineers here at the office is an AISC member so he is printing those Design Guides for me now.

Again thanks for the help!

 

[ChipB]

You're Welcome

 

[J1D]

A thought to deal the shear+tension.

How about make the surface connecting the brace in a slope to be just perpendicular to the resultant force? Then use a set of anchor bolts in slope as well. No more shear on the interface.

We once design the pier of the anchor block of stack wires this way.

 

[CowboyBill74]

JID -

Good thought and I may use that connection detail in the future, it would work well if the only force present was the brace force. However my brace is connecting to a column that is taking axial load from the roof and if I sloped my concrete pier to be perpendicular to the brace member then I would have my axial load on the column becoming the shear force at the connection point.

Thanks for the reply though. I may look at it in this configuration to see which gives me the best results.

 

[Gumpmaster]

Just another example of the impracticality of ACI 318 Appendix D.

 

[J1D]

What do you mean impractical? Just because the anchor bolts are not vertical? I don't see the clause against this application, once you have the piers properly dimensioned and reinforced.

 

[CowboyBill74]

J1D

I dont think he was calling your layout impractical, I think he was refering to the very conservative results that you get from ACI 318 Appendix D, because I was getting these huge piers needed to with stand the forces. Because trust me I have gone through the design with smaller loads and I still get a pier, in my judgement, that is way oversized for the loads.

 

[sundale]

Appendix D consistantly yields CRAZY conservative results. I am learning to size my foundation elements based on the anchor rod embed first... Yet another instance of "publish or perish" academia and the code peddling institutes leading our profession by the nose. In comparing the old UBC formulae to the Appendix D stuff, they do appear a little too simplistic, but I never heard of any major anchor rod failures.

Consider a concrete SMRF where one could perhaps see tension and shear forces of these magnitudes in a column. When detailing the joints, are you mandated to check esoteric failures like "side face blowout" or "concrete splitting"? No, you detail the proper confinement stirrups and ties, lap splice the rebars and/or check development lengths and go on with life.

There are indeed differences between a rebar dowel at a construction cold joint and an anchor rod but there are many similarities too. Once it becomes an "embedment", ACI has gone over the top with Appendix D.

 

[LGARG]

Hi guys.

Do you have a steel column that is bearing on that pier that you said? if so is that connection pinned?

LG