Concrete Pier Design for Large Shear Force 1

[Grant M]

Hi all,

I am working on a project where I received some column loading from a prefabricated metal building supplier, and I just need to design the building foundation system. Due to the client requesting no interior columns for a 75'-0" clear span, the loading coming in from these columns is significant. I am getting a factored shear force of 86.6 kips from these columns, which is acting away from the building structure. The immediate issue was getting the anchorage to work, so we looked into providing supplemental anchorage reinforcing for shear per ACI D.6.2.9, however, the pier is elevated from the building floor slab (see attached section and plan view), so it was starting to become a bit of a mess trying to transfer the shear from the anchors to the anchor reinforcing to the hairpin reinforcing to the slab and the required pier dimensions were getting a bit ridiculous.

My coworker suggested the idea of embedding a wide flange member into the pier with a welded plate on top that would be flush with the top of concrete. The building column anchors would then bolt to the plate/wide flange and we would have hairpins wrapped around the wide flange to transfer the shear into the floor slab (also shown in attachment). We discussed needing to check the beam for shear and moment and to ensure the slab could handle the loading.

I just thought I would put this out there to get any feedback on the proposed beam embed as this would seem to be the easier, more cost-effective option of transferring the shear. Or to open it up to other suggestions on ways to do this.

Thank you!

 

[phamENG]

Are you planning on using a hairpin to transfer 86kips of shear to the slab reinforcing? I only use hairpins to prevent breakout of the concrete. I've made it standard to put discreet tension ties at all rigid frames, regardless of the shear force. I thicken the slab below it and encase bars in that so they aren't part of the slab. While your method can technically work, I've seen PEMBs designed that way and built without the slab so the future tenants could build it out. The hairpins were sticking out of the piers and bent straight up to "prevent a tripping hazard." I know you'll say "they're not doing that with this one"...but the EOR for that one said "they weren't going to do that!" when he was told.

Not sure if that will help your situation all that much. The embedded steel seems a bit much to me. How have you been analyzing the pier?

 

[BridgeSmith]

You could consider concrete filled steel tubes (CFT), AKA permanent casing.

Concrete Filled Steel Tube Bridge Pier Connections - An ABC Solution

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Grant M]

Yes, based on my calculations I would need two hairpins to transfer the shear load. I haven't used the tension ties before in a design, would this be similar to providing a grade beam spanning between the frame piers? The issue I am seeing is the facto that the pier is set 8" above the floor slab, so the tension ties and hairpins would be located significantly below the anchor point for the columns to the piers.

I haven't gotten around to analyzing the pier yet as I have pretty much been focusing on the anchorage and determining how to transfer the lateral load out of the pier.

Thank you!

 

[HTURKAK]

The factored shear load of 86.6 kips is substantial amount.. The sketch which you have posted the break out zones implies that the transfer of shear is considered with two anchor bolts at left side...

I do not know the concrete quality, size and material of anchor bolts, factored design moments and axial loads etc..

But i would like to remind some options ;

- Consider the frictional resistance of base plate together with anchor bolt shear strength . For this, weld the washers to the base plate.

- Consider to increase the concrete strength,

- Provide hairpin reinf. similar to Fig. R17.5.2.9a and b .

- Finally, you may consider to provide anchor angle or anchor plate similar to the following sketch ( excerpt from PIP STE05121 )

I hope my post answers to your questions.

 

[Enable]

Agree with phamENG. It may not be your problem per se but you'll still be dragged in when things go wrong after a demo contractor chips out the slab for whatever reason not anticipating that you are relying on the slab for shear resistance at the column base. It's not a typical design so you may be found at least partially liable. EDIT - it may be commonplace in other jurisdictions though and so if that's the case where you are I suppose it wouldn't be a problem from a liability perspective. However, still quite hazardous for the fellow that wants to install a pit or something next to a column and unknowingly takes out your shear support.

I would much prefer to resolve the shear at the column stand in a very distinct / unmistakable way (i.e. not slab embedments). The solution phamENG suggests might work as would pilasters of sorts below the SOG depth or grade beams acting as tension ties, etc. I actually like your W-flange idea given the distance between the ties (whatever they end up being) and the column/pilaster interface. But I might (A) bolt it to the footing and (B) extend the footing / pilaster towards the inside of the building and encase a gusset / studs or rods extending off the W-section into a concrete mass. You only need roughly 16m3 of concrete (less amount already contained in your footing which I don't know) / column which isn't that much. Maybe an extra $1000 - $2000/ column?