ACI appendix D and pre-engineered metal bldg pedestals 3

[braves25]

When it comes to the design of pre-engineered metal building foundations, I find that app. D is causing me grief!!

How do y'all handle the uplift forces and the horizontal forces transferred from the anchor bolts into a concrete pedestal without having the pedestal be very large? For one case (at braced frames where I have a large horizontal shear perp. to the main frame column) combined with uplift, I find it almost impossible to use pedestals. AM I missing something? Thanks!

 

[hawkaz]

I have had the same problem. To resolve this, I have taken the bolts all the way through the pedestal and embedded them into the footing below using hex-head anchors
Contractors hate this- but I haven't found a good alternate yet.

 

[msquared48]

I have never understood why a pedestal is ever used considering the extra concrete, formwork, and the concrete pour and curing time considerations, unless soil retention is involved. Even them, its problematical to me. It's just so much cheaper to do a mono-pour of the slab and footings, and place the steel on the anchor bolts, either set into the slab, or after-set.

Mike McCann, PE, SE (WA)

 

[braves25]

I too have done this in the past, but for this project, I have conditions where my footings at 7 feet below the slab.

 

[msquared48]

Why? Soil retention? Frost?

Mike McCann, PE, SE (WA)

 

[braves25]

Grading at the exterior of the building...grade slopes pretty quickly. Mono pours will not work. Even so, you still have to deal with uplift forces on your anchor bolts that in these types of buildings are very near the edge of the slab.

 

[jittles]

I take all of the 'outs' in appendix D with regards to using anchor reinforcing.

I run the anchor rods down into the footing so tension bypasses the pedestal.

I then check the pedestal ties for their capacity to take the shear load, using a strut and tie sort of methodology. The geometry is critical in this calculation. This method is laid out in the attachment.

 

[MrHershey]

I've had a few jobs where due to uplift I have the footing buried deeper, which requires a pedestal. I've seen people use shallower footings and then tie it in with the slab and use slab for uplift but the only problem with that is you then have to design your slab for that point load, which seems to be rarely done. You don't get to use the whole slab if your slab can't transmit the load far enough to justify using it.

I've done two ways:
- In addition to an base plate, include an embed plate with welded bars. Clamp baseplate to welded plate with nuts and then lap welded bars with pedestal rebar. Whether that's using straight laps or hooks or headed anchors on welded bars/pedestal bars or whatever.
- You could also include pedestal bars as supplementary steel for tension and then also include closed ties around the anchor bolts close enough to satisfy supplementary steel requirements in appendix D. If your anchors are long enough and your ties are close enough, I would be hardpressed to not see the anchor and pedestal assembly work together similar to a beam/column.

I've also seen white papers where people use strut and tie analysis with concrete between anchor and ties as the 'strut' and then the ties as 'ties'. But haven't done that one myself.

Keep in mind that Appendix D wasn't really developed with this situation in mind. Appendix D was originally developed for anchorage into plain concrete and then later added supplementary steel, but still isn't really intended for structural steel frame to structural concrete frame connections like would be used here.

 

[hokie66]

What do you mean by "very large"? Where you have a large force, you need a large resisting member.

 

[ztengguy]

can you lap the anchor bolts with rebar into the footing, check pedestal for bending and be done?

 

[steellion]

For uplift/moment at a column base on a pedestal, I make sure that the vertical pedestal reinforcement meets the ACI App. D requirements for tension reinforcement so that concrete breakout calculations can be avoided. The concrete breakout failure mode is usually the toughest to satisfy. Then I design for the remaining App. D failure modes. I am not aware of a code provision that allows a "lap splice" between an anchor bolt and a rebar. Even if lap splices are permissible, for a substantial force, the anchor rod diameter would require the lap splice to be several feet long.

 

[braves25]

In one case, I have a factored load of: uplift = 56.2 kips and shear (perp. to frame) = 37.4 kips. My main concern is the transfer of load from the anchor bolt to the pedestal.

 

[DaveAtkins]

I have done what ztengguy suggests. I use very long anchor rods, lapped with the reinforcing bars in the concrete pier--you can hook the reinforcing bars at the top of the pier if necessary. It has been awhile since I looked at ACI Appendix D, but I think there are ways to justify this approach.

DaveAtkins

 

[braves25]

I too have done this approach (use the pedestal vertical bars to take the tension into the footing). But what about the large shear loads?

 

[braves25]

Would adding a shear lug to the bottom of the base plate take care of the shear? If so, the design of the lug and the bearing on the concrete are pretty straight forward; however, what about the concrete check to make sure the block of concrete resisting the shear remains in place.

 

[charliealphabravo]

Appendix D is essentially a design method for plain concrete. It will indeed cause grief when applied to pedestals. Appendix D gives only the slightest credit for supplemental reinforcement.

Longer anchor rods will only solve the uplift portion of the problem. In addition to Appendix D you will need a strut and tie method to solve the lateral case as jittles pointed out.

Also consider rigidly connecting/welding the base plate to the anchor rods to get full credit for lateral resistance under Appendix D. Perhaps shear lugs are a better way to achieve this.

 

[precast78]

If your Appendix value is not good enough, that means you must transfer the load to vertical reinforcement in the pedestal. That means you can just throw Appendix D out of the window and make sure your load can transfer to your rebars. Make sure you have enough development length to splice your anchor to your vertical steel.

 

[braves25]

Agreed, but my main question at this point is what about the shear? Is the strut and tie method the best, or are there better options? Appreciate all the great comments and feedback.

 

[MrHershey]

You can do strut and tie. I've also reinforced the pedestal like a reinforced concrete column, design it for the shear load, and then provided closed ties/cross ties in the top 5" that are sized to take 100% of the shear load. To get this to work you either have to set anchors back from the edge a bit or weld rod to nut to base plate so you can use furthest anchor from edge for breakout.

Look at the breakout cone and make sure you have stirrups crossing it. With the stirrups crossing you physically cannot break out the concrete without either failing the stirrups or somehow debonding the concrete from them.

This form of supplementary reinforcing isn't specifically addressed in Appendix D, which again is more for plain concrete. They'd have you hook hairpins around the anchors to hold them in, which isn't really applicable here. But I would argue that if you've got closed ties around the anchors and pedestal longitudinal bars and those closed ties are sized for the shear load and pass through the breakout plane, then you really can't break out with failing those bars.

 

[JedClampett]

I see two problems with anchorage of PEMB.
1) ACI 318 Appendix D titled "Anchoring to Concrete" is not really readily applicable to these structures. I see a lot of suggestions in the discussion above that seem logical, but are not addressed in Appendix D. For instance, lapping the anchor bolts with the reinforcing seems sensible, but unless you consider this included in D.4.2, it's not really mentioned.
and 2), the PEMB industry shows no interest in this issue. They develop the loads and it's up to someone else to anchor the building. For example, I've seen a 4 inch by 5 inch bolt pattern for 20 years. Why can't the PEMB give enough daylight between the bolts so maybe all of them are effective, instead of only one?
The post installed anchor industry has obviously taken an interest in this issue to make sure their interests are protected. Why can't the PEMB industry?