ACI appendix D and pre-engineered metal bldg pedestals
ACI appendix D and pre-engineered metal bldg pedestals
(OP)
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!
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!






RE: ACI appendix D and pre-engineered metal bldg pedestals
Contractors hate this- but I haven't found a good alternate yet.
RE: ACI appendix D and pre-engineered metal bldg pedestals
Mike McCann, PE, SE (WA)
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
Mike McCann, PE, SE (WA)
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
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.
RE: ACI appendix D and pre-engineered metal bldg pedestals
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.
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
DaveAtkins
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
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.
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
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.
RE: ACI appendix D and pre-engineered metal bldg pedestals
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?
RE: ACI appendix D and pre-engineered metal bldg pedestals
ASCE Design Guide 8?? Not sure, but it covered this very well and at least represents a national consensus.
Bob
RE: ACI appendix D and pre-engineered metal bldg pedestals
I know how I’ve handled it in the past…..just curious for opinions.
RE: ACI appendix D and pre-engineered metal bldg pedestals
RE: ACI appendix D and pre-engineered metal bldg pedestals
"We shape our buildings, thereafter they shape us." -WSC
RE: ACI appendix D and pre-engineered metal bldg pedestals
Mike McCann, PE, SE (WA)