Post-Frame Building with Pinned Column Base Connection

[MrEngineerUS]

I could use a little guidance concerning a Post-Frame Building analysis. I am looking at doing an analysis with the sole purpose of finding post reaction loads. The posts will be connected to a concrete knee-wall that runs the perimeter of the building. I've see in the NFBA manual a type of u-bracket with two offset bolts that is for this type of surface connection to a foundation. The NFBA 1999 manual goes through the analysis procedure but it seems to only do so for a post type that is modeled as fixed at the base. From other online research this u-bracket type connection seems to be considered as "pinned".

If the base of the post is pinned it appears that the design procedure in NFBA is not valid. Is this the case? Any guidance on how I should proceed with this analysis?

Thank you!

 

[KootK]

You're right, the vast majority of commercially available post brackets are explicitly designated as pinned connections. Most of the catalogs (simpson etc) include tabulated capacities assuming a pinned joint. I imagine that, were you to use the NFBA method, the procedure would be conservative for a pinned connection, perhaps excessively so. I scanned the manual quickly and wasn't able to locate the U-bracket information that you mentioned. Can you supply the section or page number?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MrEngineerUS]

Can you supply the section or page number?

The 1999 manual doesn't appear to have an example but on the NFBA website there are examples of these brackets:

http://www.postframeadvantage.com/design/foundations/poured-in-place-concrete-pier

http://www.postframeadvantage.com/design/foundations/pre-cast-reinforced-concrete-columns

And an excellent photo in this one. The only difference is my bracket would be epoxy doweled into a precast perimeter knee wall:

http://www.postframeadvantage.com/design/foundations/treated-wood

Some more specifics: The project concerns the design of a precast panel basement, essentially. The perimeter knee wall is likely 1-2ft high. The tricky part here is I only have some idea as to the dimensions or specifics of the post-frame buildings that will be supported by this knee wall. I am just required to determine a hypothetical, conservative case that will encompass all possible uses (within the limits I specify, as in, the type of connections at the knee wall).

Thanks for any further help!

 

[KootK]

Darn, I was really hoping to get a boo at those connection calculations that you mentioned. I'm trying to do something similar connection wise.

I think that your answer depends less on the connection detail and more on the overall decisions regarding what the lateral system for the building will be. If there will be cross braces in the wall, or the walls will be designed a shear walls, or the columns form pinned based moment connections with the roof trusses, then you can consider the bases pinned and your knee wall design should be a breeze. If the lateral system will be the posts acting as flag poled columns, then you'll need to consider the bases fixed and your design will be more challenging.

There's fellow hear by the name PostframeSE who seems to do a ton of this stuff. Too bad there's not a way for you to contact him directly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

This is not a product endorsement. A quick internet search with keywords "post frame column base" yielded this manufacture of column bases designed for moments

https://www.permacolumn.com/.

They look intriguing. It should be pretty easy to calc the capacity to verify the published values (or design something similar on your own).

 

[MrEngineerUS]

I did contact Permacolumn to ask them some questions about their brackets. According to the technical resource I spoke with the only brackets that will transfer moment are the wet-set brackets with the four dowels that get set in the concrete foundation. The post-installed Sturdi-Wall brackets only are designed to transfer shear and axial loads and are the ones they recommended for the case I've described. In this case all lateral loads would need to be resisted by the lateral resisting bracing or shearwall/diaphragm action.

So, basically, I guess the NFBA design method is unusable for the analysis I've described since I have pinned connections at the base of my posts, agreed?

 

[KootK]

Were you guys actually able to find capacities on the Permacolumn site? I was not. A lot of biblical quotations however. Maybe they rely on divine intervention for moment capacity.

So, basically, I guess the NFBA design method is unusable for the analysis I've described since I have pinned connections at the base of my posts, agreed?

How do we know that you have pinned connections? It's a difficult assignment if nobody's going to tell you what the lateral system for the building is. If the connection may or may not be pinned, you may have to be very conservative.

Somebody must be giving you some kind of information to work off of here, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

KootK

The model with DBAs has published moment capacities:

https://www.permacolumn.com/wet-set-models

It would not be too hard to calc the moment capacity governed by the bolts through the wood. The connection to the concrete can be calc'd like any other base plates. It seems like this type of connection would not be too difficult to analyze or fabricate. I wonder how big it would need to be to develop the moment capacity of the post (But, I am not interested enough to do the calculations at this time).

 

[KootK]

Thanks Wannabe. I've actually been tinkering with trying to assess the same kind of bracket for moment capacity in both directions. The other way is quite a bit trickier so I was keen to at least have some values to compare with going the easy way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MrEngineerUS]

Somebody must be giving you some kind of information to work off of here, right?

That's hilarious! Yeah, my company likes the "figure it out for yourself" style of mentorship the best. I essentially just need to come up with a conservative, all encompassing post-frame barn structure to analyze and to determine foundation loads. If that involves lateral bracing and pinned column/foundation connections then I can go that route or if that involves moment resisting connections and diaphragm action per NFBA then that is fine too. I just need to be comfortable with which way I'm going to go and right now I'm not comfortable with either way.

I suppose I need to now research the barn structures themselves to get a better idea on the type of bracing that is used, if any. Maybe that will provide me with a clue because I can't seem to get a strait answer out of anyone at my office, the NFBA, Permacolumn, or here and the clock is ticking.

 

[KootK]

I think that you go with a fixed base assumption then. It should be conservative and is a fairly common superstructure design strategy I believe.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BUGGAR]

When we do post frame buildings (pole buildings) with pinned base poles, we cross-strap one or two bays with strapping wrapped around the poles and nailed, similar to a metal bulding bracing system.
Are you putting lateral loads on the knee walls?