Post Frame Building Design

[pioneer09]

Have designed post framed building before using the NFBA Post-Frame manual. All prior applications have used embedded posts so following the manual was rather straight forward. A recent job has come up with the request to have the posts attached to the concrete at grade without a fixed connection. This would result in diaphragm action being the sole resistance for frame side sway. This is very common practice as noted by many constructed projects. My question is with regards to modeling and analyzing the frames. Since the frames have no horizontal resistance (pinned at base and at top of column), what is used to keep the frame stable during analysis? The NFBA manual uses a program called DAFI that gives the diaphragm load and horizontal displacement for each frame. My only thought is to provide a spring force at the eave of the frame model which could be established based on the diaphragm load and horizontal displacement. Does this seem like a reasonable approach for design?

 

[jayrod12]

that's how I've approached it.

Although typically one or both of the end walls have such large doors in them that they can't be counted on for shearwalls.

 

[RPMG]

If this is a 2D model, the diaphragm is modeled with a pin. The diaphragm must be designed to transfer those forces to shear walls.

Flexible diaphragms transfer load based on trib area, so shear walls are required at each wall, hence jayrod's point.

Also, the frames must be braced during construction because they are unstable without sheathing.

 

[Gnique]

The beam model for bending moment for an embedded post is the same as for a simply supported beam (wL^2/8). Get the post attached to the slab with a suitable post bracket and design as usual. With the post attached to the slab at the bottom and to the roof diaphragm at the post/truss connection, you have a (vertical) simply supported beam with a compression load at the top and a distributed load (wind from the girts) along the full length. The only caution is that you DO have to have adequate shear panels on all four walls. The posts, the diaphragm and all of the secondary framing work just like a regular old pole building. I have contractors that use this method quite often in Eastern Oregon where there is LOTS of thick, igneous rock right at ground level. This "slab on grade" method keeps the builder from having to drill into solid rock. It should be noted that the metal sheathing on a post frame building works just like the wood sheathing on a stick frame building. The post concentrates the roof and wall loading and, therefore, allows for highly efficient material usage in the secondary members but it is the diaphragm that provides the vast majority of the LFRS. You might have a look here:

http://www.aeoregon.com/index.php