The Simple Cabin structural assembly showing posts, girts, joists, and rafters. |

There are seven girts. The top girt, three loft level girts, and three first level girts. The top girt holds 16,963 pounds, the two outside loft-level-girts carry 33,926 pounds each, while the inside loft-level-girt carries 16,963 pounds, and the three first-floor-girts carry 16,963 pounds each.

The

*National Forest Products Association*provides wood structural design data. The safe load for a single 12" x 12" beam spaced twelve feet apart is 16,898 pounds. Since my beams are spaced ten feet apart, I can assume a 12" x 12" beam would suffice.

Except for the two outside loft-level-girts. They carry 33,926 pounds each, and there is no beam specified in the

*National Forest Products Association*wood design table. The problem here has an easy solution.

Double up the girts. A double 2" x 12" that is well spiked together has slightly more than double the load bearing strength of a single 2" x 12" beam.

Since a 12" x 12" beam that spans 10 feet can carry 20,278 pounds, I only need to compensate for 13,648 pounds. I could spike five 2" x 12" beams onto the 12" x 12" beam, and therefore meet the load requirements. This solution is not very efficient, and a single glulam or structural steel would carry the weight easier. If I wished to use only 2" x 12" beams, then I could make beams of five 2" x 12" spiked together, and place them independently on opposite sides of the poles.

This assumes I use finished, dimensional lumber purchased from a lumber yard. I plan on milling my own lumber, so I can create 5" x 12" beams to spike into a girt assembly. This solution is simpler, and more efficient.

Now that I have the girt sizes, I can size the joists and rafters. Again the

*National Forest Products Association*provides an easy table for sizing joists and rafters. I can use 2" x 8" joists for the first floor, and the loft floor, and they can be place 24" apart. The table sets the span limit at 10'-2", so I am well within the design guidelines because the span is only ten feet, and the girts will decrease the span even more.

The table again suggests a 2" x 10" rafter placed twenty four inches apart is necessary because the horizontal span is 13'. The entire rafter could be purchased as a single element, but it is yet to be determined where I will acquire the structural rafters.

With the pole selection, girt selection, joist and rafter selection, I have specified the most important structural elements for the cabin. If I get it wrong it could fall down, but given the meaty beams specified, I don't think it is likely. The construction quality must be sufficient though.

I have the basics for the Simple Cabin, and I am enthusiastic about starting the project. I will certainly need help in the future, so volunteers apply within! If you have any ideas or suggestions on how to improve the design, then leave a comment below.