Structural System for Pole Buildings
Positive bending moment along the length/height of the pole/column never control. Even for a propped cantilever, the negative moment at the column base is approximately twice the positive moment along the column length/height. The negative moment at the base of the column is the controlling factor. So, the analysis of the pole building shall be as follows:
- Each pair of poles/ columns, one each side of the side walls, could be analyzed together. This means that each column will support:
- its share of the vertical dead, snow and/or live load if any;
- Half of the uniform wind load from wall and half the concentrated wind load from the roof.
- The poles/columns shall be considered to be fixed at their base and free at the top.
- Since the poles/columns are connected to each other at the top through double trusses, these trusses can transfer an axial horizontal force from the loaded pole/column on one side of the building to the unloaded pole/column on the other side of the buildings.
- Since the two trusses are required to transfer the horizontal force from the loaded pole/column to the unloaded pole/column, their bottom chords must be braced so they can support this axial compression force. In our details, we do two things:
- The two bottom chords are blocked each four feet. This blocking enables the two chords to act as a compression horizontal strut.
- We provide lateral bracing between all trusses bottom chords at 20’-0” on centers.
How to Size the Pole/Column
The buckling length factor for cantilever post could be reduced from 2.1, default value in the Wood Work software, to say 1.6 or so. This is because the post is not completely free or completely restrained. The pole is partially restrained as follow:
- Parallel to the wall, the column is partially restrained by the girts.
- Perpendicular to the wall, the column is partially restrained by the several bolts and nails that is connected to the truss
Remember since we have two poles/columns, you need to divide the uniform wind load by 2. Dead, live, and snow load is calculated per single pole/column and need NOT be divided by 2.
Remember since we have two poles/columns, you need to divide the concentrated wind load at the pole/column by 2. Dead, live, and snow load is calculated per single pole/column and need NOT be divided by 2.
- Multiple of 2x lumber such as PT 4- 2×8 or PT 4-2×10 and so on.
- Solid timber pole such as 6×6, 6×8, 6×10 and so on.
- Combination pole of the above. Such as PT solid timber, say 6×8, and PT lumber, say 2-2×8, composite post.
This is an excellent option since the high moment at the column/ pole base die quickly above the building floor. So we can use say PT 6×6 and add one PT 2×6 each side. The additional 2x6s could be only 8 feet long, 4 feet below grade and 4 feet above grade. We have a standard detail for your use if you wand to use it. Please consult with the client/contractor. Remember to use at least 2-16d at 6 inches on centers to fasten the 2x lumber to the timber post/ pole to inure that the 2x will not buckle under axial or bending stresses.
- combination of PT solid timber pole and two 2x PT lumber one in the front of the post, inside the building, and one in the back of the post, outside the building.
- Combination of PT solid timber pole and one 2x PT lumber in the back of the post, outside the building. The is a good idea since the 2x lumber will occupy the same space as he girt.
When the composite pole in items d and e- above is subjected to bending moment, a slippage between the timber and the 2x lumber will occur unless mechanical fasteners are used to resist this slippage.
- Pressure treated, PT, Glulam pole.
- Round Poles like telephone poles.