Roof, Trusses Support and Foundation
Roof for Pole Buildings
Purlins design for pole buildings
- n most cases, the roof consists of simply supported purlins stacked and lapped over the trusses. In rare cases the purlin are hanged with a joist hangers between the trusses.
- Purlins span between the trusses. Since in most cases we have two trusses one each side of the pole/ column, the purlin span could be 6 inches less than the center to center spacing between the poles/columns.
Trusses for Pole buildings
- Most of the projects will use Pre-Engineered trusses. So we do not design them.
- In rare cases, the client will ask us to design the trusses. In this case, try to use plywood as gusset plates. Remember that steel gusset plates need a machine to press it into the wood and can’t be used for on site construction of the trusses.
Support for the Trusses in Pole Buildings
- Trusses need to be supported for
- Vertical downward loads such as dead loads and snow load.
- Also, trusses need to be restrained against wind uplift forces.
- Trusses support for downward vertical dead and snow loads:
- The trusses are usually anchored to the pole/column by a 7/8 diameter bolt.
- Also, the truss is supported on a 2x block that is fastened to the pole by two means. A 7/8 diameter bolt and twenty or more 16d nails.
- Trusses restrain against uplift wind forces: Above the truss, we use a 2x block that is fastened to the pole/column by several 16d nails to restrain the truss. To calculate the number of nails:
- Calculate the upward wind load.
- Multiply the upward wind force by the half the tributary area of the truss, half will go to each end of the truss.
- Calculate how many 16d nails are required to resist the uplift forces.
Foundation for the Pole/Column
- The equation for the embedment length of the pole/column in the Code is based on a cantilever pole/column that is:
- Fixed at the base;
- Free at the top;
- Subjected to a concentrated horizontal force at the top.
- We always use 2’-0” diameter concrete footing around the pole/column. The depth starts from 4 feet but can go as much as needed from the hand calculations or computer printout as out lined below.
First method of calculating the pole embedment, Hand calculations
Get the moment at the base of the pole/column from computer.
- Divide this moment by the pole/column height from top of foundation to eave of the roof to get the equivalent concentrated horizontal force at the fop of the post that caused the moment you got from the computer
- For pole buildings that do not have a concrete floor slab, the pole/column is considered no constrained, IBC section1805.7.2.1 to calculate the embedment length.
- For pole buildings that do have a concrete slab, the pole/column is considered restrained/ constrained, Use the equations in UBC section 1806.8.2.2 equation 6-1 or IBC section section1805.7.2.2 equation 18-2 or 18-3 to calculate the embedment length.
Second method, using software
- Get the moment at the base of the pole/column from computer.
- Divide this moment by the pole/column height from top of foundation to eave of the roof to get the equivalent concentrated horizontal force at the fop of the post that caused the moment you got from the computer.
Use any spreadsheet software to calculate the embedment depth such as Engineering International Spread Sheets, engineering-international.com