How to Calculate Loads on Wooden Trusses

Written by josh turner
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How to Calculate Loads on Wooden Trusses
Calculating the load capacity for a truss is difficult, but helps with construction. (roof trusses inside house image by Adrian fortune from Fotolia.com)

The wooden truss is one of the most important structures in modern engineering. Proof of the truss's versatility and strength comes in the form of bridges, high-rise buildings and residential roofing. Trusses consist of members, the lengths of wood that make up the structure and joints or "nodes," the fittings that connect the members together. Truss loads exert all of their force on the joints by way of the members. By carefully following the process of how to calculate the load capacity of a wooden truss, you can compute the maximum load capacity that a truss can handle before building the structure.

Skill level:
Easy

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Things you need

  • Tape measure
  • Paper and pencil
  • Scientific calculator
  • Tensile and compressive strength tables

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Instructions

  1. 1

    Test the stability of the truss. A statically sound truss follows the equation:

    2 X j < m+3

    Where" j" = the number of joints and "m" = the number of members

    (2 times the number of joints is less than the number of members plus 3)

  2. 2

    Choose a load to apply to the truss. This gives a reference to use for calculating the truss's strength.

  3. 3

    Compute the load distribution with the equation:

    Load per joint = Total load/ total number of joints

  4. 4

    Draw one triangle of the truss on a piece of paper. Measure the lengths of the sides and hypotenuse of the actual triangle. Label the corresponding measurements on the drawing.

    How to Calculate Loads on Wooden Trusses
    Isolate the truss into a series of triangles and draw them on a piece of paper. (triangle image by Zbigniew Nowak from Fotolia.com)
  5. 5

    Draw "x" and "y" axes on the paper and decide on the positive and negative vertical directions for the axes. Likewise, decide on a positive and negative horizontal direction. This will determine if the force is applied in a positive or negative direction.

    How to Calculate Loads on Wooden Trusses
    Label positive and negative "x" and "y" axes to use in evaluating the forces. (stock graph drawing image by .shock from Fotolia.com)
  6. 6

    Choose one angle of the triangle and calculate the sine and cosine of the angle with the equations:

    Sine of angle = length of leg opposite the angle/length of hypotenuse

    Cosine of angle = length of leg adjacent to the angle/length of hypotenuse

  7. 7

    Compute the opposing force exerted by the hypotenuse of the triangle with the equation:

    Load on joint/sine of angle

  8. 8

    Compute the force exerted by the vertical member of the triangle with the equation:

    Force on hypotenuse X sine of angle

    Compute the force exerted by the horizontal member of the triangle with the equation:

    Force on hypotenuse X cosine of angle

  9. 9

    Repeat all of the steps for the remaining triangles of the truss. Create a table of the forces exerted on each joint. Negative forces signify compression and positive forces signify tension.

  10. 10

    Record the tensile and compressive strengths of each member from "length versus strength" graphs for the wood that makes up the truss.

  11. 11

    Calculate the factor of safety for each member with the equation:

    Tensile or compressive strength of member (depending on whether the member is in tension or compression)/calculated force of the member.

    Add each member's factor of safety to the strength charts.

  12. 12

    Redo the entire process for larger and larger loads until the factor of safety becomes less than two -- at that point the truss will not be able to hold the required load.

Tips and warnings

  • Make sure to keep to the chosen positive and negative signs; if you choose the downward force to be negative all upward forces will remain positive.
  • Always follow all construction codes and safety regulations when dealing with any type of truss.

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