Scissor Truss Design Table for Building Projects and Designs

A scissor truss design table is a valuable tool for building projects and designs. It helps determine the optimal design for a scissor truss, which is a type of roof truss that is commonly used in residential and commercial construction.

The design table is based on the principle of triangulation, which provides stability and strength to the truss. Scissor trusses are typically used for spans up to 60 feet, but can be designed for longer spans with the right materials and configuration.

The design table takes into account various factors, including the span, load, and material properties of the truss. This information is used to determine the optimal size and configuration of the truss, ensuring it can support the weight of the roof and any additional loads.

Design Parameters are crucial in defining the scissor truss geometry. The top chord pitch is a required design parameter that defines the vertical rise divided by horizontal run, typically expressed as a ratio of incline (in.) to each 12 inches of run, such as 3:12, 4:12, or 5:12.

The scissor truss type is also a required design parameter, which can be selected from a list of common truss types that can be modeled within the plugin. The out-to-out span, or the distance between bearing walls that the truss is spanning, must also be specified in feet.

The overhang distance, which is the distance the truss projects beyond the bearing wall outside, can be set independently for left and right sides, ranging from 0 to 42 inches. The top chord size, or the depth of the top chord members, must be set in inches, limited to nominal lumber sizes.

The bottom chord size, or the depth of the bottom chord members, and the web size, or the depth of the web members, must also be set in inches, limited to nominal lumber sizes. The ply thickness, or the width of the truss members, can be set in inches, with a default value of 1.5 inches for 2x lumber.

Here is a summary of the required design parameters:

To design a scissor truss, you need to consider the truss geometry, which includes the top chord pitch. The top chord pitch is the vertical rise divided by the horizontal run, typically expressed as a ratio of incline to each 12 inches of run, such as 3:12 or 4:12.

The scissor truss type is a common truss type that can be modeled within the plugin. The plugin limits the roof pitch from 2.5:12 to 12:12 for some truss types.

The depth of the top chord members of the truss, known as the TC Size, is a crucial design parameter. It sets the depth of the top chord members of the truss in inches, with values limited to nominal lumber sizes.

Here are some common truss types that can be modeled within the plugin:

The overhang, which is the distance the truss projects beyond the bearing wall, can be set independently for left and right sides, and can range from 0 to 42 inches.

When designing a truss, it's essential to consider the snow load that will be acting on it. The snow load on a truss can be significant, especially in areas with heavy snowfall.

The snow load is calculated based on the terrain category, exposure condition, and thermal condition of the roof. For a garage in an open space, the terrain category is set to "B", exposure condition to "Fully Exposed", and thermal condition to "Unheated and open air structures".

The roofing material used can also impact the snow load, with G.I. sheet being considered a "Slippery" surface. This affects the snow load calculation, resulting in a higher load on certain areas of the truss.

In the case of a symmetric gable roof, the balanced snow load is equal to 0.23 kPa. However, the unbalanced case for the location results in a higher snow load on one side of the truss, specifically 0.42 kPa.

To understand the load on the truss, it's crucial to convert the snow load from a horizontal projection to an inclined load acting on the top chord of the roof truss. This conversion is necessary to accurately determine the load on the truss.

When designing a truss, it's essential to consider the truss type, which can be selected from a list of common truss types that can be modeled within the plugin.

The out-to-out span is the distance between the bearing walls, typically measured in feet. This is a crucial parameter in truss design, as it affects the overall structure's stability and load-bearing capacity.

To define the truss geometry, you'll need to input the top chord pitch, which is the vertical rise divided by the horizontal run, usually expressed as a ratio of incline to each 12 inches of run, such as 3:12 or 4:12. The plugin limits the roof pitch from 2.5:12 to 12:12 for some truss types.

You'll also need to specify the overhang, which is the distance the truss projects beyond the bearing wall, measured in inches. The overhang can range from 0 to 42 inches and can be set independently for the left and right sides.

The truss design also involves selecting the size of the top chord, bottom chord, and web members, which are typically measured in inches and limited to nominal lumber sizes.

Here are the typical design parameters that can be modified to define the truss geometry:

Remember, the size and orientation of truss members directly affect the results of the calculations, so it's essential to select the right cross-section for accurate analysis.