Michael Nehme

Pitch

The intended users / The need: This tool is intended for designers working on projects that include a roof truss system. The main need it addresses is early-stage decision-making. At the beginning of a project, designers often need to compare different roof truss options, but it can be difficult to quickly understand which option is better in terms of weight, cost, embodied carbon, and structural performance. Truss It acts as a preliminary design guide by giving the user a fast comparison between several truss geometries. It helps designers make more informed decisions about the roof system before moving into a more detailed design phase.

The inputs: The inputs are mainly related to the geometry, loading, material, section type, and section database. The user can define the span, rise, number of panels, uniform roof load, steel yield strength, cost per kilogram, carbon factor, and whether circular or rectangular HSS sections should be used. The user also provides the CSV file path for the section database, so the tool can select from available steel sections.

The logic: The model takes the user inputs into a Python component. The script first reads the HSS section database from the CSV file. It then generates five different truss geometries, including a flat Pratt truss, Howe truss, Fink truss, Scissor truss, and Hip truss. For each truss type, the uniform roof load is converted into nodal loads along the top chord. The script then performs a simplified structural analysis using the direct stiffness method and calculates the axial force in each member.

After the member forces are found, the script checks the available HSS sections from the database. It searches for the lightest (smallest area) section that can satisfy the demand in both tension and compression. The compression check includes global buckling behaviour using AISC 360 equations. Once a valid section is found, the same section is assigned to all members of that truss type for a simplified preliminary comparison.

The outputs: After selecting a section for each truss type, the tool outputs different truss options. The results include the selected HSS section, total steel weight, estimated cost, estimated embodied carbon, maximum utilization ratio, and an approximate deflection value. The tool also produces a visual output in Rhino/Grasshopper, where the different trusses can be compared side by side. Members in tension and compression are shown with different colours, making it easier for the user to understand the structural behaviour of each truss.

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Screenshot of the Grasshopper script :

Use the following link to download all that is related to this project (.gh file, .csv AISC data base) :

Note : The AISC data base can be found in the zip file, but if it doesn’t work, instructions on how to find and use the data base online are on the Grasshopper Graph (orange panel).

Documentation for how to use the tool :

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Teaser image | Expected output

As shown in the images below, the expected outputs are organized separately for each truss type. For each truss, the tool reports:

• The selected HSS section
• The section dimensions:
• For circular HSS sections: the outside diameter d and radius r=d/2, in mm
• For rectangular HSS sections: the total depth d and width, in mm
• The approximate total weight of the truss
• The approximate material cost of the truss
• The approximate embodied carbon
• The maximum utilization ratio

For circular HSS sections :

For rectangular HSS sections:

This tool is intended as a basic preliminary design tool, so it only includes basic design criteria from AISC 360 and does not perform all the checks required for the final design of an actual truss. Some important checks are missing. For example, serviceability was not considered (check the deflection against an allowable limit). This could have been added as another design criterion, but it would have made the project longer and more complex.

Another important limitation is that the tool does not check the overall stability of the truss system. It also does not include detailed connection design, non-uniform load patters, lateral bracing requirements, and more…. Therefore, the results should be understood as an early-stage comparison between different truss options, not as a final structural design.