The Hidden Costs of Optimization, Part II

Optimizing truss jobs is intended to lead to a reduction in lineal feet of lumber used in a project, but that reduction in lumber may make the trusses’ ability to distribute forces through connections more critical. It’s important to understand how the computer software automatically loads a truss. Without that knowledge, there can be unintended consequences to optimizing trusses for the loading that is automatically applied, which can result in hidden costs on a project.

Question

How can a Truss Designer avoid system design errors such as when structural fascia loads are missing?

Answer

Example: Loads Missing from Structural Fascia  

Structural fascia is a beam supporting the end and side jacks of a structure. According to ASCE-07: Minimum Design Loads for Buildings and Other Structures,  structural fascia must be designed to support loading, such as snow, wind, dead and live loads, that occurs on the overhang of a roof.

The load supported by fascia travels through the structure by the following load path:

It is the responsibility of the Truss Designer to understand ASCE-07, the computer’s automated loading, and a specific structure’s layout, in order to determine if trusses have been loaded correctly. (Note: specifying the applied loads for the roof system are the responsibility of the Owner, the Owner’s Agent or the Building Designer, per the building code. The loading conditions should be found on the construction documents.) When fascia loads are missing from a project, there is incorrect loading on the jack trusses, which results in incorrect loading of the sub girders, and subsequently, the corner girder/hip jack. This is also significant because the method of analysis for the jack and girder trusses are dependent on the structural fascia. Therefore, missing structural fascia loads on a project can lead to both missing loads occurring on the overhang of a roof, and also incorrect analysis of the structure supporting that overhang.

The layout and associated diagrams below show an example of structural fascia, its load path, and how to properly analyze the structural fascia, jacks (with and without exterior wall bearing), and sub girder (see attached pdf for larger view). It is critical to understand how the structural components bear on each other to transfer load. 

Designing structural fascia is an iterative process that requires changing where the structural fascia ties back to the structure. As shown in the sub girder diagram, not extending the structural fascia far enough into the structure can cause too much uplift. If the structural fascia doesn’t extend far enough into the structure, the connection between the sub girder and corner girder can be unattainable.          

In order to efficiently design structural fascia, perform the following:

If a structure is designed without structural fascia loads in the software, the following analysis needs to be completed for the project:

As you can see, missing loads can lead to extensive repairs and may even require a revised layout. To prevent this from occurring, Truss Designers need to know the automated loading assumptions the computer program is using and whether or not it is correct per ASCE-07 and the specific layout of the structure. Furthermore, understanding the bearing conditions of the jacks and girders, bearing capacities of top plates, and hanger connections are also essential to ensure that the overall structure is analyzed correctly. By developing a greater understanding of loading requirements, and how to verify that they are applied and analyzed correctly in the software, the Truss Designer can more accurately optimize trusses and remove any unintended downstream repairs and potential back-charges.