Results from Truss Observations
Analysis and commentary

Arch 324, Introduction to Structural Design, University of Virginia
Last updated Friday, April 12, 1996, at 9:55 AM Copyright © 1996, Kirk Martini

## The Raw Data

The following is a scatter plot based on the truss examples. The following data points were removed from the sample

 Span Depth Reason 5989 35 Stiffening truss for suspension bridge 1440 50 Trussed arch rib 598 35 Mulitple spans 1 1 Error

## Interpretation

The graph below shows trendline plotted on the graph (in red) using linear regression (it minimizes the sum of the squares of the differences between the approximating line and the actual data points). The blue lines correspond to common rules of thumb for truss span-to-depth ratios.

Rounding off the coefficients, the equation for the line has the following form:

depth = (span/14) + 5 feet
This result is effectively a rule of thumb for estimating truss depth, but it is clearly not very good for short span trusses, since is clearly influenced by the longer span examples, and does not apply well to short spans.

Limiting the examples to those with a span less than 100 feet gives the following result.

Rounding off the coefficients, the equation for the line has the following form:

depth = (span/6) + 9 inches
Here are a few conclusions:
• Data collection often includes errors.

• Short span trusses are often much deeper than common rules of thumb predict, and probably deeper than necessary for structural purposes. A likely contributing factor is that there are other factors determining geometry. The geometry of roof trusses and bridges where traffic passes through a truss are are determined by those factors rather than structure.

• A rule of thumb is not a law of nature.

• Observation is important. Expertise is gained both through logical application of fundamental principles (deduction) and comparison and application with a set of existing cases (induction).

Last updated Friday, April 12, 1996, at 9:55 AM