Setting the correct deflection ratio for cantilevers.

Setting the correct cantilever deflection limit depends on using the correct ratio and understanding the difference between the properties in BC Framer® software and the notation used on the Boise Member Report (and BC Calc® software.)

The deflection properties in BC Framer use the straight L/__ format. For example: L/240. This is true both for spans supported at each end and cantilever spans which have support at one end only. 

In contrast, the Boise Member Report and BC Calc use a different notation for cantilevers, shown like this: 2xL/__ 
For example: 2xL/480.

This causes some confusion because the input in BC Framer uses one notation, but the output is shown in another. They are both showing the same deflection ratio.

Here is a Live Load Cantilever Ratio setting of L/240 in BC Framer.


A joist designed looks like this on the Boise Member Report. In this case the controlling deflection is at the cantilever end and just barely passes because it meets the 2xL/480 criteria for the Live Load Cantilever Ratio.


The example above illustrates the input in BC Framer, the output on the Boise Member Report, and the fact that these two ratios are equal:

L/240 = 2xL/480

48/240 = (2x48)/480 = 96/480


Here is a related article explaining the cantilever deflection ratio in BC Calc.
What does 2xL/480 mean? - The meaning of the "2xL/" deflection control when designing a member with a cantilever. 


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