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® and the notation used on the Boise Member Report (and BC Calc®)

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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