General > The IRC: International Residential Code and Good Practices

802.5 Rafter Ties and Rafter Span Adjustment Factor

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Code Experts....

Table 802.5.1 specifies a Rafter Span Adjustment Factor in Note a. 

Hr     Factor
1/3   0.67
1/4   0.76
1/5   0.83
1/6   0.90

The upper limit shown is 1/3 the way up, above the wall plate.  There are no restrictions that say you cannot place the rafter tie above 1/3.  But could you?  The 0.17 to 0.33 range is a Fibonacci Sequence (3,5,8....) that can be extended to:

802                  802   
Frac   Decimal   Fib   Factor      Fac Incr
1/2   0.46            13   0.60       0.07
1/3   0.33              8   0.67          0.07
1/4   0.25              5   0.76          0.07
1/5   0.20              3   0.83          0.07
1/6   0.17              2   0.9   

We know that using the span adjustment factor reduces the allowable span because you are multiplying the table spans by the decimal factor.
If I wanted to place the rafter tie 1/3rd the way up, I would have to find a appropriate span, species, spacing, thickness. etc. using the factor.

But, If I wanted to go above 1/3rd, could I use Fibonacci calculations to further reduce the span and be in compliance with the code?
I would not be interested in 1/2, but I might be in the area between 1/3 and 1/2. 

Thanks, Dave 

I was looking at that sequence for the drive on a trebuchet last week, funny how often that shows up. Short answer... not without an engineer's blessing. You can use the WFCM to go up to 1/2 height though. I posted a link a month or two ago but can't lay my hands on it, maybe someone bookmarked it and will chime in. The heeljoint connection becomes critical as you rise.

(funny how often that shows up)
Fibonacci sequence is a natural progression (found in nature).  Large groups of people will naturally act/behave in accordance with the Fibonacci sequence.  Also the Fibonacci sequence can be seen in the branching in trees, sprouts of a pineapple, arrangement of a pine cone's bracts, etc.  I use Fibonacci retracement in stock trading. 

Found the WFCM online.  I'll see what it says. 

It's only marginally more explicit in IRC 2015 Figure 802.5.1 where it says Hc/Hr = 1/3 max.  That's the only place I saw that says you can't extend the sequence.


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