General > The IRC: International Residential Code and Good Practices

802.5 Rafter Ties and Rafter Span Adjustment Factor

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DaveBell:
Code Experts....

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

Hc
---
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

Don_P:
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.

DaveBell:
(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.

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

ChugiakTinkerer:
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.