Rafter spans and footnotes

[Squirl]

Ok.  With the cold weather, I've been spending a lot of time reading the charts and practicing design ideas.

http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_sec002.htm

All rafterspan tables have a Footnote a to them:

a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the bearing walls, such as rafter ties, is provided at that location. When ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the factors given below:

HC/HR Rafter Span Adjustment Factor
1/3 0.67
1/4 0.76
1/5 0.83
1/6 0.90
1/7.5 or less 1.00

where:

HC = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls.
HR = Height of roof ridge measured vertically above the top of the rafter support walls.

So, If I moved the rafter tie from the bottom of the rafter to 1/3 up from the bottom of the rafter I decrease the rafters span?
Example: TABLE R802.5.1(1) 2x4 spf no. 2 12" O/C can span 124 inches according to the chart.  Now if I take that 124 inch "rafter spans" and "shall be multiplied by the factors given below", which is .67, it equals 83 inches. 
Is this because the weight of the ceiling is bearing more on rafters?

I also noticed TABLE R802.5.1(9)

h. Tabulated heel joint connection requirements assume that ceiling joists or rafterties are located at the bottom of the attic space. When ceiling joists or rafterties are located higher in the attic, heel joint connection requirements shall be increased by the following factors:

HC/HR Heel Joint Connection Adjustment Factor
1/3 1.5
1/4 1.33
1/5 1.25
1/6 1.2
1/10 or less 1.11

where:

HC = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls.
HR = Height of roof ridge measured vertically above the top of the rafter support walls.

So raising the rafter tie to 1/3  puts more pressure on the connection and you need to put 50% more nails?

Did I read these right?  Comments?

[MountainDon]

Yes, decrease rated span or in another view of that increase the depth of the rafter so when corrected it covers the span, OR decrease the spacing of the rafters.

And Yes on the heel joint connections, more force means more nails. Note they specify nail size as being common nails, those are thicker than most nail gun nails.

There's a topic here on nails which gives actual dimensions of the nail sizes.

The footnotes to the tables in the IRC are equally as important as the information in the tables themselves.

[Squirl]

Yes, I remember about the nails from when I went shopping for a nailer.

http://countryplans.com/smf/index.php?topic=12197.msg157604#msg157604

My larger gripe is not just that the thickness of nailer nails do not measure up, but the length they sell is almost always 1/8" to 1/4" shorter.  When I went shopping, none of the big box stores carried full length 3.5" full head nails in either galvanized or regular.  Some guns don't even shoot 3.5" nails.

[Don_P]

On nails in shear 10 diameters penetration into the main member develops full shear values. A smaller diameter nail requires less penetration to develop its' full shear values, use more of them, think of a truss plate, many small nails. Withdrawal is a function of diameter, density AND depth of penetration.

This calc gives the forces in a conventional flat ceilinged roof, if you do some comparisons to IRC tables, this engineering professor was pretty conservative.
http://www.timbertoolbox.com/Calcs/slopedrafter.htm

As the tie is raised the forces in the tie increase. Think about the reverse, a nutcracker. Put the nut out there under your hand and squeeze the handles, the nut won't crack. Stick the nut up near at the hinge and squeeze, your force is multiplied many times. A raised tie is trying to restrain those increased forces.
This calc give the thrust forces in a roof at the plate;
http://www.timbertoolbox.com/Calcs/RafterThrust.htm
This calc give the forces in a raised tie;
http://www.timbertoolbox.com/Calcs/raisedtiethrust.htm
that takes care of the heeljoint problem.

As the tie is raised it is also placing a bending point load on the rafter at the tie's connection. The rafter is already under a bending load from snow or wind outside and now there is a connection pulling on that rafter in span from the inside. That load takes an increasingly deep rafter to resist.