irc rafter requirements

huangyhg

irc rafter requirements
does anyone understand the rational behind the rafter span adjustment factor given in note a of irc 2000 tables r802.5.1(1) thru r802.5.1(8)?  is this a strength reduction consideration when a rafter tie is placed in the rafter space rather than at the top of wall?  a fea model of the situation i did seems to indicate that the rafter strength is actually increased when adding a rafter tie in the rafter space above the top of wall.  or is this requirement there to protect the walls from the outward push force as there is still some outward push (albeit considerably less than with no rafter tie at all) when using a rafter tie above the top of wall height?  if this is the case, how is this accomplished by reducing the allowable rafter span for a given size and spacing of the rafter?
i don't know the code you are referring to but from what you say it seems to me you are wondering why a rafter can span less distance when the tie is placed above the support point than when it is at the support point. the reason would then be that with a tie above the support point there is some additional bending in the rafter assuming the wall is unable to resist any lateral thrust (a normal assumption for brick or block walls). the additional bending is the lateral thrust component multiplied by the height above the support point of the tie.
carl bauer
when you did you model what did you consider the restraints at the wall.  if you correctly had no restraint in the s direction you will see why the reduction.  by moving the tie upward you increase the momeny in the rafter because the unrestained in is the same as a cantilever.
does that make it clear?
these span tables and adjustment factors are taken directly from af&pa's wood frame construction manual (wfcm) for one- and two-family dwellings, 2001 edition. there is a commentary to the wfcm that shows example problems for each of the tables. it will give an idea of the assumptions made for these factors (see commentary for table 2.14a). the previous 2 responses are correct, in that raising the rafter tie increases the moment in the rafters, thus the reduction.
here's a link for more info on the wfcm: