In message , Roger
writes
The message
from Tim Lamb contains these words:

The sheeting is rigid so I find it difficult to visualise how the load
would be shared between the wall plate, an intermediate purlin and the
ridge. More study:-)

No one seems keen to help you out so I thought I would at least try
using some rusty knowledge that hasn't been gainfully employed for at
least some 35 years. No doubt someone better acquainted will then come
along and tear holes in what I have to say.

There's brave:-)

I think it is easiest to start by considering a flat roof. A flat roof
with no intermediate supports and uniform roofing has a uniformly
distributed load along the length of each rafter and bearing loads of
50% of the total weight at each end. Put an intermediate support in the
centre and the weight distribution becomes 25% at each end and 50% on
the intermediate support. If the intermediate support is not central
then the loadings will be such that the moments about any point are
zero.

OK to here.

The stiffness of a rafter would have very little effect on this
except in extremis (purlin failure) as it is positioned unstressed and
probably not even allowed to sag under its own weight. This conveniently
allows us to ignore the extra stiffness that might be imparted by a
rigid roofing sheet.

Umm..

If the roof is at an angle not a lot changes. The weight distribution
remains the same and the weight continues to act downwards but what
might be called the ladder effect needs to be considered. At the upper
(ridge) end on a symmetrical structure each rafter would be opposed
horizontally by another but it is only the ridge pole that is supporting
the upper ends and that is carrying the same loading as the purlins
(rafter each side). Any sag at the ridge pole will result in some sort
of spreading movement in both purlin and wall plate.

OK again. So provided the rafters/sheets are rigid and the wall plate
prevented from spreading by the truss tie, the purlin and ridge are
mainly decorative?

In reality they are not fully rigid. The manufacturer will supply data
sheets on span/deflection/loading but I don't know how to apply this to
the Superbeam software.

regards


--
Tim Lamb