for x to be in the rafter, the positive root must be taken. It
is now possible to determine the required Mpfrom Eqn. 3and hence H
from Eqn. 1. It is also now possible to draw the bending moment
diagram forthe whole frame.
With the required Mp known, it is possible to obtain preliminary
sizes for the members of the portal frameusing data in section
tables. Further checks are then necessary to ensure that the
various buckling modesthat frames are susceptible to are prevented,
either by the chosen members having sufficient resistance orby
providing restraint to buckling. This restraint will often be
provided by the purlins that attach the
roofing and cladding to a portal frame.To speed up the initial
sizing of portal frames, very often design charts based on the
above equations areused. These are normally presented in a
normalized form so that they are applicable to frames of
anydimensions and subject to any loading. A spreadsheet is
available on WebCt with the calculations presentedabove used to
produce such charts, which are also available in many design
guidance documents.
