Summary:
Portal- frame buildings have become very popular.
They make up a
large percentage of the small to medium size single-storey
industrial
buildings in current use.6.2 I-section portal frames
The rafters and columns of I-section portal frames consist
of rolled I-sections, with therafter ends being haunched
and site-bolted to the columns, as was shown in detail
(c) ofFig 5.1. The column section is heavier than the
rafter section, so that the relative columnhaunchrafter
strengths roughly follow the shape of the bending moment
diagram up thecolumn and across the rafter.
The frames
are usually designed plastically, so themoments referred
to are the plastic moments under gravity loading, with
plastic hinges developing either in the column top or
in the rafter at the haunch and in the rafter near the
apex.With the reduction in roof live loading specified
in SABS 0160-1989 (viz. 0,3 kPa instead of 0,5 kPa), and
with the more favourable load combination factor for dead
load, the design loading for the dead plus live load combination
for a typical portal frame is now only about 65 per cent
of what it was previously. This means, of course, that
lighter rafter and column sections can be used, but if
as a consequence these sections are also made shallower,
the deflection of the frame could be greater than it was
for the heavier loading.
Where deflection was critical under the old loading, full
advantage could in such a case not be taken of the reduction
in load and the required plastic modulus would be considerably
more than 65 per cent of the old value.
Format:
Pages : 11
Size: 56 kb
Author :Southern African Institute of Steel Construction
(SAISC)
Download:Portal-Frame Buildings
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