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Steel Designers' Manual - 6th Edition (2003)
Standing seam systems
The traditional forms of construction described above suffer from the inherent disadvantage
of having to be fixed by screw-type fasteners penetrating the sheet.
Traditional fixing methods also limit the length of sheet that can be handled even
if, in theory, long lengths can be rolled; thus laps are required.
The need for weathertightness at the lap constrains the minimum slope. A
5000m 2 traditional roof has 20 000 through fasteners and has to resist around
1 million gallons of water a year. The difficulty in ensuring that this large number
of fasteners is watertight demonstrates the desirability of minimizing the number
of penetrations. This has led to the development of systems having concealed fastenings
and the ability to roll and fix long lengths. In order to cater for the thermal
expansion in sheets, which may be 30 m long, the fastenings are in the form of clips
which, while holding down the sheeting, allow it to move longitudinally.As discussed
elsewhere, this may reduce the restraint available to the purlins and affect their
design. When used in double-skin configuration the liner panel is normally conventionally
fastened and provides sufficient restraint. The available permutations are
too numerous to give general rules but purlin manufacturers will give advice.
It is necessary to fasten the sheets to the structure at one point to resist downslope
forces and progressive movement during expansion and contraction. With the
through fasteners reduced to the minimum and laps eliminated or specially detailed,
roof slopes as low as 1° (after deflection) can be utilized. The roofs must be properly
maintained since accumulation of debris is more likely and ponding leads to a
reduced coating life.
Standing seam systems are used to replace the traditional trapezoidal outer sheets
in single- and double-skin arrangements as described earlier.
Composite panels
Design of common structural forms 39
This most recent development in cladding systems provides solutions for many of
the potential problems with metal roofing. The insulating foam is integral with the
sheets and so totally fills the cavity, and with good detailing at the joints condensation
can be eliminated in most environments.
The strength of the panel is dependent on the composite action of the two metal
skins in conjunction with the foam. Theoretical calculations are possible although
there are no codified design procedures. Since both steel and foam properties can
vary, and these are predetermined by the manufacturer, it is a question of selecting
the panels from load tables provided rather than individual design. In addition to
having to resist external loads, the effects of temperature differential must be taken
into account. The critical combinations are wind suction with summer temperatures
and snow acting with winter temperatures. The range of temperature considered
is dependent on the colour and hence heat absorption of the outer skin; darker
colours for roofs should only be considered in conjunction with the manufacturer,
if at all.