Span tables: the folly of believing one size fits all

By Dennis White, Director SAMCRA

In recent years there has been an increase in the incidents of both roof and side cladding being blown off during storms. Whilst a
number of these storms have been more ferocious than normal, many have not and on investigation it has been found that the damage was the result of inadequate spacing of the purlins and girts. For over fifty years the spacing of purlins and girts on structures have been based on spanning tables published by the manufacturers of the various cladding systems (profiled sheet together with fasteners/anchoring clips, etc.) irrespective of the geometry, permeability and location of the structure. Prior to the
publication of SANS 10160 Part-3 in 2010 loading resulting from wind action was invariably viewed from the point of overall forces acting on the cardinal elevations of the structure as a whole and local effects were generally ignored. Regrettably, a large number of designers still adopt this process.

It is important to note that the majority of cladding systems available in the local market have been imported, including the
span tables. Pierce-fixed systems originated from Europe and concealed-fixed from Australia. The European systems were developed between the late 1800s (corrugated iron) and 1950s (trapezoidal) and designed to be used on roofs inclined at ten to fifteen degrees and to support mainly downward forces. The concealed-fixed systems were developed in the 1960s and designed to be used on flat roofs i.e. roofs inclined at less than five degrees, which are subject to high uplift forces.

During the late 80s and early 90s many international wind loading codes were substantially changed and improved to incorporate new scientific research. Whilst most reputable manufacturers modified their span tables, based on the results of performance testing, current tables remain based on the one size fits all principle.

A critical point often overlooked by designers is that span tables often have qualifications pertaining to the ultimate uplift load a system can withstand or geometrical and permeability criteria that have to be met. Both of these qualifications are often less than that required or relevant to the structure in question. The positioning of louvres, large overhangs, contents and large plant stacked or located immediately behind large openings as well as temporary conditions during construction such as missing shopfronts, regularly produce forces greater than those considered when developing the span tables.

It is important to note that span tables cannot be reliably derived from mathematical models and have to be based on performance tests. When developing span tables different loading conditions have to be considered.

PDF/ Print version