FEATURES
All larger buildings , and those to which the public has access , are to be designed under the supervision of professional engineers applying rational design criteria to satisfy engineering first principals .
Timber framed structures in compliance with SANS 10082 are deemed to satisfy the National Building Regulations and in compliance with SANS 10400 , which covers conventional construction methods and processes . Only deviations from the specified design and construction requirements of SANS 10082 will demand engineering intervention and the provision of a rational design .
DESIGNING FOR PUBLIC BUILDINGS
Approval has , in general , been given by municipalities to prefabricated truss fabricators that use a suite of rational design programmes supplied by ‘ system suppliers ’ to design roofs up to 10m in span for non-public buildings . All these system suppliers are accredited and approved by the Institute for Timber Construction South Africa ( ITC-SA ).
All larger buildings , and those to which the public has access , are to be designed under the supervision of professional engineers applying rational design criteria to satisfy engineering first principals . Engineers who have access to engineering design software can use these design programmes to satisfy the rational design requirements . Timber structures designed by a competent person in accordance with Part B of the National Building Regulations are not limited to the size , height , span , pitch and geometric configuration of structures as specified in SANS 10082 or Part L of SANS 10400 .
TIMBER GRADING AND TREATMENT
The ITC-SA works closely with the Technical Committees of the South African Bureau of Standards ( SABS ) on grading and treatment specifications for structural timber , including design codes and standards for structural timber as well as on matters affecting the National Building Regulations and their application .
The ITC-SA has also instituted a Certificate of Competence scheme for timber frame builders and truss fabricators ( truss plants ) that design , manufacture and supply prefabricated nail-plated trusses . Stringent auditing of both the professions ’ operations and key personnel is done before the awarding of such Certificates of Competence . This is an assurance of quality on timber structures for both the specifier and the general public .
TIMBER QUALITY
All timber used in a structural application should be structural South African pine that complies with the requirements of SANS 1783-2 / 1460 / 10149 and bears the full standardisation mark .
Timber used for roof battens should comply with SANS 1783-4 and should also bear the full standardisation mark .
Timber used for the construction of timber structures on site must be ordered in the dimensions in which it will be used and should not be re-sawn into smaller crosssectional sizes on site , as this will cause the grade , strength and dimensional tolerances to change .
TIMBER SPECIFICATION
The various grades of timber are defined by different strength properties and allowable design stresses . The grades commercially available are : S5 , S7 and S10 . ( Note : there is a limited availability of grade S10 .)
TIMBER TREATMENT
In certain magisterial districts in South Africa , it is illegal to use timber which has not been treated against biological attack for structural purposes . Treatment can be completed using either CCA or Boron in accordance with SANS 10005 ‘ Treatment of Timber ’.
STORAGE OF TIMBER
Structural timber stored on site should be stacked on level ground on bearers and adequately protected against the weather by being covered with a waterproof material . Air must be allowed to circulate through the timber stacks and strapping around bundles of battens should not be removed until the battens are to be fixed , to prevent excessive warping .
FACTORY ENGINEERED TRUSSES
A network of prefabricated timber roof truss manufacturers can be found throughout South Africa . These fabricators operate under license to suppliers of nail-plate connectors and use computer design programmes devised by professional engineers . When placing orders with a truss fabricator , or when a quotation is required , the fabricator must be provided with the following minimum information of a detailed drawing :
• Overall span of roof
• Eaves overhang
• Pitch of roof
• Cantilever distance ( if any )
• Truss spacing ( can be optimised by the truss engineer ) normally 760mm for concrete tiles and 1 100mm for sheeting
• Roofing materials plus any special loads ( solar panels )
• Ceiling materials plus any special loads ( airconditioning )
• Geyser position and capacity
• Hatch openings , size and position
• Special eaves detail
• Other details which may affect the design
For more information about specifying timber for construction or to find an ITC-SA accredited fabricator member near you , visit : www . itc-sa . org .
44 JUNE / JULY 2017 //