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Timber Allowable Stress Design

The timber design module supports three allowable stress design codes:

  • SANS 10163-2:2003
  • SABS 0163 – 2001
  • BS 5268 – 1991

Essentially there are two design philosophies to follow when confronted with timber design; allowable stress design (ASD) or limit state design (LSD). ASD compares actual and allowable stresses, while LSD compares the required strength to actual strengths. LSD was introduced to compensate for the fact that ASD tends to produce overly conservative designs for structures with predictable loads while producing unconservative designs for structures with less predictable loads.

Younger users are typically only familiar with LSD procedures, while older users will recall the ASD procedures. LSD is relatively new to timber and many engineers stick to the ASD procedure because it is what they are used to. It is for this reason that PROKON does not yet support LSD for timber design. Modern codes that allow for LSD might be added to the program in the future.

When reading data from an analysis output file, you may see this message:

Figure 1: Allowable stress design pop-up window.

This means that a load factor larger than 1 is specified for a given load combination, which is in conflict with the load factor of 1 required when doing allowable stress design.

The solution is to define an additional load combination in the analysis software with a load factor of 1 assigned for timber design purposes.