When using Prokon’s Timber Design Module, it is important to note that the Load Duration Factor, k1, has not been included within the Design Parameters input dialog.
When specifying the use of the SABS 0163-2: 2001 Allowable Stress Design code, the code states that the allowable stress, f, is equal to the grade stress, p, multiplied by the modification factor, k.
The program adjusts the allowable stress for all modification factors except k1, as this is dependent on the type of loading. Prokon’s Analysis Modules (SUMO and Frame) allow the user to define the type of loading on the structure and therefore it is up to the user to modify the loading at the analysis stage in accordance with the code.
When a member is subjected to bending and an axial force (either compression or tension), the calculated stress is required to be adjusted to accommodate for either of the following interaction equations:
σbx and σby -Calculated bending stress about either the x or y-axis;
σc and σt - Calculated compressive or tensile stress parallel to the grain;
fbx and fby - Allowable bending stress about either the x or y-axis;
fc and ft - Allowable compressive and tensile stress parallel to the grain.
The calculated bending, tensile and compressive stress results for the above-mentioned interaction equations are obtained from Prokon’s Analysis packages. Therefore, the following derivation needs to be applied to the above-mentioned interaction equation to evaluate how the loads can be adjusted in conjunction with the load modification factor, k1, described in the code.
W – Type of load (self-weight, imposed, wind etc.);
A – Area subjected loading;
p – Material grade stress;
Cf – Load duration coefficient.
Therefor from the above derivation, to have the required effect on the interaction equation, the type of load, W, must be multiplied by the load duration coefficient, Cf, described in the table below (Refer to 6.3.3 of SABS 0163-2: 2001).