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For the interaction equations for combined axial and bending resistance from O86 6.5.9, 7.5.12, 8.4.6, and 15.3.3.10, for sawn lumber, glulam, CLT and SCL, respectively. for both tension and compression, the progam applies the same duration factor KD to both moment resistance Mr and the axial resistance Pr or Trr derived from the shortest duration of loads (highest factor) for either direction of stress. In other words, the program examines a load combination for combined design, and uses the load duration factor corresponding to that combination for both axial and bending resistance, regardless of which load types within the combination contribute to axial and to bending stress.
For example, for a column under concentric compressive dead load and lateral live load the program uses KD factor of 1.0 for both axial Pr and lateral Mr
This procedure is shown in the CWC Wood Design Manual, Section 5.1, Example 1 - Glulam Column and in the CWC’s Introduction to Wood Design 10.1, Example 10.1 Column subjected to snow, wind and dead loads.
For the case that O86 5.3.2.2 is used to determine a KD for long-term loading, the program applies the highest factor so calculated to both directions (see Long-term Load Factor, above.)