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O86 5.3.2.2 says that when the long-term load PL is greater than the standard-term load Ps , KD = 1.0 - 0.5 log (PL/Ps )
Load Types Used for Ps and PL
PL is calculated by considering the effects of the permanent loads D, Ds, Ls, Lt, and H.
Ps is calculated by considering the effects of S and L. When combined, they are combined as S + 0.5L or L + 0.5S according to whether S or L is the companion factor in the load combination used.
Calculation of Permanent KD for Different Criteria
In its determination of Ps and PL, rather than the loads on the member themselves, the program uses the effect of the constituent load types on the shear, bending moment, compression, combined axial and bending, and compression design criteria. It does so as follows:
Shear - KD values are chosen on each span using the ratio of the average shear on the span for Ps vs. "D" loads in 5.3.2.2.
Moment - KD values are calculated at each of the three evaluation points on a span – the ends, and point of maximum moment at mid-span. The absolute values of moment due to "D" loads and due to Ps at these points are used for 5.3.2.2
Axial Compression - The axial PL is compared with axial Ps load.
Bearing - At each support, the component of the reaction due to PL load and to Ps load is compared.
Combined Axial and Bending - The program considers the effect of loads in the lateral direction for Mf (including eccentric axial loads), and the effects of loads in the axial direction for Pf, and uses the highest KD from either direction to the calculation of both Mr and Pr. See Column Design, Combined Axial and Bending, below, for more information.
Short-term Loads
The permanent factor from 5.2.3.1 is not applied in the presence of any short-term loads, so that the 1.15 factor is always used when there is wind or earthquake loading.