I have a question on the use of the 0.6 factor on dead load in the basic ASD load combination 0.6D + 0.6W, Equation 16-15 in the 2012 IBC (assuming no horizontal earth pressure, H). I have always thought that the 40% reduction in the dead load was attributed to the conservative nature of the loads we oftentimes assume for dead load and also the likelihood of dead load (like finishes) being removed. However, I have recently been told that this was extremely conservative since foundations will always be in place and the weight of concrete is well determined.

Do you know of any exception to the 0.6 factor for the case of foundations? I am aware that 2012 IBC Section 1605.3.2 (Alternative Basic Load Combinations) specifically requires that a two-third reduction be taken.

Answer:

To answer your question, let us start with the strength design counterpart of that combination: 0.9D + 1.0W, Equation 16-6 in the 2012 IBC. The high 0.9 factor on D reflects confidence that D can be determined reasonably accurately.

The 0.6D in Equation 16-15 provides parity between strength design and ASD. 90% of the dead load counteracting strength-level wind forces and 60% of the dead load counteracting service-level wind forces result in substantially the same designs. That’s where the 0.6 factor on D comes from.

In the alternative basic load combinations of Section 1605.3.2, there is no explicit combination with just dead load resisting wind. That combination has to be derived from Equation 16-18 by assuming the design live load to be zero. That combination then becomes: D + ωW. This is where the line you are referring to comes in: “for load combinations that include counteracting effects of dead and wind loads, only two-thirds of the minimum dead load likely to be in place during a design wind event shall be used.” This statement changes the above equation to 0.67D + ωW, very similar to what you find in the basic ASD load combination.

Finally, the alternative basic load combinations are a carry-over from the UBC. The sentence quoted above evolved from the following provision in Section 1621.1 of the 1997 UBC:

“The base overturning moment for the entire structure, or for any one of its individual primary lateral-resisting elements, shall not exceed two thirds of the dead-load-resisting moment. For an entire structure with a height-to-width ratio of 0.5 or less in the wind direction and a maximum height of 60 feet (18 290 mm), the combination of the effects of uplift and overturning may be reduced by one third. The weight of earth superimposed over footings may be used to calculate the dead-load-resisting moment.”