rifkamattman wrote:you know, I just glanced the IBC chapter 16 and can't find where these numbers might be, but I am positive that this is what Thaddeus is teaching. Mike, someone, where are these safety factors published just for curiosity sake? I'm still talking about the 1.5 for retaining walls and 1.67 for buildings.
1.5 is in the foundation chapter of IBC. 1.67 = 1/0.60 and is found in the load combinations 0.6DL + WL See my 702 Guide for more information
The higher safety factor has nothing to do with the people in the building but rather with correlating ASD and USD design.
When to apply SF: the only time you need to consider SF is when checking stability of a structure. Note that stability is a serviceability issue not at strength issue. Now to address the original question in the thread:
When designing a member you first perform an analysis to determine the ACTUAL shears and moments in the members for each load case. Then you combine the loads using the appropriate load combinations from chapter 16, either ASD or LRFD. This provides the maximum shear and moment for the design of the member (think of this as "how fast is the car moving" not "how fast CAN the car go")
Finally you look at the capacity of the member "how fast can the car go". For ASD (wood) the code provides allowable stress for different grades of wood. THEY have incorporated the SF into the allow stress and no further SF is required.
For USD and LRFD the load combinations increase the load to ultimate strength, based on probability and statistics. The member capacity is shown as phi-Mn or phi-Vn. Mn and Vn represents failure points. The phi factor provides a slight reduction due to imperfections in the construction process but are not considered safety factors
I feel like I have answered a lot of questions but am not sure if I answered the ones that are asked. Let me know.