Converting ASD to simple Factor of Safety
Converting ASD to simple Factor of Safety
(OP)
Hello,
I am a mech. engineer who often uses AISC design guides for designing frames and other steel structures. In our industry we are required by mil. spec to have a factor of safety of 5X to ultimate on the equipment we design/build. When using the ASD method what are your thoughts on coming up with ASD inputs corresponding to a simple FOS? For example, what I typically do is take Load*5/1.67 as my input. As I understand this would account for the 5X factor of safety with an omega=1.67.
Thanks
I am a mech. engineer who often uses AISC design guides for designing frames and other steel structures. In our industry we are required by mil. spec to have a factor of safety of 5X to ultimate on the equipment we design/build. When using the ASD method what are your thoughts on coming up with ASD inputs corresponding to a simple FOS? For example, what I typically do is take Load*5/1.67 as my input. As I understand this would account for the 5X factor of safety with an omega=1.67.
Thanks






RE: Converting ASD to simple Factor of Safety
The real question is if using that ratio reasonably approximates the goal of providing the same probability of failure per the Mil spec. I am not familiar with the Mil spec, but I suspect your method is reasonable in most cases.
One in the hand is worth two in the bush.
RE: Converting ASD to simple Factor of Safety
The only way to check that is to calculate the failure load.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Converting ASD to simple Factor of Safety
RE: Converting ASD to simple Factor of Safety
RE: Converting ASD to simple Factor of Safety
For A36 steel and using Fu = 58 ksi then 58/5 = 11.6 ksi and 5/1.67 = 2.99 or 3 and 36/3 = 12 ksi 5:1 on ultimate = 3:1 on yield. Note the reciprocal of 1.67 is the 0.6 in Fb = .6 Fy.
For A992 steel or other Gr. 50 material where Fu = 65 then 65/5 = 13 ksi and 50/3.85 = 13 ksi. 5:1 on ultimate = 3.85:1 on yield.
Attached is a chart showing AISI (cold formed steel) safety factors. I have not seen a similar chart for AISC or ever tried to derive one.
https://www.aisc.org/globalassets/modern-steel/ste... is a link to a pdf article that has tables showing Fy and Fu for different grades of steel and fasteners.
RE: Converting ASD to simple Factor of Safety
RE: Converting ASD to simple Factor of Safety
One in the hand is worth two in the bush.
RE: Converting ASD to simple Factor of Safety
Part copy: