charliealphabravo
Structural
- May 7, 2003
- 796
I'm wondering if anyone has tried to reverse engineer an actual failure load for a wood screw loaded in shear or withdrawal using the NDS ASD design methodology. The problem is that the NDS doesn't seem to clearly indicate which factors are "safety" factors and which ones are simply to more accurately account for environmental and loading conditions (for example Z, Z', Z'alpah, Rd, Cd, Cm, Ct, Cg etc).
Edit:
For shear, if I consider the reference design value Z as the failure load sans safety factors then the Cd adjustment factor to obtain Z' is problematic as it can increase the design load (say Cd of 1.25 for construction loads). If the remaining factors (Cd, Cm, Ct, Cg, etc) don't apply to my case then I haven't really applied a safety factor at all.
I'm thinking that safety factors must be buried inside the yield modes Z or in the Rd reduction term. If Rd is the safety factor then for fasteners under 1/4" this would look like Rd = 2.2. For 1" diameter fasteners then Rd could be as high as 5. A safety factor of 5 seems to ring a bell. Like I think Simpson StrongTie connectors use something like the lower/greater of testing and calculation with a safety factor of 5.
A consideration of the withdrawal formula is no more clear. I have W=2850 * G^2 * D which is multiplied by the adjustment factors to get W'. In other words the safety factor must be buried in the 2850 term.
TIA
I found some commentary that answers some of my questions. For withdrawal it looks like the 2850 term includes a 1/5 reduction (safety factor) from the ultimate failure load.
Edit:
For shear, if I consider the reference design value Z as the failure load sans safety factors then the Cd adjustment factor to obtain Z' is problematic as it can increase the design load (say Cd of 1.25 for construction loads). If the remaining factors (Cd, Cm, Ct, Cg, etc) don't apply to my case then I haven't really applied a safety factor at all.
I'm thinking that safety factors must be buried inside the yield modes Z or in the Rd reduction term. If Rd is the safety factor then for fasteners under 1/4" this would look like Rd = 2.2. For 1" diameter fasteners then Rd could be as high as 5. A safety factor of 5 seems to ring a bell. Like I think Simpson StrongTie connectors use something like the lower/greater of testing and calculation with a safety factor of 5.
A consideration of the withdrawal formula is no more clear. I have W=2850 * G^2 * D which is multiplied by the adjustment factors to get W'. In other words the safety factor must be buried in the 2850 term.
TIA
I found some commentary that answers some of my questions. For withdrawal it looks like the 2850 term includes a 1/5 reduction (safety factor) from the ultimate failure load.