Simpson Tie Down Values, LRFD

[JStructsteel]

So, my wind load is per ASCE 7-10, so Im taking as ultimate load. For connection tie downs, the NDS has a 3.32 conversion factor for the capacity of the connector, and a phi factor 0.65.

Question about Simpson catalog has the capacities with the duration of load at 160%. Do I leave that 160%, and then just increase my capacity of the connector for the 3.32*065?

Always confused about this.

 

[phamENG]

You'd likely need to convert it back to a 1.0 load duration factor. Cd is ASD only.

Curious though. Why? I've never met a US engineer that uses LRFD for wood design.

 

[JStructsteel]

phamENG, I understand the wind design from ASCE7-10 is a ultimate load now.

 

[phamENG]

That it is...but that's why we have ASD load combinations. 0.6W and all that.

Notice that 7-05 and earlier used 1.6W for LRFD and 1.0W for ASD....7-10 and later use 1.0W for LRFD and 0.6W for ASD.

 

[phamENG]

The difference in the wind speeds is all return period based. So for a RC2 building (like a house), 7-05 and earlier were based on a 50 year MRI and 7-10 and later are based on a 700 year MRI. The 0.6 roughly approximates the 50 year MRI wind load from the 700 year MRI wind load.

 

[JStructsteel]

PhamENG, gotcha. That makes sense, will use the ASD load combos. Strange the NDS has conversion factors.

 

[phamENG]

Well there is an LRFD procedure for wood...it just isn't widely used in the states. As I understand Canada uses Limit State Design (their version of LRFD) almost exclusively.