Factors of Safety for Wood Plank for Scaffold Deck

[machinma]

I am trying to determine a suitable wood deck to be used for scaffold access. The required factor of safety for scaffolding components is 4:1 per OSHA 1926.451(a). Should the adjusted design value for bending (ie Fb') be divided by 4 to get an allowable stress to compare to the actual stress? Does the adjusted design value (Fb') have a built-in factor of safety? I am having difficulty trying to determine what the wood's allowable stress value actually is due to the fact that it is not clear in the NDS Manual. If Fb' is an ultimate value for bending stress, would this also apply to the other adjusted design values (ie Fc', Fv', Ft', etc.)?

 

[woodman88]

Per my WWPA Western Lumber Grading Rules 2011 Section 100 "... Bending stresses (Fb) for Scaffold Plank grades have incorporated a scaffold use factor according to the American Standards Institute Stardard A10.8. This factor modifies the allowable bending stresses to the equivalent safety level of four times the design load without failure..."
Please note that this applies to the Scaffold No. 1 and 2 grades, per the WWPA, and these grades are not given in the NDS. Because of this I would use the Scaffold grades and values per the WWPA or other lumber grading association with their design values, not the NDS.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[MiketheEngineer]

See OSHA 1926 Subpart L Appendix E

Basically says you should use 2x10s SPIB DNS IND 65 or SCAF PLK D Fir S Dry for Western Woods.

That said - it has always been my understanding is the lumber has a SF of about 1.6 to 2. I would take the Fb x 1.6 and use that number divided by 4.

Deflection is also limited to 1/60. Also should overlap bears by no less than 6'' and no longer than 12'' and secured in place.

This US only - Canada has different regs.

Bottom line - go read OSHA 1926 - 450 and 451