Frequently Asked Questions
For LRFD in the 2005 NDS®, why does the φ factor cancel out when adjusting nominal resistances using the LRFD Format Conversion Factor, KF , and the resistance factor, φ?
The resistance factor, φ is used to adjust the LRFD design value for variability. However, reference design values in the NDS are based on near-minimum values and thus are already adjusted for variability. In order to be used with conventional LRFD design procedures, the NDS reference design values (ASD based), must be divided by the resistance factor. The NDS Format Conversion Factor, KF , explicitly divides a constant conversion factor by the resistance factor consistent with the format used in ASTM D5457 Standard Specification for Computing Reference Resistance of Wood-Based Materials and Structural Connections for Load and Resistance Factor Design. However, explicitly dividing by the resistance factor in the calculation of the format conversion factor, and then multiplying by the resistance factor in LRFD design procedures, has proven to be disconcerting to many NDS users. For this reason, revision of the format conversion factor is being considered whereby final values of KF will be tabulated in-lieu of equations having the format of:
KF = 2.16/φ or KF = 1.875/φ
An example of what is meant by “cancel out” is shown as follows for determination of the adjusted LRFD bending design value, Fb’:
Fb’ = (Fb)(2.16/φb)(φb)(all other applicable adjustments)
where
Fb = reference bending design value in the NDS
2.16/φb = Format conversion factor for bending
φb = resistance factor for bending
Wood Products
- Why are the wind speeds in Minimum Design Loads for Buildings and Other Structures, ASCE 7-10 so much higher than those in ASCE 7-05?
- Where can I find information on federal specifications and standards for lumber and wood products?
- What are the design manuals for wood construction in high wind areas?
- What resources are available for wood construction in high seismic areas and why build with wood in earthquake-prone areas?
NDS
- What is the difference between 2012 NDS 3.5 and 2012 IBC Table 1604.3 Footnote d for adjusting long-term deflection of wood members?
- What do the column headers for bolt design value tables 11 A-K in the NDS mean?
- Where can I find background on In-Grade Testing for the NDS supplement?
- Where can I find more information on all of the NDS references?
- What are the safety factors (factor of safety) that are used to derive allowable design values for wood fasteners, lumber design values, etc. referenced in the NDS?
- Why did shear parallel to grain (horizontal shear) design values increase in the 2001 NDS?
- If there's an existing structure where the MC<19% for southern pine timbers, could the designer back out the wet service adjustment from the NDS published design values in table 4D?
- For structural composite lumber (SCL), should the volume factor, CV, be applied to Fb* in calculation of the beam stability factor, CL, or is it intended that CV be applied following calculation of CL?
LRFD
- What is LRFD?
- What is the purpose of the NDS Format Conversion Factor, KF?
- For LRFD in the 2005 NDS®, why does the φ factor cancel out when adjusting nominal resistances using the LRFD Format Conversion Factor, KF , and the resistance factor, φ?
- For Load and Resistance Factor Designs (LRFD), many of the design examples and much of the discussion focuses on elastic stress analysis. I thought that one of the benefits of LRFD was its ability to accommodate plastic (non-linear) analysis. Why don't you cover plastic design considerations in your standards?
WFCM