Canadian Wood Design - Load Duration Factor (KD)

[skeletron]

CSA O86-19 Clause 5.3.2 discussed the load duration factor (KD). For short-term loading, KD=1.15. The Clause also states "...shall be multiplied by a load-duration factor, KD, in accordance with Table 5.1 but not exceeding 1.15..."

The Canadian Wood Council's "Engineering Guide for Wood Frame Construction (2014 Edition)" Clause 3.4.2 also provides insight into the load duration factor (KD). This clause states "...the specified strengths and resistances may be multiplied by a load duration factor KD=1.25 for wind and earthquake design..." This document provides tabular solutions and capacities for common residential, light-frame situations. Reviewing some of these numbers indicates that the KD=1.25 factor has been applied.

One of the key clauses of Part 9 (CL 9.4.1.1) indicates that along with conforming to Part 9 requirements, the design shall be in accordance with "...good engineering practice such as provided in [CWC's guide]..."

What is everyone's interpretation of this?
(a) Does this mean you can apply KD=1.25 (as per CWC's guide) throughout the Part 9 structure (nail connections, shearwall capacities, etc.)?
(b) Or does this mean you apply KD=1.15 (as per CSA O86) throughout, unless using solutions straight out of the tables or interpolated from the tables?

The situation is primarily for force transfer elements under seismic loading (e.g. framing connections like nails from bottom plate to rimboard, framing clips to connect between vertical LFRS, etc.) and trying to attain the +20% factor as per CSA O86-19 CL. 11.8.6. I've always applied (b) and rarely dip into the tables because they are metric. The benefit of using KD=1.15 is that that is the standard short-term factor applied in manufacturer's tables as well (ie. Simpson or Mitek). But, allowing myself to get into the weeds and check certain calculations, I noticed that this increased KD value is being applied and that it could be beneficial to utilize this so that I can maintain common spacings (32", 24", 16", 12", etc.) rather than always having to bump up. I think this is a nitpicky curiosity thing on my end, but I'd be curious to know how others approach this or if they even think about it at all.

 

[TLHS]

I thought this might be a left over legacy thing, but even the 2001 wood handbook I have has a 1.15 limit. Looking at the Engineering Guide for Wood Frame Construction, they explicitly acknowledge that they're doing this (Clause 3.4.2 in the 2014 edition).

You're certainly fine using the tabulated values and methods in the Engineering Guide if you're specifically within the scope that they have in section C and Clause 1 in section B of the Engineering Guide (less than 3 stories, under 600m^2, various span, load and occupancy limits, etc). The guide is incorporated as acceptable engineering practice for Part 9 wood structures in the code. You could use the 1.25 KD with any of those.

Once you're doing your own calculations out of the wood code, you'd be stretching to pull that KD in, I think, unless it's filling in a gap in the Engineering Guide tables. I think you're in solid judgement call territory though depending on the scope of the item you're looking at. The Engineering Handbook is basically saying that they're trying to bridge the prescriptive parts of Part 9 and the rational engineering parts of Part 4 without getting weird looking designs. This is presumably the same basis that is used for the more recent prescriptive lateral resistance parts that have been added in part 9. If you're still in the realm of Part 9 styles of prescriptive construction (well defined diaphragms, lots of shear wall area with the walls stacked and spread through the building, etc) then you've got a better argument than if you're doing lots of unusual load paths. You could potentially argue that you're in the scope of the Engineering Guide and following their written design criteria to match their standard of care.

 

[skeletron]

Ok. I'm on board with your interpretation @TLHS.

I would probably be using this at most to fill-in info and get nail capacities (toe nails, plate nails to rim, etc) since the CWC Guide does not specifically call-out nail diameters.

I wouldn't use this to get shearwall capacity. If you look into CSA O86-19 CL 11.6.2.2, the nail embedment strength already gives you a +8% bump regardless of CSP or DFP. Smudging this with the standard KD factor = 1.08 x 1.15 = 1.24 ---> *almost* suggesting that the 1.25 factor is already built-in. I'm not saying it is, but the simplification does suggest there is a bit of tolerance.