The Canadian Wood Council publishes a book called the Engineering Guide for Wood Frame Construction. The latest edition was published in 2009. The reason that this book is so important to engineers is that it defines good engineering practice with respect to Part 9 buildings. (See our post on Part 9 vs. Part 4 of the Building Code.) The CWC Engineering Guide (CWC) is referenced twice in the current building code. Once in 9.4.1 Structural Design Requirements and Application Limitations and also in 9.23.13 Bracing to Resist Lateral Loads Due to Wind and Earthquakes. In the later, the 2012 BC Building Code (BCBC) requires all Part 9 buildings, including single and two family dwellings, to be designed to resist lateral loads due to wind and earthquake using ONE of the following methods:
- Prescriptive requirements of Part 9 (9.23.13.) of the 2012 BC Building Code, or
- good engineering practice such as that provided in the “Engineering Guide for Wood Frame Construction 2009 Edition” (CWC 2009) published by the Canadian Wood Council, or
- Part 4 of the 2012 BC Building Code.
Part 4 of the BC Building Code deals with buildings larger than 600 square metres in footprint or higher than 3 stories. Most structural engineers are very familiar with designing to Part 4. It is what they do all day. Why bother with the CWC Engineering Guide?
Well, the answer is money.
The CWC Guidelines allow for simpler, less expensive designs.
For example, in CWC 4.4.2 Seismic Design Considerations for Shearwalls and Diaphragms, it states, “Seismic design provisions of Clause 9.8 of CSA O86 need not be considered for buildings that fall within the scope of this Guide.” Clause 9.8 requires that holddowns have 20% excess capacity so CWC allows for less expensive holddowns.
Seismic loads for residential houses are based upon the weight of the house and the roof snow load . In CWC 4.2.2. the snow load is stipulated to be 55% of the ground snow load plus the rain load. In Part 4 the roof snow load is 80% of the ground snow load plus the rain load. Seismic loads under CWC are smaller than under Part 4. Less load, less money.
In CWC 4.4.4 it says diaphragms amy be assumed to be flexible and torsional moments need not be considered. This allows for a less involved design resulting in lower engineering fees.
In CWC 6.6.3 lumber beams are permitted larger system factors. Larger system factors translates to smaller and less expensive beams than permitted in Part 4.