State of Redesign
Not a lot has happened over the last week as I needed to do some bookkeeping and tax return preparation. With most of that behind me this morning I have now turned my attention to the redesign of the structure to meet the requirements of Part C of the CWC’s Engineering Guide for Wood Frame Construction.
My biggest fear was that I would loose my only window on the first floor at the front of the house. Fortunately, this will not be the case, but what I will have to do however is move many of the windows so that they are no longer centred from a ‘view’ point of view (usually the centre of the internal space) to allow averaging of panel lengths. I had made use of several 24″ panels on the end of walls and 30″ panels between door and window clusters. Now, I need 48″ min length panels in all locations unless I want to use the alternative procedure identified in Part C. This allows for a min 34.25″ panel for a 9ft ceiling, BUT then requires a larger percentage of the band to be a braced wall panel compared to the original standard. So where I only need 40% of the band to be lateral load supportive for the first floor, if I elect this optional narrower panel I then need 66% of the band to be solid.
Another change between this Guide and the BC Building Code Part 9, is that the guide measures the panel lengths to the centre lines of the perpendicular panels where part 9 measures to the outside edges. Fortunately, the panels are still measured to the outside edge, so this actually relaxes the requirements a bit over Part 9.
The most annoying part of this new design requirement is that even the guide admits that there is nothing wrong with Part 9 and that the prescriptive requirements of the Guide and Part 4 designs are typically over-engineered.
“Using this approach [engineering calculations], many wood frame buildings based only on the Part 9 prescriptive requirements would appear to be inadequate for resisting lateral loads; however, performance history indicates that this is not the case. The performance history of small wood frame structures cannot be explained completely by Part 4 structural calculations using simple 2-dimensional load path assumptions. Traditional wood-frame construction is difficult to model mathematically due to the many load paths in the indeterminate structural system and the contributions of “non-structural” elements. Various aspects of building performance have been investigated experimentally during the past fifty years (for example Dorey and Schriever 1957, Boughton and Reardon, 1982, Boughton, 1988, Ceccotti, 1990, Fischer et al. 2001, Paevere, 2002, UWO 2002, Doudak, 2005). Most of these studies have focused on one-and two-story structures on rigid foundations. These reports gave more insight into the mechanisms of structural deformation including the importance of load sharing among the structural and “non-structural” elements within the structure:
1. Interior finishes and many types of exterior cladding contribute to the lateral resistance of the structure. Both the ultimate load capacity and the lateral stiffness are improved by the addition of architectural components.
2. Non-Ioadbearing partitions stiffen and strengthen the structure so that the building acts as a rigid box rather than a series of diaphragms and shearwalls.
3. Other non-structural elements such as stairs, closets and cabinetry also contribute to the lateral resistance of the building.
4. The performance of wood light frame systems is enhanced by the load sharing and composite actions. The overturning resistance of a wall is enhanced through “corner effects” that engage adjacent walls. Roof and floor diaphragms, if adequately connected, will transfer lateral wind and earthquake loads to all supporting walls, including walls parallel and perpendicular to the direction of loading that normally may not be considered in design.
Although to a large extent the structural stability of Part 9 buildings relies on these non-structural elements, to date, this action has not been quantified in a systematic manner suitable for use in structural design. The numerous wood frame buildings throughout Canada and elsewhere represent countless “prototypes” subject to field-testing over many decades.
The damage from the 1989 earthquake in the San Francisco area was studied
by Canadian researchers (Rainer, Jablonski, Law and Allen, 1990), leading to the following observations concerning the performance of wood frame construction:
1. Foundation walls weak in racking resistance-such as cripple-stud walls-led to failure of buildings.
2. Openings for doors in the ground floor of multiple storey buildings created “weak storeys” which led to damage of the buildings.
3. Most of the serious structural failures that occurred to residential construction were due to deficiencies prohibited by California building codes and reflected in the 1985 Uniform Building Code (UBC).”
It really sucks having to go through these steps when it is clear they are not really needed!
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