Saturday, July 23, 2011

Conventional Construction: Selecting framing lumber

Wood is the most conventional of materials, and lumber is the wood used in conventional construction. Not just any lumber, though – as noted previously, lumber must be standardized and pre-approved in several ways. Also, allowable size and spacing of framing members are prescribed. The building code (CBC) simplifies the selection of conventional structural lumber by providing tables, organized by intended use. One thing you'll notice about the tables is that only a few wood species and sizes are included. For other choices, an online selection calculator from the American Wood Council is available here. If you use this tool, be sure to verify your selection with your inspector or building official.

The image at left shows the title of the first of two tables for selection of floor joists. The difference between the two tables is enclosed in parentheses below the title: (Residential sleeping areas, live load = 30psf…). The other table is for: (Residential living areas, live load = 40psf…). This split table is a change from the previous code; using different live load (mainly people and furniture) values for living and sleeping areas.

Just below the title is the header section of the table:
The header shows several important variables. The first is dead load, which is the weight of installed materials supported by the joists. The old code had a table giving weights of many different materials, but there’s really only one common situation that might call for using a dead load = 20psf – that’s heavy flooring like mortar-set tile. Another situation where using 20ps might be a good idea is if you know the room is going to be used for something like weight-lifting. In the following example, we'll use only the 10psf side of the table.

To use the table, start with the desired span, then move out to the edges to find size, wood species and spacing. For example, say you have a 14’-0” span. On the left half of the table (dead load = 10psf), find the span numbers closest to but not less than 14’-0”. For simplicity, we’ll stick to the most common (in Santa Cruz) wood species, Douglas fir (D.F.). Grade #3 is not commonly available, so we’ll ignore that also.
As a designer, I would conclude that the three best conventional size/grade/spacing options are:
2x8 #2 @ 16” (rectangles added to highlight choices)

2x8 SS @ 19.2”

2x10 #2 @ 24”

At this point, you can plug these three into your estimator software along with current prices and go with the most cost-effective choice (note also that, for the contractor, current prices and/or labor costs might point to a choice other than these three). Cost aside, sometimes there are design reasons for preferring one size or spacing over another, but that’s a topic for another time.