Span Problems

Variant II kitchen (previous blog entry) requires that I span a distance formerly supported by the interior wall. This is probably best done by a glulam - an engineered, laminated, wood beam. Mechanical engineers have already puzzled out what size of glulam is required to span a distance under some standard load conditions. The only problem is figuring out how to interpret the "span tables" that give this information.

I'm currently reading through the glulam span table at
but it's not too clear how to use it. Fortunately, there's plenty of others online. Google for "span table glulam" for a bevy of them.

It is blindingly obvious by inspection that this interior wall doesn't support the same load as the exterior walls. For one, the exterior wall have the roof to support. It does, however, support one end of the 2x6 ceiling joists. This fact doesn't make life simpler.

The game, as it was explained to me, is to find the equivalent glulam that would be used for a single story exterior wall and use that. That way it's unequivocably built to code, should questions later arise. Code does allow the use of more appropriately sized beams (smaller, in my case), but only after a structural engineer has blessed the calculation that says what that smaller size ought to be.

A cool thing about glulams is that they can be purchased in "architectural" quality for use as an exposed beam. If you're not going to see it, then less a expensive "framing" or "industrial" quality is available.

Some manufacturers offer "balanced" glulams which have equal resistance to positive and negative bending moments. "Unbalanced" glulams have a distinct top and a bottom; they have the best timber on the bottom in order to resist tension forces which result from their span.

Camber is a small amount of bow that is used to offset anticipated sag or make roof drainange easier. Glulams are also available without camber (my choice).

This publication
has a likely looking table on page 7.

Variant II has an open span of about 14'. Ceiling joints on one side are 12'5" and joints on the other side are 10'8" long, for a total house width of 24' or less. According to the table, I can use a 3.125" x 12" or a 5.125" x 9" architectural glulam. A 3.5" x 10.5" or 5.5" x 9" framing gluam is also OK.

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