Q: If you have two identical length floor joist spans, from an exterior foundation wall to a center beam and then on to the opposite exterior foundation wall, is there a significant difference between framing this as two simple spans with the joists lapped over the center beam (the most common arrangement), or using one continuous joist?
A: Mark McKenzie, an engineer from Brewster, Mass., responds: There is a significant difference between the two framing methods. In engineering terms it would be considered a simple-span beam versus multiple-span continuous beam analysis. When you have multiple-span continuous joists, the design moments (bending forces) are reduced and the deflection (sag) of the joists decreases. In other words, with regard to deflection and vibration, continuous joists are stiffer than two simple spans because the two coupled spans act together.
But it is also true that with multiple-span continuous joists, the reactions (loads at supports) concentrate at the center span and decrease at the ends. With simple single-span joists that are overlapped at the center beam or girder, the reactions are equal at each end of the joists. The result is that you can use smaller-sized stock (be it conventional lumber or I-joists) for multiple-span continuous joists than with simple single- span joists. But with the continuous joists, the center beam has more load on it, so you may be required to upsize that beam from the standard tables in the code. In other words, with continuous joists the size of the center beam might have to go from three 2x10s to three 2x12s, or the lally columns might need to be spaced closer together to support the added load.
These days you really only see this problem with engineered joists because it is difficult to find conventional lumber in lengths greater than 24 feet, which is pretty much the minimum width of a standard, modern-day house. Engineered joists come in lengths of up to 40 feet, so they are often used when you want to run full-span joists over a main carrying beam. By the way, this is only an issue with low girders (beams that are below the joists). Obviously, if joists are installed flush framed with the center beam, those joists would be considered simple single-span joists.