Practical Engineering: Sizing Stiff Floor Girders

We received a call recently from a homeowner who had a problem he wanted us to solve. He had just built a garage with living space above, and the upstairs floor was too bouncy. The odd thing was that the homeowner had specifically asked the building designer to ensure that the floor was very stiff, so the designer had specified floor joists (wood I-joists, in this case) with an L/720 liveload deflection limit. So why was the floor so springy? The problem, as it turned out, was the midspan girder that the joists crossed. Although the girder had been sized correctly by “code” for strength and stiffness, its deflection limit was much greater than that of the