Design Criteria for Wood Floor Systems

The criterion for wood floor systems is defined as 1/360th of the span. Tests were conducted to determine how floors designed to this criterion actually do perform. These tests show that floors designed on the basis of FHA wood properties and the assumption that only the joists resist deflection actually deflect more than the predicted amount if only the joists act to resist. However, the effect of floor sheathing and connections stiffens the floors sufficiently to meet the criterion. The addition of hardwood flooring adds about 10 percent to floor stiffness while the effect of wall weight above the end supports is negligible. The composite action between joists and nailed plywood sheathing increases stiffness by about 60 percent. The authors conclude that FHA values of modulus of elasticity are too low, but the practice of ignoring composite action approximately offsets the error this produces. They recommend that tests of floors must simulate the composite-action effect of the entire system. A “Recommended Method of Test for Joists and Rafters” developed by the authors is presented. It is pointed out that material for floor joists is rarely stressed to the allowable bending strength of the grade, inasmuch as elastic modulus and deflection control the design. It is recommended that cost reduction without performance loss can be achieved through reconsideration of lumber grade requirements and the composite-action effect of sheathing and flooring.

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