Volume 25, Issue 2 (March 1997)
Effect of Construction Adhesive and Joist Variability on the Deflection Behavior of Light-Frame Wood Floors
A total of 16,000 light-frame, structural floors of a geometry consistent with those used in residential construction throughout North America was numerically analyzed to determine deflection characteristics under a constant uniformly distributed load of 40 Ibf/ft2 (59.5 Pa). The intent of these analyses was to determine the effect of joist variability on the deflection behavior of light-frame floors and to quantify the contribution of elastomeric construction adhesives (ECA) in reducing the deflection of such systems. The joist property (moudlus of elasticity, MOE) was allowed to vary in a systematic fashion between 750 (5.17) and 3,000 (20.07) ksi (MPa) in increments of 250 ksi (1.72 MPa). In addition, the variability of the joists (at each value of MOE) varied between coefficient of variation (COV) values of 0.05 to 0.26 in increments of 0.03. One hundred floors were evaluated at each mean MOE value and at each COV value for the joists. All other sheathing and connector properties [nail slip modulus = 25,500 Ibf/in (4466 N/mm)] remained constant. However, after the 100 floors were analyzed using only nailed connections between joist and sheathing, the same 100 floors were analyzed with ECA used with the nails.
The results indicated that the contribution of the ECA to reduced floor deflection under uniformly distributed loads was dependent in large part on the mean stiffness of the floor joists and to a much lesser extent on the COV of the joists. For joists of lower stiffness, the contribution of ECA had a more pronounced effect on mitigating the deflection level of the floors. Reductions of nearly 12% were realized when the mean joist MOE was 750 ksi. For floors made with stiffer joists (mean MOE = 3,000 ksi), floor deflections were reduced by only about 6% when adhesive was used. The presence of ECA between joist and sheathing also had the effect of reducing the variability of floor behavior. When components were connected only with nails, the COV of floor deflections varied between 0.02 and 0.11. However, when ECA were incorporated into the assembly, the COV of floor deflections ranged from less than 0.01 to only about 0.04.
It can be concluded that the presence of ECA of the type used in this analysis can reduce light-frame wood floor deflection by 6 to 12% depending on joist stiffness.