Volume 27, Issue 3 (May 1999)
Effect of Construction Adhesive and Joist Variability on the Deflection Behavior of Industrial Wood Floors
A total of 19 440 structural floors of a geometry and material configuration consistent with those used in industrial construction in North America were numerically analyzed to determine their deflection characteristics under a constant uniformly distributed load of 145 lbf/ft2 (215 Pa). The intent of these analyses was to determine the effect of joist modulus of elasticity (MOE) (mean and variability) on the deflection behavior of industrial floors (made with glue-laminated (glulam) structural members covered by plywood sheathing) and to quantify the contribution of elastomeric construction adhesives (ECA) in reducing the deflection of such systems. The joist MOE was allowed to vary in a systematic fashion between 1250 (8.62) and 3250 (22.4) ksi (MPa) in increments of 250 ksi (1.72 MPa). In addition, the joist variability (at each MOE value) varied between coefficient of variation (COV) values of 0.025 to 0.150 in increments of 0.025. One hundred-eighty floors were evaluated at each mean MOE value and COV value for the joists. MOE values for each of the eleven joists in each floor were obtained via a statistical simulation process. All other sheathing and connector properties [nail slip modulus = 25 500 lbf/in. (4466 N/mm)] remained constant. After the 180 floors were analyzed using only nailed connections between joist and sheathing, new random numbers were simulated (with the same mean and standard deviation) to produce 180 new floors for analysis with increased connection stiffness to reflect the contribution of the ECA used with the nails to attach sheathing to joists.
The results indicated that ECA used with nails contributes positively to floor system stiffness. The magnitude of the contribution of ECA to reducing 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 floor deflections. Reductions of nearly 6.3% were realized when the mean joist MOE was 1250 ksi. For floors made with stiffer joists (mean MOE = 3250 ksi), floor deflections were reduced by only about 3.3% when adhesive was used. The presence of ECA between joist and sheathing did not have much effect on reducing the variability of floor behavior. When components were connected only with nails, the COV of floor deflections varied between 0.01 and 0.09. When ECA were incorporated into the assembly, the COV of floor deflections had very much the same range.
It can be concluded that the presence of ECA can reduce the deflection of this particular industrial floor configuration by approximately 3.0 to 6.3% depending on joist stiffness.