Determination of Deflection Coefficients for Laminated Decking

Structural tests were performed on laminated roof decks for the purpose of obtaining strength and stiffness data to compare with theoretical deflection values, code or recommended design deflection coefficients, and existing data from other decking tests. A check of deflection coefficients was obtained from 6 tests: 12-, 14-, and 16-foot simple spans, and 3-, 12-, 14-, and 16-foot multiple continuous, spans. The major test variables included span, decking thickness, simple span, and continuous spans. Maximum theoretical deflections for the outer spans of a continuous three-span homogeneous beam, loaded uniformly, are predicted by delta = 1/145 WL4/EI. This value is more than 13 times the theoretical deflection that occurs in the center span. The average deflection coefficients obtained in each test for the end spans ranged from 1/148 to 1/130. These values were from 4 to 6-1/2 times greater than the coefficients for the center spans as determined from the tests. This would indicate that the roof deck did not actually deflect as a continuous beam of uniform thickness. The presence of end joints affects the strength and stiffness, as the section modulus is reduced in proportion to the number and location of these end joints. The test results indicated that the theoretical deflection equation can be used conservatively to predict stiffness characteristics of simple span decks fabricated under actual construction conditions. Maximum deflections of the random length three-span systems which occurred on the outer spans were about equal to that predicted by theoretical equations This would indicate that the deflection coefficients, 1/116 or 1/100, now recommended are too conservative. A coefficient as low as 1/130 could be used to allow longer spans or higher loads. Load versus deflection curves for both the simple span lay-ups and the random three-span lay-ups indicated elastic behavior for the laminated decking roof panels. The allowable design bending stress value for laminated decking of 2,000 psi appears safe and reasonable.

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