Civil MDC

Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections 2

Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections


Three experimental flat plat structures were erected at the Division of Building Research, Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia. The investigations were carried out under field conditions, the structures being completely exposed to the weather. Structure Mark I consisted of an expanded shale concrete slab, 3-l/2 in. thick, spanning three bays of 9 ft in one direction and three bays of 12 ft in the other, with cantilevers 4 ft6 in. long in this direction. The reinforcement was conventional individual plain round bars and was designed by the empirical design method given in ACI 318-56.

The slab was carried on 16steel columns of box section with a grillage type shear connec-tion. The significant features of this structure were (1) span/depth ratios of 41 in one direction and 31 in the other; (2) the ratio 4:3 of the sides of the panels; and (3) the steel columns. of lightweight aggregateconcrete was also an important Further, no edge beams or torsion reinforcement near the edge columns was used. The long-term deflections reached “annoying” proportions.The slab was allowed to stand under its own weight for 8 months ,during which time the deflection at the center of the middlepanel increased by O.62 in.

This was 12 times the initial elas-tic deflection of O.O5 in.In a study of the long-term deformation of this structure it was suggested that about 2O% of theincrease at the center of the middle panel was due to differential settlement of inner and outer columns, about 4O% was due tofurther cracking causing a reduction in stiffness, and to localbond slip, and about 4O% to creep. This analysis also suggestedthat the increment of deflection due to creep was about 85% ofthe immediate deflection of a completely cracked slabs.

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