ACI 435.8R-85 Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections.
A test load of 1OO psf, corresponding to the design live load, was applied over two bays of the 24th floor. Bricks were stacked on the slab 3-l/2 months after the concrete was placed and the load was left on the slab for 6 months. The graph show- ing the measured deflection plotted against time, along with other deflection measurements, is shown in Fig. 4. It can be seen that the deflection that occurred when the test load was applied (O.13 in.) was much less than the maximum deflection that occurred during the construction period (O.33 in.) Also, at 28O days when the test load was removed, the recovery was about the same as the elastic deflection that took place at the time of application of this test load, whereas the measured recovery at the time of removal of the construction loading was only O.O1 in. The difference in these recoveries could be accounted for by the fact that the stress/strength ratio was higher at the time of application of the construction loading than when the test load was applied, and more cracking occurred during the initial appli- cation of load than during the application of test load.
In both figures (6b and 6d), there is a considerable scatter around the arithmetic average of the measured deflections. The size and time dependence of the deflection are affected by several individually varying factors: for instance, different concrete ages at removal of formwork, different standards of workmanship and different loading conditions. It was observed that the deflections were considerably less for the top floor slabs. This difference was mainly due to different loading con- ditions, since the top floors did not support any floors above during the erection of the buildings. Since the span and the span-to-thickness ratio were approximately the same for the two buildings studied, a comparison might be of interest. A direct comparison of the average deflections from the two test series is made in Fig. 6 e and it can be seen that the deflection curves only deviate slightly from each other.
Tests were carried out on a panel on the fourth level of a building comprising five levels of flat plate floor, four bays wide and seven bays long (Fig. 7a). The column grid was 21 ft 3 in. x 18 ft 10 in. with columns 22 in. square. The slab was designed to be 9 in. thick to carry 12O psf live load. There were no column capitals or drop panels, but bent-up bars were provided to produce the desired shear capacity. The test panel was surrounded by other panels on all sides; however, two adja- cent panels on the western side were only half size panels. Measurements of the slab depth indicated an average thickness of 9.57 in. The maximum span to depth ratio was therefore 26.7. Pre-mixed concrete was used, with a specified minimum compressive strength of 3 ksi at 28 days. At 1O8 days, when some tests were conducted, the average strength from four cylinders was 4.12 ksi. The experiments for short-term behavior, which were carried out on the test panel, were a series of loadings from zero to 140 psf uniformly distributed load. The south-west quadrant of the loaded panel was instrumented at various locations to measure various deformations. Deflection readings were taken during construction to record the effects of stripping of formwork from the slab, the sequence of propping, and the loading history of the slab. Deflection readings were continued into the service life of the structure. At the time of writing of the paper (Ref. F.l), the slab was 27 months old and the full service load had been in place for one year.ACI 435.8R pdf download.