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DISCUSSION OF RESULTS Structural behavior of the t e s t structure may be divided i n t o three cate- gories: (a) service load behavior, (b) strength under overload, and (c) post - f a i l u r e behavior. Service load i s defined as added dead load plus one l i v e load. Behavior vinder overload i s defined as that under load greater than service load; i t includes the load at which the f i r s t punch- ing f a i l u r e occurred i n each t e s t . Post-failure behavior i s defined as that a f t e r the f i r s t punching f a i l u r e had occurred. The tests were short-term, covering a period of about two days f o r each t e s t . Long-term effects were not measured. However, the b u i l d i n g was about two years old at the time of t e s t . The structure showe_d evidence of dam- age apparently caused by shrinkage and d i f f e r e n t i a l settlement. These long-term effects may have influenced r e s u l t s of the short-term t e s t s . SERVICE LOAD BEHAVIOR Response t o service load was detemined by measuring strains i n both the reinforcement and the concrete and by measuring deflections. Prior t o application of the t e s t loads, the concrete was severely cracked. However, no additional cracking was observed i n loading t o dead plus one l i v e load. Cracks that were present remained quite narrow. Strain gages were located where maximum stresses were expected. Measured strains showed that reinforcement stresses were low at service load. Max- imum change i n stress under application of added dead load plus l i v e load was 13,500 p s i . Most strains recorded indicated an increase i n stee l stresses less than 10,000 p s i . Measured and computed deflections at the center of loaded panels are shown i n Fig. 53 f o r a l l three t e s t s . I t i s seen th a t the two quantities agree
reasonably w e l l . Deflection l i m i t a t i o n s are given i n Chapter 9 of the ACI Code (ACI 3l8-63).(20) por roofs intended t o support other con- st r u c t i o n l i k e l y t o be damaged by large deflections, the allowable l i m i t f o r deflections i s L/360. This allowable de f l e c t i o n includes immediate deflection from l i v e load plus de f l e c t i o n due t o creep and shrinkage. For the panels tested the value of L/360 ranged from O.9O i n . t o O.96 i n . when L was taken as the short span dimension. Deflections measiired i n the three t e s t s a t 350 psf (added dead plus about 1.0 l i v e load) ranged from 0.15 t o 0.37 i n . These deflections are about O.166 and 0.333 of the ACI l i m i t . Even with time-dependent ef f e c t s added, the structure should easily s a t i s f y the c r i t e r i o n f o r deflections given i n Chapter 9 of "the ACI Code. (20) Chapter 2 of the ACI Code^^^ contains provisions f o r load t e s t s of struc- tures. When a load t e s t i s required by a bu i l d i n g o f f i c i a l , superimposed load s h a l l be equal to 0.3 times the dead load plus 1..7 times the service l i v e load. For the te s t structure t h i s i s 590 psf. Under t h i s load, max- imum defl e c t i o n should not exceed: L^/20,000 t (6) where L = the shorter span of slabs t = t o t a l depth of member I f the maximum deflection exceeds L^/20,000 t , recovery of deflection w i t h i n 2k hours a f t e r load i s removed s h a l l be at least 75 percent of the maximum deflection. The t e s t load must remain i n positio n f o r 2k hours. I f recovery of deflection i s less than 75 percent, a second t e s t may be conducted 72 hours a f t e r removal of the f i r s t t e s t load. Measured deflections and deflections computed by Eq. 6 are l i s t e d i n Table X I I . Measured deflections at 590 psf were taken from load-deflection curves. Zero deflection f o r f i r s t loading was that at zero applied load at the s t a r t of each t e s t . Zero deflection f o r second loading was that at zero applied load following application of load t o added dead plus I.5 l i v e loads. I t i s seen that measured deflections were greater than those computed l.k8
by Eq. 6. I n Tests I I and I I I , the measured deflections were about twice those specified by the Code. The t e s t load was not held f o r 2k hours. However, because of age of the structure and previous loading, additional deflection from creep over a 24-hour period would not be large. TABLE X I I CENTER OF PAMEL DEFLECTIONS Measured Recovery, Test Panel F i r s t , X I I . Second L^/20,000 t . F i r s t =11 b Second No. No. Loading Loading i n . Loading Loading 3,iÌ -B,C o.6o o.hk 0.24 67 93 T 4,5-B,C 0.i^6 0.36 0.24 92 92 1 3,U-C,D 0.42 0.37 0.22 86 95 It,5-C,D 0.29 0.29 0.22 86 93 2,3-D,E 0.66 0.59 0.25 67 87 I I 3,U-D,E 0.65 0.64 0.25 82 83 lf,5-D,E â â 0.25 ââ¢â â I I I 5,6-C,D 0.49 0.46 0.22 97 98 Measured recovery of deflection from the short term load i s also l i s t e d i n Table X I I . Recovery i s given i n percent of the measured deflection at the center of the panel. I n Panels 3,4-B,C and 2,3-D,E recovery was less than 75 percent of the maximum deflection f o r f i r s t loading. However, a l l panels exhibited recovery more thaji 75 percent f o r the second loading. Although comparisons made are based on short term deflection data, indications are th a t the structure would s a t i s f y load t e s t requirements of the 1963 ACI Code.(20) Deflections computed according to frame analysis(l5) agreed s a t i s f a c t o r i l y with t e s t r e s u l t s . This method was developed f o r slabs under uniform load- ing. However, f o r these t e s t s i t appears that the procedure i s applicable also when only a portion of the structure i s loaded. By considering frame rotations at the ends of loaded spans, ef f e c t s of adjacent unloaded areas 1-49
