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Test I I I . This t e s t was intended t o investigate the influence of i n - plane forces on the strength of the slab. Ockleston(5) and other invest- igators have shown th a t in-plane forces are substantial. Ockleston found that f a i l u r e loads were 2.5 t o 3 times the loads computed by y i e l d - l i n e analysis. This excess strength was a t t r i b u t e d t o arching action. In t h i s single-panel t e s t , the slab exhibited some i n e l a s t i c behavior p r i o r t o punching at Col\amn C5. Load versus both deflection and s t r a i n relationships became non-linear under an applied load of 1500 psf. Yield of reinforcement i n the positive moment regions indicated i n c i p i e n t formation of a y i e l d - l i n e mechanism. Shear f a i l v i r e at Column C5 occurred at 1972 psf (added dead load plus 6.4 l i v e loads). Subsequently, shear f a i l i i r e occurred at Column D5 when applied load was again increased t o 1449 psf. Shear capacity at Colimin C5 was not adequately predicted by e i t h e r Moe's equation or the ACI method. However, good agreement was obtained by mod- i f y i n g the ACI method. In t h i s modification, l i v e load was considered to enter the column only through the two sides j o i n i n g the loaded area. The manner i n which load was transmitted t o Column C5 appeared t o be similar t o that f o r a comer colvmin. POST-FAILURE BEHAVIOR Af t e r i n i t i a l f a i l u r e , loading was continued i n a l l three tests t o determine the c a p a b i l i t y of the structure t o continue to carry load. I n each case, the structure had a capacity t o support some load a f t e r f i r s t f a i l u r e . Table X I I I l i s t s applied load i n t e n s i t y at f a i l u r e and the maximum applied load a f t e r f a i l u r e f o r each of the three t e s t s . Data f o r Test I I are from the second part of the t e s t . TABLE X I I I POST-FAILURE STRENGTH Test No. Maximimi Applied Load, psf Maximum Applied Load A f t e r Failure, psf I 895 579 I I 728 594 I I I 1972 1449 1-53
I n a l l three t e s t s , load was d i s t r i b u t e d t o adjacent supports a f t e r f a i l u r e . However, load-deflection curves f o r the three t e s t s show that large deforma- t i o n s r e s u l t when a coltmm support i s l o s t t h r o u ^ shear f a i l u r e . I n Test I I I , load capacity dropped substantially a f t e r shear f a i l u r e at both Colimins C5 and D5. Shear f a i l t i r e s had already occurred at adjacent columns i n Line 4. Consequently, an applied load of only 644 psf could be maintained. This low load capacity was due t o lack of load r e d i s t r i b u t i o n since nearby supports had already punched through. Because the loading system was hydraulic, load dropped o f f when a shear f a i l u r e occurred. A hydraulic system i s not capable of immediate response to instantaneous deflections. I f -the structure had been subjected t o gravity loads, severe damage would have resulted a f t e r the f i r s t column punched through the slab. In each of the t e s t s , only a part of the structure was loaded. VJhen dis- tress occurred load was d i s t r i b u t e d t o adjacent parts of the structure. This d i s t r i b u t i o n could not take place i f the whole roof area were sub- jected t o a uniform gravity load. After the f i r s t column punched through, capacity at adjacent columns wovild be exceeded and t o t a l collapse wo\ild occur. 1- 54
