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COMPARISON OF MEASURED AMD COMPUTED SHEAR STRENGTHS Test I . Ratios of t e s t to calculated values are l i s t e d i n Table IX for the f i r s t column that punched through during each t e s t . I t i s seen that shear strength a t Column Cl* i s conservatively predicted by the ACI Code method. Strength predicted by Moe's equation i s somewhat higher than that measured. This, however, i s not beyond the range of sc a t t e r encountered in the t e s t s eval\xated by Moe. Moe's equation i s based on a " b e s t - f i t " curve and not a lower bound curve. Test I I . Colimuis E3 smd E^^ were subjected to the same magnitude of force in the t e s t s . Although Column E3 had a somewhat greater cross section than Coltann Ek, shear f a i l u r e occurred f i r s t a t E3. The f a i l u r e load f o r E3 was s l i g h t l y l e s s than that f o r Ek. Observed shear strength a t E3 was about 50 percent of that predicted by Moe's equation and about U5 percent of that using the ACI equation. Shear strength at Elt- was also l e s s than that predicted by both methods; i t was nearly 60 percent of that predicted by Moe and about 55 percent of the ACI prediction. Application of Moe's Eq. 2 indicated that the e f f e c t on shear strength of moment tr a n s f e r was small. Shear capacity was 20 kips l e s s at E3 and 30 kips l e s s at Eh than that for "concentrically" loaded columns. The low strength exhibited i n the t e s t s at E3 and Ek was probably inflvienced by the condition of the structure. As shown i n F i g . k, diagonal cracks were v i s i b l e a t E3 p r i o r to t e s t i n g . Although looA from settlement was added to the t e s t value for E3, t h i s increased the column load a t f a i l u r e by only about 10 percent. Tensile s t r e s s e s i n the plane of the slab r e s u l t i n g from settlement and shrinkage may also have contributed to the unusually low strength. Diagonal cracking at Columns E3 and Ek i s shown i n F i g . 38 and kO. Since l i t t l e moment was transferred to the columns, s t r e s s e s from torsion were not s i g n i f i c a n t . The small Influence of torsion can be seen by comparing the 1-ltO
f a i l u r e surface of the t e s t slab with that of a laboratory specimen designed to be weak in torsion. Cracking at the free edge of t h i s laboratory speci- men i s shown i n Fig. 57* Diagonal cracks run i n the opposite d i r e c t i o n from those of the t e s t structure shown in F i g . 38 and 40. In the laboratory , specimen, the diagonal cracks are outward from the column beginning at the junction of the column and the top of the slab. These cracks are the r e s u l t of t o r s i o n a l moment. Observed behavior of the slab i n Test I I indicated one-way action. There was l i t t l e b i a x i a l bending at the edge colimins, and moments across Line E were small. Panel loads were transferred to the colianns p r i n c i p a l l y through the edge beam along column Line E. Consequently, shear strength i s more clo s e l y represented by a beam than a slab. Shear strength for the edge beam was computed using Eq. 17-2 of the ACI Code. (20) Ultimate shear strength i s given by: = hdi[l.9yfj + 2500(PâVd/M) ] (4) where A = Area of tension reinforcement s b = width of compression face of f l e x u r a l member d = distance from extreme compression f i b e r to centroid of tension reinforcement fc = compressive strength of concrete M = bending moment Pâ = As/bd V = t o t a l shear at section = t o t a l ultimate shear Strength due to v e r t i c a l s t i r r u p s was computed by using Eq. 17-4. This equation i s : = (Avfyd)/s (5) where A^ = t o t a l area of web reinforcement i n tension within a distance, s, measured i n the d i r e c t i o n p a r a l l e l to the longitudinal reinforcement fy = y i e l d strength of reinforcement l - 4 i .
