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The author is correct: the punching strength is a function of the opening of a critical shear crack in the slab. Nevertheless, the position of this crack can not be predicted through the slab rotation, hence can not be considered as an independent variable of the phenomenon.
REFERENCES Brown, M. D.; Bayrak, O.; and Jirsa, J. O., 2006, “Design for Shear Based on Loading Conditions,” ACI Structural Journal, V. 103, No. 4, Brown, M. D., and Bayrak, O., 2007, “Investigation of Deep Beams with July-Aug., pp. 541-550. Various Load Configurations,” ACI Structural Journal, V. 104, No. 5, Cross, H., 1952, Engineers and Ivory Towers, McGraw-Hill, New York, Sept.-Oct., pp. 611-620. 142 pp. Disc. 105-S42/From the July-August 2008 ACI Structural Journal, p. 440 Punching Shear Strength of Reinforced Concrete Slabs without Transverse Reinforcement. Paper by Aurelio Muttoni Discussion by Andor Windisch ACI member, PhD, Karlsfeld, Germany The author is to be complimented for his new failure criterion The contributions of shear friction and dowel action can be for punching shear based on the critical shear crack theory. neglected, too. (The size effect originates from the limited The failure procedure is explained12 as follows: “the shear extent of the process zone in fracture mechanics and must be strength is reduced by the presence of a critical shear crack taken into account.) that propagates through the slab into the inclined compression The shear strength formulas in the different codes, that is, strut carrying the shear force to the column.” Eq. (1), (2), and (4), referring to the slab depth d and the Regarding Fig. 2, this explanation shall be complemented arbitrary control perimeter, smear the different contributions. as follows: at the critical shear crack shown in Fig. 2(a), the The smeared, mechanically inconsistent material characteristic continuous thick line consists of (at least) two different is than approximated with fc′1/2 or fc′1/3, which have no real sections—the upper part is a typical flexural-shear crack and physical meaning; they are relatively close to the calculated the lower part is a sliding surface across the compression figures only. zone of the slab around the column, that is, this part cannot The author’s interesting new failure criterion based on the be considered an “ordinary” crack. The theoretical strut rotation of the slab must be opposed due to the two load-rotation depicted in Fig. 2(b) cannot exist as described. It does not curves shown in Fig. 3(a). The detrimental effect of the develop around the critical shear crack, nor develops the critical supplementary reinforcing ring db12 (one No. 4) cannot be crack across the strut. The source of the inclined compressive predicted by the rotation. Menétrey29 found similar force in this strut, as shown in Fig. 2(b), is not clear either. jeopardizing influence of reinforcing rings in his tests. How does it develop on the top of the slab? The discusser The author is correct: th
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