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
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: the punching strength is a function
would like to assume that the author had similar doubts. The of the opening of a critical shear crack in the slab. Nevertheless,
elbowed-shaped strut in Fig. 2(c) confirms this feeling. The
the position of this crack can not be predicted through the
sliding surface part of the critical shear crack crosses the
slab rotation, hence ψ can not be considered as an independent
node where both struts join. The truss model with the elbow-
variable of the phenomenon.
shaped strut shown in Fig. 2(c) is completely irrelevant (as is
the entire strut-and-tie model). How would a shifting of the The paper gives very valuable impacts for looking for a
loading, for example toward the column, influence the truss? mechanically sound model on punching shear strength.
And concerning the load-bearing capacity of the triple lines,
the ties that should had been of concrete: who cares? The REFERENCES
strut-and-tie model shows its limits very clearly. 28. Windisch, A., “Towards a Consistent Design Model for Punching
Shear Capacity,” International Workshop on Punching Shear Capacity of