Fafafaffa

STRUCTURAL CONCRETE BUILDING CODE/COMMENTARY

318-155

CHAPTER 11 — SHEAR AND TORSION
CODE
11.1 — Shear strength
11.1.1 — Except for members designed in accordance with Appendix A, design of cross sections subject to shear shall be based on φVn ≥ Vu (11-1)

COMMENTARY
R11.1 — Shear strength
This chapter includes shear and torsion provisions for both nonprestressed and prestressedconcrete members. The shear-friction concept (11.6) is particularly applicable to design of reinforcement details in precast structures. Provisions are included for deep flexural members (11.7), brackets and corbels (11.8), and shear walls (11.9). Shear provisions for slabs and footings are given in 11.11. The shear strength is based on an average shear stress on the full effective cross section bw d.In a member without shear reinforcement, shear is assumed to be carried by the concrete web. In a member with shear reinforcement, a portion of the shear strength is assumed to be provided by the concrete and the remainder by the shear reinforcement. The shear strength provided by concrete Vc is assumed to be the same for beams with and without shear reinforcement and is taken as the shear causingsignificant inclined cracking. These assumptions are discussed in References 11.1, 11.2, and 11.3. Appendix A allows the use of strut-and-tie models in the shear design of disturbed regions. The traditional shear design procedures, which ignore D-regions, are acceptable in shear spans that include B-regions.

where Vu is the factored shear force at the section considered and Vn is nominal shearstrength computed by V n = Vc + V s (11-2)

where Vc is nominal shear strength provided by concrete calculated in accordance with 11.2, 11.3, or 11.11, and Vs is nominal shear strength provided by shear reinforcement calculated in accordance with 11.4, 11.9.9, or 11.11.

11.1.1.1 — In determining Vn , the effect of any openings in members shall be considered.

R11.1.1.1 — Openings in theweb of a member can reduce its shear strength. The effects of openings are discussed in Section 4.7 of Reference 11.1 and in References 11.4 and 11.5. R11.1.1.2 — In a member of variable depth, the internal shear at any section is increased or decreased by the vertical component of the inclined flexural stresses. Computation methods are outlined in various textbooks and in the 1940 Joint CommitteeReport.11.6 R11.1.2 — Because of a lack of test data and practical experience with concretes having compressive strengths greater than 10,000 psi, the 1989 edition of the Code imposed a maximum value of 100 psi on f c′ for use in the calculation of shear strength of concrete beams, joists, and slabs. Exceptions to this limit were permitted in beams and joists when the transverse reinforcementsatisfied an increased value for the minimum amount of web reinforcement. There are limited test data on the two-way shear strength of high-strength concrete slabs. Until more experience is obtained for two-way slabs built with concretes that have strengths greater than 10,000 psi, it is prudent to limit f c′ to 100 psi for the calculation of shear strength.

11.1.1.2 — In determining Vc , wheneverapplicable, effects of axial tension due to creep and shrinkage in restrained members shall be considered and effects of inclined flexural compression in variable depth members shall be permitted to be included. 11.1.2 — The values of f c′ used in this chapter shall not exceed 100 psi except as allowed in 11.1.2.1.

318-156

MANUAL OF CONCRETE PRACTICE

CODE
11.1.2.1 — Values of f c ′greater than 100 psi shall be permitted in computing Vc , Vci , and Vcw for reinforced or prestressed concrete beams and concrete joist construction having minimum web reinforcement in accordance with 11.4.6.3, 11.4.6.4, or 11.5.5.2.

COMMENTARY
R11.1.2.1 — Based on the test results in References 11.7, 11.8, 11.9, 11.10, and 11.11, an increase in the minimum amount of transverse reinforcement is...