Design of Deep Beams Using Strut-and-Tie Models-Part I: Evaluating U.S. Provisions and Part II: Design Recommendations. Papers by Michael D. Brown and Oguzhan Bayrak/AUTHORS' CLOSURE by ProQuest

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									                                                                                                                                                      DISCUSSION
Disc. 105-S37/From the July-August 2008 ACI Structural Journal, p. 395 and p. 405

Design of Deep Beams Using Strut-and-Tie Models—Part I: Evaluating U.S. Provisions and Part II: Design
Recommendations. Papers by Michael D. Brown and Oguzhan Bayrak

Discussion by Dipak Kumar Sahoo, Bhupinder Singh, and Pradeep Bhargava
Research Scholar, Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, India; Assistant Professor, Department of Civil Engineering, Indian Institute of
Technology Roorkee; Professor, Department of Civil Engineering, Indian Institute of Technology Roorkee



   The authors are complimented for the exhaustive compilation                                                                          1
and meticulous evaluation of a large database of 596 reinforced                                                                να -------------
                                                                                                                                              -
                                                                                                                                             k2
                                                                                                                                                                               (C)
concrete beams with a/d ratios less than 2 to identify the                                                                        ( fc ′ )
unconservatism in the strut-and-tie model (STM) provisions
of ACI 318-05 and the AASHTO LRFD. Recently, Yang                                             where k1 and k2 are two independent exponents that can be
and Ashour (2008) analyzed a large shear database, part of                                    evaluated by suitably grouping the database considering one
which is also used by the authors, and observed similar                                       parameter at a time while keeping the other parameter
unconservatism in the STM provisions of the ACI 318-05                                        constant. Finally, combining Eq. (B) and (C), the composite
and the Eurocode 2 (2004) as well, with these provisions                                      model relating the strut efficiency factor ν to the a/d and fc′
being even more unsafe, particularly for continuous deep                                      will become
beams. The discussers would like to draw the authors’ attention
to two points:                                                                                                                            –k1                –k2
   1. With reference to the findings of Part I, it may be noted                                                      ν = k0 ( a ⁄ d )             ( fc ′ )                     (D)
that the cylinder compressive strengths of concrete used in
the 596 beams of the database fall in a wide range—28 to                                      where k0 is a constant coefficient that can also be evaluated
83 MPa (4000 to 12,000 psi)—whereas the applicability of                                      from the database.
the STM provisions of ACI 318-05, Appendix A, is limited                                        The discussers feel that unless the values of both k1 and k2
to concrete with compressive strengths up to 41 MPa (6000 psi).                               (evaluated using the database as discussed) are close enough
The literature suggests that the strut efficiency factor                                      to unity, two independent exponents may be preferred.
decreases with an increase in the concrete compressive
strength and, therefore, it should be expected that the higher-
                                                                                                                            REFERENCES
strength concrete beams will be unsafe when evaluated using                                     EN 1992-1-1:2004, 2004, “Eurocode 2: Design of Concrete Structures—
the ACI 318-05 provisions; and                                                                Part 1-1:
								
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