RP438-concrete design for torsion

RP438-concrete design for torsion - Torsion Design of...

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Torsion Design of Structural Concrete Based on ACI 318-05 By Mahmoud E. Kamara, Ph.D., and Basile G. Rabbat, Ph.D., S.E. PROFESSIONAL DEVELOPMENT SERIES September 2007
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2 PDH Special Advertising Section — Portland Cement Association T orsional moment develops in structural concrete members as a result of asymmetrical loading or member geometry, or as a result of structural fram- ing. For example, spandrel beams built integrally with the floor slab are subject to torsional moment resulting from the restraining negative bending moment at the exte- rior end of the slab. The restraining moment is proportional to the torsional stiffness of the spandrel beam. In complex structures such as helical stairways, curved beams, and eccen- trically loaded box beams, torsional effects dominate the structural behavior. Torsional moment tends to twist the structural member around its longitudinal axis, inducing shear stresses. However, structural members are rarely subjected to pure torsional moment. In most cases, torsional moments act concurrently with bending moment and shear forces. During the first half of the twentieth century, structural codes were silent regarding torsion design. Torsion was looked at as a secondary effect that was covered in the factor of safety considered in the design. Demand for more complex struc- tures, improved methods of analysis, new design approaches, and the need for more economical design required a better understanding of the behavior of reinforced concrete members subjected to torsion. In the second half of the twentieth century, research activities helped engineers understand many aspects of behavior of concrete members under torsion. This article focuses on torsion in solid and hollow closed sections. Thin, open C- and U-shaped sections subject to torsion suffer distortions (referred to as Vlasov torsion), and are not covered in this article. The procedure presented herein reflects the provisions of the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318-05) (Reference 1) and those of the soon-to-be-published ACI 318-08. All section numbers within this article refer to ACI 318-05. Note that the fourth edition of the American Association of State Highway and Transportation Officials’ Load and Resistance Factor Design (AASHTO LRFD) Bridge Design Specifications prescribes torsion design approaches for structural concrete members slightly different from those of ACI 318. Equilibrium versus compatibility torsion It is important for designers to distinguish between two types of torsions: equilibrium torsion and compatibil- ity torsion (References 2 and 3). Equilibrium torsion occurs when the torsional resistance is required to maintain static equilibrium. For this case, if sufficient torsional resistance is not provided, the structure will become unstable and collapse. External loads have no alternative load path and must be resisted by torsion.
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RP438-concrete design for torsion - Torsion Design of...

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