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Title: Shrinkage Restraint and Loading History Effects on Deflections of Flexural Members

Author(s): Andrew Scanlon and Peter H. Bischoff

Publication: Commentary Reference



Appears on pages(s): 498-506

Keywords: beam; deflection; loading history; shrinkage restraint; slab


Date: 7/1/2008

The effects of shrinkage restraint cracking and loading history on deflection of reinforced concrete flexural members are discussed. It is shown that deflections of lightly reinforced members (less than 0.8% reinforcement) are highly sensitive to both shrinkage restraint cracking and loading history, whereas deflections of more heavily reinforced members are insensitive to these effects at full service load. Results of a deflection example are presented along with recommendations for changes to the ACI 318 Building Code. Proposed changes include a) adoption of a cracking moment equal to two-thirds the value currently specified in the Code to account for shrinkage restraint stresses; and b) evaluation of deflection using an effective moment of inertia based on the full dead plus live service load to account for preloading from construction loads prior to installation of nonstructural elements. Both effects are evaluated using an alternative formulation for the effective moment of inertia that works well over a wide range of reinforcing ratios and for fiber-reinforced polymer (FRP)-reinforced concrete.