Code Provisions for Deflection Control in Concrete Structures (ACI SP-203) contains 11 papers. Designers, constructors, and codifying jurisdictions will benefit from reviewing deflection-control provisions in codes from other countries and international entities. Other topics include span/thickness limitations for deflection control, effect of flange width on deflection and deflection predictions, bending stiffness effects, and time-dependent deflection and cracking effects on flat slabs. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP203

To determine the structural response to service load of reinforced concrete structures and structural components, the consequences of cracking on the effective stiffness must be considered. Usual methods to determine structural response of tall reinforced concrete structures involve codified simplifications and global reductions in effective stiffness of the beams and columns. Previous work (I), (2) has developed a method to determine the effective stiffness of members within a structure which can be extended to the analysis of structural frames. This method considers the consequences of cracking in tall buildings using a member element stiffness reduction model. The model is based on the ratio of the area of the member moment diagram where the applied moment exceeds the cracking moment, to the total area of the moment diagram. From this ratio the effective stiffness of the member can be estimated. A practical cracking analysis system has been established by integrating the proposed stiffness reduction model with an iterative algorithm and the commercial package of a linear finite element analysis. With this procedure, each member can be assigned their appropriate stiffness as a function of the applied load. and the response of the structure to service loading determined. Verification of this method of analysis, and its application to structural frames, has been achieved from and extensive experimental program considering flexural members. shear wall members and large-scale structures.

AC1 Building Code requirements for deflection control are critically reviewed. Provisions for minimum thickness, deflection computations, and permissible computed deflections are reviewed. Differences in the approaches to deflection control for one-way and two-way construction are identified. Limitations in the application of the prescribed deflection calculation method are discussed. Results of a survey of consulting firms concerning deflection control in design offices are presented. The paper concludes by suggesting possible directions for future changes in building code requirements for deflection control.

An experimental program of long-term testing of large-scale reinforced concrete flat slab structures is described and the results from the first series of tests on five continuous flat slab specimens are presented. Each specimen was subjected to sustained service loads for periods up to 500 days and the deflection, extent of cracking and column loads were monitored throughout. The measured long-term deflection is many times the initial short-term deflection, due primarily to the loss of stiffness associated with time-dependent cracking under the combined influences of transverse load and drying shrinkage. This effect is not accounted for in the current code approaches for deflection calculation and control. Recently proposed procedures to improve deflection calculation (1) are evaluated against the test results and good agreement between the measured and calculated deflections is obtained.

This paper describes the behavior of reinforced and prestressed concrete flexural members under sustained service loads and outlines recent developments in the design of concrete structures for the serviceability limit states, particularly with regard to deflection and crack control. The effects of concrete cracking, creep and shrinkage on cross-sectional stresses and deformation are demonstrated and discussed for a wide range of actions and reinforcement layouts. Recent amendments to the serviceability provisions of the Australian Standard for Concrete Structures AS3600 are presented and the background to, and reasons for, the proposed changes are explained. The paper also highlights the inadequacies of the existing deflection calculation procedure in AC1 3 I8M-99 and suggests ways to improve it. A method is proposed for calculating the time-dependent deflection of reinforced and prestressed concrete members taking into account the time-dependent effects of creep and shrinkage, including the loss of stiffness caused by shrinkage induced cracking and the breakdown of tension stiffening with time. The method is illustrated by several examples.