Amin Ghali

Punching Shear Design of Earthquake-Resistant Slab-Column Connections

ACI Structural Journal/September-October 2000 ACI Structural Journal, V. 97, No. 5, September-October 2000. MS No. 99-202 received September 9, 1999, and reviewed under Institute publication policies. Copyright  2000, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the July-August 2001 ACI Structural Journal if received by March 1, 2001. ACI STRUCTURAL JOURNAL TECHNICAL PAPER

Design of Stud-Shear Reinforcement For Slabs

Extensive tests presented earlier have establidhed the effectiveness of stud-shear reinforcement in increasing the punching-shear strength and ductility of slab-column connections transfering axial force V and bending moment M. The shear reinforcement is composed of vertical rods mechanically anchored at their top and bottom ends. This paper presents rules to design and detail stud-shear reinforcement in accordance with the 1989 ACI Building Code (ACI 318-89). Three numerical examples illustrate the design of shear studs and their distribution in slabs in the vicinity of interior, edge, and corner columns transferring V and M. Because of effective anchorage, design rules that reduce the amount of shear reinforcement are suggested and applied. Rules for the design according to the Canadian standard CAN3-A23.3-M84 are also given.

Tests on Concrete Slab-Column Connections With Stud-Shear Reinforcement Subjected to Shear-Moment Transfer

Test results of jive full-scale reinforced concrete flat-plate connections with interior columns subjected to shear-moment transfer are reported. One specimen had no shear reinforcement, and the remaining jour contained various arrangements of stud-shear reinforcement (vertical rods mechanically anchored at their top and bottom). The results confirmed the effectiveness of this type of shear reinforcement in increasing the shear strength and ductility of the connection. Code provisions suggested earlier for the design of shear studs are verified. Requirements for the dimensions of the anchor heads are suggested. The design of shear-stud reinforcement is demonstrated by a numerical example.

CONTROL OF THERMAL CRACKING OF CONCRETE STRUCTURES

Studies have proven that tensile stresses due to temperature can be high enough to produce cracking in concrete structures, and that thermal stresses are substantially alleviated when cracking occurs, causing a drop in stiffness. The authors review the Analysis of thermal stresses in statically determinate and indeterminate concrete structures and describe an analytical model for determining the effects of progressive reduction in stiffness as cracks form on thermal stresses and internal forces in continuous structures. Design criteria and equations are presented for determining the minimum amount of reinforcement necessary to control cracking due to temperature.

Connection of Flat Plates to Edge Columns

Test results of 6 full-scale reinforced concrete flat plate connections with edge columns subjected to shear-moment transfer are reported. Two specimens had no shear reinforcement and the remaining 4 had various arrangements of stud-shear reinforcement. The results confirmed the effectivenesss of this type of shear reinfrocement in improvig shear strength and ductility. Code provisions and rules for the design of shear-stud reinforcement are compared with the test results.

A HYPOTHESIS ON MECHANISM OF CREEP OF CONCRETE WITH REFERENCE TO MULTIAXIAL COMPRESSION

IT IS SUGGESTED THAT CREEP IS DUE TO ORIENTED INTERNAL MOISTURE DIFFUSION CAUSED BY A FREE ENERGY GRADIENT AND TO SLOW DEFORMATION OF THE ELASTIC SKELETON OF THE GEL INDUCED BY VISCOUS DEFORMATION OF ADSORBED WATER. SHRINKAGE ACTUALLY REALIZED IS AFFECTED BY THE RELIEF OF THE STRESS IN THE ELASTIC SKELETON BY CREEP. THE NATURE OF MOISTURE DIFFUSION SUGGESTS THAT CREEP STRAINS MAY NOT BE SUPERPOSABLE UNDER MULTIAXIAL STRESS. CREEP RECOVERY IS SHOWN TO DEPEND BOTH ON CREEP AT THE TIME OF UNLOADING AND ON MAGNITUDE OF ELASTIC STRAIN RECOVERY. A RHEOLOGICAL MODEL IS PROPOSED. SOME SUPPORTING EXPERIMENTAL DATA ARE PRESENTED. /AUTHOR/

Seismic-Resistant Joints of Interior Columns with Prestressed Slabs

This study was undertaken to contribute to the development of design recommendations for post-tensioned flat plates that can endure the lateral displacements induced by earthquakes. The authors developed a procedure for the punching shear design of post-tensioned slab-column connections. The procedure is based on existing design recommendations for nonprestressed slabs, on a series of seven full-scale tests presented in this paper of interior column connections with post-tensioned slabs and on a companion similar series on edge column-slab connections. In the seven tests, the ratio of prestressed to nonprestressed reinforcement was varied, while flexural strength was maintained the same. Columns were subjected to a constant axial force, simulating gravity load effect, combined with cyclic displacement reversals of increasing amplitude up to failure. The slabs were reinforced with stud shear reinforcement. The authors found that the existing design recommendations for nonprestressed slabs can also be used for post-tensioned slabs having an average effective prestress of 0.4 to 1.4 MPa (60 to 200 psi). They conclude that prestressing mitigates degradation of the stiffness; however, energy dissipation is better in nonprestressed connections. The prestressing increases the unbalanced moment strength of the connections.

Headed Studs in Concrete: State of the Art

Headed studs are smooth or deformed bars, usually short compared to the lengths of concrete members, with forged or welded heads for anchorage at one or both ends; they are used instead of conventional reinforcing bars anchored by hooks and bends. This article discusses several practical applications where the use of headed studs is appropriate. The authors note that the main advantages are simpler installation, less congestion of reinforcement, improved confinement, and more effective anchorage. The authors discuss applications for headed studs in slabs and footings, beams with thin webs, crossties in columns and walls, precast beams, deep beams and pile caps, and in beam-column joints. They conclude that use of a headed bar, as opposed to a bar with a hook, is advantageous in applications where there is demand for the yield strength at a section of the bar close to its end.

DESIGN CONSIDERATIONS FOR SLAB-COLUMN CONNECTIONS IN SEISMIC ZONES

The transfer of shearing forces and moments between concrete flat slabs and columns can produce brittle punching failure. Slab-column connections must satisfy adequate strength against punching failure. In seismic zones, the connections are expected to undergo deformations into the inelastic range and, hence, it is necesary to design connections with adequate strength and ductility. In addition, the connections must be able to undergo a specified limit of interstory drift without punching failure. The ductility and drift requirements that must be adhered to in design are discussed. Tests reported in the literature show that the strength under cyclic moment transfer is less than the strength under monotonic loading. For an earthquake-resistant structure, the design must be based on the strength under cyclic loading. Test results are reviewed and discussed. As an application, a hypothetical structure is designed according to American Concrete Institute (ACI) Building Code (ACI 318-89). The structure is subjected to the 1940 Elcentro ground motion, and time-history dynamic analysis is performed. The results show that to obtain adequate strength, drift capacity, and ductile behavior of a slab-column connection without shear reinforcement, it is necessary in most cases to increase substantially the slab thickness in the connection region beyond the minimum thickness required to control deflections by providing shear capitals. The disadvantages of shear captials can be avoided by using shear reinforcement.

Deflection Prediction of Members of Any Concrete Strength

This paper presents a summary of a method of analysis to predict the immediate and time-dependent strain in reinforced concrete sections with or without prestressing. The main time-dependent parameters required for the analysis are the creep coefficient, free shrinkage, modulus of elasticity of concrete, and relaxation of the prestressed steel. Some codes give guidance on the values of the first three parameters as a fraction of the specified concrete strength. The analysis procedure is applied to different examples to show the influence of varying the concrete strength on the predicted deflections. The analysis procedure is verified by comparisons of predicted deflections with published experimental data.