International Concrete Abstracts Portal

A part from column-slab connections, almost all reinforced concrete connections can be analyzed and designed using plastic strut and tie models. The strut and tie model provides a simple, rational and highly transparent explanation for the flow of forces within a connection. By examining a unique substructure within a column-slab connection, Alexander and Simmonds (1) develop what amounts to a plastic strut and tie model for concentrically loaded connections between interior columns and two-way slabs with orthogonal reinforcement. On the basis for this model, a general design procedure for gravity-loaded column-slab connections has been developed. The resulting design procedure is simple and it handles column-slab connection problems that are not easily analyzed by existing code provisions. This paper outlines the design procedure and the important features of the model upon which it is based. The model is compared both to existing test results in the literature and to the ACI code design procedure. Two design examples are included.

Traditionally, U.S. Government agencies have developed and maintained manuals for the design of structures to resist severe dynamic loads, I.e. blast effects. However, such manuals have been primarily directed toward structures of a military nature, and relatively little attention has been given to the design of civilian buildings to resist blast effects. The lack of concern for the blast resistance of buildings is no surprising in that the threat has been minimal. Although some design guidance for blast resistance has been available to the general public, the primary users have been petro-chemical industries that are aware of potential accidental explosions related to their normal operations (I.e., chemical plants). Historically, general design guidance, such as that of the American Concrete Institute's Committee 318 (ACI, 1995) (1) has served the public well. However, two recent events, the World Trade Center and the Alfred P. Murrah explosions, have heightened awareness in the United States of the potential need to consider blast effects in the design of some buildings. The discussion presented herein summarizes existing blast-resistant design approaches and addresses issues that are critical to the development of buildings with improved resistance to severe dynamic loads. Emphasis is given to the design and behavior of reinforce concrete structures.

Design of connections of columns to flat slabs to ensure safety against punching failure is presented. The connections transfer shearing forces and moments between the columns and slabs. The objective is to cover the design procedure in most practical situations including: interior, edge and corner columns, prestressed and nonprestressed slabs, slabs with openings and slabs with shear reinforcement. The ACI 318-95 code requirements are adhered to where applicable. The designs are demonstrated numerical examples. Design of shear reinforcement in raft slabs, footings and walls subjected to concentrated horizontal forces is also discussed.

The yield line theory for the determination of the ultimate load for slab structures is a well documented method of analysis. The basics of the method, which can be implemented using either equations of equilibrium or virtual work equations, are briefly reviewed, using a rectangular panel with all edges supported. A more complex single panel is then considered, followed by a brief review of multi-panel failure mechanisms. The potential importance of in-place forces, both compression and tension, is noted. These forces, which can be thought of in arch or dome terms for compression and catenaries for tension, have led to slab failure loads much greater than can be explained on the basis of flexure alone in many test. This phase of behavior is seldom usable for normal design of civil structures, but may be very useful and helpful in trying to understand the behavior of and design structures to resist blast loadings.

This paper review the requirements of the upper-and-lower-bound theorems of plasticity as they apply to continuous reinforced concrete slabs. The background and assumptions leading to Johansen's yield line theory (upper-bound) and Hillerborg's strip methods (lower-bond) are presented and the advantages and disadvantages of these two methods are discussed. The segment equilibrium method proposed by Wiesinger is described and presented as an alternative procedure. It is concluded that the theory of plasticity provides a practical solution for the design of continuous reinforced concrete slabs, particularly for slab systems with irregular support geometry.