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A large amount of prestressed concrete is being used in high-rise building construction. Unlike the prestressed concrete channel liners in Italy where concrete is transferred into a fully flexible material (see Fig. l), and unlike the finely detailed, thin-webbed girders used in a hanger con-struction in England as shown in Fig. 2, the prestressed concrete used in high-rise construction where used for office and commercial buildings consists mainly of shallow slabs and beams, one-way joists, skipped joists 1 or waffle slabs. In residential buildings,the more common type of pre-L stressed member is the flat plate. Plain and simple-looking to the outside, the flat plate is nevertheless quite a sophisticated structure, presenting to the structural engineer a challenge as far as its structural behavior is concerned. In fact, a visual expression of what is happening within the flat plate is given by the analogues and very sophisticated and different-looking structure: The Structural Membrane. The following gives a short description of the prestressed structural membrane,helping the engineer to understand the design of prestressed flat plates. The main characteristic of the structural membrane is its ability to carry uniformly-distributed loads to point supports through uniform stresses within its thin-shell surface. In the structural mem-brane, the resisting bending moments are developed by this uniform stress times the variable distance of the membrane to the thrust surface. The thrust surface is defined as the surface in which all the prestressing ten-dons lie. The analogy between the structural membrane and the mild steel-reinforced flat plate consists of the fact that in the flat plate, the bending moment capacity or the force couple has a level arm, or distance II II D , which is constant and a stress that is variable. In a prestressed flat plate,one further feature to be recognized is that a portion of the load is balanced by the prestressing cables. This portion, often 80% of the dead load, has the same characteristics as that of a structural1 membrane; that is, the prestress in the tendons stays constant as does the concrete stress, but the level arm of the draped tendon varies. The other portion of the load (often 20% of the dead load and all of the superimposed dead load and live load) causes stresses to build up within the plate much in the same manner as it does within the mild steel-reinforced flat plate. These con-ditions will be discussed later in this paper. It should be noted that high peak moments occur near the column supports corresponding to the funnels in the structural membrane.

A simplified method to calculate the moment mag-nification factor and the critical buckling load for a laterally unsupported story of a building is discussed. The method is a hand calculation technique which has nearly the same results as a computerized P A method. The method is a substitute for the effective length method. A practical designer% definition of a braced frame is given. A practical designer's definition of frame instability is given. Several design examples are given toillustrate the application of the procedures.

Stability of tall frames has been studied for elastic structures by a number of authors. A few references are: (5, 8, 9, 10) 3 . These methods have usually considered the elastic buckling of structures. Most structures have been found to buckle in a lurching mode at some story, usually near the bottom. Plastic design of multi-story frames considers the beams to yield, with a resultant loss of stiffness. When these hinges form, the structure may buckle in its entirety within a region several stories high. Stability of structures with nonlinear stiffness has been studied for structures of a limited size (2, 3, 7, 8). For large structures, it is usually hoped that adequate required strength is predicted by accounting for the geometric nonlinearity in the analysis and by including a load factor on the vertical load (11). This paper presents a theory which can be applied to structures which have sufficient drift to cause yielding of their bracing elements. Large structures with a large number of sequentially yielding elements can be readily solved using this method.

As the speed of construction of concrete frame structures has increased and the sophistication of design has improved, there has been an increased need for a more thorough understanding as to the way construction loads are disbursed into the structure. During the 60's and 70's, several designers and researchers proposed methods of analyzing the loads in multi-story structures during construction. A computer program employing one of these methods has been developed. In the 1982 PCA conference,the author used the results of this program to show how the number of levels of equipment, cycle time, and attained concrete strength affected the number of levels of reshores required. This paper describes in detail the process used to calculate the reshoring requirements for a 359-story flat plate structure built using a three-day construction cycle. The discussion includes the practical implications of providing reshoring for a mild steel structure. The hand calculation procedure presented parallels the computer program and is sufficiently detailed to provide the reader a practical procedure that can be used on the next project.

This text is the product of a collaborative effort by the concrete construction services staff of Ceco Industries, inc. Many staff members contributed their findings and recommendations, which were organized and integrated by William R. Anthony, PE, Manager, Market Development. Through its subsidiaries, Ceco Industries is a national formwork subcontractor, and provides the preconstruction services of value engineering, systems analysis and budget pricing. In a growing number of markets, Ceco also provides a total concrete building frame contracting service.