Prestressed Concrete in High-Rise Construction


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Title: Prestressed Concrete in High-Rise Construction

Author(s): Kolbjorn Saether

Publication: Special Publication

Volume: 97


Appears on pages(s): 159-198


Date: 2/1/1987

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.