Post-Compression -- Prestressing Technique


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Title: Post-Compression -- Prestressing Technique

Author(s): Michael R. Johns

Publication: Concrete International

Volume: 1

Issue: 3

Appears on pages(s): 29-31

Keywords: beams (supports); bending; bridges (structures); columns (supports); compression; high-strength steels: loads (forces); prestressed concrete; prestressing; reinforced concrete; unbonded prestressing.

Date: 3/1/1979

The article describes a prestressing technique designed to increase the carrying capacity of concrete and reinforced concrete members in compression or combined compression and bending. It is also suggested that the system, combined with conventional post-tensioning, can be applied to members in bending. The technique takes advantage of the difference in nature of dead load and live load, dead load being assumed to be irreversible. The system, by applying a tensile force to a dead load pre-loaded member, eliminates some, or all, of the dead oad or dead load and moment and leaves the member free to accept a fluctuating live load. The ratio of DL/LL governs the value of the reduction, the ideal ratio being i.o. The tensile force is provided by a prestressing bar or wire bundle contained within the member but debonded from it. The bar or wire bundle core is compressed by jacking (temporarily tied to the anchor plate), and transmitted by the anchor which is tied into the structural member. In the event of application to an axially loaded compression member, the anchor is central, but in the event of the application to a member in compression and bending, the anchor is given an eccentricity. The system using composite with "post-tensioning" can be applied to members in bending only. Typical applications are: columns or concrete walls; arch bridges; tunnel linings; arch dams; and composite with "post-tensioning" to beams. The application to beams, however, would appear to offer economy only in limited cases. Anchors and "de-bonding" methods are described in the papers. Experiments in the laboratory have shown the system to work in principle; and economy in size and cost of the members has been demonstrated in theory.