BDCS - Prestressed Concrete

The Initial Walnut Lane Memorial Bridge Sketch

What is prestressed concrete? Simple. It is the preloading of a concrete member, before the application of service loads, so as to improve its service load behavior (cracking and deflection).

The single most important event that led to the dramatic launch of the precast prestressed concrete industry in North America was the construction of the technically innovative Walnut Lane Memorial Bridge in Fairmont Park in Philadelphia, Pennsylvania in 1949 and 1950. Designed by noted Belgian architect Gustav Magnel, the Walnut Lane Memorial Bridge is arguably less famous than its arched cousin (also called the Memorial Bridge, but it was - to builders across the world - by far the most economical usage of concrete in its time.

Among the advantages of prestressed concrete to owners and designers are low initial costs, low maintenance needs and longer life expectancy of bridges. This is reflected in the increasing market share of prestressed concrete, which has grown from zero in 1950 to more than 55 percent today.

Below are some slides I've put together detailing the construction of prestressed elements in design.  In my next post I will detail the structural analysis of prestressed concrete members.  As should be expected, since high strength steel is used for this form of construction, the stress/strain diagram across the beam/column will be dramatically different from that of a typical reinforced concrete beam/column. This distinction allows designers to do many things, not the least of which is reach spans that until 1950 were previously unheard of.

Typical Assemblage of Prestressed Elements

Sandwich Wall Panels (or Prestressed Walls)

Types of Wall Panels

The Hollow Core Slab

Double Tees and Topped Slabs

BDCS Notes - Reinforcing Steel

Structural Concrete: Concrete members subjected to tension reinforced with reinforcing steel bars. (Conventional or prestressed reinforcing)

Conventional Reinforcing: Stresses fluctuate with loads on the structure. No special requirements on the steel.

Prestressed Reinforcement: Steel is under continuous tension. Stress relaxation will reduce the effectiveness of the reinforcement, thus steel bars are required.

Manufactured Forms:

Plain Bars – Round, without surface deformations (not used for tension or bending)

Deformed Bars – Have protrusions (deformations) at the surface. Ensures a good bond between the bar and the concrete. Deformed surface of bond prevents slipping, allowing unit to work as one. (Used in concrete beams, slabs, columns, walls, footings, pavements, concrete structures, etc.)

Wire Fabrics (Plain and Deformed) – flat sheets in which wires pass each other at right angles, and one set of elements is parallel to the fabric axis. (Used in concrete slabs and pavements, to resist temperature and shrinkage stresses).

1. Plain Wire Fabrics – Develop anchorage in concrete at the welded intersections.

2. Deformed Wire Fabrics – Develop anchorage through deformations and at the welded intersections.

Kinds of Reinforcing Steel:

A615 – Billet (most widely used)

A616 – Rail

A617 – Axle

A706 – Low-Alloy

Four Grades of Reinforcing Steel:

40 – 276 MPa

50 – 345 MPa

60 – 414 MPa

75 – 517 MPa

Prestressed Concrete – Requires special wires, strands, cables and bars. Steel for prestressed concrete reinforcement must have high strength and low relaxation properties. High-carbon steels and high-strength alloy steels are used for this purpose.