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

PPP Notes - Retaining Walls

SITE DEVELOPMENT

 

RETAINING WALLS

 

• Retaining walls are designed and constructed to resist the thrust of the soil, which can cause the wall to fail by overturning, sliding or setting.
• In stone walls, resistance to soil thrust can be helped by battering the stonework (i.e. recessing or sloping the masonry back in successive courses).
• Garden-type retaining walls, usually no higher than 4 ft., are generally made from small building units of stone, masonry, or wood.
• For higher walls, reinforced concrete is more commonly used.
• Terracing may be built with walls of wood, stone, brick, or concrete.
• Walls less than 2 ft. high do not require drains or weepholes.
• Preservative-treated wood is recommended for any design in which wood comes in contact with the ground. 
• Redwood may be substituted if desired.
• Stagger vertical joints from course to course 6 in. minimum horizontally.  The thickness of the wall at any point should not be less than half the distance from that point to the top of the wall.

 

CAST-IN-PLACE CONCRETE RETAINING WALLS

 

• When designing cast-in-place concrete retaining walls, keep these guidelines in mind:
• Provide control and/or construction joints in concrete retaining walls approximately every 25 ft.  Every fourth control and/or construction joint should be an expansion joint.  Coated dowels should be used if average wall height on either side of a joint is different.
• Consult with a structural engineer for final design of all concrete retaining walls.
• Concrete keys may be required below retaining wall footing to prevent sliding in high walls and those built on moist clay.