Insights worth sharing

Motives for better Engineering

Coulomb's wedge theory
of earth pressure

Explore horizontal earth pressure,
Coulomb's theory, and its applications.
Compare geotechnical results and
understand the trial wedge method's nuances.

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Baltimore bridge collapse:
What structural engineers can do

Explore the technical content on vessel collision
to calculate the annual frequency of bridge component collapse.

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Seismic Isolation in Structural Design:
Concepts and Applications

Introducing the concept of seismic isolation design.

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Understanding Time Dependent Material in Prestressed Concrete

Benjamin BlasenOct 20, 2023

When we talk about prestressed concrete, the things that we are mostly concerned about are the compressive strength gain with respect to time, and the prestressing tendon relaxation with respect to time. Figure 1 shows various time-dependent effects for concrete including creep and shrinkage.

 

The factors that affect the creep rate include water/cement ratio, age and strength of the concrete when it is subjected to stress, and ambient temperature and humidity. Creep rate also depends on many other factors related to the quality of the concrete and conditions of exposure such as the type, amount, and maximum size of aggregate; type of cement; amount of cement paste; size and shape of the concrete mass; amount of steel reinforcement; and curing conditions (Robert Salca, tech support, midas UK).

 

For shrinkage, its rate decreases much faster with time compared with creep as shown in figure 1. Finer aggregates and finer gels result in increased shrinkage, the moisture content of the concrete and the relative humidity of the ambient medium have a big influence on carbonation shrinkage, and harder aggregates with higher modulus of elasticity decrease shrinkage. 

Advanced Composite Section Analysis in Structural Design

Composite Section Analysis

 

Finally we came to the composite section analysis. One of the best examples is from Dr. Gilbert and Dr. Ranzi (Example 5.10, Time-dependent behavious of concrete structures, CRC Press, 2010.)