Prestressed Concrete Beams Deflections
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Questions and Answers

What is the primary consequence of failure to predict and control creep deformation in prestressed concrete members?

  • Increased flexural rigidity
  • Reducing the moment-curvature relationship
  • Concave surfaces
  • High reverse deflection (correct)
  • What is the primary design consideration in prestressed concrete members?

  • Proportioning the structural member for the limit state of flexural stresses at service load (correct)
  • Moisture protection of the concrete
  • Serviceability of reinforced concrete elements
  • Anchorage development strength
  • Why are prestressed concrete members more critical in terms of deflection control than reinforced concrete elements?

  • Because they are stronger and can withstand more load
  • Because they are more slender and their behavior is more affected by flexural cracking (correct)
  • Because they are less prone to corrosion
  • Because they are more sustainable
  • What is the consequence of excessive deflection in prestressed concrete members in buildings?

    <p>Cracking of partitions and misalignment of windows and doors</p> Signup and view all the answers

    What is the term used to describe the deformation of prestressed concrete members over time due to sustained eccentric compression?

    <p>Creep</p> Signup and view all the answers

    What is the primary effect of the prestressed force on prestressed concrete members?

    <p>Seriously affecting their long-term creep deformation performance</p> Signup and view all the answers

    What is the primary factor governing short-term deflections of prestressed members?

    <p>Bending moment distribution along the span and the flexural rigidity</p> Signup and view all the answers

    In the post-cracking stage, a prestressed concrete beam behaves similarly to a

    <p>Reinforced concrete beam</p> Signup and view all the answers

    Which of the following is NOT a factor considered in Mohr's moment area theorems?

    <p>Tendon profile</p> Signup and view all the answers

    What is the primary reason for locating tendons with eccentricities towards the soffit of beams in prestressed beams?

    <p>To counteract the sagging bending moments due to transverse loads</p> Signup and view all the answers

    What is the relationship between moment-curvature and the post-cracking behavior of a prestressed concrete beam?

    <p>The moment-curvature relationship is used to estimate deflections in the post-cracking stage</p> Signup and view all the answers

    What is the purpose of using Mohr's theorems in the computation of short-term deflections?

    <p>To estimate the deflections due to the prestressing force, self-weight, and imposed loads</p> Signup and view all the answers

    What is the primary reason for controlling deflections in structural concrete members?

    <p>To ensure the strength and serviceability of the structure</p> Signup and view all the answers

    Which of the following factors does NOT influence the deflections of prestressed concrete members?

    <p>Duration of the load application</p> Signup and view all the answers

    In the pre-cracking stage, what is used to compute the deflections?

    <p>Second moment of area of the gross concrete section</p> Signup and view all the answers

    What is the consequence of large deflections under dynamic effects and variable loads?

    <p>Discomfort to the users</p> Signup and view all the answers

    What is the term used to describe the sagging of principal structural members?

    <p>Deflection</p> Signup and view all the answers

    What is the purpose of designing structural concrete members with adequate stiffness?

    <p>To limit deflections and ensure strength and serviceability</p> Signup and view all the answers

    Study Notes

    Deflections in Prestressed Concrete Design

    • Serviceability of prestressed concrete members is critical in design, considering deflection and cracking behavior, similar to reinforced concrete elements.

    Importance of Deflection Control

    • Excessive sagging of principal structural members can make the floor unsuitable for intended use.
    • Large deflections under dynamic effects and variable loads can cause discomfort to users.
    • Excessive deflections can cause damage to finishes, partitions, and associated structures.

    Factors Influencing Deflections

    • Imposed load and self-weight
    • Magnitude of the prestressing force
    • Cable profile
    • Second moment of area of cross-section
    • Modulus of elasticity of concrete
    • Shrinkage, creep, and relaxation of steel stress
    • Span of the member
    • Fixity conditions

    Short-Term Deflections of Uncracked Members

    • Governed by bending moment distribution along the span and flexural rigidity of the members.
    • Mohr's moment area theorems can be used to estimate deflections due to prestressing force, self-weight, and imposed loads.

    Effect of Tendon Profile on Deflections

    • Tendons are located with eccentricities towards the soffit of beams to counteract sagging bending moments due to transverse loads.

    Computation of Deflections in Post-Cracking Stage

    • Prestressed concrete beam behaves similarly to a reinforced concrete beam.
    • Moment-curvature relationships involving section properties of the cracked beam are used to compute deflections.

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    Description

    Understand the computation of short-term deflections in prestressed concrete beams, including the use of Mohr's theorems and moment-curvature relationships in the post-cracking stage.

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