18 Questions
What is the primary consequence of failure to predict and control creep deformation in prestressed concrete members?
High reverse deflection
What is the primary design consideration in prestressed concrete members?
Proportioning the structural member for the limit state of flexural stresses at service load
Why are prestressed concrete members more critical in terms of deflection control than reinforced concrete elements?
Because they are more slender and their behavior is more affected by flexural cracking
What is the consequence of excessive deflection in prestressed concrete members in buildings?
Cracking of partitions and misalignment of windows and doors
What is the term used to describe the deformation of prestressed concrete members over time due to sustained eccentric compression?
Creep
What is the primary effect of the prestressed force on prestressed concrete members?
Seriously affecting their long-term creep deformation performance
What is the primary factor governing short-term deflections of prestressed members?
Bending moment distribution along the span and the flexural rigidity
In the post-cracking stage, a prestressed concrete beam behaves similarly to a
Reinforced concrete beam
Which of the following is NOT a factor considered in Mohr's moment area theorems?
Tendon profile
What is the primary reason for locating tendons with eccentricities towards the soffit of beams in prestressed beams?
To counteract the sagging bending moments due to transverse loads
What is the relationship between moment-curvature and the post-cracking behavior of a prestressed concrete beam?
The moment-curvature relationship is used to estimate deflections in the post-cracking stage
What is the purpose of using Mohr's theorems in the computation of short-term deflections?
To estimate the deflections due to the prestressing force, self-weight, and imposed loads
What is the primary reason for controlling deflections in structural concrete members?
To ensure the strength and serviceability of the structure
Which of the following factors does NOT influence the deflections of prestressed concrete members?
Duration of the load application
In the pre-cracking stage, what is used to compute the deflections?
Second moment of area of the gross concrete section
What is the consequence of large deflections under dynamic effects and variable loads?
Discomfort to the users
What is the term used to describe the sagging of principal structural members?
Deflection
What is the purpose of designing structural concrete members with adequate stiffness?
To limit deflections and ensure strength and serviceability
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.
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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