Prestressed Concrete Structures Multiple Choice Questions on “Design for Compression and Bending”.
1. Most compression members, such as long columns and piles are subjected to ____________
a) Bending moment and cracking forces
b) Bending moment and tension forces
c) Bending moment and axial forces
d) Bending moment and compression forces
Clarification: Due to handling in some members like portal frames and masts the sections are subjected to compression and bending and most compression members such as long columns and piles are subjected to bending moment and axial forces.
2. The load moment interaction diagrams are more or less similar to ____________
a) Prestressed columns
b) Reinforced concrete columns
c) Aluminium columns
d) Steel columns
Clarification: The load moment interaction diagrams are more or less similar to prestressed columns those of reinforced columns expect that precompression exists in prestressed concrete columns and the prestressed columns are much advantageous than any other elements which are replicable.
3. The compression failure mode develops under?
a) Static loading
b) Moment loading
c) Concentric loading
d) Tensile loading
Clarification: The compression failure mode develops under concentric loading and this type of failure mode develops under concentric loads and the section is considered to have failed when the concrete strain ε0 reaches a value of 0.02.
4. The balanced failure develops when there is simultaneous ____________
a) Tension yielding
b) Compression yielding
c) Flexure yielding
d) Prestress yielding
Clarification: Balanced failure develops when there is simultaneously tension yielding of prestressing steel and crushing of concrete, the eccentricity of the axial load is defined as balanced eccentricity Eb. and this eccentricity factor is used for various failures considering top and bottom fibers.
5. The charts proposed by Bennett are useful in dimensioning columns of ____________
a) L section
b) I section
c) T section
d) Edge section
Clarification: Bennett has proposed design charts with dimensionless parameters expressed in terms of the service loads and moments, and section properties and permissible stresses in concrete expressed as a fraction of the characteristic strength these charts are useful in dimensioning columns of I section with non uniform prestress and allowing desirable tensile stresses in concrete as in class 3 type members.
6. The steps involved in design of biaxially loaded column are ____________
Clarification: The load contour method of analysis, detailed by Nawy and generally termed Bresler-Parme counter method is ideally suited for the design of biaxially loaded columns and the design procedure is outlined the following steps: give the ultimate moments, determine the larger of the equivalent required, assumed cross section, verify the ultimate load carrying capacity, calculate the actual nominal moment capacity, the moment value.
7. The Prestressed concrete compression members should have a minimum average effective prestress of not less than?
Clarification: According to ACI 318-1989, a minimum non prestressed reinforcement ratio of one percent should be provided in compression members with an effective prestress of lower than 1.55n/mm2 and the American code specifies the various effective prestress in prestressed concrete members considering all the factors and makes them economical.
8. The Spirals are particularly useful in increasing the ____________
a) Tensile strength
Clarification: Closely spaced spirals reinforcement increases the ultimate load capacity of the column due to confinement of concrete in the core and spirals are particularly useful in increasing the ductility of the member and hence are preferred in high earthquake zones.
9. The pitch of spiral is computed as v
a) S = 4as(Dc – ds)/Dc2 ρs
b) S = 2as(Dc – ds)/Dc2 ρs
c) S = 6as(Dc – ds)/Dc2 ρs
d) S = 10as(Dc – ds)/Dc2 ρs
Clarification: S = 4as(Dc – ds)/Dc2 ρs, as = cross sectional area of spiral, Dc = core of the column measured to the outside diameter of the helix, ds = diameter of spiral wire, ρs = ratio of the volume of helical reinforcement.
10. The pitch of spirals is limited to a range of ____________
a) 40 to 45mm
b) 25 to75mm
c) 15 to 30mm
d) 10 to 30mm
Clarification: The pitch of spirals is limited to a range of 25 to 75mm and the spiral should be well anchored by providing at least 11/2 extra turns when splicing rather than welding of spirals is used and the pitches of the spirals are limited to certain ranges.