Prestressed Concrete Structures Multiple Choice Questions on “Analysis of Composite Sections”.
1. The dimensioning of composite sections involves determining the required size of ____________
a) Precast sections
b) Precast beams
c) Composite sections
d) Prestressed beams
Clarification: The dimensioning of composite sections involves determining the required size of the composite section using a standard precast prestressed beam of known section properties in order to support the required design service loads.
2. What is necessary to design a precast prestressed section?
b) Section modulus
c) Factor safety
d) Reinforcement details
Clarification: It may become necessary to determine the section modulus of the precast prestressed section for a composite slab of given depth and either case, formulae relating the section moduli of the precast prestressed and composite section loading on the member, permissible stresses in the concrete and loss ratio may be developed by considering various stages of loading.
3. The critical stress condition generally occurs at ____________
Clarification: The critical stress condition generally occurs at the soffit of the precast prestresssed beam is calculated in order to support the required design service load calculate the overall depth of composite slab by assuming the trial depth and add the trial depth to the depth of standard prestressed beam.
4. The first two factors considered in design considerations of composite sections are ____________
a) Sectional properties and overall depth
b) Elevation properties and overall depth
c) Design properties and overall depth
d) Construction properties and overall depth
Clarification: The known sectional properties of the precast prestressed beam is calculated in order to support the required design service load, calculated the overall depth of composite slab by assuming the trial depth and add the trial depth to the depth of standard prestressed beam.
5. The design considerations of composite section in step 3 and step 4 are ____________
a) Alignment and forces
b) Self weight and moments
c) Area and moments
d) Deflection and moments
Clarification: Calculate the self weight of the precast beam and insitu concrete self weight of precast beam W = overall depth x width x unit weight of concrete, w = d x b x 24 and Calculate the moment due to self weight and live load moment due to self weight M = 0,125xwxl2. Moment due to live load Ml = 0.125xBxPxL2 according to the specification the permissible compressive stress in concrete = 0.5fct, fci = compressive strength of precast pretensioned member.
6. Under minimum and maximum moments the critical stresses occur at ____________
Clarification: In Step 5 under minimum and maximum moments we calculate the critical stresses that occur at the soffit of the precast prestressed element. The stress conditions are (Pinf – Mmin/b) < Pct
(ɳPinf-M/Zb-Mb/Zb) > or equal to pth.
7. In typical detail of expansion joint the open cell compression seal is dependant upon its ability to maintain ____________
Clarification: In typical detail of expansion joint the open cell compression seal is dependent upon its ability to maintain Pressure on the joint side walls with varying degree of stress and generally elastometric (Neoprene) compression seals for expansion joints in bridge decks and they are made of polychloroprene otherwise known as Neoprene.
8. The coupling units are used in prestressing steel for ____________
Clarification: The coupling units are used in prestressing steel for coupling units used for joining of high tensile wires should have an ultimate strength of not less than the individual strengths of the wires or bars being joined and welding is not permitted for joining of high tensile wires or bars.
9. The prestressing steel, sheathing and anchorages should be stored at ____________
Clarification: The prestressing steel, sheathing and anchorages should be stored at site in such in such a way as to provide them with adequate corrosion protection and after stressing the steel in the sheath, it should be provided with permanent protection as soon as possible preferably within one week and while providing protection by pressure grouting of cement, care should be taken that the neighboring cables are penetrated by grout.
10. The prestressing tendons are not grouted in the case of ____________
a) Nuclear pressure vessels
b) Earth vessels
Clarification: the prestressing tendons are not grouted in the case of nuclear pressure vessels and protection against corrosion is ensured by filling the ducts with petroleum based jelly and the unbounded tendons facilitate re tensioning operations whenever required and the force in the tendons can be checked at periodical intervals.
11. The bottom fiber of the prestressed beams is expressed as ____________
a) Ptw – Mmin/Zt
b) Ptw + Mmin/Zt
c) Ptw – Zt/Mmin
d) Ptw + Zt/Mmin
Clarification: The required top and bottom fibers of the precast prestressed beams are calculated in 7 step they are expressed as:
Pt > or less than (Ptw/ɳ + M/ɳZb + Ml/ɳZb‘)
Pb > or less than (Ptw – Mmin/Zt) where Pt = characteristic tensile strength of concrete (n/mm2), Pb = stress at bottom fiber (n/mm2).
12. The maximum eccentricity in the design of composite sections is given as ____________
a) e = ZtZb(Pt) / A(PbZt+PtZb)
b) e = ZtZb(Pt-Pb) / A(PbZt+PtZb)
c) e = ZtZb(Pt+Pb) / A(PbZt+PtZb)
d) e = ZtZb / A(PbZt-PtZb)
Clarification: The maximum eccentricity for composite sections is given in step 8 is
e = ZtZb(Pt-Pb) / A(PbZt+PtZb) where Zt = section modulus of the top fiber, Zb section modulus of bottom fiber, the minimum prestressing force is given as W = A(PtZb+PbZt) / Zt+Zb.