Prestressed Concrete Structures Multiple Choice Questions on “Construction Techniques”.
1. Cantilever construction eliminates the use of ____________
Clarification: Most of the long span bridges are built using prestressed concrete and those built by the cantilever method developed by Finister walder, demonstrate the latest refinements of the construction techniques and this method eliminates the use of expensive formwork and scaffolding especially for bridges in deep valleys and rivers with large depth of water.
2. The major methods of cantilever construction are classified into how many types?
a) 2 tyres
b) 3 types
c) 4 types
d) 6 types
Clarification: There are two major methods of cantilever construction techniques are classified as:
Cast in situ construction: In this method the bridge in cat in situ with sections 3-6m long, cantilevering symmetrically on both sides of pier
Construction using precast segments: In this method type of construction, the bridge segments comprising structural elements (mainly segmental single or multi celled box girders) are cast in a casing yard using special forms and they are transported to the work site.
3. In cat insitu construction the formwork is supported by ____________
a) Aluminium frame work
b) Wood frame work
c) Steel frame work
d) Cloth formwork
Clarification: The form work for cat in situ construction is supported by steel frame work attached to the completed part of the bridge and the formwork moves from one complete section to the next part and the sequential operations in this method are: fabrication of steel truss, placing of reinforcement, concreting using concrete mix, curing the concrete, threading the high tensile cables and anchoring and grouting of the cables, releasing formwork.
4. One of the example of cast insitu construction.
a) Boussens bridge, France
b) Larens bridge, Uganda
c) Lithcher bridge, Spain
d) Rouli bridge, Africa
Clarification: Boussens bridge over the garnne river in France having spans of 49-96m and the supporting form work to facilitate the concreteing of the cantilever portion of the bridge abuts the previously constructed section and the typical cross section comprises box girders of constant or variable depth with cables running in the ribs and flanges and a notable example of cast in situ cantilever construction of the basin creek bridge by gammon India company at Bombay.
5. The main advantage of using precast segmental units is they can be cast on ____________
Clarification: The main advantage of using precast segmental units is that they can be cast on ground near the work site well in advance and the quality of units will be better than those which are cast in situ and another advantage is the units can be curved to achieve their full strength before bringing them to assemble at worksite.
6. In the cost insitu method the least time required to move formwork is?
Clarification: In the cast in situ method at least a week’s time is required to move the formwork to the next incremental length and the precast segment system, the units can be bought to site and lifted by cranes to join them to the previous units by using temporary stressing cables and the rate of construction will be faster in the precast method than in the cast in situ method and in both methods a typical cross section would be a box girder of constant or variable depth.
7. The cantilever method has been successfully used in the span range of ____________
Clarification: The cantilever method has been successfully used in the span range of 50-200m and for small spans of less than 50m and for elevated roads or flyovers were scaffolding beneath the structure must be avoided precast segmental construction is preferred and at present for spans over 70m, prestressed concrete single or multi cell box girders complete successfully with steel construction.
8. Which one of the following method of construction is adopted when low clearance is required below the deck?
a) Erection method
b) Stagging method
c) Elongation method
d) Longitudinal method
Clarification: The staging method is adopted when low clearances required below the deck and supporting formwork does not interfere with the traffic and this method facilitates rapid construction by maintaining correct geometry of the structure with relatively low cost.
9. Which type of construction is particularly advantageous in long via duct structures, a segmental?
a) Span to span
b) Edge to edge
c) Column to column
d) Beam to beam
Clarification: In long span via duct structures, a segmental span by span (stage by stage construction, is particularly advantageous and the movable formwork may be supported from the ground and the traveler consists of a steel super structure which is moved from the completed portion of the structure to the next span to facilitate the casting or supporting of the precast units.
10. In push out technique each unit in cast directly against the ____________
a) Previous unit
b) Tensile unit
c) Last units
d) Progressive units
Clarification: Segments of the bridge super structure are cast at site in lengths of 10-30m in stationary forms located behind the abutments and each unit is cast directly against the previous unit and after the concrete attains the desired strength the new unit is joined to the previous unit by post tensioning and the assembly of units is pushed forward in a step wise manner to permit casting of the succeeding segments.
11. The construction starts at one end of the structure and proceeds continuously to the other end is?
a) Progressive placement method
b) Transverse placement method
c) Horizontal placement method
d) Linear placement method
Clarification: In the method the construction starts at one end of the structure and proceeds continuously to the other end and in contrast to the balanced cantilevered construction in which the super structure proceeds or both sides of the pier, in the progressive placement technique, the precast segments are placed from one owned of the structure to the other in successive cantilevers on the same side of the various piers and at present this method has been found to be practicable and economical in the span range of 30-90m.
12. What is the main feature of this method comprises a moveable temporary arrangement to limit?
a) Feasible stress
b) Cantilever stress
c) Rigidity stress
d) Bond stress
Clarification: The main feature of this method comprises moveable temporary arrangement to limit the cantilever stresses during construction to a reasonable level and the precast segmental units are transported over the completed portion of the deck to the tip of the cantilever span under construction, where they are positioned by swivel crane that moves over the deck.
13. The construction procedure must be planned using ____________
a) Sequential computations
b) Reverse computations
c) Complicated computations
d) Aligned computations
Clarification: Construction techniques developed have shown major progress towards simplification and reduction of erection equipment and the construction procedure must however be planned using sequential computations for the alignment forces, exact lengths and angles considering temperature and creep influence which depends on seasonal, climatic and daily environmental conditions.
14. In incremental launching or pull out technique to allow the super structure to move forward, what are provided?
a) Sliding bearings
b) Rolling bearings
c) Strut bearings
d) Tensile bearings
Clarification: Normally a work cycle of one week is required for casting and launching the segments and to allow the super structure to move forward, special low friction sliding bearings are provided at the top of various piers with proper lateral guides and the main problem is to ensure the safety of stresses in the super structure under its own self weight during all stages of launching at various critical sections.
15. If the spans are large, they can be sub divided by means of ____________
Clarification: If the spans are large, they can be sub divided by means of temporary piers to control the magnitude of bending moments with in safe limits and according to Raina this construction technique has been applied to spans up to 60m without the use of temporary false work bents and also spans up to 100m have been built using temporary supporting bents and the main girders must have a constant depth generally varying from 1/12 to 1/16 of the longest span.