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Bridge Engineering (6) Superstructure
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1. Bridge Engineering (6) Superstructure – Concrete Bridges *Determination of concrete bridge superstructure:
1.1 the geometry of the bridge;
1.2 the span length; and
1.3 the method of construction
*Bridge types and suitability
2.1 Non pre-stressed concrete slab bridges
2.1.1 The simplest type in which the deck slab also serves as the principal load carrying element.
2.1.2 The solid, cored, or ribbed concrete slab is supported directly on the substructure.
2.1.3 For spans below about 12m, a cast-in-place simple spans or continuous span solid slab may be used; and for spans over 12m, voided slabs become more economical.
2.1.4 Pre-cast slab decks usu. consist of a number of pre-cast voided slab sections placed side by side.
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2. Bridge Engineering (6) Superstructure – Concrete Bridges Bridge types and suitability
2.1 Non pre-stressed concrete slab bridges
2.1.5 The individual slab units are made to act integrally, either by the use of longitudinal shear key or by casting a concrete slab on top.
2.1.6 For simple spans of less than 15m, pre-cast slabs are usu. used; for longer spans, box sections are preferable.
2.2 Non pre-stressed concrete girder bridges
2.2.1 This type consists of a deck slab supported by non pre-stressed concrete longitudinal girders.
2.2.2 The two most common forms are cast-in-place T-beams and box girders.
2.2.3 The spans may be simply supported or continuous.
__ simple spans are generally prismatic (of constant cross-section);
__ continuous spans may be haunched;
2 Saturday, April 14, 2012 Shear key method: ?«??????»137??Shear key method: ?«??????»137??
3. Bridge Engineering (6) Superstructure – Concrete Bridges __ diaphragms are normally provided at piers and abutments;
__ intermediate diaphragms may be used to improve load distribution particularly at mid-span.
2.2.4 The analysis should take into consideration the interaction of the axial, flexural and torsional stiffness of the longitudinal girders and transverse components (deck slab, floor beams and/or diaphragms)
2.3 Pre-stressed concrete slab bridges
2.3.1 For expressway interchanges:
____ because the inherent torsional stiffness of slabs bridges suitable for curved continuous structures, and structures with single column intermediate supports;
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4. Bridge Engineering (6) Superstructure – Concrete Bridges
__ solid slabs for the short spans; round voided slabs for the intermediate range; and rectangular voided slabs for the longer range.
2.3.2 For simple spans, but more commonly for continuous spans
2.3.3 Other issues in design and construction
__ For post-tensioned voided slabs, the section should be solid over the piers and the abutment bearings.
__ At fixed piers the slabs are generally cast integral with the piers.
__ At expansion piers sliding pot bearings are used.
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5. Bridge Engineering (6) Superstructure – Concrete Bridges __ Continuous multi-span decks used to reduce the number of deck expansion joints, and thereby minimize maintenance problems.
__ Structures on horizontal curves or single column supports require particular attention to torsional stresses.
2.3.4 Pre-cast pre-tensioned concrete slabs, like pre-cast non pre-stressed concrete slabs are usually limited to straight square spans and are not adaptable to varied deck geometries;
2.3.5 Simple spans may be made partially continuous for live load in the same way as pre-cast beams or box girders.
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6. Bridge Engineering (6) Superstructure – Concrete Bridges
2.4 *Prestressed concrete girder bridges
2.4.1 In the short span range, pre-cast AASHTO beams with a composite cast-in-place non-pre-stressed concrete slab or closed box girders with pre-cast top slabs are frequently used for simple spans;
2.4.2 In the medium span range, I-girders and closed box girders with pre-cast top slabs are used for partially continuous spans;
2.4.3 In the medium to long span range, continuous pre-cast segmental trapezoidal open box girders with the deck cast-in-place are common;
2.4.4 The longest spans are generally cast-in-place segmental box girders.
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7. Bridge Engineering (6) Superstructure – Concrete Bridges 3. Design and Construction Techniques
__ For cast-in-place construction, the girders (T-beams and box girders) and slab are formed together and cast before the removal of formwork and supports.
__ the usage of continuous spans, and thinner transverse post-tensioned slabs and a reduced number of longitudinal girders at a larger spacing;
__ the importance of temperature effects for box girders due to the possibility of large differential temperatures between the top and bottom slabs;
__ the necessity of knowing exact construction loads for cast-in-place segmental construction built by the balanced cantilever method, in order to calculate stresses and deformations at each stage.
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8. Bridge Engineering (6) Superstructure – Concrete Bridges __ the essentiality of knowing the creep characteristics of the concrete for calculating deformations after the addition of each segment, and for calculating the redistribution of moments after completion and final stressing;
__ the usage of intermediate diaphragms;
__ the same design and analysis items as given for reinforced concrete girder bridges;
__ construction procedures for composite beams and slabs: erecting beams; placing forms for the slabs between beams; casting a reinforced concrete slab
__ The slab and beams act compositely for superimposed dead load and live load.
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9. Bridge Engineering (6) Superstructure – Concrete Bridges
__ the advantage of reducing the number of deck joints by using partially continuous pre-cast pre-stressed beams for multi-span bridges;
__ the development of support moments due to superimposed dead load, live load, differential temperature, shrinkage and creep.
__ the achievement of continuity for superimposed dead load and for live load by casting diaphragms at the time the deck concrete is placed.
__ the placement of longitudinal reinforced steel in the deck slab across the intermediate pier in order to resist the tension from negative moments at the supports.
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10. Bridge Engineering (6) Superstructure – Concrete Bridges __ the connecting of the bottom flanges of adjacent beams at the diaphragms to resist the tensile stress due to positive moments generated by differential temperature, shrinkage and creep;
__ the application of segmental pre-cast pre-tensioned girders to achieve continuous spans, beyond the range of pre-cast girders;
__ the employment of single or multiple cell boxes with transverse segments post-tensioned together longitudinally;
__ the use of pre-cast segmental construction for medium spans
__ the use of the balanced cantilever method for longer spans where each segment is successively stressed after erection;
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11. Bridge Engineering (6) Superstructure – Concrete Bridges __ being aware of the time difference between segment pre-casting and erecting against the deformation of the structure;
__ the application of elastic analysis and beam theory in the design of segmental box girder structures.
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