Analysis and Design of Continuous T-Beam Girder Type Superstructure

Abstract

One of the most important aspects of design, which can affect structure life and maintenance costs, is the reduction or elimination of roadway expansion joints and associated expansion bearings. Unfortunately, this is too often overlooked or avoided. Joints and bearings are expensive to buy, install, maintain, repair and more costly to replace. The most frequently encountered corrosion problem involves leaking expansion joints and seals that permit salt-laden run-off water form the roadway surface to attack the girder ends, bearings and supporting reinforced concrete substructures. Elastomeric glands get filled with dirt, rocks and trash, and ultimately fail to function. Many of our most costly maintenance problems originated with leaky joints.

Bridge deck joints are subjected to continual wear and heavy impact from repeated live loads as well as continual stages of movement from expansion and contraction caused by temperature changes, and or creep and shrinkage or long term movement effects such as settlement and soil pressure. Joints are sometimes subjected to impact loadings which can exceed their design capacity. Elastomeric bearings can split and rupture due to unanticipated movements or ratchet out of position.

Jointless bridges are performing well in service. Integral abutment and jointless bridges my cost more to construct but require less maintenance cost then equivalent bridges with expansion joints. Due to jointless support bearing are less affected by atmospheric agencies, which increase its life, reducing maintenance cost.

An attempt is made to reduce to construction joints by providing continuity to the bridge. But as pwer the government agency of India, Maximum Length of bridge after wich expansion joints is to be provided is 100m. For providing continuity at support tow alternatives are available, Cap cable or reinforcement steel.

The Dissertation includes parametric study of 80m and 100m, continuous bridges with various equal spans for suitability of Cap cable and reinforcement steel at support.

Analysis and design is carried out for 80m two span continuous bridge of equal span. The analysis is carried out according to constructions stages. The following stages are considered for analysis and design.

1. Girder is casted insitu: - 1st stage prestressing is done for simply supported girder self weight. 2. Diaphragm and slab are constructed insitu: - 2nd stage prestressing is carried out for simply supported slab and diaphragm load. 3. Support diaphragm is casted with cap cable or negative reinforcement steel. Continuity is provided only for live load and SIDL. Analysis is again carried out for continuous bridge, along with upward prestressed force as UDL. The stresses before continuity and after continuity should be within permissible limits. So girder is design for dead load only & SIDL and live load are not considered for design, Bacause after giving continuity BM due to self weight + slab + Diaphragm weight. 4. Continuity at support: - For Support hogging bending moment two alternatives are designed a) Cap Cable: - Two cables of 19T13 are provided at top. The cables are extended only upto Neutral axis of girder and at distance “d” from point of contra – flexure. End block is provided at neutral axis for prestressing. b) Reinforcement steel: - Reinforcement is provided at top of slab. 5. Construction of footpath, wearing coat and parapet 6. Prestressing of Cap Cable.

In this piece of work, an attempt is made to study the behavior of continuous as well as simply supported bridge. Cost of superstructure per meter is carried out by considering only concrete, reinforcement steel and prestressing steel. The graphical representation for BM envelop is developed to study the behavior. Other indirect benefits of continuous bridge not considered for costing are a) Depth of girder can be reduced due to continuity. b) If the bridge is situated in congested area, where cost of land for construction very high, saving can be achieved in approach length. c) Maintenance cost is less d) Cross-sectional area of pier can be reduced, as only single bearing is to be placed below diaphragm.