Sesmic Retrofitting of Bridges

Retrofitting of RCC bridges, in other words can be defined as strengthening of existing bridges in order to increase its seismicforce resistance level,so that it can successfully withstand any severe future earthquakes. There are two situations that necessitate retrofitting(I)seismically deficient existing bridges which have not yet experience deny earthquakes and have not meeting current code requirements (ii)bridges that are damaged in earthquakes. In the second case both repair and retrofitting is required. Seismic retro fitting is natively new concept in bridge engineering and was motivated by the damage sustained by highway bridges during the1971San Fernando earthquake.The earthquake clearly pointed out the existence of a number of deficiencies in the then-current bridge design specifications. It also focused on the fact that numerous existing bridges may be expected to failing some major way during the is remaining life if subjected to strong seismic loads. The extensive damage of bridges all over the world in recent earthquakes has been the motivation in significant advancement in the earthquake resistant design and retrofitting of bridges. The bridges constructed prior to 1970 were not designed for adequate seismic resistance as the ductility provisions were not incorporated in the seismic codes till then. As a result the bridges designed before this year lack in earthquake resistance and ductility and may be vulnerable to significant damage even from moderate earthquake. The post earthquake damage surveys in recent earthquakes have confirmed this view. It has also been revealed from damage surveys that many of the damages that occurred in bridges and flyovers could be prevented by proactive measures of seismic retrofitting prior to earthquakes. Many of the reinforced concrete piers designed earlier had inadequate shear capacity due to lack of transverse steel and confinement, inadequately lapped longitudinal steel, and premature termination of longitudinal steel. The superstructures were vulnerable to falling down in the absence of restraining devices; bearings were deficient in accommodating large seismic displacement and bearing seat was inadequate. These deficiencies had an adverse impact on the performance of bridges. An existing bridge can be replaced by a newly designed bridge to meet earthquake demands or upgraded in its strength by appropriate retrofitting measures.

Waghmare P. (2011):- have studiedTo increase concrete confinement by transverse fibre reinforcement, especially for circular cross-sectional columns, To increase shear strength by transverse fibre reinforcement, To increase flexural strength by longitudinal fibre reinforcement provided. They were anchored at critical sections. Transverse fibre should be wrapped all around the entire circumference of the members possessing close loops sufficiently overlapped or welded in order to increase concrete confinement and shear strength. This is how members with circular cross-section will get better confinement than member with rectangular cross-section. Where square or rectangular cross-sections are to be jacketed, circular/oval/ elliptical jackets are

by virtue of their shape to the splice region proving to be more effective. Kazuhiko Kawashima.(2000) have studied and proposed a new concept The traditional seismic coefficient method is being replaced with the ductility design method, and linear/nonlinear dynamic response analysis is used on routine basis in design of bridges with complex structural response.Verification of ductility evaluation of reinforced concrete/steel single/frame columns is being conducted in various regions. Verification for the effectiveness of steel jacket was conducted, and use of new composite materials such as the carbon fiber sheet is being studied. For preventing the flexure-shear failure at midheight, it is required to wrap CFS in not only horizontal direction but also vertical direction. Probably the effect of bilateral excitation for bridges may be considered in terms of difference of ductility capacity of piers. Benito Pacheco, Kazuhiko Kawashima, Romeo Estañero(2009) have studied anReinforced concrete columns& piers that were designed without taking into account the importance of plastic deformation and ductility are commonly deficient in flexural ductility, flexural strength, and/or shear strength under strong seismicexcitation.Seismic isolation reduces the force demands on the substructure andsuperstructure, but increases the displacement demands.Providing a jacket around an existing column that has insufficient ductility and strength capacity is effective to prevent premature failure. The jacket resists not only tension and compression but also shears of the column, as well as provides lateral confinement to the core concrete.Seismic isolation reduces the force demands on the substructure and superstructure, but increases the displacement demand.

III. METHODOLOGY

The bridges need retrofitting primarily because of two reasons (i) these were designed for smaller forces than that can occur, and (ii) these may be lacking in ductility in the absence of ductile detailing of reinforcement. The structural deficiencies in many of the existing bridges have been observed from seismic behavior in recent earthquakes. Several bridges constructed prior to existence of modern seismic codes fail to meet requirement of safety. Seismic retrofitting is required for protection of such bridges in future earthquakes. It is possible to retrofit many of the existing bridges against falling of spans and other distress by simple retrofitting measures. The key issues in retrofitting are: retrofit philosophy, seismic assessment, retrofit techniques, validating effectiveness of retrofit measures, application of composite materials and smart materials, in retrofitting. The bridge retrofit program is a necessity for the country and should be undertaken for vulnerable bridges that have not yet experienced earthquakes. The retrofitting of existing bridges has direct relevance towards mitigating disaster caused by earthquakes. The need of seismic retrofitting in existing bridges can arise due to any of the following reasons. ● Upgrading of seismic coefficients as a result of revised seismic zone

code ● Bridges not designed to seismic force ● Bridges damaged in earthquakes ● Deterioration and agine

1. Preliminary Screening The seismically deficient bridge sari identified by preliminary screening. The screening procedure is mainly based on (a) Seismicity (b) Vulnerability and(c) Importance. The prioritization of bridges to be retrofitted can be made on the basis of rating procedures based on above factors. 2. Detailed Seismic Assessment The detailed seismic evaluation of expected performance of existing bridges necessary to determine seismic capacity, weake r sections and mode of failure. The strength evaluation can be made according to codes. It is normally desired that assessment procedure should be more precise than code. 3. Selection and Design of Retrofit Measures On the basis of detailed seismic assessment it should be confirmed when the individual component level retrofit such a tending seating width, providing restrainer so global retrofit of complete bridge is to be undertaken. The global retrofit may include: jacketing of bridge piers, replacement of bearings and retrofit of foundations. 4. Checking of Design of Retro fitted structure Re-analysis of retro fitted structure will be required using dynamic method of analysis. The checking of design of retrofitted structure should be based on current design codes.

The observation of performance of bridges in past earthquakes world over has highlighted following deficiencies in bridges: Superstructure: Traditionally there is no linkage provided between two adjacent spans in the case of multi–span simply supported bridges as a result spans are dislodged bearing failures. The bearing failure occurs in fixed bearings when these are unable to withstand the seismic force generated in the superstructure. In many cases, spans fall down from their supports resulting into irrepairable damage. Superstructure deficiency is also associated with inadequate seat length at expansion joints on the supports or at the abutments resulting into unseating of span.

The traditional rocker and roller bearings have not shown satisfactory performance in earthquakes. There have been problems of jumping, inadequacy of bearings in accommodating displacements. Inadequate seat width on supports: The inadequacy of seat width on bearing supports or at expansion joints have caused unseating of span.

The various types of deficiencies observed in RC columns and piers are (i) Lack of flexural strength, (ii) Lack of shear strength (iii) Insufficient transverse reinforcement and confinement (iv) In adequate lap splicing of longitudinal steel (v) In adequate ductile detailing in plastic hinge region of columns (vi) Premature termination of longitudinal steel in piers (vii) Insufficient strength of joints between pile cap beams.

Superstructurefailsmainlyduetocollapseofspan.Thisoccursmainlyduetothe following reasons: Unseating of spanat the unrestrained movement joints. Pounding of adjacent segments Liquefaction of the ground Fault rupture under neath the bridges

Vertical jumping of the girders. Case study of Static analysis of bridge component part in IRC 70R Loading in STAAD PRO Software.

Rectangular column size: - 0.3 x 0.3 m. Rectangular beam size: - 0.3 x 0.3 m. Deck slab: - 0.2 m plate thickness.

Node displacement summery

It is observed that bridges, designed, detailed and constructed with appropriate seismic considerations can with stand any severe earthquakes ,although there may be some repairable damage .The bridges need retrofitting primarily because of two reasons that these were(i)designed for smaller forces than that can occur ,and(ii)lacking in ductility in the absence of ductile detailing of reinforcement.Several bridges constructed prior to existence of modern seismic codes fail to meet requirement of

such bridges in future occurring earthquakes. It is possible to retrofit many of the existing bridges against falling of spans and other distress by simple retrofitting measures .The key issue in retrofitting are ,retrofit philosophy ,seismic assessment ,retrofit techniques ,testing effectiveness of retrofit measures ,application of composite materials and smart materials,in retrofitting.The bridge retrofit program is necessary and should be undertaken for vulnerable bridges that have not yet experienced earthquakes.

BIS(1993).IS13920-1993:Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forcest,Bureau of Indian Standards, New Delhi. Colomb F., Tobbi H., Ferrier E. (2007). ―Seismic Retrofit of Reinforced Concrete Short Columns by CFRP Materials‖ GMT Construction, DITC, Elsevier. Floren Amy, Mohammadi Jamshid (2001). ―Performance-Based Design Approach in Seismic Analysis of Bridges‖,Journal Of Bridge Engineering37ASCE. HA (1996). BA 42/96: The Design ofIntegralBridges,Design Manual forRoads and Bridges, Volume1, Section 3, Part 12, HighwaysAgency, U.K. IRC(2000).IRC6-2000: Standard Specifications and Code of Practice for Road Bridges, SectionI ILoads and Stresses,Indian Roads Congress, NewDelhi.

Acharya Shrimannarayan Polytechnic,Pipri(M), Wardha –Maharashtra.

Assistant Professor, Civil Engineering Department, JSPM‘s ICOER, Wagholi, Pune, Maharashtra, India