Scope and purposes
Structure of study
An extended portion of this study was dedicated to the literature reappraisal. Though really the most interesting portion is the one which concerns to the consequences of the exposure of span of the instance survey, it is non admissible to pretermit all the general considerations based on an extensively research in proficient text, through which was possible to find the specific construction, the method and the stages of building, the type of analysis to be carried out and in which manner the consequences have to be read. Indeed close attending is needed in construing the consequences due to restrictions of the specific instance under consideration and at the same clip because of the demand to generalise the specific consequences for usage on a larger graduated table of undertakings.
In the “ literature reappraisal ” subdivision of the study a general survey about Bridgess has been developed. This has been done look intoing the history, the development, every bit good as the engineerings and tendencies of span design, with peculiar attending to recent developments in California. Construction techniques have besides been analyzed in more inside informations mentioning to many of import undertakings, based on international literature, scientific diaries and proficient books.
Then the “ temblor ” subdivision was covered.
In the subdivision about Codes of pattern an overview of the most of import facets has been made and few tabular arraies of interested have been attached to warrant the picks made in the instance survey.
Type of Analysis of old documents
The 2nd portion, elaborated with the support of the package SAP2000 BRIDGE Modeler, follows the development of the undertaking of a common span in California harmonizing to the local and currents codifications of pattern.
An extended literature reappraisal was conducted to understand which is the province of the research and the involvement of the codifications in assured public presentation behaviour of the Bridgess during stages of building.
Among all the technology scientific disciplines, span technology is one of the most complex because involves in itself a batch of subjects, from proficient to aesthetic, from environmental to societal, from economic to political facets. Without any uncertainty the proficient and the economic factors are those that strongly act upon applied scientist ‘s picks in planing a span, but they do non cover all the design procedure. Using Troitsky words “ Planning and planing Bridgess is portion art and portion via media ” ( Chen, Lian Duan, 1999 ) .
For understanding recent tendencies in span design and building it is of high importance to see the development that span technology has undergone during centuries. History illustrates that societal and economical alterations in a state have been reflected on span technology development. At different historical minutes, assorted types of Bridgess have been built for many intents with the new engineerings at each clip. From the nineteenth century, due to the large industrial growing, Bridgess have been built basically as portion of the transit system. During the twentieth century, span technology has been characterized by large alterations in the structural solutions and methods of buildings because of the diffusion of the strengthened concrete. The interior decorators had eventually a big possibility of picks between stuffs, methods of buildings and engineerings of analysis. This led to the existent scenario where multiple type of Bridgess are in operation foregrounding the broad creativeness of span applied scientists.
Planing a span, the chief of import parametric quantities to take in history in taking the best solution are: location, span, stuff, type of foundation, strategy of the span, type of superstructure, type of supports, method of building. Sing stuffs, nowdays timber is used merely for impermanent Bridgess, so for ordinary Bridgess the pick is between strengthened concrete and steel. Depending on the span of the construction one stuff is to be preferred to the other. For spans between 65ft and 330ft reinforced concrete gives the best via media, for spans greater than 330 ft steel is recommended. The span influences besides the system of the span: for little and average spans a beam span is normally used, for spans longer than 160 foots an arch system could be adviced and a suspended span can be the solution for really long span Bridgess. These are merely indicant, and every individual instance has to be evalued sing restrictions due to location, codifications, cost and typical span of the country. It is a good regulation of pattern to see typical undertakings recent designed in the country in which there is the intent of insert a new span construction, if the medium-span is the 1 of involvement. ( Chen, Lian Duan, 1999 ) .
Figura 1 – Comparative Bridge Costss in California January 2010. ( Caltrans Construction Statistics )
Figura 2 – 2009 Bridge Trends in California. ( Caltrans Construction Statistics )
4.2 Structural Types
This paragraph intends to supply a list of the several normally used types of Bridgess, foregrounding the differences for geometry, analysis, economic system. This list is non to be intended complete, as other type of Bridgess may be design, in which instance, specific deep surveies have to be done in order to analyze the peculiar solution.
Harmonizing to Raina it is possible to categorise Bridgess in six different types of superstructure sing the stuff which is made of.
Materials normally used for constructing the superstructure of lasting Bridgess are:
Mix of steel and reinforced concrete or mix of strengthened concrete and prestressed concrete
Particular superstructures with overseas telegrams
For each stuff there are possibilities of different sorts of subdivisions.
1. Reinforced Concrete Superstructures can be simple span or uninterrupted span ; balanced cantilever, arch or frame, tipically utilised for short span Bridgess. Some parts may be precast.
Solid Slab – Used for spans between 5 and 14 metres
Slab and girder ( T-beam Bridgess ) – Used for spans between 14 and 25 metres
Hollow box girder – Used for spans between 25 and 70 metres – Characterized by a high torsional opposition, is suited for curving waies.
2. Prestressed Concrete Superstructures can be simple, balanced or free cantilever, or uninterrupted span. It is possible to hold segmental dramatis personae in situ or precast solutions. Prestressed Concrete superstructures cover medium spans.
Hollow slab – Used for spans between 10 and 25 metres
Slab and girder ( Girder Bridgess )
Hollow box girder
3. Masonry superstructures have been associated with arch Bridgess in the past centuries.
This type of Bridgess is non of relevant importance for this survey, which is comparative to new Bridgess under building.
4. Steel Superstructure typically consists in a steel truss deck and screens long spans.
5. Composite superstructures can hold the undermentioned deck types:
Longitudinal home base girder and transverse beam girder with concrete slab
Longitudinal and cross beam girder with concrete slab
Longitudinal box girder with concrete slab
The deck agreement can be simple span or uninterrupted span, or arch and screens medium and long spans.
5. Particular superstructure
Cable stayed Bridgess
These constructions are of comparatively new construct and in last century have been used for long span solutions.
( V.K.Raina, 1994 ) .
Figura 3 – a ) Reinforced Concrete solid slab ; B ) Voided Slab
Figura 5 –
4.3 Methods of building
Figura 6 – Balanced CAntilever Segmental Construction
Conventional building: utilizing falsework
Cast in topographic point segmental building
Precast segmental building
Figura 7 – Precast beam method
Figura 8 – Full moon span precast method
Figura 9 – Span by span hard-on on falseworks
Figura 10 – Balanced cantilever method with establishing gauntry
Figura 11 – Span by span hard-on with establishing gauntry
Figura 12 – Balanced cantilever method with Cranes
Figura 13 – Balanced cantilever method with raising frames
Figura 14 – Form traveler method
Figura 15 – Incremental launchig method
4.4 New tendencies
Harmonizing to Caltran ‘s informations, in California the bulk of Bridgess have been built with the cast-in-place ( CIP ) technique ( See besides fig.x? ) . This method of building provides a good via media between span cost and seismal public presentation of the construction. Commonly CIP system requires large attempt in design and readying at the site ; frequently needs model to back up and long clip for projecting and completing. An ever-increasing petitions of new manner of transit has been registered due to the rapid growing of population and the high economic criterions, added to the ripening of substructures and the debut of new seismal design standards. The transit contrivers are seeking for new solutions that can speed up main road and span building in option to the traditional 1s. Caltrans applied scientists are looking at precasting to seek to accomplish the Accelerating Bridge Construction ( ABC ) ; they are developing research and surveies to understand the effectivity of precast solutions in cut downing building clip on site and holds in the decongestion of traffic. In the beginning, Caltrans was concerned about the behaviour of the precast constructions in high seismal zones, because there was non a long tradition in the temblor prone countries. Precasting has been normally considered holding excessively many points of connexion which are failings for seismal public presentation capablenesss. The University of California-San Diego conducted trials on the seismal behaviour of precast segmental span and the consequences showed that this sort of method gives desirable public presentations besides for high seismal zones. ( Aspire, Spring 2007 ) In the last 3 old ages few undertakings have been completed successfully in California with prefabricated elements, and even if they costed 30 % more than it they were built with traditional CIP method, the decreased time-on-site warrants a overall addition.
Figura 16 – Sections wait in storage behind the completed spans – Otay River Bridge, San Diego ( 2007 )
4.5 Economic analysis and Quantity tendencies
Figura 17 – Bridge Construction Index Trend in California. ( Caltrans Construction Statistics )
In the figure above is diagrammatically shown the tendency of the Bridge Construction Cost Index in California over the last 45 old ages. It is apparent how the index cost has been increasing over the decennaries, although sometimes decreases have been registered. This is perchance due to periods of arrested developments in the economic system of the State or due to betterments in assorted proficient facets of building and accordingly in the acceptance of new, cheaper and better designed solutions.
It is really hard to calculate the cost of span technology plants, as they are related to a batch of uncertainness parametric quantities like economic state of affairs, rising prices, local environment. By the manner an attempt in this sense has been done to hold some costs of
4.7 Seismic Codes
WHICH SEISMIC CODE? ? ? ? ?
AASHTO Standards Specifications give commissariats of lower limit demands for conventional Bridgess with span non transcending 300ft. There are some general indicants about cable-stayed Bridgess, suspended Bridgess, arch Bridgess but these are non to the full covered.
The cardinal rules on which the Standards are based on are the undermentioned points:
Structural public presentation should be ensured without important harm ( elastic scope ) under little and moderate temblors
Design has to be carried out using realistic land gesture accelerations
Large temblor should non do prostration
The Codes are valid for all United States and the seismal hazard varies a batch through the Country. Reading the acceleration contours on the jeopardy map ( fig.20? ) the acceleration coefficient A, comparative to the topographic point of involvement, is determined spliting by 100 the value read ; the map is based on a return period of 475 old ages for events with 10 % chance of exceedence in 50 old ages. Four Seismic Performance Categories ( SPC ) from Angstrom to D ( AASHTO SDC ) or from 1 to 4 ( AASHTO LRFD ) are defined ( fig.20? ) on the base of the Acceleration Coefficient ( A ) and two Importance Classification ( IC ) parametric quantities categorize every span as Essential ( I ) or Critical ( II ) sing its importance ( fig. 22? ) . To take into history the dirt conditions a Site Coefficient ( S ) has to be used in the design procedure for all type of foundation for approximates the consequence of the dirt alteration on the structural behaviour ( fig.22? ) . S coefficient varies from 1 to 2 for dirts from stone to soft clays or silts. For those instances in which is non possible to find the word picture of the site, a dirt Type II with S=1.2 should be assigned. With this specifications the elastic seismal response coefficient Csm has to be calculate for the Tm period of quiver of the construction that corresponds to the mth manner. Csm is defined by the Code ( 188.8.131.52 AASHTO LRFD ) as a map of A, S and Tm. Seismic design forces for infrastructure and connexions have to be determined spliting the elastic forces by the appropriate Response Modification Factor given for each Importance Category of span. To see the variableness of the waies in which the temblor may happen, two combinations of extraneous seismal forces have to be apply to the superstructure of the span with the proportion of 100 % and 30 % in each way. The Code ( AASHTO LRFD ) defines a perpendicular support as a column if the ratio of the clear high to the maximal dimension of the cross subdivision is non less than 2.5. If the ratio is less than 2.5, the demands for wharfs have to be satisfied. A wharf can be analysed as a wharf in its strong way and as a column in the weak one.
Figura 20 – Seismic public presentation zones ( Table 3.10.4-1, AASHTO LRFD )
Caltrans estimates infrastructure ‘s capacity through non additive “ push-over ” analysis.
Figura 21 – Seismic public presentation zones ( Table 3.10.4-1, AASHTO LRFD )
Figura 22 – Bridge importane classs ( AASHTO LRFD )
Figura 23 – Dirt profile ( AASHTO LRFD )
Figura 24 – Site coefficient ( AASHTO LRFD )
4.8 Analysis Method
The aim of seismal design is to specify forces which constructions are capable to, cipher elastic and inelastic distortions, study the malleable behaviour of the construction and verify the ability of the individual elements to defy. Different methods of analysis were found in the literature reappraisal with different premises and for different Scopess. Briefly it will be given a list of the methods covered by the codifications.
ESA – Equivalent Inactive Analysis
EDA – Elastic Dynamic Analysis
ISA – Inelastic Inactive Analysis
Structural System Global Analysis
Stand-Alone “ Local ” Analysis
Cross Stand-Alone Analysis
Longitudinal Stand-Alone analysis
ESA – Equivalent Inactive Analysis – can be used to gauge the supplanting demands of those construction with unvarying stiffness, reacting by the prevailing manner of quiver, where dynamic analysis would non give significantly different consequences. ( Cetinkaya, Nakamura, Takahashi,2005 )
EDA – Elastic Dynamic Analysis – can be used to gauge the supplanting demands of those constructions where ESA does non give a good response of the dynamic behaviour. The consequence of using design spectral acceleration likely consequences in emphasiss transcending the additive scope. This is due to the part of the dirt, giving in the structural elements, enlargement of the articulations.
Harmonizing to recent Caltrans specifications ( Caltrans SDC ( v.1.5 ) ) an Ordinary Bridge can be analyzed either with Equivalent Static Method or Linear Elastic Dynamic Method for gauging the supplanting demands. For set up the displacement capacity of the elements a nonlinear analysis is required to take into history the ductileness of the construction.
The instance survey has been chosen after measuring the tendencies in span building in California in the last 10 old ages. The tendency shows that the bulk of Bridgess designed in recent yesteryear are of medium span, built with the dramatis personae in topographic point technique. A really high per centum of these are prestressed box girder built by sections with the balanced cantilever hard-on method. Since the involvement of this survey is aimed at foretelling the likely behaviour of the Bridgess that will be likely built in the following hereafter in California, the pick was made in that way. Precast! ! ! ! ! New Tendencies
The chief aim of this survey is to analyze the exposure of the span under the seismal burden if the temblor is traveling to happen during the building phases. Before making this, nevertheless, is cardinal to be certain that the span ( instance of survey ) , after work completion and during life clip under unrecorded tonss, will react fulfilling all safety standards required by the current local codifications.
We are speaking about little temblors if the public presentation behaviour of the construction has to remain in the elastic scope, because this is a rule that governs the span design harmonizing to the codifications of pattern.
5.1 Structural System
The solution adopted consists in a prestressed dramatis personae on site concrete box girder constructed with the segmental balanced cantilever method, with a assorted system of prestressing overseas telegrams in post-tension.
5.2 Phases of Construction
One of the most important facet of the span under consideration, that affects the all the design stages, is the definition of the span behaviour during building in the consecutive patterned advance of structural constellations. Therefore, issues associating to structural analysis during building and those due to the actions in the concluding stage, have been examined in deepness.
The mold has been conducted
The capacity demand of perpendicular supports is a complex map of a batch of variables including:
Land gesture feature
Required design degree
Time period of quiver of the construction
Elastic damping coefficient
Soil status and foundation type
The geology of the land and the morphology of the site play a cardinal function in the design phase and regulate the picks to be made in footings of foundations, maximal span length and building type.
A subsurface probe in the locality of hemorrhoids and abutments is necessary to place a suited foundation type. The economic facet is besides affected because, depending on the geological state of affairs, in instance of high hazard of liquefaction or incline stableness the cost of foundations can change greatly and make really high costs in proportion to the entire cost of the construction. In the instance survey taken into consideration, there was no possibility to find the needed type of the foundations due to the deficiency in cognition of the geology of the dirt.
The executable foundation options that could be proposed are many and, depending on the peculiar one chosen, they could significantly alter the behavior of the construction under seismal burden. It was hence preferable to go forth the type of foundations as vague and analyze the behavior of the superstructure ( wharfs and abutments ) by delegating a good grade of restraint at the base of the infrastructure. Basically in the analysis theoretical account the base of the infrastructure has been to the full restrained pretermiting the possibility of distortion. This determination is based on the premise that a to the full fixed restraint ( as the one simulated/assigned in the analysis theoretical account ) will ne’er happen in world, irrespective the type of foundation chosen ; this means that the reactions at the base obtained from the theoretical account will be higher than the 1s expected in world, guarantying a conservative attack of the survey. On the other manus the range of this survey is non the full design of the span, but the fluctuation of the seismal exposure during the building procedure, therefore the old premises, changeless during the survey of all building phases, do non impact the concluding consequences. This means that doing certain that the restrictions deducing from these premises are taken into consideration, it has been possible to construe the consequences suitably as described in the undermentioned paragraphs.
5.4 Analysis process
The undertaking of the span should be executable in the close hereafter at the location indicated and hence the completed construction has to be able to defy to all the tonss defined by the codifications, including seismal tonss. Therefore, the first stage of this survey involved the design of the span subdivisions by analysing bending, shear and tortuosity in superstructure and infrastructure.
Normally, while carry oning a push-over analysis, the construction is pushed till prostration is reached. This means that the construction would be allowed to go through the additive province and undergo to the non-linear 1 with the progressive formation of plastic flexible joints. Plastic flexible joints are a illustration of checking and initial amendss in the construction. Wherever they occur, is non admissible to hold clefts in the construction during building of the span. For this ground the capacity of the wharfs to defy to seismic tonss during building has been evaluated through “ additive ” pushover analysis. Therefore the construction behaviour has ever been considered additive and when plastic flexible joints started happening, which means that the construction was dispersing energy while checking, the pushover has been stopped because the wharfs were considered damaged.
Since the construction was reproduced with a three-dimensional theoretical account, two different pushover analysis were carried on: one in the longitudinal way and one in the cross way. The two chosen waies correspond to the first and the 2nd manner of quiver of the span. Longitudinal pushover analysis was performed using a easy increasing seismal burden on the superstructure of the span in the same way of the span, and transverse pushover analysis likewise in the perpendicular way.
Table OF MODES OF VIBRATION AND PERIODS FROM SAP
6.1 Summary of pushover analysis consequences
6.2 Discussion of pushover consequences
The push-over graphs show
The consequence of push-over way
6.3 Relative exposure relationships
Summary and Conclusion
The treatment in this survey was carried out looking at a individual span and doing some simplifications ( premises of fastness supports, two extraneous waies of seismal forces, no admissible harm, additive behaviour ) to restrict the figure of variables that could impact the analysis. The old push-over consequences refer to different theoretical accounts for the different building stages of the same span. It would be risky coming to the decision that these consequences are representative of all span behaviours. Surely this survey can be considered a good starting point for farther probe to be carried out on Bridgess with different features for comparison and group them in categories with linear behaviour to make a generals decisions about seismal exposure during building.