COMMISSION 6: PREFABRICATION
TASK GROUP 6.3. PRECAST BRIDGES
STATE OF THE ART REPORT
PRECAST BRIDGES
COMMISSION 6: PREFABRICATION
TASK GROUP 6.3. PRECAST BRIDGES
STATE OF THE ART REPORT: PRECAST BRIDGES
INDEX:1. SCOPE
2. DEFINITIONS
3. HISTORY. PAST EXPERIENCES
4. TYPOLOGY. PRESENT TECHNOLOGY. 4.1. ELEMENTS
4.2. ISOSTATIC BRIDGES
4.3. PARTIAL CONTINUITY BRIDGES 4.4. STRUCTURAL CONTINUITY BRIDGES 4.5. INTEGRAL BRIDGES
4.6. SEGMENTAL BRIDGES 4.7. SPECIAL BRIDGES
4.8. CABLE STAYED BRIDGES 5. AESTHETICS 6. DESIGN 6.1. SPECIFIC ASPECTS 6.2. DETAILS 6.3. DURABILITY 6.4. SEISMIC ASPECTS 7. EXECUTION 7.1. PRODUCTION PROCESS 7.2. QUALITY CONTROL
7.3. TRANSPORT AND ERECTION 7.4. SITE WORK
7.5. TESTING
7.6. MAINTENANCE, INSPECTION AND ASSESMENT 8. BIBLIOGRAPHY AND PICTURE CREDITS
1. Scope
The intention of this paper is to provide a description of the present status of development of solutions for precast concrete bridges, of which there is now an astounding variety.
The document also attempts to focus attention on other areas where specific research is needed to clarify real or fictitious problems arising in the development of such structures.
The development of precast concrete bridges actually has rather a long history, with the first designs dating from practically the initiation of prefabrication itself. Solutions dating from the thirties may be found in most developed countries, although admittedly for short span bridges and generally restricted to small works.
It was not until the fifties that these structural solutions began to develop more intensely. The vigorous development that took place from that time on was largely driven by advances in the ways and means available for transport and erection the precast members as well as by progress in research on prestressing with steel wires and strands.
While in some countries the Ministries of Transport or equivalent bodies draft codes on standard series of prefabricated bridges, generally limiting such regulations to the transversal sections and leaving manufacturers free to determine tendon placement and dimensioning, most countries have not proceeded in this manner, allowing precast concrete manufacturers to develop their own solutions. At present the prevailing approach is to refrain from regulating these members, leaving the development of new types of solutions to manufacturers' imagination and research.
Development has been continuous, speedy and quite spectacular since the fifties. The three basic reasons for the success of prefabricated bridges are as follows:
- Speedy construction
- High cost-competitiveness compared to cast-in-situ solutions
- Minimum disturbance of traffic in the event of bridges to be built over railways or roads. The primarily beam - solutions initially developed for these structural members were fiercely criticised because of their obvious aesthetic limitations, the inherent difficulties in reasonably adapting them to curved bridges and the problems ensuing from the large number of joints, which caused user discomfort as well as maintenance problems.
Other criticism levelled at prefabricated bridges at the time was less justified and falls within the usual repertoire of objections to prefabricated structures. This criticism hinged essentially on two issues:
- The idea that prefabricated structures are monotonous and ugly.
- Although not generally expressed clearly and explicitly, rather a large number of designers offered considerable resistance to prefabrication, and in particular to prefabricated bridges, because they saw this type of industry to be competition that would reduce the demand for the services of engineering firms engaging in design.
This latter objection, veiled and unexpressed, has been latent in numerous countries for a good many years.
Certain specific technical issues were also referred, such as the initial doubts that arose with regard to the performance of prestressing tendon transfer length, particularly in the case of tendons with large diameters subjected to fatigue stress - typically in the case of railroad bridges - or about the effectiveness of the shear strength at the interface between the cast-in-situ concrete and the prefabricated members, also especially in the event of fatigue stress.
The prefabricating industry's response to these problems has been speedy, spectacular and conclusive.
It has responded to the accusation that its structures are monotonous and ugly by creating an extraordinarily wide variety of forms, often with greater freedom than in cast-in-situ bridges, developing members with variable depths, solving the problem of curved plan bridges and providing for hyperstatic continuity which not only confers structural advantages but eliminates the need for joints, thereby precluding maintenance and discomfort issues.
Surfaces, edges and surface textures that are impossible to achieve via cast-in-situ execution are within reach in prefabricated construction. The visual appearance of the concrete in precast members, moreover, is more apt to be of a high quality than in cast-in-situ solutions.
The veiled criticism based on the fear of losing design work has been eradicated by the power of reality that always underlies practical evidence.
Moreover, the development of cranes, launching cradles and assembly methods in general, along with the increased length and loading capacity of transport vehicles, have enhanced prefabrication potential enormously, both from the technical (longer spans) and financial standpoints. All this has led to the existence of prefabricated structures that can readily bridge spans of 100 m; and, as discussed below, in the light of the combination of these advantages and other modern techniques, there is no appreciable difference between the potential of precast and cast-in-situ concrete. Indeed, solutions involving a precast deck, generally comprising large members, and a stayed bridge system, used not only on the grounds of its structural effectiveness but as an evolutive system, have eliminated the specific span limitations affecting prefabrication and increased its competitiveness enormously.
The truly beneficial advances in recent years in transversal deck constitution using strut bracing and precast members in general have made it possible to build very wide decks with lightweight as well as highly aesthetic monotube beams.
All the forgoing has substantially enhanced the three essential advantages listed at the beginning of this section.
The prefabrication industry has, in a period of barely 50 years, proved able to respond to important challenges, solving many problems and creating technology that has led to solutions that are:
- of greater technological excellence - more cost-competitive
- of higher aesthetic quality - faster to build.
2. Definitions
Bridge: Civil engineering construction works mainly intended to carry loads related to
comunication over a natural obstacle or a communication line. This includes all types of bridges, especially road bridges, footbridges, railway bridges, etc.
Abutment: Any end support of a bridge usually without rigid continuity with the deck.
Rigid abutments and flexible abutments should be distinguished where relevant.
Pier: Intermediate support of a bridge, situated under the deck.
Bearing: Structural device located between the deck and an abutment or pier of the bridge
and transferring loads from the deck to the abutment or pier.
Prestress: Permanent effect due to controlled forces and/or controlled deformations
imposed on a structure. Various types of prestress shall be distinguished from each other as relevant (for example prestress by tendons, prestress by imposed deformation at supports).
Headroom: Free height available for traffic.
Continuous bridge: Bridge with no expansion joints between adjacent intermediate
spans, with or without structural continuity
Integral bridge: Bridge with no expansions joints - neither between adjacent intermediate
spans nor between end spans and abutments
Diaphragm: Transverse deck stiffening beam of insitu or precast concrete construction Crosshead: Transverse support beam at an intermediate deck support
Sagging moment: Bending moment inducing tension in the bottom fibres (positive
moment)
Hogging moment: Bending moment inducing tension in the top fibres (negative moment)
Internal bonded tendon: Post-tensioning tendon contained within a ribbed ducting which
3. History. Past Experiences.
3.1. Germany
The first prestressed concrete bridge built with prefabricated beams was executed in Germany in 1938 by WAYSS & FREYTAG. The underpass at Oelde in Westfalen crossed the Autobahn without intermediate piers. It was realised only two years after the beginning of the construction of prestressed concrete bridges. The length of the I-shaped concrete beams was 33 M. They were cast on site in a bed and then moved into their final position.
1938: First prestressed concrete bridge „Underpass at Oelde in Westfalen„ (WAYSS & FREYTAG)
According to Freyssinet the beams were cast in two phases: The lower flange was cast first and then posttensioned, then the web together with the upper flange was cast. The hardening process was accelerated by steam curing. [3.1]
Nearly two decades later the industrialised production of prestressed precast beams for bridges started in West-Germany, initiated by the Federal Railway Administration. The need for shorter erection times and for the least hindering of traffic led to the design of the first standardised railway-crossings.
Two main production methods were developed for the railway-bridges:
HOCHTIEF used prestressed precast I-beams, weight up to 8,0 tons. After positioning cross posttensioning cables in the upper flange and in the ribs an orthotropic slab was created. The joints were cast in situ (wet-joints).
DYCHERHOFF & WIDMANN used a „contact“ system. The precast elements were box girders with smooth side walls. The cross posttensioning cables in the lower and in the upper flange realised by pure contact the orthotropic system (dry-joints).
In East-Germany several standardised precast beams, mainly hollow-box-sections, were developed for railway and road-bridges, all were posttensioned in the transverse direction.
Later elements with sufficient torsion stiffness were used and transverse posttensioning could be omitted.
The advantage of the high degree of shop precasting brought some disadvantages in the construction of road-bridges:
* inaccuracies could be corrected only within the screeding, which was unsatisfactory for the demanded high comfort required for the road surface,
* the isolation-layer could not always be placed accurately, *many joints produced problems for maintenance (icing), *continuos bridges could not be constructed.
In the 1960’s precast bridges without transverse posttensioning were designed as so-called „mixed systems“, e.g. precast beams with structural in-situ concrete on top of the beams and in-situ concrete for cross beams at support. This method allowed for tolerances, no transverse posttensioning was required and the orthotropic behaviour was easily achieved. Several variants of these „mixed systems“ developped:
*beams placed close to each other. The in-situ concrete slab had no joints. (Classical mixed system).
* precast beams placed with gaps in between and in-situ concrete slab without joints. * precast beams placed with smal gaps and in-situ parts in between, which take bending moments.
In 1979 guidelines for design and execution of bridges with precast beams were published [3.2]. The guidelines include design rules, examples and details specific for precast bridges. These recommendations are valid still to-day and only some minor amendments had to be made due to the progress of the state of the art.
The latest developments in the construction of precast bridges include continuous bridges which are prestressed for transportation and after placing continuously posttensioned. Up to 200 m length (up to 7 spans) were continuously posttensioned.
4. Typology. Present Technology
4.1. Elements
4.1.1 - Decks having precast beams as main resistent elements.
Some types of precast beams used in bridges are sketched in fig. 4.1: - square beams (fig.4.1 a)
- I beams (fig. 4.1 b, c, ) - T beams (fig. 4.1 d)
- inverted T beams (fig. 4.1 e, 4.1f) - U beams, V beams (fig. 4.1 g, h)
a) b) c) d)
e) f) g) h)
Cast in situ concrete
The following sub types of decks result:
Precast slab (or formwork) Cast in situ slab
Figure 4.1.2 Bridge Decks. Precast beams, completed by a cast in place concrete topping
Precast beam In–situ slab Precast beam In–situ slab a) b)
Figure 4.1.4 One or more U beams, completed by cast in place topping
cross section view
connections
cast in-situ slab
precast beam
Fig. 4.1.6. segmental beams, completed by cast in place topping
Precast elements Transverse reinforcement Transverse reinforcement Precast elements a) b)
Fig 4.1.8. Infilled precast beams
4.1.2. - Solid slabs
Decks formed by precast slabs on the entire span, having longitudinal shear keys, completed by cast on site topping or postensioned transversally. Solid slabs are usually used on minor spans.
Figure 4.1.11 – Partially precast slabs
4.1.3. - Precast piers and crosshead.
4.1.3.1. Piers and crossheads in portal frame - Solid crosshead
- Hollow crosshead
- Crosshead formed by two walls
4.1.3.2. Single pier
1. Box beam with single support - Prismatic
- Prism trunk - With column head 2. Box beam with double support
- Hammer - Palm or/and Y - Solid prismatic block - Frame
Figure 4.1.13 Precast single pier and box beams in Spain.
3. With hollow crosshead, for beam double T, launder or caisson - Single pier
Figure 4.1.14 Precast single Pier for I beams in Spain. 4.1.3.3. Pile pier, raymond piles
Cast on site foundation
Figure 4.1.15 – Precast piers and crosshead
4.1.4. - Precast abutments
In some cases precast elements are used to form bridge abutments. The elements are full height, modular width, and usually have, on the buried side, one or more webs from the top to
tie can be used to form a truss structure. The elements are placed on site near each other and are completed by cast on site foundations and a top beam.
Cast in situ footing Cast in situ top beam Precast abutment
Figure 4.1.16 – Precast abutments
Figure 4.1.17. Precast Abutment and wall in Spain.
4.1.5. - Precast arches or vaults
The vaults are of curved or poligonal guideline structures of different types:
Domed structures with more than two elements. These have an arch supported on two curved side walls that are fixed to a raft foundation or directly supported on the ground with precast strip footings incorporated to the precast piece.
Domed structures with two elements which are composed of two curved lateral walls. Domed structures with one element, with the bottom slab built into the precast or in-situ element.
III.- Domed structure with two elements, commonly known as triarticulated. The
connection to the foundations is a simple articulation; the foundations can be a bedplate or a footing .
IV.- Domed structure of an element which can be fixed to the foundations or biarticulated. The different elements are usually of reinforced concrete, with rectangular section or ribbed section. The aspect from inside the passage is usually plain, although there may be solutions in which the lateral walls are ribbed inside (particularly in the second type of
structure with large clearances). The arches generally have a rectangular section, although for large spans (L>10 metres) are usually ribbed. In the case of vaults with two elements, the support points may be raised with some walls to give more clearance.
Multi-arch passages may be achieved by making two vaults share the same foot or wall
Continuous cast in place concrete footing
Precast concrete Arch elements Precast collar wall
Wing wall Wing wall
4.2. Isostatic bridges
This chapter is mainly concerned with bridge decks constructed using factory made prestressed concrete bridge beams and designed isostatically with simple supports.
The beams are generally used as
follows:-a) Beam and slab decks – prestressed beams positioned at centres and connected with a cast in-situ reinforced concrete deck slab.
b) Solid infill decks – prestressed beams positioned contiguously and encased in cast in-situ concrete such that a solid deck is formed. Transverse reinforcement is positioned through holes in the webs of the prestressed beams. The in-situ concrete generally covers the tops of the beams by approximately 75mm allowing the positioning of a top mesh cover down.
Beam and slab decks are popular in the span range of 20 – 40m and solid infill decks are popular in the smaller span range of less than 20m.
From the early days of prestressed bridge beam manufacture it was considered logical to design bridge decks as simply supported and with end joints. Joints were incorporated between intermediate spans and between end spans and abutments.
Beams would be erected onto individual bearings – one at each beam end – and the joints would be dimensioned to allow thermal movement of the concrete within the bridge decks.
Isostatic bridge design also suited the nature of prestressed concrete bridge beams in that deformities due to creep , shrinkage and temperature could occur in relative isolation. In the same way, differential settlement of deck supports was easily accommodated.
Many thousands of bridges have been built as described, and the bridge beams have proved highly durable.
The main reasons for the high durability are as follows:
- The consistent use of high strength, low water/cement ratio concrete.
- The prestress design providing an absence of cracking under working loads.
- An almost guaranteed provision of specified concrete cover to secondary reinforcement (links).
Although the beams themselves have proved highly successful, there are disadvantages inherent in simply supported deck design.
Bearings, as previously stated, are generally required at each end of each beam. They are costly and eventually have to be replaced.
However, the main problem with simply supported deck design is that of the joints. This problem became apparent following the practice of using de-icing salt on the road/bridge surfacing in winter. The salt solution was then able to penetrate the joints in the bridge decks and corrode the bearings and the concrete support structure.
Good detailing at the tops of piers and abutments can help delay the onset of this corrosion. The following are examples:
However, it was realised that the best solution to de-icing salt penetration would be the elimination of all joints within the bridge deck.
The elimination of joints between intermediate decks would require continuous bridges. The elimination of joints between the intermediate decks and also between end spans and abutments would require integral bridges.
Fig 4.2.2 – Access gallery 18 0 0 10 00
4.3. Partial continuity bridges
Partial continuity is a method of providing continuity between intermediate bridge decks such that no distribution of vertical load effects between the intermediate bridge decks occurs. This applies to all vertical loads – dead, superimposed dead and live.
Two methods of providing partial continuity in beam and slab decks will be described. A third method, providing ‘notional’ continuity is also described.
Typical features:
1. Separate bearings and diaphragms are provided for each span
2. Deck slab is separated from support beams for a short length to provide rotational flexibility
3. There is no continuity reinforcement between ends of beams and there is no moment continuity between spans
Figure 4.3.1 – Partial Continuity detail type 1 – Continuous separate slabs
This continuity detail confines itself to the deck slab only, which flexes to accommodate the rotations of the simply supported deck beams. The beams are erected in the conventional manner onto individual bearings.
To permit this flexure, the deck slab is separated from the support beams for a length of about 1.5m by a layer of compressible material.
Typical features:
1. The tie reinforcement at mid-depth of the slab is debonded for a short length either side of the joint to permit deck rotation. There is no moment continuity between spans.
2. Slabs between spans are separated using compressible joint fillers but deck waterproofing and deck surfacing are continuous and special seals are provided over the joint for double protection.
3. Separate bearings and end diaphragms are provided for each span.
Figure 4.3.2 – Continuity detail type 2 – Tied deck slab
The bridge decks are designed and constructed in the conventional multispan simply supported manner with slab trimmer diaphragms at the beams ends. As with type 1, the beam ends are carried on two parallel rows of bearings on the piers.
Long connecting reinforcement dowels are incorporated at the slab mid-depth to tie the slabs together over the pier, eliminating expansion movement at deck level and permitting the use of a buried deck rotation joint. To accommodate this rotation, the dowels are debonded and sleeved from the surrounding slab concrete over short lengths either side of the joint. Also, the slab and trimmer beam downstands are “necked” using compressible joint filler below and above the dowel connection.
Types 1 and 2 continuity are the logical extension to providing simply supported decks with continuity. They offer the minimum of extra design and construction effort.
4.4. Structural continuity bridges
Typical features:
1. Beams are erected on temporary supports generally off pier foundations 2. Permanent bearings are in single line
3. Continuity reinforcement is provided in the slab and at the top and bottom of bridge beams. The lapping of reinforcement is normally not difficult.
Figure 4.4.1 – Continuity detail type 3 – Wide in situ integral crosshead
This type of continuity detail uses prestressed beams significantly shorter than the spans between support piers. The beams are usually supported on temporary trestles built off the pier foundations. The wide in-situ integral crosshead over the pier is then cast between and around the beams to provide about 1m embedment. Longitudinal continuity is accomplished by reinforcement within the continuous composite deck slab, generally supplemented by reinforcement, and pretensioning strand extending from the top and bottom of the embedded beams. Transverse strength is provided by either prestressing tendons or reinforcement, some of which may pass through holes in the ends of the precast beams. The crosshead is
supported on a single row of bearings set centrally on the pier.
Although more complex to design and more expensive to construct than any of the other methods, type 3 continuity offers more advantages.
Plan curvature can be readily accommodated by varying the width of the integral
crosshead to form a plan trapezium shape. This permits the use of a standard length beam per span.
Vertical curvature problems can be reduced by vertically curving the top and bottom surfaces of the crosshead. This reduces the increased slab thickness at midspan required to take up the vertical curvature above the straight chorded precast beams.
A single central row of bearings is required. This immediately halves the number of bearings required for simply supported construction, although individual bearing size will increase.
Piers are thinner, not only because a single line of bearings takes up less room at the pier top, but because the dead and live load moments applied to the piers by off centre pairs of bearings are removed.
Full width piers are not required. The integral crosshead can be designed to allow
considerable deck cantilevering outside the pier. This also provides a further reduction in the number of bearings.
Typical features:
1. Temporary supports are not required
2. Permanent bearings may be in single or twin line
3. Continuity reinforcement is provided in the slab and at the bottom of bridge beams The lapping of reinforcement is difficult.
Figure 4.4.2 – Continuity detail type 4 – Narrow in situ integral crosshead
The prestressed beams are long enough to be erected onto two parallel rows of temporary or permanent bearings on the pier tops. The in-situ integral crosshead over the pier is then cast between and around the beams to provide about 1m embedment. The crosshead is, however, narrower than type 1 because of the small gap between the beams. This same narrow gap makes adequate bottom flange reinforcement connection difficult between beams.
Longitudinal hogging bending continuity is again readily established by top reinforcement within and extending well into the continuous composite deck slab. Transverse strength of the crosshead is generally provided for by reinforcement, some of which passes through holes in the ends of the beams.
Where twin rows of temporary bearings are used, a central row of permanent bearings located under the crosshead is brought into use by removing the temporary bearings after the crosshead concrete has gained sufficient strength. Some examples use a wide single
permanent rubber bearing which acts as a seating for both beams.
This type of continuity is relatively easier to construct than type 3 but cannot offer the advantages of extra span or curvature.
The greatest advantage lies in the ease of beam erection directly onto the pier bearings. Nevertheless, adequate connection between bottom flange reinforcement is difficult.
Typical features:
1. Beams are supported on stage 1 crosshead during erection. 2. Crosshead to be monolithic with pier
3. Crosshead soffit is normally lower than beam soffit
4. Reinforcement is similar to types 1 and 2 depending on the cross-section of the stage 1 crosshead.
Figure 4.4.3 – Continuity detail type 5 – Integral crosshead cast in two stages
This type of continuity detail is a variant of types 3 and 4 where the integral crosshead is cast in two stages. The crosshead is of greater depth than the main deck beams and the bottom section is cast first to support these beams, generally on thin mortar beds.
The second stage cast completes the integral crosshead in the manner described for type 3. The advantage of this type of continuity is the complete elimination of bearings.
The disadvantage is that the downstand half of the crosshead is obstructive, both aesthetically and in terms of headroom.
Type 5 continuity is also described as framed construction in that the piers are monolithic with the deck and consequently contribute with the moment distribution.
Figure 4.4.4 Continuous precast rectangular beams bridge in Germany.
Figure 4.4.5 Detail of Continuity of precast rectangular beams bridge in Germany.
Figure 4.4.7. Continuous precast rectangular beams bridge in Germany.
Figure 4.4.9. Continuous precast I beams bridge in France.
Figure 4.4.12 Continuous precast Box beams bridge and Piers in Spain.
4.5. Integral bridges
Integral bridges are designed with no expansion joints – neither between adjacent intermediate spans nor between end spans and abutments.
Chapters 5 and 6 have described various methods of deck continuity – whether partial or structural.
This chapter describes the various types of integral abutments and associated design issues. In the same way that bridge decks can be continuous over intermediate supports but still require bearings, it is possible for there to be no expansion joints at abutments and bearings still be provided. This type of construction is referred to as semi-integral and is particularly suited to prestressed beam bridges in that the bearings eliminate the problems associated with moment continuity and rotation due to creep and thermal effects.
Abutments of integral bridges are attached to the bridge, and so have to move horizontally in response to temperature fluctuations in the bridge. The abutments must be designed to allow this movement to occur, at the same time as being able to resist longitudinal traffic loads. The design of integral abutments involves different considerations from the design of conventional fixed abutments. Similarly, the design of integral abutments for bridges built using prestressed concrete bridge beams also requires special considerations which do not arise in integral bridges using other forms of construction.
Various types of integral and semi-integral abutments are shown in Figure 4.5.1
Some countries recommend a limit to the overall length and skew of bridges designed integrally. (U.K. – maximum overall length = 60m, maximum skew = 30°).
In America, integral bridges have been designed and constructed successfully to overall lengths in excess of 200m.
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4.6. Segmental bridges
A segmental deck is a structure formed by a number of precast segments having a length of the same order of magnitude as the depth of the deck, connected together by joints transverse to the direction of the span (fig. 4.6.1). The performance of the joints between elements influences the way the structure works, there being discontinuity in the passive reinforcement.
Figure 4.6.1 – Precast segment
4.6.1 Joints
The joints between segments may be provided in three ways:
Mortar joints: a mortar joint is provided with a width of several centimetres.
Glued joints: joints where, before closure, a layer of epoxy or other synthetic resin is applied on the surface.
Dry joints: joints where there is no material between the segments in contact. Such type of joint is not covered by this Standard.
There are some conditioning factors related to the use of each type of joint:
a) In case of mortar joints, the postensioning cannot be applied until the mortar has reached sufficient strength;
b) Glued joints require that adjacent concrete surface match.This is generally achieved by using as mould the surface of the adjacent segment (method of match casting).
4.6.2 Keys
The joint between segments has to be capable to transmit forces parallel to its plan: shear and torsion.
In order to increase the capacity of load transmission salient shear keys can be provided. The keys may be of a large size in small quantities or multiple and of a small size (the latter is preferred).
The single keys are designed as short brackets in order to transmit the total shear, while the multiple keys are not reinforced and the shear transmission capacity is checked using an enhanced coefficient of friction.
4.7. Special bridges
Besides normal precast bridges with beams, there are also other possibilities for bridges, like arch, external postensioning or the use of struts for long span bridges.
Figure 4.7.1 Precast Arch bridge in Spain.
Figure 4.7.3 Precast bridge with side cantilevers and single pier in Spain.
Figure 4.7.5 Precast bridge with external postensioning.
Figure 4.7.7 Precast variable depth elements bridge
4.8. Cable stayed bridges
It is possible to use stays to reach longer spans in the construction of precast bridges. Spans up to 400 metres can be achieved by building with precast decks in cable stayed bridges.
Precast decks can be designed for two planes of stays with a box girder under each plane of stays. It is also possible to design a deck with a single plane of stays with one or two box girders joined by a transverse beam at each anchorage of the stays.
For shorter spans, up to 120 metres, other kind of bridges can be designed. Bridges supported with extradosed prestressing or with additional struts.
Figure 4.8.1 Precast cable stayed bridge in Germany.
Figure 4.8.3 Centenario Precast cable stayed bridge in Spain.
5.
Aesthetics
References should be made mainly to fib-Guide to good practice (bulletin No. 9, Guidance to good bridge design, July 2000), chapter 1.4.2. [5.1]
Amendments are required for some specific aspects of precast bridges:
Decisions in favour of or against a design with precast elements should not be based only on technical and economical reasons. They result very often from prejudices and a thorough investigation proves frequently that the differences in pure construction cost are only small.
Average figures are nearly identical for precast and in-situ structures: as an example one can assume that for a bridge with slenderness l/h ≈ 20 the following values per m² are to be taken into account:
* Concrete: 0,50 m³, Prestressing Steel: 15 kg, Steel S500: 70 kg.
* Cost for transport and erection comply mostly with cost for scaffolding. Economical advantages can be achieved when other aspects are taken into account:
* existing traffic (road or rail) which does not allow the placing of scaffolding at the required area and which would require additional costly measures like moving or lifting of structure into final position
* erection of new structures for existing traffic which allows only very short interruptions
* unfavourable soil conditions in between the abutments * reduction of construction time
* construction is rather independent from unfavourable weather conditions
* economical advantages due to rate of capacity utilisation at the manufacturer The only seeming disadvantage of precast girder bridge decks is the fact that the surface is often considerably larger than for an in-situ design. However this is compensated by substantial higher density and quality of the outer concrete shell which leads to a better durability and a longer expected service life.
The options for aesthetic detailing are a little bit more limited when precast elements are used.
[5.1]: The rules for proportions, transparency, slenderness, unity and harmony are valid for all bridge structures be they in-situ or precast.
There are however many possibilities to influence the design in a positive manner:
* Careful attention must be given to the design of details and secondary elements. The slenderness con be accented by painting the rim of the corbels in a bright warm colour and leaving the web of the beams in a dark colour.
* Other alternatives are shown in slides No. 14 and 15. There are examples where the outer edge beam became special shape to accent the slenderness
The layout of the beams can nowadays easily be curved in order to follow the gradient of the road. Special attention must be given to some aspects in the design and erection of these beams, but by solving these problems allows precast bridges to be more competitive in use.
measures. The detailing of the structure can only be successful and sensible if the final view is taken into consideration.
Example of shaped edge beam: (principle):
Fig 5.1 Precast bridge in Spain
painted with bright colour
Fig 5.4 Precast bridge in France
Fig 5.6 Precast bridge in Holand.
6. Design
6.1.
Specific Aspects
The integral advantage of the combination precast + high quality materials require the clarification of many specific regulations. Between them should be appointed:
* II.1. Values of γc and γf to apply in the case of precast members.
* II.2. Contribution of the concrete to the strength to shear at the interface in function of the fatigue level.
* II.3. Influence of the form and position of the connection reinforcement for shear at the interface.
* II.4. Strength of compressed struts at shear, especially with HPC. * II.5. Transfer length of tendons with HPC.
As in any other structural typology, the development of prefabricated bridges is hindered, in a way, by the need to solve problems via research, which delays the extension of the use of such solutions.
In our case, attention should be drawn to the following.
a) Obviously, in the case of the manufacture of bending members, the fundamental part of prefabricated bridges, prefabrication affords higher quality than cast-in-situ execution in many respects.
- Cutting, bending and placing reinforcement bars and tendons.
- Low pouring height together with very powerful concrete compaction procedures. - Extraordinarily efficient curing systems.
All this means, in short, that the quality of the members, in particular bending members, is much higher in prefabrication than in most cases of cast-in-situ execution.
Since, broadly speaking, under the deterministic approach the bending safety factor is expressed by the equation
f s s kγ γ
ε =
where k is a coefficient that varies very little from one in the case of prestressing, the only way to establish a suitable procedure to reward the highest prefabrication quality is, obviously, to make it truly dependent on the load factor
γ
f by means of the quality control towhich the structure is subject.
The idea frequently invoked to simply reduce the
γ
c coefficient entails a morepsychological than actual reward. The reduction of the
γ
c coefficient, indeed, only rewardscases as columns or where compression on struts is limited in the event of shear, but in all other respects its importance for bending members is essentially negligible. It is only by
b) The limitation on compressed struts had evolved pessimistically in recent years in the Model Code as well as in the successive versions of the Eurocodes, both with respect to the regulations dealing with concrete in general and those relating to precast concrete in particular.
Recent research (1) has shown that such reductions were unwarranted: Fortunately, new proposals to revise Eurocode EC-2 have re-established a more realistic solution.
c) The anchoring conditions for prestressed tendons, especially in high strength concrete, need to be analysed in greater depth, not only to establish the transfer length but to provide for a more accurate evaluation of the distribution of stress throughout the transfer zone to improve shear stress studies in these areas of members.
The shear strength of conection with “in situ” slab is 25% to 33% higher in b) than in a) Fig. 6.1.1.
d) The shear interface problem calls for supplementary research in two directions. On the one hand the use of normal strength cast-in-situ concrete in conjunction with high strength prefabricated members will become more and more frequent, which places special demands on engineering for shear at the interface.
Secondly, the present forms of computing the contribution of the connection reinforcement between cast-in-situ and precast concrete take account of the ratio of the area of reinforcement only. However, the way these bars are bent and placed has a substantial effect on shear strength and should be lent more attention in future (Fig. 6.1.1)
6.3.
Durability
6.3.1. Introduction
Durability is the capability of a structure, assembly or component to maintain minimum performance and adequate levels of stability and serviceability over at least a specified time under the influence of degradation factors with anticipated maintenance but without excessive unforeseen maintenance.
The requirement of an adequately durable structure is met if, throughout its required life, a structure fulfils its function with respect to serviceability, strength and stability without significant loss of utility.
Durability of concrete bridges is mainly governed by the quality of materials. The quality of the concrete, the bearings, the joints and the sealing of the surface are the essential parts which together determin the length of the service life.
At present the codes do not explicitly ask for the definition of the required lifetime of a structure. Generally the overall durability, as defined above, depends of the intended use of the structure together with load specifications.
In some countries maintenance programmes are set up for bridges. These programs form part of the requirements for durability. They are regulated in specific codes other than design codes and aim at the minimisation of maintenance during life time of the structures. Thorough knowledge of degradation mechanism in a structure which could develop to future failure is essential to secure serviceablity and to reduce cost due to developing faults.
Durability may be affected both by direct actions and also by consequential indirect effects inherent in the performance of the structure (e.g. deformations, cracking, water absorption, etc.). The possible significance of both direct and indirect effects is to be considered.
The load-bearing capacity of bridges can be violated by the degradation of concrete and reinforcement. Therefore bridges (and all other structures) must be designed in a way that the minimum safety level is secured during the intended service life despite degradation and ageing of materials.
As long as systematic durability design methods are still missing and common rules are not yet available as design tools, the general approach is to observe border conditions to improve performance of the structure and its components to avoid unexpected and premature failure.
These stipulations are met when specific design rules, special quality demands for materials, material compositions, working conditions, structural dimensions etc. are observed. Durability calculations which could result in the use of modified safety factors in combination with traditional mechanical design will be developed in the future [1]. Until these methods are available durability design has to ensure a high standard of all parameters which could influence service life of the structure in general.
To ensure an adequately durable structure, the following inter-related factors are to be considered:
* Degradation Factors
* Influence Of Environmental Conditions
* Composition, Properties And Performance Of The Materials * Shape Of Members And The Structural Detailing
* Quality Of Workmanship, And Level Of Control * Particular Protective Measures
6.3.2. Degradation Factors
The following factors may have negative long-term effects on the load-bearing capacity of concrete structures:
* corrosion due to chloride penetration * corrosion due to carbonation
* mechanical abrasion * salt weathering * surface deterioration * frost attack
For reinforced concrete, corrosion protection for any type of reinforcement is provided by taking care of the following aspects:
Concrete Stress conditions: Compliance e.g. with the requirements contained in the clause 4.4.1 of the ENV 1992-1-1.
Excessive compressive stresses in concrete may promote the formation of cracks.
Steel stress conditions: Compliance e.g. with the requirements in the clause 4.4.2 of the ENV 1992-1-1 results in limitation of cracks under SLS.
To avoid complex crack-width calculations the following simplification is often used: Concrete stresses calculated at the uncracked section must not exceed the value σb:
3 2 46 , 0 WN b β σ ≤ (σb and βWN in MN/m²).
In cross-sections in reinforced concrete (e.g. webs or base plates for transverse flexural loading) the transverse flexural-tensile stresses (calculated in accordance as uncracked section) may not exceed the values of σb either.
6.3.3. Influence of Environmental Conditions
Environment, in this context, means those chemical and physical actions, to which the structure as a whole, the individual elements, and the concrete itself is exposed, and which results in effects not included in the loading conditions considered in structural design.
Durability parameters:
* depth of deterioration of concrete and corrosion of reinforcement * concrete cover
* diameter of rebars
* Factors to be taken into account: * strength of concrete * permeability of concrete * type of cement * curing method * type of reinforcemenet * structural dimensions
For values of concrete cover and acceptable crack-width reference could be made to design codes e.g. ENV 1992-1-1.
design and detailing, standards of workmanship and construction, and intended maintenance regimes - to produce the required level of intended maintenance regimes - to produce the required level of performance for the structure throughout its service life.
Standard values can generally be increased by an allowance (∆h) for tolerances, which for precast elements may be assumed in the range 0 ≤∆h ≤ 5 mm depending on the standard of workmanship and quality control.
In some countries the values for concrete cover may be reduced when prefabrication in a factory is ensured.
6.3.5. Shape of Members and Structural Detailing
Early in the design process, the effects and possible significance of the environmental conditions is to be considered in relation to the durability requirements.
Reference should be made to the design criteria in point 3 above and to the requirements for concrete cover to reinforcement in point 4.
Other factors to be considered in design and detailing, in order to achieve the required level of performance, should include the following aspects:
the adoption of a structural form which will minimise the uptake of water or exposure to moisture.
the size, shape and design details of exposed elements or structures should be such as to promote good drainage and to avoid run off or standing pools of water. Care should be taken to minimise any cracks that may collect or transfer water. In the presence of cracks across a complete section, additional protective measures (coated bars, coatings, etc.)may be necessary.
for the drain of the deck slab surface, the sealing surface must be detailed carefully. attention in design and detailing, to the different aspects of indirect effects.
Expansion joints between bridge decks can seriously effect the long term durability of the structure. They can lead to serious corrosion of the r.c. elements of the deck and substructure due to leakage of salt solution through the joints.
Several methods of eliminating joints in continuous decks are acceptable.
6.3.6. Quality of Workmanship and Level of Control
The standard of workmanship on site shall be such as to ensure that the required durability of the structure will be obtained. The combination of materials and procedures used in making, placing and curing the concrete shall be such as to achieve satisfactory resistance to aggressive media for both concrete and steel.
During construction, adequate measures shall be taken, by means of supervision and quality control, to ensure that the required properties of the materials and standards of workmanship are achieved.
Quality Control:
An other important factor to ensure the obtaining of the required durability is the quality control in the different phases of the construction.
7. Protective Measures
In addition to all the facts that must be considered to ensure the required durability, other protective measures should also be taken into account.
Sealing: Joints between precast elements must be sealed.
Tendons placed in sheaths or ducts in the concrete, couplers and anchorage device shall be protected against corrosion.
Should the delay between stressing and grouting exceed the time permitted, then protection of the tendons shall continue until grouting takes place.
Where temporary protection is provided, the material used shall have an approval document and shall not have a deleterious effect on the prestressing steel or on the cement grout.
Written instructions shall be provided for the site or the works for the preparation and execution of the grouting.
It frost is liable to occur, measures shall be taken to prevent freezing of water in sheaths which are not yet grouted. After a period of frost, the sheaths should be free from ice before grouting is started.
Tendons may be protected by materials based on bitumen, epoxy resins, rubber, etc., provided that there are not detrimental effects on bond, fire resistance, and other essential properties.
6.3.8. Control and Maintenance of the Completed Structure.
A planned control programme should specify the control measures (inspections) to be carried out in service where long term compliance with the basic requirements for the project is not adequately ensured.
All the information required for the structure’s utilisation in service and its maintenance should be made available to the person who assumes responsibility for the complete structure.
6.4.
Seismic Aspects
6.4.1.Objectives, requirements and confeormity criteria
6.4.1.1. Objectives
With respect to bridges, national policies for prevention of accidents associated with major hazards such as earthquakes involve ensuring the acceptability of the risk of failures that would interrupt traffic. This acceptability is assessed in terms of the socio-economic repercussions of such failures.
Emergency transport must be maintained with an appropriate degree of reliability after the design seismic event has occurred.
The design seismic event depends on local seismicity and other contingencies. 6.4.1.2. Requirements
National Authorities can define two fundamental requirements for different kinds of structures, depending on the possible consequences of their collapse.
a) No-collapse requirement (ultimate limit state)
After occurrence of the design seismic event, the bridge must retain its structural integrity and adequate residual load bearing capacity. It must be capable of accepting damage (intended to help dissipate energy) during the earthquake and be able to carry emergency traffic and be inspected and repaired as required. Formation of plastic hinges in the piers is generally acceptable, but is prohibited in the deck which must also be fitted with devices to prevent it coming off its supports under the effect of extreme seismic displacement.
b)Damage limitation requirement (serviceability limit state)
Seismic action with a high probability of occurrence during the design service lifetime of the bridge must cause nothing more than minor damage to the parts of the structure designed to help dissipate energy during the seismic event, and engender neither traffic restrictions nor a need for immediate repair.
6.4.1.3. Conformity criteria
The criteria aimed at explicitly meeting the no-collapse requirement implicitly cover the damage limitation requirement.
a) Intended seismic behaviour
Bridges must be designed to be ductile (inelastic behaviour) or have limited ductility (essentially elastic behaviour) under their design seismic actions.
For both economic and safety reasons it is preferable to design bridges to have ductile behaviour in zones of moderate to high seismicity. This is achieved by ensuring that plastic flexural hinges are formed (normally in the piers, and normally accessible for inspection and repair) to dissipate energy, or by using isolation systems. Tall piers are generally designed to remain in the elastic range. Generally speaking, the bridge deck must also remain in the
b) Capacity design
The capacity design method involves ranking the energy-dissipation zones and is aimed at ensuring the formation of flexural plastic hinges in the piers.
The shear strength of the plastic hinges and the shear and flexural strength of all other regions must be checked in order to avoid any risk of brittle failure and to guarantee the elastic behaviour of standard areas.
The capacity design method is used for bridges with ductile behaviour. For bridges with limited ductility the method is not indispensable, and the conventional design process for sections under seismic loading can be used.
c) Connections and displacement control
Equivalent linear analysis methods must be capable of determining the displacements engendered by the design seismic action with an appropriate degree of accuracy. For members with plastic hinges (this does not concern bridge decks), the secant stiffness at the theoretical elastic limit is generally adopted.
Connections between supporting members (piers) and supported members (deck) must be designed appropriately so as to ensure structural integrity and to prevent unseating as a result of extreme seismic displacement.
To ensure satisfactory overall ductility, structural and non-structural detailing must ensure appropriate behaviour of the bridge and its component parts, and appropriate clearances must be provided to protect important or critical structural members. These clearances must correspond to the calculated total displacement under the effect of seismic conditions:
where:
dE is the design seismic displacement
dG is the long-term displacement due to permanent and quasi-permanent actions
(shrinkage and creep)
dTs is the displacement due to thermal movement for a representative value, Ts, of
temperature variation considered to be appropriate for the combination with seismic effects. It can be assumed that dTs is 40% of the maximum thermal movement.
Displacements due to second-order effects are to be added if they make a significant contribution.
Protection must be provided against major impacts caused by exceptional pounding between members of major structural importance, by using ductile members or dampers. The clearance in these elements must be at least the displacement dEd.
At connections in rail bridges, differential transverse displacements must be either prevented or limited to appropriate values in order to prevent derailments.
6.4.2. Design principles
6.4.2.1. General design of the project
Ts G E
Ed d d d
Choosing ductile behaviour is generally appropriate in areas of moderate to high seismicity. In this case, the following factors generally need to be taken into account:
Choose piers and abutments to withstand seismic forces in the longitudinal and transverse directions.
Greater deformability reduces the level of the design seismic action but increases movements at joints and deformable supports, which can induce substantial second-order effects.
In the case of continuous-deck bridges, where the transversal stiffness of the abutments and the shorter piers adjacent to them is great relative to that of the other piers, it may be preferable to use deformable bearings or bearings sliding transversally on top of the abutments and short piers.
The locations of energy–dissipation systems must be carefully chosen to ensure they are accessible for inspection and repair.
With exceptionally long bridges or bridges on heterogeneous soils, it will be necessary to decide on the number and locations of intermediate expansion joints.
With bridges spanning potentially active tectonic faults, the probable discontinuity of ground displacements will generally have to be assessed and taken into account by means of structural flexibility or by appropriate layout of expansion joints.
6.4.2.2. Bridge site
Bridges should not be sited near (less than 500 m) an active fault where earthquakes are particularly violent and poorly represented by the regulatory spectra.
The bridge site can be called into question if it is discovered that there is a risk of liquefaction of the subsoil. If the site cannot be changed, it is vital that the bridge be founded on soils of appropriate characteristics beneath the liquefiable soils, and that adequate drainage be provided.
6.4.2.3. Distribution of spans – Location of supports
The location of supports and the distribution of spans must be based on classical analysis of the gap, taking account of the following particularities.
Span uplift :
If the end spans are short (0.5 to 0.6 times the length of adjacent spans) and cannot be made longer, the end crossbeams on the abutments can be ballasted, or a hold-down device can be provided. End spans should systematically be extended in the case of highly skewed bridges (<70 grades).
Balanced spans:
Bridges with symmetrical spans and support systems have better seismic behaviour. Every effort should be made to limit the distance between the centre of mass of the deck and the centre of elastic stiffness of the supports. For a straight bridge, when this distance is zero, the deck and supports undergo no rotation about their vertical axis.
Skew of decks:
The possibility of the deck pounding against the abutments is a major risk for skew bridges. Special seismic stops must be used to secure the structure, by directing impact forces parallel to the longitudinal axis of the bridge and thus preventing the deck sliding off its abutments. The behaviour of highly skewed bridges (<70 grades) under vertical seismic
also be specially studied from the seismic-risk viewpoint. The choice of connections between the deck and the piers and abutments is the major factor to be considered by the designer.
Deck:
The seismic behaviour of bridge decks and integral precast underpasses (box culverts, buried portals) generally falls in the elastic (or quasi-elastic) range and therefore poses no particular problem.
In order to ensure sound mechanical behaviour in an earthquake, the design of standard bridge decks, on the other hand, must take certain arrangements into account:
Limit displacement of the deck relative to its supports so that it is not unseated. Provide a sufficiently large bearing surface and, as an ultimate security measure, seismic stops at the abutments, unless the deck is restrained transversally at the abutments as a matter of course.
Prevent the deck pounding the abutment and piers, or localize impact in a specially designed area, by means of a sacrificial zone (e.g. expansion joint), so as not to restrict deck movement during earthquake, or by using an impact-damping zone with a laminated elastomeric bearing as a transverse and/or longitudinal stop. Care should be taken to ensure that any impact does not involve sensitive areas (tendon anchorages, for instance).
Prevent brittle failure of any part of the bridge (particularly the bottom of piers) due to lack of ductility. Prevent buckling of longitudinal reinforcement in compression and ensure sufficient anchorage length of longitudinal reinforcement.
In highly seismic areas, prestressed concrete bridge decks must have additional checks performed with respect to the vertical component of earthquakes.
6.4.2.4. Structural choices
a) Bridge decks independent of supports
The criteria regarding choice of the type of bridge are the same as in non-seismic zones, but consideration of the seismic hazard will deal with the following in particular:
Lightness:
Reducing the weight of bridge decks in order to diminish actions on the supports is only of interest for slender piers (>15 m).
Mechanical continuity of structures:
Mechanical continuity improves a structure’s strength and ductility. Deck bridges have to be made continuous (integral crosshead or continuous separate slabs). Deck segments can be tied together over several spans. Cantilever spans should be avoided.
Fixing piers to the deck:
Fixing the deck to two piers which are not far apart—thus creating a portal frame—can be one sort of seismic design solution in highly seismic zones or where the piers are very high. This option should be compared with the classical design where the deck is simply supported on the piers.
b) Underpasses
Integral precast underpasses (box culverts, buried portals) generally display good seismic behaviour. Taking seismic hazard into consideration makes little change to the design
The main objectives in choosing a bridge bearing for seismic conditions are to limit deck displacement and to limit forces in the piers and abutments.
Using classical bearings such as laminated elastomeric or pot types which keep the deck independent of the piers is a conventional design option. Special devices such as dampers are only justified in exceptional circumstances (they are complex and costly).
Reinforced hinges made by reducing the sectional area of concrete members (Freyssinet type) are not recommended in seismic zones (limitation of the angle of reaction).
6.4.3.2. Taking account of longitudinal earthquake movement, the main design options are as follows:
Prefer elastomeric laminated bearing pads for road bridges. Ground movements are filtered by these bearings which act as soft isolating springs. The deck undergoes quite considerable movement relative to the ground. If, due to the use of laminated pads, the periods of vibration of the structure are quite long, the horizontal forces will remain reasonable.
Restrain displacement of the deck by using fixed bearings or stops on one or more piers. The horizontal forces will remain reasonable from the moment a high behaviour factor can be used. “Pseudo-elastic” analysis assumes the structure to be perfectly elastic, the forces calculated being divided by the behaviour factor which limits the bending moment by creating a plastic hinge at the bottom of the pier.
Fix the deck to some of the piers (adjacent and flexible). By creating a portal frame there is no need for relatively expensive special bearings, and in the case of rail bridges the displacements due to train startup and braking are also limited.
Restrain the deck at an abutment (rigid) or at a massive pier (for rail bridges only, and only in areas with low seismicity, because of the considerable forces engendered). Special bearings have to be installed so that the bridge can have substantial horizontal reactions relative to the vertical reactions without moving relative to the abutments.
6.4.3.3. Taking account of transverse earthquake movement
For standard bridges less than 15 metres above the ground, it is neither necessary nor possible to use a behaviour factor since the piers are rigid (pier walls or columns designed for vehicle impact).
The design will take account of the following points:
Control transverse movement of the bridge on its abutments. This is indispensable when the deck carries pipes (gas, etc.). The deck must be restrained at all the supports (if possible) to prevent train derailment.
Avoid rotation of the deck about its vertical axis by (if possible) using the same type of bearing for piers of similar stiffness, or by installing sliding (or laminated) bearings on the stiffest piers in order to isolate the deck.
For standard bridges, take the transverse earthquake at the abutments, either by installing one-way bearings or by installing laminated bearings together with limiting stops.
For short bridges (<40 m) with expansion joints with good transverse behaviour and not carrying major services, an alternative solution involves using the same kind of transverse bearing (laminated type) on the piers and abutments, without transversal restraint of the structure.
In the example in the following figure, transverse seismic action is highly unfavourable because of the very great transverse stiffness of the abutments and their adjacent piers relative
to the other piers. In this case it may be preferable to use bearings that deform or slide transversally at the top of the abutments and the shorter piers.
Figure 6.4.1. Example of irregular bridge (elevation): end piers too short
6.4.3.4 Criteria for choosing between fixed bearings and elastomeric laminated bearings
With respect to forces, choosing the bearing system involves examining the site of the
bridge and the intensity of the earthquake. For sites in moderately seismic zones, isolation of the structure by elastomeric laminated bearings seems to be the inevitable solution. Restraining the deck at one or two piers appears to be an interesting solution too, especially if the piers are designed with a behaviour factor, or in highly seismic zones.
With respect to displacement control, a solution whereby the deck is restrained at its
supports is naturally more effective and merits investigation, particularly in highly seismic zones or with poor-quality subsoils.
6.4.4. Detailed design
Construction details must be consistent with the way the structure works in an earthquake. 6.4.4.1 Support length
The overlap of the deck on its support must be long enough. The minimum support length must be sufficient for the support function to be maintained in the event of extreme seismic displacement. It can be calculated with the following formula:
d is the differential displacement of the ground between the barycentre of the fixed support and the support concerned.
D is the displacement of the deck on the support as a result of seismic combinations.
Figure 6.4.2. Bearing definition. tablier = deck
culee = abutment
appareil d'appui = bearing
tablier deplace sous seisme = deck displaced by earthquake appareil d'appui distordu = distorted bearing
6.4.4.2. Stops
Stops can be made of steel or reinforced concrete. They are generally not very ductile. There are two kinds of stops :
Safety stops designed to prevent the deck leaving its supports while allowing free
distortion of the bearings in response to seismic action.
Limit stops (which also act as safety stops) designed to severely restrict relative
displacement between the deck and its supports due to seismic action.
The faces of the stops must be oriented appropriately to limit rotation of skew bridges about the vertical axis.
a) Longitudinal stops
In general it is not necessary to install longitudinal safety stops because of the safety margin contributed by abutment backfill.
Longitudinal limit stops can be envisaged to complement elastomeric laminated bearings in some rare special cases (e.g. long bridge on piers of the same characteristics for which a behaviour factor is to be applied). The gap around the stops must be adjusted so as to limit the effects of impact on the supports.
Transverse safety or limit stops (for rail bridges) must be provided to limit relative displacement between the bridge and its supports and to prevent the deck falling.
It is recommended that limit stops be installed on abutments, with a small gap (1 to 2 cm) which will allow the bridge to “work” in service and will limit the effects of pounding in an earthquake.
In the case of a deck whose transverse movement is restricted at two supports, transverse stops are not generally required on the other supports.
Figure 6.4.3. Support with stops Tablier = deck
Appareil d'appui = bearing
Bossages d'appui = bearing shims Appui = support
The figure above shows one possible arrangement. Restraint is obtained by the reinforced concrete tenons on the support or on the underside of the deck. The tenons overlap each other by about 10 cm. The safety stop so formed works only transversely.
6.4.4.3. Design of ends of road bridge
The longitudinal movements predicted by the analysis model must not be hindered by the abutment backwall. For “sacrificial” joints, preference should be given to fillers such as sealants or preformed rubber systems. This makes it unnecessary to over-size the abutment foundations.
It is preferable to use the following solutions, depending on the class of bridge:
If the bridges are to remain usable after the earthquake, it may be appropriate to choose a solution with fixed bearings on the piers to limit displacements and, consequently, the joint opening (joints that can withstand ultimate seismic deformation without damage are very expensive).
If the earthquake-induced displacement of the deck is slight (± 2 cm), it may be possible to have expansion joints designed for the design seismic event. If the seismic displacement of the deck is greater, then the expansion joints should be designed with the combination:
