INTEGRAL BRIDGESViral Panchal1 and Chaitanya S. Sanghvi2

1M.E. (CASAD) Student, Applied Mechanics Department, L.D.College Of Engineering, India.

E-mail: viralcivileng@yahoo.co.in2 Associate Professor, Applied Mechanics Department, L.D.College Of Engineering, India.

E-mail: cssanghvi@rediffmail.com

ABSTRACT

The increase in demand for complex roadway alignments, advances in construction technology and availability

of computing power for bridges design, are some of the factors for developments in bridge engineering. Concept

of “Integral Bridges” is one of these developments. Due to ease & economy in construction and maintenance, it

is also getting popular in India. Integral bridge concept is also widely adopted in marine structures where many

times foundations are flexible like pile foundation. Main reasons for increasing popularity of integral concept in

marine structures are efforts of minimizing use of bearings and to resist large lateral forces. Integral bridge

requires flexible foundation to accommodate thermal stresses and stresses produced from lateral forces like

waves, current, wind, seismic etc. There are many advantages to jointless bridges as many are performing well

in service. There are long term benefits to adopting integral bridges concept and therefore there should be

greater use of integral bridge construction. Integral abutment and jointless bridges cost less to construct and

require less maintenance then equivalent bridges with expansion joints. This paper explains why we should use

integral bridges and discusses some of the recommended practices for integral abutment and jointless bridges.

Keywords

Integral bridge, bearing, seismic, expansion joint, abutment, soil structure interaction.

Why Integral Bridges?

One of the most important aspects of design which can affect structure life and maintenance costs is the

reduction or elimination of roadway expansion joints and associated expansion bearings. Unfortunately, this is

too often overlooked or avoided. Joints and bearings are expensive to buy, install, maintain and repair and more

costly to replace. The most frequently encountered corrosion problem involves leaking expansion joints and

seals that permit salt-laden run-off water from the roadway surface to attack the girder ends, bearings and

supporting reinforced concrete substructures. Elastomeric glands get filled with dirt, rocks and trash, and

ultimately fail to function. Many of our most costly maintenance problems originated with leaky joints. Bridge

deck joints are subjected to continual wear and heavy impact from repeated live loads as well as continual stages

of movement from expansion and contraction caused by temperature changes, and or creep and shrinkage or

long term movement effects such as settlement and soil pressure. Joints are sometimes subjected to impact

loadings which can exceed their design capacity.

Deck joints are routinely one of the last items installed on a bridge and are sometimes not given the

necessary attention it deserves to ensure the desired performance. While usually not a significant item based on

cost, bridge deck joints can have a significant impact on a bridge performance. A wide variety of joints have

been developed over the years to accommodate a wide range of movements, and promises of long lasting,

durable, effective joints have led States to try many of them. Some joint types perform better than others, but all

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joints can cause maintenance problems. The problems arising from provision of bearings and expansion joints

can be summarized as:

Increased incidence of inspection and maintenance required, bridge durability is often impaired.

Necessity of replacement during the service life of the bridge since their design life is lesser than that

of the rest of the bridge elements.

Decrease in redundancy and difficulties in providing adequate ductility for resisting earthquake

effects, leading to larger earthquake design forces.

Surajbari new bridge superstructure shifted in the transverse direction.

Bridge between Surajbari & Bhachau – Violent shaking has resulted in pier head being damaged due to

pounding of deck

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Possibilities of dislodgement of superstructure during accidental loads, especially those due to

earthquakes, is a clear danger requiring expensive and clumsy attachments. The latest amendments to the Indian

Road Congress codes require the positive measures such as restrainers be provided so that girders do not get

dislodged during earthquake.

Bridges presents soft target for terrorists who could put them out of service with little difficulty.

What is An Integral Bridge?

Because of above mentioned problems, use of integral or integral abutment bridge is being increased

all over the world. Integral bridges are bridges where the superstructure is continuous and connected

monolithically with the substructure with a moment-resisting connection. As an effect we obtain a structure

acting as one unit. However, simply supported bridges are still popular in India. The main reason for their

popularity is that these structures are simple to design and execute. The sub-structural design is also greatly

simplified because of the determinate nature of the structure. Sometimes there are situations where

bearings/simply supported spans/expansion joints can not be altogether avoided because of the length of the

bridge. In such cases intermediate joints will be provided with bearings to allow horizontal movements. But

these joints will be lesser in numbers as compared to simply supported bridges. On the other hand, monolithic

joints and redundancy of the structural system do result in savings in the cost of the construction and

maintenance. Elimination of bearings improves the structural performance during earthquakes. Finally, integral

form of construction will require lesser inspection and maintenance efforts. Several urban structures in India

have been built with this concept. However no national standards or uniform policy regarding the permissible

bridge length, skews and design procedures have been clearly established, although certain general concepts

become common in practice.

The advisory note BA 42/96 recommends that all bridges need to be integral if overall length exceeds

60 m and skews less than 30 deg. The longitudinal movement in the bridge abutment is limited to 20mm from

the position at time of restraint during construction. Integral bridges are designed for same range of temperatures

as other bridges. According to IAJB 2005, the range of design criteria for selection of integral bridge is

summarized below.

Steel girders Concrete

Maximum span (ft) 65-300 60-200

Total length (ft) 150-650 150-1175

Maximum skew (degree) 15-70 15-70

Maximum curvature 0-10 0-10

Some of the common features of monolithic bridge construction include:

i) Elimination of the pier cap which improves bridge aesthetics.

ii) Heavily reinforced slender piers

iii) Change in the structural system.

Integral bridges accommodate superstructure movements without conventional expansion joints. With

the superstructure rigidly connected to the substructure and with flexible substructure piling, the superstructure

is permitted to expand and contract. Such bridges are the answer for small and medium length bridges where

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bearings and expansion joints can be either eliminated altogether or reduced to a minimum. By incorporation of

intermediate expansion joints, the integral bridge concept can be extended to long bridges and viaducts too.

Integral bridges are designed to provide resistance to thermal movements, breaking forces, seismic forces and

winds by the stiffness of the soil abutting the end supports and the intermediate supports. A typical three span

integral abutment bridge is shown in Fig.1.

Fig.2 shows three principle methods by which an integral bridge can accommodate movements of the

super structure. Fig.3 shows different types of end supports used for integral bridges. The main types of the end

supports can be categorized and described as:

a). Frame abutment:- Full height frame abutments are suitable for short single-span bridges. The

horizontal movements will only be small, so the earth pressures should not be very high.

b). Embedded wall abutment:- Embedded wall abutments are also suitable for short single-span

integral bridges.

c). Piled abutment with reinforced soil wall :- A piled abutment with reinforced soil abutment wall

and wing walls is a form of construction that should have a wide application.

d). End screen (semi integral) :- Semi-integral construction with bearings on top of a rigid retaining

wall is a design method that can be used for full-height abutments for bridges of any length. Jacking of the deck

can result in soil movement under the abutment soffit. This can obstruct the deck from returning to its original

level.

e). Piled bank seat :- Piled bank seats are recommended for widespread use. The piles prevent

settlement while allowing horizontal movement and rotation.

f). Piled bank seat with end screen (semi integral):- Bank seats can be designed as semi-integral

abutments. The footing is not required to move horizontally and piled or spread footings can be used.

g). Bank pad abutment :- Shallow abutments on spread footings are only considered to be suitable

for situations where the foundation is very stiff and there can be no settlement problems. A granular fill layer

should be placed below the footing to allow sliding.

Benefits of Integral Bridges

Some of the advantages of adopting Integral bridges over that of the conventional bridges are summarized

below:

i. Simplified Construction- The simple characteristics of integral bridges make for rapid and

economical construction. For example, there is no need to construct cofferdams, make footing

excavations, place backfill, remove cofferdams, and prepare bridge seats, place bearings, back

walls, and deck joints. Instead, integral construction generally results in just four concrete

placement days. After the embankments, piles and pile caps have been placed and deck stringers

erected, deck slabs, continuity connections, and approach slabs can follow in rapid succession.

ii. No bearings and Joints- Integral bridges can be built without bearings and deck joints. Not only

will this result in savings in initial costs, the absence of joints and bearings will reduce

maintenance efforts. This is an important benefit because presently available deck joint sealing

devices have such short effective service lives. Smooth jointless construction improves vehicular

riding quality and diminishes vehicular impact stress levels.

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iii. Improved Design efficiency- Tangible efficiencies are achieved in substructure design due to an

increase in the number of supports over which longitudinal and transverse superstructure loads

may be distributed. Built-in abutments can be designed to accommodate some bending moment

capacity, reducing end span bending moments with possible savings in end span girders. Due to

rigid connection between superstructure and substructure, bending moments are considerably less

thus resulting in smaller sections and economy in reinforcement and concrete.

iv. Enhanced load distribution- One of the most important attributes of integral bridges is their

substantial reserve strength capacity. The integrity of their unified structural system makes them

extremely resistant to the potentially damaging effects of illegal super imposed loads, pressures

generated by the restrained growth of jointed rigid pavements, earthquakes, and debris laden flood

flows. A joint less bridge with integral abutments will have a higher degree or redundancy that

may be beneficial in earthquake zones. The problem of retaining the superstructure on its bearing

during seismic events is eliminated and the inherent damping of the integral bridge structural

system allows it to better absorb energy and limit damage.

v. Added redundancy and capacity for catastrophic events - Integral abutments provide added

redundancy and capacity for catastrophic events. Joints introduce a potential collapse mechanism

into the overall bridge structure. Integral abutments eliminate the most common cause of damage

to bridges in seismic events, loss of girder support. Integral abutments have consistently performed

well in actual seismic events and significantly reduced or avoided problems such as back wall and

bearing damage, associated with seat type jointed abutments. Jointless design is preferable for

highly seismic regions.

The reasons for adopting integral bridges in India and elsewhere could be quite different. When

earthquake forces like predominant or when considerations like increased resistance to blast are to be reckoned

with or there is a strong need of incorporating reduced cost of inspection & maintenance integral bridge concept

is an excellent option.

Problems and Uncertainties

Despite the significant advantages of integral abutment bridges, there are some problems and

uncertainties associated with them. Many articles, mentioned that the main problem connected with integral

bridges are consequences of temperature variations and traffic loads, which cause horizontal bridge movements.

Horizontal movements and rotations of the abutment cause settlement of the approach fill, resulting in a void

near abutment if the bridge has approach slabs. Effects of lateral movements of integral abutments under cyclic

loadings are obvious problem which demands solving, but positive aspect in this case is that temperature

induced displacements in the traditional bridge is over twice bigger than displacement at the end of (considering

objects with the same span length) integrated structure because of symmetrical nature of the thermal effects as

illustrated below..

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The other uncertainties connected with designing and performance of integral abutment bridges are:

The elimination of intermediate joints in multiple spans results in a structural continuity that may

induce secondary stresses in the superstructure. These forces due to shrinkage, creep, thermal gradients,

differential settlement, differential deflections, and earth pressure can cause cracks in concrete bridge

abutments. Wingwalls can crack due to rotation and contraction of the superstructure. Also, differential

settlement of the substructure can cause more damage in case of integral bridges as compared to traditional

briges.

Integral bridges should be provided with approach slabs to prevent vehicular traffic from consolidating

backfill adjacent to abutments, to eliminate live load surcharging of backfill, and to minimize the adverse effect

of consolidating backfill and approach embankments on movement of vehicular traffic. For bridges with closed

decks (curbs, barriers, etc.), approach slabs should be provided with curbs to confine and carry deck drainage

across backfill to the approaches and prevent erosion, or saturation and freezing of the backfill.

The piles that support the abutments may be subjected to high stresses as a result of cyclic elongation

and contraction of the bridge structure. These stresses can cause formation of plastic hinges in the piles and may

reduce their axial load capacities.

The application of integral bridge concept has few other limitations. Integral bridges can not be used

with weak embankments or subsoil, and they can only be used for limited lengths, although the maximum length

is still somewhat unclear. Integral bridges are suitable if the expected temperature induced moment at each

abutment is certain value specified by suitable authorities in every country, and somewhat larger moments can

be tolerable.

Recommended Design Details for Integral Abutments

Use embankment and stub-type abutments.

Use single row of flexible piles and orient piles for weak axis bending.

Use steel piles for maximum ductility and durability.

Embed piles at least two pile sizes into the pile cap to achieve pile fixedly to abutment.

Provide abutment stem wide enough to allow for some misalignment of piles.

Provide an earth bench near superstructure to minimize abutment depth and wingwall lengths.

Provide minimum penetration of abutment into embankment.

Make wingwalls as small as practicable to minimize the amount of structure and earth that have to

move with the abutment during thermal expansion of the deck.

For shallow superstructures, use cantilevered turn-back wingwalls (parallel to center line of

roadway) instead of transverse wingwalls.

Provide loose backfill beneath cantilevered wingwalls.

Provide well-drained granular backfill to accommodate the imposed expansion and contraction.

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Provide under-drains under and around abutment and around wingwalls.

Encase stringers completely by end-diaphragm concrete.

Paint ends of girders.

Caulk interface between beam and backwall.

Provide holes in steel beam ends to thread through longitudinal abutment reinforcement.

Provide temporary support bolts anchored into the pile cap to support beams in lieu of cast bridge

seats.

Tie approach slabs to abutments with hinge type reinforcing.

Use generous shrinkage reinforcement in the deck slab above the abutment.

Pile length should not be less than 10 ft. to provide sufficient flexibility.

Provide pre bored holes to a depth of 10 feet for piles if necessary for dense and/or cohesive soils

to allow for flexing as the superstructure translates.

Provide pavement joints to allow bridge cyclic movements and pavement growth.

Focus on entire bridge and not just its abutments.

Provide symmetry on integral bridges to minimize potential longitudinal forces on piers and to

equalize longitudinal pressure on abutments.

Provide two layers of polyethylene sheets or a fabric under the approach slab to minimize friction

against horizontal movement.

Limit use of integral abutment to bridges with skew less than 30 degree to minimize the magnitude

and lateral eccentricity of potential longitudinal forces.

Summary

There are many advantages to jointless bridges as many are performing well in

service. There are long-term benefits to adopting integral bridge design concepts and

therefore there should be greater use of integral bridge construction. Due to limited

funding sources for bridge maintenance, it is desirable to establish strategies for

eliminating joints as much as possible and converting/retrofitting bridges with

troublesome joints to jointless design.

Now various organizations and authorities have adopted integral abutment

bridges as structures of choice when conditions allow. Many of them are now building

integral and/or semi-integral abutment type of bridges. Recently in India, this concept is

widely used in Delhi Metro Rail bridges.

While superstructures with deck-end joints still predominate, the trend appears to

be moving toward integral. Although no general agreement, regarding a maximum safe-

length for integral abutment and jointless bridges, exists among standards or

organisations, the study has shown that design practices followed by the most

organizations are conservative and longer jointless bridges could be constructed.

Continuity and elimination of joints, besides providing a more maintenance free

durable structure, can lead the way to more innovative and aesthetically pleasing

solutions to bridge design. As bridge designers we should never take the easy way out,

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but consider the needs of our customer, the motoring public first. Providing a joint free

and maintenance free bridge should be our ultimate goal. The best joint is no joint.

References

1) Babu P.V.Mayur and Bhandari N.M. A Comparative Study Of Integral Bridges Versus Simply

Supported Bridge.

2) Chen Wai Fah and Duan Lian, Bridge Engineering Handbook, CRC Press.

3) Connal John, Integral Abutment Bridges – Australian And US Practice

4) Flener Esra Bayoglu, Soil Structure Interaction in Integral Bridges

5) O’brien Eugene J. and Keogh Damien L., Design Details Of Integral Bridges.

6) Raina V.K., Concrete Bridge Practice, The McGraw-Hill Publishing Company Liminted, Second

Edition.

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Fig.1 Sketch Of A Typical 3 Span Integral Bridge

Fig.2

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(a) (b)

(c) (d)

(e) (f)

(g) (g)

Fig.3

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