Composite construction in Bridge Deck systems A Solution in City Environment for Speedy Construction with Industrialized

Full Length Prefabrication

By,

Suhas Khedkar , Kishore Saxena

Abstract Rapid construction of flyovers in congested cities has become necessary, considering the present traffic trends, and anticipated growth in future. Construction has to be done without closure of roads and with minimum disturbance to the existing traffic. Use of Precast and Prefabricated components for superstructure help in reduction of construction time, as superstructure components can be kept ready elsewhere, when construction of foundations and substructure is in progress. Erection of such components can be planned during night hours, when the traffic is minimal; thus causing least hardships to the general public. Many flyovers have been constructed in our country using full length precast prestressed girders, and cast in situ deck slab. Flyovers with full length prefabricated girders in structural steel and composite RCC deck slab are very few. This paper presents advantages of such structural system for congested cities. Key Words: Structural Steel Bridge Girders, full length Industrialized Prefabrication , working window, girder transportation , site erection

1. General Many of our cities are very busy, over populated, and congested for both living conditions, and traffic situation. The existing infrastructure is already overtaxed. This is not only true in case of the major metropolitan cities, but also in case of second and third level cities, particularly those in the vicinity of major cities. All these cities and urban areas need better infrastructure for decongesting traffic. While the metropolitan cities are moving from the era of flyover construction, to construction to that of Elevated Transportation Corridors, covering multiple junctions, and longer routes measuring in kilometres; smaller developing cities and towns need flyover bridges over busy traffic junctions to help in decongestion of traffic. The typical site conditions pose many constraints to any new construction. Although there are site specific problems, many constraints are common. Engineers can adopt standard solutions in such situations. The same can be modified suitably to address site specific issues, by a judicious mix of different solutions.

2. Typical constraints in construction of flyovers in congested cities Construction of flyovers in cities must be done in busy and congested city environment, respecting all aspects: viz. No / minimal disturbance to traffic, safety of the citizens,

availability of small working windows, coordination with local bodies etc. All these need to fit into the tight construction schedule, which is the basic demand of the present times. The alignment of flyovers many a time passes through area with heavy encroachment, further restricting available working space for construction. This needs suitable consideration in designs, which may demand basic changes such as different type of structure and construction materials. Sometimes the site constraints may dictate choice of structural steel instead of Prestressed concrete (which is a normal choice for construction); considering limitation of transportation and erection of heavy segments and girders. It may also be a chosen solution by the owners to ensure speedy and easier construction.

3. Possible solutions, Pros and Cons

Conventional method is to use cast in situ superstructure. However, it is time consuming; as the construction is strictly sequential - Foundation, Substructure, and then Superstructure by using scaffolding and formwork under each span. Construction time can be reduced if precast superstructure elements are used, as precasting can be done as a parallel activity when construction of foundations and substructure is in progress at site. Such construction does not need staging under each span, if the precast elements are erected using launching equipment.

Main advantage of using Precast / prefabricated structural elements being utilising time required for construction of foundations and substructure for casting / fabricating superstructure elements in a Precast Yard / Fabrication shop. Such construction helps in achieving much better quality control, as it happens in a dedicated , specially designed casting yard / fabrication shop; in controlled conditions and factory like environment; which is another major advantage of such construction. Thus precasting / prefabrication is called as Industrialised Construction.

Normal methods of superstructure construction of such urban flyovers, therefore; is to use full length concrete girders or concrete box girder segments; precast in a dedicated Casting Yard for the purpose, normally distant from the site; transported to respective spans using a specially designed trailer, and erected using high capacity cranes or a Launching Gantry, designed to suit the size and weight of segments, and span length. The method is well established, and many flyovers and elevated roads have been constructed using the method in various parts of our country.

However, weight of precast concrete elements, whether full length girders or box girder segments typically ranges from 45 t upwards; and needs large capacity trailers for transportation of the girders / segments and heavy erection equipment – Launching Gantries or High Capacity cranes. When the alignment passes through roads of very restricted width; deployment of such equipment is not possible. In such situations, engineers have to look for further alternatives. These can be use of cast in situ construction, or use of structural steel composite superstructure, which facilitates reduction in weight of prefabricated girders; and obviates need for using high capacity

erection equipment.

There is a general feeling against use of structural steel girders in composite bridge decks mainly due to two reasons: Higher cost and Corrosion Performance. However, in certain situations, use of structural steel girders may be dictated by site constraints; and the cost, if higher needs to be provided for in budgeting. The second issue of corrosion performance can be addressed by a suitable choice of girder section, which will enable easy maintenance.

4. Steel composite Bridge deck systems

Structural Steel Composite Bridge Decks consist of structural Steel girders with Reinforced Concrete Deck Slabs. The deck slab transfers deck loads by transverse spanning to the main girders spanning longitudinally. The system is designed to develop composite action in longitudinal direction between the RCC deck and Steel girders by providing shear connectors between the flange of the steel girders and deck slab. The deck slab is cast in situ. The action is illustrated in Fig. 1 below:

Fig.1 Composite Action of Structural Steel Girders with RCC Deck Slab

Steel girders can be of various types; viz. Plate Girders, Box Girder, Multiple Box girders, Steel Trusses or multiple Steel Trusses in place of individual girders spanning between piers. The different types are illustrated in the figures below:

Fig. 2 Multiple Plate Girders

Fig. 3 Multiple Plate Girders Examples of Constructed bridges

Fig. 4 Twin Plate Girders with Transverse Composite Deck Slab

Fig. 5 Twin Plate Girders with Stringer Beams and Composite Deck Slab

Fiig.6 Composite Tapered Box Girder with Very Wide RCC Deck Slab

Fiig.7 Composite Twin Tapered Box Girder with Wide RCC Deck Slab

Fig.8 Composite Action of Structural Steel Trusses with RCC Deck Slab

Fig. 9 Modern Aesthetic Steel Girder Types With Composite Deck Slab

Fig. 10 Multiple Triangular Steel Trusses With Composite Deck Slab

Fiig.11 Composite Multicell Tapered Box Girder with Very Wide RCC Deck Slab

5. Suitable Choice for an Urban flyover

Urban flyovers are normally designed for two lanes or three lanes of traffic, without

footpath. Normal span range is from 20 m to about 30 m. Obligatory spans are of longer

span length of 40 m and beyond. Choice of the deck system chosen for normal spans

needs to be changed for obligatory spans. This is normally the case even for flyovers

constructed using Prestressed concrete Superstructure. The carriageway width is 7.5 m

(Two Lane) or 11.25 m (Three Lane). Adding for Crash Barriers on both sides; the total

deck width is normally 8.5 m or 12.25 m. If Multiple Plate girders or Truss Type Girders

are adopted, a minimum of 4 girders will be needed for Two Lane flyovers and a

minimum of 5, preferably 6 girders will be needed for 3 lane flyovers. The girders can be

prefabricated in a separate fabrication shop, suitably located away from the site, and

brought to site on low bed trailers. Girders are erected using cranes, single or two

cranes operating in tandem. Deck system with even number of girders are technically

superior; as two girders can be braced together with intermediate diaphragms before

erection. This ensures lateral stability of girders during erection.

Single box for superstructure will be comparatively wider, and may pose difficulties for

transportation and handling, especially if fabricated in full length. To restrict weight and

dimensions for transportation, the girder may have to be fabricated in three or four

segments. All the span will therefore need intermediate staging supports; where

smaller prefabricated box segments can be erected. Joints and splices between the

segments will have to be done in situ. This will increase cycle time for erection of the

box girders.; and will also jeopardize fabrication quality compared to full scale

fabrication in fabrication shop.

It is possible to adopt Twin box girders or Three box girders under the deck. With this

arrangement, it will be more feasible to fabricate full length box girders, which can be

transported from the fabrication yard, and erected in full length, using a single crane or

two cranes in tandem of suitable capacity. Three box girders may be more suitable,

considering preference of IRC specifications for a minimum of three longitudinal girders

in bridge spans. The weight of individual girder will be much smaller, if three girders per

span are adopted. This will be helpful as smaller capacity cranes can be deployed for the

job, which will be more suitable for operation within the restricted working space

available in the congested area, although girder transportation and erection will have to

be necessarily done during night hours, when the traffic is expected to be minimal, and

diversion / stoppage of the same will not pose problems, or inconvenience to the

general public.

Box girders can be meticulously fabricated such that after completion of fabrication, the

girders can be given proper anticorrosive coatings from inside before closure from the

top and ends. After closure, since the inside of the girders is hermetically sealed, no

painting may be needed from inside during service life. It should be enough to paint

only outside surfaces of the box girders.

Box girders as such have following distinct advantages over plate girders, considering

structural behaviour , aesthetics and maintenance, in Indian environment.

Box girders have better torsional behavior; better lateral and torsional stability,

compared to plate girders. Single box girders can therefore be handled and erected

without lateral bracings for longer lengths.

In Plate girders all the stiffeners are visible. In box girders all the stiffeners can be

provided inside the box; and hidden from the view. No external stiffeners are

needed. Thus external surfaces are clean.

External surfaces do not have horizontal projections, unlike plate girders. The

horizontal flanges are potential places to gather dust, and become eye sores later

during service life. Cleaning of these areas cause additional maintenance

requirements.

Depths of box girders can be lower than the depths of plate girders; and will result

in a sleek superstructure. However, adopting very sleek sections (L/D = 20)

normally requires higher thickness of the plates in the lower flange and webs. This

will need special precautions during fabrication, and especially during welding.

Quality control on fabrication and welding will have to be more stringent, needing

special tests and frequent inspection. Rejection of fabricated components can also

be expected. Considering that Steel Composite flyover systems in our country is still

in the process of development. It is advisable to restrict the L/D ratio to about 16,

which will result in plate thicknesses which are quite moderate, and fabrication will

be easier.

For obligatory spans also, two or three box girders can be adopted, instead of a

single box girder, for uniformity in appearance. However, considering larger length;

full length girder may not be feasible for these spans. A couple of staging towers

will be necessary for erection of fabricated units / segments; which will have to be

spliced in situ. The staging towers will have to be protected by temporary

barricading, and use of sand filled barrels with rubber tires around them, to take

care of accidental impact due to vehicles. If multiple box girders are adopted, it will

be necessary to provide intermediate diaphragms connecting the girders in

transverse direction; for improved structural behavior, considering longer lengths

of obligatory spans. These girders can also be in the shape of box girders, so that

general appearance of the superstructure is not vitiated by the transverse girders.

Box girders give batter aesthetic appearance. These can be with tapered webs.;

which may be preferred for aesthetics. However, if the alignment is curved, and

radius of curvature is smaller; curved box girders (in plan) may have to be adopted.

It will also improve aesthetic appearance considerably. With tapered webs,

fabrication of curved box girders will be quite complicated, needing much more

precision, and development of more detailed fabrication / shop drawings.

Proportion of wastage in fabrication will also increase, leading to increased cost for

fabrication. This point should also be considered while choosing type of box girders,

whether with tapered or with vertical webs.

6. Conclusion

This paper discusses different types of deck systems for urban flyovers, and advantages

of Structural Steel Composite bridge decks over prestressed concrete decks, in specific

urban areas where alignment passes through very congested locality and with many

restrictions on access route in transportation and erection of precast / prefabricated

structural components, for location, maneuvering and operation of erection

equipment. Solution for ease in maintenance by choosing suitable girder profiles is also

suggested.

7. Acknowledgement

The authors are grateful to the organizers for extending invitation to contribute paper

for this National Conference CEMCON 2014 on Composite Construction & Modern

Concrete.