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The Arch Bridge
An arch has been recognized as a safe, durable, economical, and aesthetic structure form for centuries and many such structures are still in existence. Till stone and brick masonry were common building materials, the arch form was regularly adopted, even for bridges carrying heavy loads.
The BEBO® Precast Concrete Arch Bridge
With the development of reinforced concrete, masonry arches were phased out. Ease and economics of casting straight precast or cast in situ concrete beams drastically reduced the use of the arch. The arch was revived in 1962 by Swiss Engineer Dr. Werner Heirerli at Massachusetts Institute of Technology.
The basic principle of support from an arch bridge is its curved design which does not push forces straight down, but are instead conveyed along the curve to the supports or abutments at each end. These supports carry the load of the entire bridge and hold the arch in its precise position. Dr. Heierli designed slender overfilled concrete arch structures which worked on the principle of the classical arch, and by virtue of its slenderness, the form flexed into the overfilled soil to invoke soil-structure interaction. Thus, the BEBO® (an abbreviation of German “BEton BOgen”, meaning concrete arch) System was conceived.
The BEBO® precast concrete arch comprises concrete strips of arch profile elements, either semi-elliptical or semi-circular. The width is generally around 1.8m which may vary according to project requirements. There may be vertical extension or curtailment to the profile. Since transportation of a complete profile would be a constraint, the elements are cast in two leaves and put together at site. The arch element supports fit within a groove on a pedestal running over the entire width on either side of the bridge. The respective pair of leaves of each element is brought together at the crown. After placing adequate connecting reinforcement to the main bars of the arch, exposed within the niche at the crown, the crown is concreted. The supports within the pedestal grooves are grouted. Theoretically, the arch is continuous at the crown and pin-jointed at the supports.
Bridge over IOCL Pipeline at NH-30
An Indian Oil Corporation Ltd (IOCL) gas pipeline crosses NH-30 between Patna and Bakhtyarpur at Ch 800+296, skewed at 49°. The alignment of the carriageway is also curved at the crossing. With the gas line in place below the ground, IOCL disallowed any surcharge on the ground at that location. This eliminated adoption of the convention cast in situ or precast concrete systems which require formwork supports. These factors made it favourable for adopting the BEBO® precast concrete arch bridge. The semi-elliptical Type E 60T/0 was selected to provide the required geometry. Clear span of the bridge at the top of the pedestal was 18.252m. Ground clearance up to soffit at the crown was 4m, which was acceptable to IOCL.
Considering the large span of the arch, a suitable area for casting yard was rented. Areas for various casting yard activities were planned out for smooth man, material, and equipment movement.
The area was rendered firm by rolling and gently sloped to ensure drainage of the site. PCC was placed over the ground to provide a firm, plane surface.
Casting and curing
The base of the mould was erected and levelled. The outer shuttering was placed in position and checked for plumb. The reinforcement cage and inserts were placed into position within the mould and the concrete leaf was cast. M45 concrete was used with 20mm downgraded aggregate. The grade of reinforcement steel was Fe500.
Concrete was vibrated by appropriately placed shutter and needle vibrators for dense concrete. During concreting and vibrating, special care was taken to ensure that lifting anchors were not displaced.
The mix design ensured that the mould could be struck by twenty four hours of casting to allow reuse, i.e., for casting the next leaf. The completed leaf was lifted and stacked on timber slats. The leaves were cured for fourteen days.
In view of weak sub-strata, the grooved pedestals for supporting the arch elements were piled. At each support 44, 1,000mm diameter bored cast in situ piles were provided to 27m depth. Piles were friction-cum-end-bearing. Piles and pile caps were constructed, while the arch leaves were cast. Both piles and pile caps were of M35 grade concrete with maximum aggregate size of 20mm.
To ensure perfect level of the arch elements, levelling pads of grout were cast within the grooves with level tolerance of 3mm.
Erection of arch leaves
The first pair of leaves, one of the end elements, was erected. Erection of the first pair required precision, since these set the starting-point for erecting other elements progressively. Timber shims were used for horizontal alignment.
After all the elements were erected, the grooves within which the arch elements were supported were grouted.
Gaps between the arches were treated to prevent seepage of water from the fill that could cause piping of fines. Coir rope was placed within the gaps and covered with jute that was saturated with cement slurry. This was topped with mortar and covered with nonwoven geotextile.
Thereafter, dowels were placed, tying exposed reinforcement within the crown. The crown was concreted with M45 concrete.
Since the essence of analysis of a BEBO® arch is soil-structure interaction, the type of soil used for backfill of the arch is significant. Soil gradation must essentially fall within the recommended envelope.
To this end, suitability and adequacy of local soils were studied. Backfilling was done on both sides of the arch, in rises of 400mm at a time on each side, to ensure balanced horizontal thrust on the structure. Compaction was done in 200mm lifts to 97% of maximum density as per the modified compaction test. Backfill was thus done to crown level. The pavement crust was constructed thereafter.
Advantages of the BEBO® arch bridge
The BEBO® system meets the challenges of cost and time constraints and superior quality requirements of highway development.
The System offers economy in design. Savings in concrete is possible up to 50% and almost 40% in reinforcement as compared to a convention bridge.
Construction is speedy. Leaves of the arch can be positioned within hours without disturbing the system being spanned. Once the backfilling is complete to 500mm above the crown, heavy construction equipment and normal traffic can ply over the bridge.
Compliance with BEBO® specifications for design and construction guarantees a quality product. Longevity of the concrete structure is assured. The structure requires virtually no maintenance considering that there are no bridge bearings, approach slabs, or decks to be maintained.
The BEBO® solution is eco-friendly, with lesser quantities of vital building materials required and virtually no maintenance.