The segmental way

Using the construction of the Hebbal-Devanahalli access controlled highway as an example, PV CHANDRAMOHAN dilates on what makes pre-cast segmental bridges an obvious choice. As part of the capacity enhancement programme of NH-7, the stretch from Hebbal to Devanahalli in Bengaluru was tendered out on BOT basis and the job was taken up by Hyderabad-based Navayuga Engineering Company. Expected to be commissioned by September 2013, the length of the stretch is 22.12 km and the existing highway there carried six-lane traffic. Since this is the major connecting point from the city to the busy Bengaluru Airport, it assumed great importance. The prerequisites The proposal involved providing for high speed uninterrupted passage of six-lane through traffic for the entire stretch. Most of this reach, being in or close to the city, is fairly well developed. At many places, there were intersections at grade and the traffic was being regulated at the crossings. The NH-7 traffic had to avoid crossings at grade, this meant that six lanes of traffic had to be carried on grade separators. But below the elevated carriageway, status quo of six lanes had to be maintained. This also meant that the elevated carriageway would have to be on single piers. Important crossings were required to provide for a mandatory span of 46 m. Since elevated carriageways are expensive, these were limited to three distinct reaches. The first one near the Devanahalli end is 571 m in length and will overpass one important junction. Next is 615 m overpassing another important crossing. The last one, the longest, is 3,726 m and will be passing over four busy crossings. The central median is 5.5 m wide at bridge locations and 2.0 m otherwise, this is to accommodate piers of the structure. Flanking the median, on both sides are two three-lane carriageways of 11 m in width. The traffic here has been protected by crash barriers outside. Covered drains have been provided beyond the crash barriers. Two service roads, with a two lane carriageway of 7 m and 1.5 m paved shoulders on either side of it to facilitate local traffic. On one flank of the highway, between the service road and the main carriageway, 5.0 m space has been set apart for a future high speed rail link. Segmental superstructure This paper focuses on the construction of the elevated structures leaving out the road portion. Bridge building will be easier and faster if the superstructure is pre-cast. This enables casting work of the elements of the superstructure to start simultaneously with foundation and substructure works. Once the pier reaches the bearing level, superstructure units, which are already cast, can be assembled above it. Assembling can be completed faster with any in-situ post tensioning and minor in-situ concreting work and this in turn helps commission the bridge in a shorter period of time. Casting of precast units requires a casting yard where elements are mass produced with precision on a large scale. One challenge with bridge structures in the city is that the space for casting yard is not available in or around the city. Being done in an interior area, hauling of the pre-cast units from the yard to the bridge site through the city's busy roads with sharp intersections is always difficult. This is regardless of the fact that most of the hauling is done during off-peak hours. If the units to be transported are of the span's length, hauling becomes more difficult. With all these difficulties pre-cast segmental bridges have become the order of the day. These are essentially box sections divided into a number of pieces longitudinally. In the case of a six lane segment, the weight of the segment governs its width. And if the carriageway is wider, segments are made narrower to reduce the weight. Single pier on six pile group However, the choice of foundation is mainly governed by the subsoil conditions. The bed rock in this site exists at an average depth of 10 m below ground level. The rock is granite with fairly high crushing strength. Magnitude of the loads is another factor. Bored cast-in-situ piles of 1,200 mm diameter were chosen. These piles were socketed into the rock and each pier was supported by six such piles. As the requirement was six lanes on the top and six lanes below, the super structure had to be supported on single piers. Supporting a 24.5 m wide section on a single pier meant designing for high bending moments brought about by the unbalanced vehicular loads from above. Post tensioning Segmental bridges are invariably post tensioned. It is very convenient to cut a girder into pieces to erect easily. Further, these are to be joined together by post tensioning over the piers. But the reinforcements will lack continuity between segments. Such problems have been solved by various means. Shear is resisted by shear keys provided between webs of the segment. Tension is induced by temperature gradient and this has to be countered by dedicated post tensioning. In this project, post tensioning is external. It reduces the weight of the segment because of a thinner web. Since the cables are external, it is easy to install and can be done after assembling the segments. Transverse stressing with strut Another important factor for reducing the weight of the segment is to reduce the thickness of the deck. Hence, to achieve lighter sections, the span of the web has to be reduced. Traditionally, this was done by an additional web, which increases the weight of the segment. But in this project, internal struts are provided to support the deck and reduce its span. Pre-cast struts are lighter and the deck is also transversely post tensioned using flat tendons. Also, following the long overhang of the deck from the web, the web is subjected to a heavy bending moment. Hence, the web also required some tensioning. Slab continuity Structurally, the girders are simply supported. But installing a structurally continuous girder is difficult and time consuming. Joints at the supports could be a discomfort to the traffic. So, the recent innovation of link slabs has been provided, making this a four-span continuous structure. The link slab will transfer translation but not moments, its stiffness being much less than that of the girder. Functionally, link slabs avoid joints and provide riding comfort. Bearings are of Pot/PTFE type. For instance, in a continuous girder, longitudinal horizontal forces are taken by fixed bearings in a module of four spans and modular movement joints are provided to accommodate the long movement of the spans in series. To ensure continuity in appearance at the sides, masking slabs are provided to conceal the gap. This enhances the structure's overall appearance. Modular construction Segments are cast with precision fabricated steel moulds. In one project, it is desirable to have only one external size for all the moulds. Structurally, the size of the segments will also be the same. But since the spans vary, the structural requirement in each span may be different. It is always a delicate exercise to decide on a common shape and vary the post tensioning forces in order to meet the structural needs. In short, the modular nature of the segmental construction is to be maintained. Segments of the larger spans will have to be accommodated in the same mould. Smallest span is 22 m, but at intersections, the mandatory span is 46 m. In this case, cantilever piers have been provided to reduce the effective span of the girder to 38 m to make it fit into the modular pattern. Launching of segments The segments are extra long, and being heavy, it requires special trailers for the haul from the casting yard. The launching girder picks up the segments one by one and suspends them in line with the span. First, they are dry matched. Cable ducts in the case of external post tensioning are nothing but HDPE pipes. They are inserted through the deviator blocks. Strands are then threaded through the pipes. Post tensioning the tendons is done next. In the case of the segmental bridge, stressing is done when the segments are in the suspended stage. As the stressing progresses, the girder is lowered on to the supports. First, it is lowered to span jacks and post the final alignment, to permanent bearings. Precision: the catch word Segmental construction technique has changed the concept of 'precision' in civil engineering. As a segmental bridge is made in the casting yard, nothing can be changed once the segments are brought to the site. In turn, these have to be adjusted to the horizontal and vertical curves of the bridge as well as the super elevation. There are two ways of casting to achieve the desired precision standards. In long line casting, moulds of the span are laid out in the casting yard to the actual alignment with camber. The important thing is that the preceding corners of the succeeding segment should match the succeeding corners of the preceding segment, and the soft moulds are twisted to achieve this. But long line method requires the entire span to be set out at the yard; this requires a long space. Another method is short line method. While this method requires special expertise, here in the succeeding side of the segment, already cast is used for match casting the succeeding segment. This is called match casting. About the author: Dr PV Chandramohan, President-Technical, Navayuga Engineering Company, Hyderabad, has been involved in the designing of several bridges and ports. He has developed a æbase load system' for the measurement of dredging, and a new type of armour block to resist wave action (awaiting patent).