New Belleair Causeway Constructed Using Incremental Launching Method by Bonnie L. Quick
Old draw bridge in background is contrasted with new piers about 35-feet tall. The new structure will have 75-feet of vertical clearance at its highest point and be about 3,400-feet long.
Back in the 1950s and ‘60s in Pinellas County, FL, as new residents flocked to the area many bridges were built to provide access to the beautiful beaches on the barrier islands. Now, 50 years later, the bridges, which were mostly drawbridges, are “coming to the end of their usefulness,” said Tony Horrnik, P.E., division engineer in charge of Pinellas County structure department and project manager for the Belleair Causeway project. The bridges, including the one at Belleair, are classified by the state as obsolete due to rising maintenance and operational costs resulting from deterioration. Type of Bridge Pinellas County carefully evaluated several alternatives and chose the one that best met the design criteria and standards outlined by the County, Florida Department of Transportation (FDOT) and the public. There were three options to choose from. Two designs were for drawbridges, one 25-feet high, and one mid-level with 45-foot clearance and a fixed-span at 74-feet high. According to Horrnik, “Although draw-bridge options create a lower construction cost, in the long run they are more costly to maintain and operate.” And a fixed-span bridge also eliminates the risk of a mechanical or electrical malfunction during an emergency evacuation. The project will provide the local community with a beautiful thoroughfare, improved boat ramp facilities and improved traffic flow. The new bridge will be built just north of the existing structure. Project improvements include 12-foot travel lanes in each direction, 8-foot sidewalks and 10-foot wide shoulders with designated 5-foot bicycle lanes. It will also include reconstruction of the boat ramp facilities at the east end of the causeway. “The old bridges are being replaced when possible,” continued Horrnik, “with structures that are less restrictive to traffic flow and are stronger and more aesthetically pleasing. The new structures are engineered to last 75 years and I think this one will outlast that estimate.” Bridge Design HDR Engineering designed the bridge, designers of more than 60 bridges in Florida, including the Howard Frankland that spans Tampa Bay connecting Tampa and St. Petersburg and the Clearwater Memorial Bridge to Sand Key. The design was done in accordance with Florida Department of Transportation (FDOT) criteria with special emphasis on aesthetics to be a gateway between downtown Clearwater and the beaches. This is a joint project between the city of Clearwater and FDOT with funding from the City, FDOT, Pinellas County and the federal government. “The bridge will not be ornate, but it will be elegant,” observed Horrnik. The Belleair Causeway will have a fixed-span with piers 150- to 175-feet apart, a roadway about 74-feet above ground at the pinnacle, with 100-feet of horizontal clearance at the center. Designed to eliminate traffic backups caused by the at-will raising and lowering of the present drawbridge for boaters, the fixed span will allow easy entrance or exit into the Gulf Intra-coastal Waterway. “We were able to go with a high level bridge on this site because we had enough land to increase the length of the bridge also, to increase the support of the structure, added Horrnik “In some of the locations it would be impossible to do because it would be both cost and socially prohibitive. In those places you have to do another drawbridge because of real-estate prices and resistance of residents to losing land for the support ramps.” Incremental Launching Method The incremental launching method (ILM) is one of the most highly mechanized erection methods used in bridge construction and although there are about 200 concrete bridges built using this method worldwide, it is new to Pinellas County. “A lot of new things are happening here,” said Horrnik. “For one thing we are going to be doing a fairly rare type of construction on the roadbed. It is called the incremental launching method of casting superstructure on abutments and then moving each section about 60 feet.” In this method segments of the bridge deck are manufactured in short sections of between 10-30 meters, on stationary formwork in a prefabrication area located behind one of the abutments. With ILM, the advantages of in-situ and pre-cast concrete construction are combined. The fabrication area is often covered to make it independent of the weather. Transport distances are short and the concentration of equipment permits almost factory-like conditions of construction and aids in producing a high quality fabrication area including the formwork, concrete mixing plant, a rail mounted lower crane, storage areas for reinforcing and pre-stressing the steel. The jacking equipment at the abutment and temporary bearings on the piers and in the fabrication areas aids in forward jacking operations. Each unit is concreted directly against the previous unit. After the pre-stressed concrete has hardened to sufficient strength the new unit is positioned against the previously cast unit. This assembly of units is then pushed forward the length of one unit (approximately 60-feet). This principal has been used for many years in the construction of steel bridges, but because concrete can withstand only small tensile stresses without damage, special methods must be employed to enable concrete sections to be slid forward. Prestressing is the most important of these measures. The ILM was used in concrete for the first time in 1962 and even today, Germany is the only country that routinely applies this method. The alignment of the bridge for this type of construction must be straight or curved with a constant rate of curvature. The constant rate of curvature applies both horizontally and vertically. The deck must have a constant depth, which is usually 1/12 to 1/16 of the longest span. To enable the superstructure to be moved forward, special low-friction bearings are provided at the piers and other support locations. Low-friction lateral guides are also provided to guide the deck in correct horizontal alignment. A real advantage of the incremental launching method is that the separate working operations occur in regular cycles so it is possible for high quality and output rates to be attained. A team of about 15 is usually trained to create a high rate of progress, usually constructing about one unit per week. This saves labor costs. Coffer Dams In addition to ILM to construct the roadway, cofferdams are used in building the under girding and support system during the construction of structures such as dams, locks, and bridges. A cofferdam commonly consists of steel sheets that are driven into the ground and supported by pile-driven posts. The walled enclosure thus formed is then pumped dry so that construction work can proceed. Cofferdams must be sturdy enough to resist the pressure of the water surrounding the cofferdam. “This is highly specialized work,” continued Horrnik, and the guys at Misener and Johnson Brothers are up to the task. “Much of the construction of columns is done in a cofferdam, a walled structure used to create a dry work space in an area otherwise covered by water.” “Foundations are driven deep enough to prevent scouring and erosion on the concrete pilings using high quality high performance concrete that contains added chemicals designed to last 75 years before needing replacement. Micro silica has such fine particles that it makes it impervious to the chlorides in the salt water and keeps it from invading the concrete, which retards the erosion process to make the structure last longer. According to Horrnik, public reaction to the project has been good so far. “It takes everyone from the designers, engineers, to the laborers to combine art and science of building.” Cory Butz, project engineer at Misener, agreed. “I have been in this business for 26 years and this is a fine effort.” “Working on this project has been outstanding,” said Max Smith, construction manager of Misener Marine. “The project is going well. It is a good joint effort for both Johnson Brothers and us. I was part of the Sand Key bridge project and have years of experience on many highway and other bridge projects. I just know if you do the right things at the bottom then it will be right at the top.”
