Incremental Launching Method Used to Build Bridge

Wed December 19, 2007 - Southeast Edition
Bonnie L. Quick



Not since the 1950s and ’60s have there been so many bridges under construction in Pinellas County, Fla. Back then, the bridges gave the new residents who were flocking to “Paradise” access to the beautiful beaches on the barrier islands.

Fifty years later, the bridges, which were mostly drawbridges, are “coming to the end of their usefulness,” said Tony Horrnik, division engineer of Pinellas County’s structure department and project manager of 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 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. Two designs were for drawbridges — one 25 ft. high (7.6 m) and one mid-level with 45 ft. (13.7 m) of clearance — and a fixed-span at 74 ft. high (22.5 m).

According to Horrnik, “Although drawbridge options create a lower construction cost, in the long run they are more costly to maintain and operate.”

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 bridge will be built just north of the existing structure.

Project improvements include: 12-ft. (3.6 m) travel lanes in each direction, 8-ft. (2.4 m) sidewalks and 10-ft.- wide (3 m) shoulders with 5-ft. (1.5 m) bicycle lanes.

It also will 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 Belleair Causeway. It has designed 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 is in accordance with FDOT criteria with special emphasis on aesthetics, as it will 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, county, state and federal levels.

“The bridge will not be ornate, but it will be elegant,” Horrnik said.

The Belleair Causeway will have a fixed-span with piers 150 to 175 ft. (45.7 to 53 m) apart, a roadway approximately 74 ft. (22.6 m) above ground at the pinnacle, and 100 ft. (30 m) of horizontal clearance at the center.

Designed to eliminate traffic backups caused by the at-will raising and lowering of the drawbridge for boaters, the fixed span will allow easy entrance or exit into the Gulf Intracoastal 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, Horrnik said “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.”

Between 160 and 170 drilled auger shafts (36, 48 or 60 in. in diameter [91.4, 122 or 152 cm]) will be poured into the sand, the reinforcing concrete adding stability to the entrance ramps. To aid in building the bridge bed, temporary steel pilings will be pounded in place to give additional support to the incremental launching method.

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 33 and 98 ft. (10 and 30 m) on stationary formwork in a prefabrication area 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 eliminate the impact of 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 and 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 ft. [18 m]).

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. Pre-stressing 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 the 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 the length of the longest span.

To enable the superstructure to move forward, special low-friction bearings are provided at the piers and other support locations. Low-friction lateral guides also are 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 people is usually trained to create a high rate of progress, usually constructing about one unit per week. This saves labor costs.

Cofferdams

In addition being used in ILM, cofferdams are used in building the under girding and support system during the construction of dams, locks and bridges.

“This is highly specialized work,” said Horrnik, adding that the crew at Misener and Johnson Brothers is 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 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 [and] engineers to the laborers to combine art and science of building.”

Cory Butz, project engineer of Misener, agreed.

“I have been in this business for 26 years and this is a fine effort.”

Max Smith, construction manager of Misener Marine, said, “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.” CEG