The Isle of Palms Connector Bridge

Project Description

The Isle of Palms Connector Bridge. Located in the wetlands just north of Mount Pleasant, is approximately 11,700-feet long consisting or two lanes. Construction of the Connector Bridge began in October, 199O and was completed during the spring of 1993. The project summary is as follows:

  • PROJECT COST: $35,000,000
    • 5'-0" IN DIAMETER
    • 170 TOTAL
    • DEPTHS FROM 110' TO 180'
    • TOTAL LF OF SHAFTS = 21,850 6'-0" IN DIAMETER
    • 62 TOTAL ' DEPTHS FROM 120' TO 180'
    • TOTAL LF OF SHAFTS = 6,750
  • PIER CAPS: 116 ( 12,000 CY )
  • DECK CONCRETE: 24 000 CY (DECK LENGTH = 11,700 FT
    • AVG. 116' LONG
    • 749 TOTAL

The project specifications had very stringent requirements that all construction equipment and access to the protect could not damage the marsh that the bridge was to c cross. This required that the contractor design a method by which an of the construction operations be above the marsh. The two main falsework systems that were implemented were the trestle system and the modular crane rail system.

Process Description

The two most important parts of the project were the falsework systems. These two systems were dependent on each other to assure continuous movement.

Trestle System

The trestle system was 339-feet long and composed of six, 56-foot-long sections. Each section had a dead load of approximately 50 tons. Each trestle section was supported on each end by two 36- inch diameter by 80-foot pipe piles. When designing the trestle, the contractor had to implement modularity whereby it could advance itself ahead upon completing a pair of trestle piles. The trestle sections were used in a leap-frogging manner as the marsh was crossed The trestle supported three tractor cranes which were all involved in the drilled shaft work and the moving of the trestle ahead

The trestle system's main objective was to drill, form and pour the caissons for the bridge itself. The lead crane used a vibrator; hammer to drive the projects 60 and 72-in diameter steel casings through 50 to 100-feet of sand and silt strata into the dense, sand- clay Cooper mar! on which the bridge bears. The second crane moved forward to drill and clean out the caissons using an auger and then placed rebar and poured concrete. The third crane was used to forward the rear-end of the trestle deck and remove the trestle piles. These caissons were 116-feet apart with the trestle system completing three caissons a week.

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Concerning trestle support, a modular system had to be used. To assure project specifications were met, the trestle sections. after the cranes advanced, were picked up by the trailing crane and moved to the front of the trestle, avoiding any contact with the marsh. The lead crane then set the trestle section on the pair or previously driven 36-inch diameter trestle piles. When a given section is removed from the rear end and reset ahead, the piles which supported the back end of the trestle section which was moved were pulled and passed to the front. At this point, the trestle system has advanced :6-feet while leaving no damage to the marsh. This cycle continued throughout the project.

The final objective for the trestle system was to place 29-inch diameter pipe piles to support the overhead rail system. The placement of these piles followed the same pattern as used to place the trestle supports. Please see Fig. 1 to help clarify the Trestle System.

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Modular Crane Rail System

The second major falsework system designed was the modular crane rail system which supported the two overhead gantry cranes. The crane rail support system, which basically straddles the bridge, was composed of the pipe piles previously driven by the trestle system located 56-feet on center on each side of the bridge. A fabricated steel girder spanned the distance between the plies with a crane rail resting on top in order for the overhead cranes to run. There were two overhead canes that were 65-feet wide and 80-feet tall whose primary purposes were to transport project materials and advance the rail system. The rail system was advanced In similarity to that of the trestle system. A truck crane located on the completed concrete deck would. remove the rear section of the rail system and set it on the deck. The overhead cranes would pick up the rear end of the rail section and move it to the front of the system. The trestle cranes would then place that section on the previously driven piles. These piles were moved ahead in a similar leap-frogging manner as was used with the rails. The main difference between the trestle system and the crane rail system was that the crane rail system ran approximately 2200 linear feet from the completed deck out to the lead end of the trestle. The trestle system was only 339-feet long. The other primary use of the overhead cranes was to transport project materials. After the trestle advanced past a pair of caissons, the overhead cranes would form and pour the pier caps. The next phase included placing the precast concrete girders on the pier caps and then form the deck. The final step was to reinforce and pour the concrete deck.

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Model Description

The Cyclone Model that was developed for the present application reflects the construction methodology previously explained. The process just described can be broken down into several stages: 1) Delivery of material to trestle, 2) trestle cycle, 3) form and pour cap, 4) place girders and 5) form and pour concrete deck. Only a deterministic approach was used for durations of these stages due to lack of historical figures.

  1. STAGE 1 - DELIVERY OF MATERIAL TO TRESTLE : This portion of the model represents material delivery to the completed deck and then on to the trestle system by the use of the gantry cranes.
  2. STAGE 2 - TRESTLE CYCLE : This portion of the model places the pipe piles for the rail system, trestle system and bridge caissons. It is also responsible for the advancement of the trestle itself
  3. STAGE 3 - FORM AND POUR CAP: This put Lion of the model requires the gantry crane and crew to form and pour the pier caps. The curing of all the pier caps is also included.
  4. STAGE 4 - GIRDER PLACEMENT: This portion of the model utilizes the gantry crane to place the prefabricated girders onto the cured pier caps.
  5. STAGE 5 - FORM AND POUR CONCRETE DECK : This portion or the model requires a crew and gantry crane to form, place rebar and pour the concrete deck. After the deck is cured' this represents one complete unit of production.

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Major Resources Utilized For the CYCLONE Model:

LABOR RESOURCES 1 Forming crew
2 Gantry Cranes
1 Concrete Bucket
1 Concrete Truck
1 Material Truck
1 Girder Supply Truck
Rebar Steel
Precast Girders
Misc. Material

Durations Used for the CYCLONE Model:

Process Duration (min)
1. Load material truck 60
2. Unload material from truck 15
3. Move material to trestle 30
4. Place two rail piles 480
5. Place two trestle piles 480
6. Place two caisson casings 480
7. Move trestle section to front 120
8. Pull piles 60
9. Drill out caissons 480
10. Place rebar and pour caisson 120
11. Form cap and place rebar 240
12. Pour cap 30
13. Load bucket with concrete 6
14. Load and move concrete truck 30
15. Cure cap 1920
16. Pickup girder 30
17. Unload girder 15
18. Load and move girder truck 60
19. Place girder 15
20. Form and place deck rebar 240
21. Pour deck 120
22. Cure deck 1920

Assumptions Made for CYCLONE Modeling

Several assumptions were made to simplify the cyclic process far modeling Assumptions are as follows:

  • Crane #1 can receive materials (i.e. - pipe piles) without the assistance of Crane #2 and Crane #3.
  • Crew members on trestle are associated with crane operations.
  • Crew members for delivery cycles are associated with the trucks.

Model Enhancement

To try and improve productivity in our model, we decided to assume high early strength admixtures were incorporated into the cast-in-place concrete (pier caps and deck). These admixtures allowed us to reduce the curing times from 1920 minutes to 600 minutes.


The deterministic run time of the model was 98,700 minutes, completing 60 cycles. One cycle refers to approximately 116 linear feet of completed deck or one full section. Thus the production of the original mode' was 0.0365 sections per hour. Assuming a 50 hour work week, this comes to 1.723 sections per week.

Massman Construction reported that they had a productivity of approximately 1.5 sections per week. Our Cyclone model produced a 15% greater productivity. There are probably several explanations for this. One reason may be that in a Cyclone model start-up times are not taken into effect and in a project such as this, it would take time each morning to carry the workers out to the work trestle. Also, the cyclone model presented does not take into account the weather which was probably responsible for some delays. As stated earlier, this model used deterministic values which create a rather ideal scenario.

In our enhanced model, the productivity of the process increased even more. By lowering the cure times the later stages of the cycle were able to be completed earlier. In 99,180 minutes the enhanced model completed 62 sections, for a productivity of 0.0375 sections per hour. This comes to 1.875 sections per week, an improvement in productivity 8.7%. It appears that if an economical high early strength mixture could be used, it would save money and time.


Each permanent construction operation on this project, whether it be drilled shafts, pier caps. precast girders or deck construction, is directly affected by the performance of each operation. A good line-of-balance is critical during these operations. A slowdown at any point in the process affects the entire project. In order that each permanent operation proceed as scheduled. the temporary construction trestle ahead of the pier cap and deck construction must have its specified scheduled work accomplished on a daily basis.

The success of the cyclic, linear operation can yield an organized effort which produces a specified end product within budge+. Special attention must be paid to the critical importance of each activity scheduled through time, material, personnel, and equipment availability to achieve this success. Such was proven in the case of the Isle of Palms Connector Bridge.