Article 2.10 Demolition
Removing Existing Segmental Concrete Bridges
By Myint Lwin (For posting 9/2003)
As bridge engineers we are more used to designing and building new segmental concrete bridges than demolishing them. Occasionally, existing bridges have to make room for improvement or expansion.
Three precast segmental concrete bridges have to be removed in the expansion program of the Florida's Fort Lauderdale-Hollywood International Airport1. The bridges, measuring 1,183 ft. (8 spans), 808 ft. (7 spans), and 477 ft. (4 spans), are only 15 years old and in good condition. These bridges were constructed by the balanced-cantilever method. However, due to the rapid growth in traffic, these two-lane bridges are already functionally obsolete.
How do we demolish segmental concrete bridges built by the balanced-cantilever method? By reversing the way the segments were installed. After studying the original as-built plans and analyzing the design forces, the design-build contractor and the design subcontractor2 decided to take down the bridges in the reverse way they were put up.
One of the three bridges was removed by making a saw-cut at the midspan, and then removing one segment at a time, alternating from one side of the cantilever to the other side. The segments were lowered to the ground and crushed by a pulverizer. Photo 1 shows the saw cutting and Photo 2 shows the lowering of a segment. This was a slow and tedious process.
Photo 1 Saw Cutting a Segment |
Photo 2 Lowering Segment for Crushing |
In the process of the removal, the contractor found that the grout in the post-tensioned tendons was sound and strong. There was no sudden release of the prestressing force. With this knowledge, the contractor was able to crush the segments in place by a pulverizer. The crushing started with the wings, then the webs as shown in Photo 3. The top and bottom slabs were then removed, as shown in Photo 4.
Photo 3 Removing the Web |
Photo 4 Removing the Bottom Slab |
Demolition of a segmental concrete bridge must be planned carefully. It should be started with an inspection and evaluation of the existing conditions of the bridge and then analyze the present state of the stresses. The stresses at various stages of the proposed demolition sequence should be checked to assure safety of the people and stability of the remaining portion of the structure.
For more information on the demolition of the three segmental concrete bridges described in this article, contact Dr. Luis Vargas of Beiswenger Hoch and Associates2 of Miami.
[DV1]
Reference:
1 August 2002 issue of Civil Engineering, Volume 72, Number 8, Page 28 - Segmental Bridge Carefully Dismantled.
2 Beiswenger Hoch and Associates, Inc., 1190 NE 163rd. Street, Suite 203, North Miami Beach, Florida 33162.
[DV1] When are the continuity tendons dismantled? Is this step 1?
