SEISMIC RETROFIT/STRENGTHENING OF BRIDGE COLUMNS/PIERS

The retrofit of bridge columns/piers is conducted by one of two methods, the winding method and the sheet method. In the winding method, pioneered by Mitsubishi Kasei and Obayashi Corp., unidirectional tape is applied to the prepared concrete surface with fiber in the longitudinal direction. Fiber tow is then wet-wound on top in the circumferential direction. Once the circumferential winding is completed and the composite is cured, a fire-resistant coat such as cement mortar, calcium silicate, or an equivalent high temperature polymer coating is applied. The unidirectional tape provides flexural reinforcement, while the circumferential tows provide confinement. The method has been used only with carbon fiber reinforcement and is actually an adaptation from the methods used on chimneys and smokestacks. A schematic of the process is shown in Figure 4.13. The sheet method involves the adhesion of sheet or tape elements in the circumferential and/or longitudinal direction, with layers being applied one on top of the other until a sufficient number of layers/thickness is attained. Steps in this process are shown schematically in Figure 4.14. The entire process is akin to the wet layup process with compaction and impregnation of resin into fiber bundles achieved through manually applying pressure using hands, rollers or squeegees. This process is common to the application of Tonen's tow sheet, Mitsubishi Chemical's Replark and Toray's Torayca cloth.

The retrofit philosophy used in Japan is different from that prevalent in the United States. The primary goal of the Japanese is strengthening the column rather than increasing ductility or deformation capacity. Also a significant number of single column bents in Japan have longitudinal reinforcement that is cut off along the height based on cantilever response without consideration of tension stiffening in flexurally cracked zones. This is the primary reason for the application of vertically aligned fibers. In the case of aramid fiber-based systems, both tape and fabric are used for retrofit of the column. In both cases, application is through the wet layup process similar to that used for carbon fiber based sheet products (Fig. 4.15a -b).

Figure 4.16 shows an example of the use of Replark sheet material for strengthening piers cracked by seismic forces. Two layers of fabric were used first in the vertical direction for shear strengthening and then in the circumferential direction for flexural strengthening through confinement. Figure 4.17 shows the use of vertically aligned Replark sheets to strengthen existing piers of an expressway and facilitate the widening of the deck to accommodate greater traffic flow.


Fig. 4.13. Schematic of the "winding" process for column retrofit.


Fig. 4.14. Steps in the "sheet" process for column retrofit.


(a) use of circumferential wraps with FITS tape


(b) use of longitudinally oriented aramid fabri
Fig. 4.15. Use of aramid-based systems for column retrofit.


Fig. 4.16. Use of Replark to retrofit cracked "Shinkansen" piers.


Fig. 4.17. Use of Replark to strengthen piers to facilitate widening of the expressway.

The use of FORCA tow sheet in the repair of a cracked section of a pier on the Yodogawa Bridge in Osaka is shown in Figures 4.18 and 4.19. Interestingly, the tow sheet (carbon) was placed in the circumferential direction as a band rather than over the entire height. The pier has changing cross-sections and 4 individual supports are connected by diaphragm walls forming the entire pier. The pier's foundations are in water. Figures 4.20 and 4.21 show the application of carbon tow sheet in the haunch capital region of a column


Fig. 4.18. Overall view of the retrofitted pier. The band shows the retrofitted area.


Fig. 4.19. Close-up of a retrofitted section showing the placement of the carbon fiber tow sheet.


Fig. 4.20. Application of FORCA carbon tow sheet in the capital region of a column of the Tsushima Viaduct in Aichi Prefecture.


Fig. 4.21. Application of FORCA carbon tow sheet on the surfaces of a column of the Tsushima Viaduct in Aichi Prefecture, retrofitted to prevent further crack growth.

Bridge column/pier retrofit comprised about 55% of the sales volume for Tonen's tow sheet in 1996, and represented 12.3% of the use of Mitsubishi Chemical's Replark and fiber products between 1987 and 1995. Table 4.5 gives examples of the use of the Torayca cloth for retrofit of bridge column/piers.

Table 4.5
Examples of the Use of Torayca Cloth on Bridge Columns/Piers

Description of Structure

Date

Number of Layers

Quantity of Fabric (m2)

Repair of cracked piers on a railway bridge, Japan Railways, Kyushu

February 1984

1

Repair of a cracked pier on a road bridge in Okinawa Prefecture

June 1984

Repair of a cracked pier on a road bridge in Kobe

1982

  

Repair of a pier on a railway bridge deteriorated due to salt induced corrosion, Japan Railways, West

December 1990

1

100

Repair of cracked piers on an expressway, Japan Highway Public Corp.

February1992

1

200

Repair of viaduct columns for the Tokai branch of Japan Railways to cover an exfoliated surface

September 1992

1

230

Seismic retrofit of bridge piers on the national highway in Osaka Prefecture

February 1995

5

2500

Seismic retrofit of viaduct piers on an expressway, Japan Highway Public Corp.

November 1995

 

1084

Seismic retrofit of viaduct piers on an expressway, Japan Highway Public Corp.

November 1995

3-4 layers vertical and horizontal

800

Reinforcement of a pier in Ibaraki Prefecture

January 1996

 

39

Reinforcement of a bridge pier in Shizuoka Prefecture

January 1996

1 vertical, & 1 horizontal layer

440

Seismic retrofit of piers on a viaduct, Japan Highway Public Corp.

February 1996

1 vertical, & 1 horizontal layer

2000

Seismic retrofit of viaduct piers in Osaka Prefecture

February 1996

4-5 layers vertical and horizontal

7650

Seismic retrofit of piers on a road bridge in Aichi Prefecture

March 1996

3 layers horizontal

521

Seismic retrofit of piers for an overbridge in Nara Prefecture

March 1996

 

201.6

Seismic retrofit of piers on a national highway bridge for a Ministry of Construction Project

May 1996

 

2270

Seismic retrofit of piers on a bypass road for a viaduct in Saitama Prefecture

May 1996

 

98.8

Repair of a cracked pier on a bridge on the national highway in Toyama Prefecture

June 1996

 

1000

Repair of cracked/exfoliated piers on a bridge in Shizuoka Prefecture

June 1996

 

520.6

Seismic retrofit of piers on a bypass road in Saitama Prefecture

July 1996

 

734.6


Published: November 1998; WTEC Hyper-Librarian