SEISMIC RETROFITTING TECHNOLOGIES FOR BRIDGES TO REDUCE THE ENVIRONMENTAL IMPACT OF EARTHQUAKE

ON EXISTING STRUCTURES

1Benito PACHECO, 2Kazuhiko KAWASHIMA, 3Romeo ESTAÑERO

1Professor and Director, Institute of Civil Engineering, UP Diliman

2Professor, Department of Civil Engineering, Tokyo Institute of Technology

3Professor (Ret), Department of Civil Engineering, De La Salle University

Abstract: In the context of the Philippines and Japan, various technologies of seismic

retrofitting of bridges are classified in this paper. The most commonly required retrofits are

those of foundations, columns and piers, superstructure joints, and bearings. The

prevalence of soft ground conditions in these Asian settings is emphasized; in this regard,

retrofit of columns and piers is shown to be a practical compromise. The use of lead rubber

bearings and sliding bearings is encouraged as a form of seismic isolation of the

superstructure, to reduce the seismic force demands on the substructure in the first place.

Key Words: seismic retrofit, foundation, liquefaction, jacketing, seismic isolation

1. GOALS, MEASURES, AND LIMITATIONS OF RETROFIT

Seismic requirements for bridge design have been upgraded periodically to render the

newer bridges less vulnerable to earthquake excitation; accordingly, retrofits have become

necessary for many existing bridges that do not meet the new level of seismic design

requirements. Yet, bridges can be retrofitted only when doing so is technically,

economically and socially feasible; those that fail to satisfy these requirements are deemed

to be better replaced with new bridges (Priestley, Seible and Calvi 1996).

Observations of the nature of earthquake damages in the past are vital in the design of

seismic retrofitting works. Following are some examples of damage in past earthquakes in

the Philippines, Japan, and Taiwan. Many of the failures of bridge structures in the

Philippines as a result of the 1990 Luzon earthquake were mainly due to lateral spreading

of the ground due to soil liquefaction. Bridge foundations were affected and bridge decks

were unseated from their bearings. Damage due to soil liquefaction is exhibited by the

failure of the Magsaysay Bridge in Pangasinan, shown in Figure 1, and the Carmen Bridge,

shown in Figure 2 (Pacheco 2000).

In Japan while retrofitting works were initiated as early as 1971 (Kawashima 1990; and

Kawashima, Unjoh and Mukai 1994), more recent ones were essentially a consequence of

the 1995 Hyogo-ken Nanbu earthquake. The failures observed comprise mostly of the

following (for detailed documentation, see Kawashima and Unjoh 1997): (1) shear failure

at column mid-heights due to premature termination of longitudinal bars in concrete

columns with insufficient development lengths at cut-off point (Figure 3); (2) flexural

failure at base of reinforced concrete columns; (3) local buckling of web and flange plates

of steel columns accompanied by rupture of welds at their junctions in some cases; (4) soil

liquefaction resulting in settlement and tilting of foundations; (5) lateral spreading of

ground that caused movements of foundations; and (6) damage to bearings and unseating

prevention devices.

Figure 3 shows an example of reinforced concrete column failures in shear or in flexure.

Such observations resulted in the retrofit of more than 40,000 bridge columns and piers

that were evaluated to have the same deficiencies (Kawashima 2007). The 1995

earthquake caused much damage on the bearings, too, especially brittle failure in the steel

bearings.

Failures of many bridges in Taiwan resulting from the 1999 Chi Chi earthquake were a

direct result of a thrust-type faulting that caused large ground displacements. Unseating of

bridge decks was of destructive consequence so that some spans of multi-span bridges

collapsed; and piers were severely damaged (Figure 4; see also Kawashima and Shoji

2000; Hsu and Fu 2004; Chang et al, 2004).

Figure 1. Tilting of pier of Magsaysay

Bridge caused by soil liquefaction and

lateral spreading due to the 1990 Luzon

earthquake

Figure 2. Collapse of superstructure of

Carmen Bridge caused by soil liquefaction

and lateral spreading due to the 1990 Luzon

earthquake

Figure 3. Collapse of 18-span viaduct

section of Route 3, Kobe Line of Hanshin

Metropolitan Expressway due to the 1995

Figure 4. Failure of Wu-Shi bridge caused

by large fault displacement due to the 1999

Chi-Chi earthquake

Hyogo-ken Nanbu earthquake

Table 1. A Study of Bridge Deficiencies in the Philippines *

Item Type of Deficiency Frequency, %

1. Foundation Pile flexural strength

Pile shear strength

Soil liquefaction

Pile axial load strength

Footing flexural strength

Footing shear strength

Soil bearing pressure

40

24

20

19

8

4

4

2. Columns and piers Flexural strength

Shear strength

Confinement

Anchorage

80

14

11

5

3. Column cap beam Flexural strength

Shear strength

5

4

4. Pier bearings and abutment Bearing forces

Pier expansion joint displacement

Abutment expansion joint displacement

Abutment displacement

67

54

39

33

* Source: Angel Lazaro & Associates

As the justification for bridge retrofit is very much dependent upon the capacity-demand

(C/D) ratios of the various bridge elements, the extent of retrofitting works for a particular

bridge initially necessitates an accurate determination of the seismic demands. This is

complemented by an equally accurate assessment of the capacities of the various bridge

structural components to meet these demands. In the Philippines, as a consequence of the

1990 Luzon earthquake and as an aftermath of the 1995 Hyogo-ken Nanbu earthquake, a

major bridge retrofit program was initiated, too. For more than 350 bridges evaluated as

requiring retrofitting works, the more predominant bridge deficiencies for which the

capacity-demand (C/D) ratios were evaluated to be less than unity are given in Table 1

(courtesy of Angel Lazaro & Associates).

Thus, the retrofitting measures may cover: (1) foundations against soil liquefaction

(Figure5); (2) footings for shear and flexure; (3) abutments and approaches (Figure 6); (4)

columns and piers (Figures 7 and Figure 8); (5) cap beams of columns and piers; (6) cap

beam/column joint regions; (7) superstructure (Figure 9); and/or (8) isolators and dampers

(Figure 10). The seismic retrofit design is usually iterative, as any attempt to retrofit

certain components of a bridge may affect the seismic demands (hence put the capacity-

demand ratios in question) for other components. Research is ongoing to confirm the

performance of retrofitted bridges or bridge components, even as newer measures of

seismic retrofitting are being explored, developed, tested, and evaluated (Yashinsky 1998;

Kawashima 2007).

This paper briefly discusses various retrofitting measures. For their advantages and

disadvantages, applicability or suitability under certain site working conditions, retrofit

construction methods and procedures related thereto, and further information, the interested

reader may see the references cited at the end of this paper, and an extended version of this

paper in the accompanying CD of Kusakabe et al (2009).

2. RETROFIT OF FOUNDATIONS

Since the retrofit of a foundation is much more costly, and has more potentially negative

impact on the environment, than the retrofit of columns and piers, therefore a strategy of

seismic retrofit that does not require the retrofit of foundation should be sought first, before

considering the techniques that are cited in this section.

2.1 Considering Active Fault Crossings or Soft Alluvial Deposits

The most difficult (but the least common) situation for seismic retrofit design is the retrofit

for active fault crossing (Billings 2007). The expected fault displacements being large,

retrofitting in consideration of fault crossing involves any combination of the following

measures: continuity between adjacent spans; restrainer systems across the superstructure

joints; seismic isolation bearings; seat extensions; and strengthening of the foundations.

Figure 5. Seismic retrofit of a pile foundation by new piles and an extension of footing

The geotechnical and foundation-related deficiencies due to a design earthquake may in

fact be very varied in Asian countries like the Philippines and Japan, where alluvial

deposits or man-made sandy fills are frequently encountered at bridge sites, whether near to

or far from a fault zone (Towhata 2008, p34). Near the earthquake source, large ground

deformations may create the critical demand on foundations through instability of the

surrounding soils. In alluvial deposits or man-made sandy fills of low density, liquefaction

may cause the subsidence or the tilting of heavy foundations.

2.2 Considering Instability of Surrounding Soils

In unstable soil conditions, whole foundations have to be retrofitted in most cases. The

first option is to construct new piles by extending the existing footing. Figure 5 (as cited in

Kawashima 2007) shows an example. The other option is to improve the soils around the

foundation so that the level of seismic risk due to soil liquefaction can be mitigated.

However, this second option is often more costly and the net risk reduction is often more

difficult to ascertain.

As for abutments, liquefaction of sandy soil or sliding of soft clayey soils is a typical cause

of instability of the underlying ground. Bearings mostly suffer extensive damage. If the tilt

of the abutment is large, unseating of the deck from the abutment occurs. Retrofit may be

conducted by enlarging the footing and constructing new piles. Enhancement of soil

strength by soil replacement or cement mixing is also widely accepted for mitigating a tilt

that is resulting from instability of the underlying ground. Earth anchors are also used to

mitigate a tilt to the front direction.

Reconstruction of the approach slab whereby it is doweled into the abutment end (Figure 6)

may be expected to minimize the relative displacement that is due to lateral spreading of

the surrounding soil.

2.2 Considering Shear and Flexure Demand on Footings or Axial Demand on Piles

A need to retrofit footings may arise, too, from an increased seismic demand as a result of

the seismic retrofit of the columns and piers. For inadequacies in shear and flexure,

footing retrofit by concrete overlay may be used.

An alternative method of shear and flexural retrofit is to introduce prestressing forces into

the footing in the vertical and horizontal directions. Vertical prestressing enhances the

shear capacity while the lateral prestressing enhances both the shear and flexural capacities

of the footings. Prestressing strands are used for lateral prestress while aramid fiber

reinforced plastic rods are preferred for vertical prestressing. (Kato et al, 2001.)

Figure 6. Dowelling the approach slab into the abutment mitigates relative displacement

due to lateral spreading (courtesy of Angel Lazaro & Associates)

For more demanding situations, construction of a new footing surrounding the existing, as

shown in Figure 5 can accommodate this requirement.

For piles that are deficient in axial capacity, additional piles are generally driven around the

existing. Because space limitation under the deck makes driving of additional piles

difficult, a combination of new pile construction and soil improvement is sometimes

adopted. Where driving piles becomes difficult due to space limitations, additional pile

requirements can be attained with micropiles.

3. RETROFIT OF COLUMNS AND PIERS Reinforced concrete columns that were designed without taking into account the

importance of plastic deformation and ductility are commonly deficient in flexural

ductility, flexural strength, and/or shear strength under strong seismic excitation. In the

Philippines, for instance, it is noted in Table 1 that a large majority of the bridges that had

to be retrofitted were deficient in the columns or piers. Column or pier retrofit techniques

are usually accomplished by jacketing with steel (Figure 7 and Figure 8), reinforced

concrete, precast concrete segments, or composite materials such as fiberglass, carbon

fiber, and aramid fiber reinforced plastics.

Figure 7. Steel jacket retrofit at (a) Metropolitan Expressway and at (b) Hanshin

Expressway in 1989 which was effective during 1995 Hyogo-ken Nanbu earthquake

Figure 8. Steel jacketed piers of (a) Nagtahan Overpass (Manila) and (b) Ortigas Flyover

(San Juan-Pasig, Metro Manila)

Providing a jacket around an existing column that has insufficient ductility and strength

capacity is effective to prevent premature failure. The jacket resists not only tension and

compression but also shear of the column, as well as provides lateral confinement to the

core concrete. In the implementation of jacket retrofit, however, it must be noted that as

the jacket enhances the shear and flexural strength and the ductility of the columns, the

enhancement consequently also increases the moment and shear demand on the foundation.

Consequently, there are essentially two options. The first is to minimize the amount of

increase of moment and shear demand on the foundation. For this purpose, it is

recommended to provide a space between the jacket and the footing or the cap beam to

avoid excessive flexural and shear enhancement of the plastic hinge. The other option is to

allow a certain amount of increase of moment and shear demand on the foundation. If the

foundation has some excess of the moment and shear capacity, it is effective in restricting

the large plastic displacement of the column; otherwise some foundation retrofit may

become required at the same time. In either option, it is appropriate to avoid excessive

residual displacement of the column.

Figure 9. Typical unseating prevention devices for the superstructure [(1), (2), and (3) from

Kawashima et al 1994; (4) from Priestley et al 1996]

(a) RC Bracket (b) Steel Bracket

(1) Extension of Seat

(a) Cable Restrainer (b) Steel Chains (c) PC Strands

(2) Connection of Deck to Substructures

(a) Steel Decks (b) PC Decks

(3) Connection between Decks

(4) Device for Support Following Unseating at Internal Movement Joints

(a) RC Bracket (b) Steel Bracket

(1) Extension of Seat

(a) Cable Restrainer (b) Steel Chains (c) PC Strands

(2) Connection of Deck to Substructures

(a) Steel Decks (b) PC Decks

(3) Connection between Decks

(4) Device for Support Following Unseating at Internal Movement Joints

Reinforced concrete jacketing is generally cheaper than other retrofit measures, and it is

favorable in the retrofit of columns in water. However there is considerable increase in the

space occupied by the retrofitted columns. Precast concrete segment jacketing has an

advantage over reinforced concrete jacketing in reducing the construction period as well as

avoiding the drying-up of foundations; it is more expedient for columns that have the same

uniform size.

When retrofit can be conducted easily and quickly without interruption of traffic by using

composite materials, the use of materials such as carbon fiber-reinforced plastics, aramid

fiber-reinforced plastics and/or fiberglass can be justified from the total cost and

performance points of view. (Ogata et al 1999; Osada et al 1999)

Jacketing is more effective for circular than for rectangular columns. If using steel jacket

on rectangular column or pier, an elliptical section is recommended. Figure 8 shows

examples in the Philippines. A rectangular steel jacket is effective for enhancing the shear

and moment capacity of a rectangular column, but it cannot provide sufficient lateral

confinement in the critical region as the size of the column increases.

For column bents or framed piers, infill shear walls are found to be effective as well as

independent jackets of the columns, provided the seismic performance of the foundation is

carefully re-evaluated, too.

4. RETROFIT OF SUPERSTRUCTURE AND BEARINGS

The superstructure may also be retrofitted (Figure 9) to prevent unseating from the piers

and abutments in the event of large-displacement demand. Other bridge components can

be enhanced by corresponding retrofitting technologies (Kusakabe et al 2009); but the final

example in this paper is the use of lead rubber bearings and sliding bearings as a form of

seismic isolation as in Figure 10.

Seismic isolation reduces the force demands on the substructure and superstructure, but

increases the displacement demands.

Figure 10. Seismic retrofit using isolation: (a) installation of lead rubber bearings and

sliding bearings, and (b) after retrofit (courtesy of Japan Highway Public Corp.)

Seismic isolation may be justified as follows. Retrofits can be minimized if seismic forces

can also be minimized. If the seismic response of a bridge can be reduced, such as by

using isolators and/or dampers, to a level that does not require any direct retrofit of

structural components, there must be tremendous benefit in the cost and period of seismic

retrofit.

One the most important gains from the past earthquake damages is the increasing use of

seismic isolation devices for bridges. Japanese engineers are relying more on the use of

seismic isolation bearings for mitigating seismic vulnerability of bridges in their current

design. Vulnerable steel bearings are replaced with seismic isolation bearings that have

proven to be effective in reducing seismic forces in buildings.

Seismic isolation bearings are placed between the superstructure and the top of piers and/or

abutments. Under normal conditions, they behave like regular bearings but in the event of

a strong earthquake, they add flexibility to the structure by elongating its period and they

dissipate energy, allowing the superstructure to oscillate at a lower frequency than its piers.

Indeed, seismic isolation is effective not only for new bridges but also for existing ones.

Figure 10 shows an implementation of seismic isolation for retrofit. Because the existing

steel bearings were vulnerable to damage they were replaced with lead rubber bearings.

The existing steel bearings were made to carry the dead weight of the superstructure by

removing the stoppers so that they functioned as sliding bearings. The lead rubber bearings

were installed to provide the lateral restoring force and energy dissipation capability. Many

bridges in Japan supported on frame piers were retrofitted using this method.

ACKNOWLEDGEMENTS

This paper is based on the section contributed jointly by K. Kawashima, R. Estanero and B.

Pacheco to the book by Kusakabe et al (eds) 2009. The book writing was supported by the

Japan Society for the Promotion of Science and the Department of Science and Technology

of the Philippines through the JSPS Core University Program. Some of the data and

figures are courtesy of Angel Lazaro & Associates and Japan Highway Public Corporation.

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