Seismic strengthening of bridge piers
US-50 (Lincoln Highway) is one of the country’s last intact transcontinental highways, stretching 3,073 miles from West Sacramento, California to Ocean City, Maryland. Originally constructed in the 1960s, a section of the highway system in the Sacramento region needed extensive improvements to meet the current code. Utilizing an innovative approach, the Multimodal Corridor Enhancement and Rehabilitation (MCER) project expanded the needs of the Sacramento community by widening seven miles of carpool lanes and reconstructing 12 bridges in the process. The project also included replacing the existing concrete pavement, restoring roadway elements, and the seismic retrofit of bridge piers using steel jacketing.
Prior to 1972, concrete bridges were designed using older design codes and standards that did not include requirements for reinforcement detailing to ensure ductile behavior. The current seismic design practice, on the other hand, promotes the use of confinement reinforcement detailing to avoid brittle failure modes such as shear or plastic hinge failure when subjected to load cycles that push the element into the inelastic range of behavior. Bridge columns that were not designed to to avoid these failure modes would not be able to accommodate inelastic displacements when subjected to high seismic forces. In adequare reinfocement detailing of column regions where plastic hinges may form during the seismic event are particularly concerning. With this in mind, seismic retrofit of the bridge columns, such as those on the US-50 corridor, is essential to ensure life safety during an earthquake event.
The seismic retrofit scope of the Multimodal Corridor Enhancement and Rehabilitation (MCER) project included seismic enhancement of bridge columns in two locations on US-50: the Southside Park Viaduct bridge and the 15th–16th Street Viaduct bridge. Both bridges consisted of Cast-in-Place (CIP) box girder superstructures supported on 4-ft diameter columns with a hyperbolic flare at the upper 10 feet. Bridge column heights vary from 19 feet to 29 feet, with about 4 feet of the columns below grade, and supported on pile foundations. Both bridges were initially specified to receive a steel solution that consisted of half-inch thick steel jackets for the full height of the columns, including the top hyperbolic flare portion. The project scope for the Southside Park Viaduct included retrofitting 42 columns, and 24 columns for the 15th–16th Street Viaduct.
Steel Jacket versus FRP Jacketing
A common technique for improving the seismic behavior of columns when retrofitting is by using steel jacketing. Although there are many structural advantages to using steel jacketing, there are also a few disadvantages. The fabrication of special steel shapes to match existing column geometry, the use of multiple steel pieces to create the jacket, full-depth Complete Joint Penetration (CJP) groove welding, jacket modification to accommodate existing conditions, and grouting of the annular space between the steel jacket and the column are some of the disadvantages of using steel jacketing. As a result, the total cost of materials, fabrication, installation, and quality control of these jackets is typically high. An effective alternative to steel jacketing that can eliminate schedule delays and higher costs, is jacketing with Fiber Reinforced Polymer (FRP).
The use of FRP is gaining popularity in the repair and retrofitting of concrete structures. FRP composites are high-performance materials that can be used for seismic rehabilitation of reinforced concrete bridges, and offer many benefits for seismic retrofit applications. Some of these benefits include:
- High strength-to-weight and stiffness-to-weight ratios
- Material properties that can be tailored for unique applications
- The ease of handling and installing FRP wraps that reduce the project schedule and minimize disruption to traffic
- Corrosion resistance with practically no maintenance costs
- Increased strength and ductility without altering the stiffness of the structure
Economical retrofitting techniques using FRP jackets can be used to improve the seismic performance of bridge columns by increasing plastic hinge confinement, to improve rotation capacity as well as avoiding lap-splice failures. It can also increase the shear resistance of columns.
Pullman, the specialty retrofit contractor, proposed to use FRP jacketing in lieu of the specified steel jacketing. FRP provided an ideal solution for seismic retrofitting of the existing columns considering their non-uniform geometries. Moreover, unlike steel jacketing, FRP can be cut and shaped on-site, and the design can be easily modified for the varying demands of different regions of the columns. Pullman engaged Structural Technologies, a specialty retrofit consultant, to develop a value-engineered option that utilized FRP systems with the main objective of reducing costs while maintaining the project schedule and eliminating challenges associated with steel jacket installation.
FRP Jacket Design Approach
The columns in question have a hyperbolic flare at the upper portion and a circular section at the bottom portion. Seismic guidelines of the California Department of Transportation (CALTRANS) did not address the seismic retrofit of this type of column using FRP jackets. Therefore, the FRP retrofit solution had to be reviewed and approved by CALTRANS’ technical team before being considered as a viable option.
A performance-based approach was used for the design of the FRP jacket. To this end, WSP, The Engineer of Record, provided Structural Technologies with the target drift demands in longitudinal and transverse directions for the columns. Structural Technologies then developed analytical models to identify the location and length of plastic hinges in order to calculate the required plastic rotation capacity to withstand target drifts. The model consisted of frame elements with pinned support at the base and fixed support at the top. Figure 1a shows existing column geometry, and Figure 1b shows the analytical model D. The locations of the plastic hinge from the non-linear analysis occurred at mid-height of the column. The location of the plastic hinge occurred at the vertical reinforcing bar splice region, at the base of the flared top of the column. The location of the plastic hinge did not correspond to the location of the maximum moment.
To address the plastic hinge limited rotation capacity, lap-splice slippage, and shear strength deficiencies, a Lumped Plasticity Model (LPM) was utilized (Priestley et al. /2007). According to this approach, the plastic hinge rotation capacity (𝜃p) and ultimate displacement (Δu) at the tip of a cantilever concrete column can be calculated using an LPM in conjunction with bilinear moment-curvature curves, as described in equations 1a and 1b.
In which, 𝜙y, and 𝜙u, are the sectional curvature at the first effective yield and ultimate, and My and Mu are the moments at the first yield and ultimate condition, respectively. H represents the column height and Lsp and LP are the strain penetration and plastic hinge length. To ensure that the reinforcement at the lap splice region would not slip during the rotation of plastic hinge region, FRP jacket thickness (tFRP) was determined to provide the additional clamping pressure required at the splice considering equation
Where D is the diameter, fl and fh are required and existing clamping pressure, 𝜓FRP is the FRP strength reduction factor, and EFRP is the FRP modulus of elasticity. After calculating the minimum FRP thickness based on the required clamping pressure, the plastic hinge rotation capacity was calculated based on the idealized M-𝜙 curve. The idealized curve was constructed based on an elastic perfectly plastic response and the idealized plastic moment capacity was obtained by balancing the areas between the actual and the idealized M-𝜙 curves as specified in section 5.3 of CALTRANS’ Seismic Design Criteria Version 2.0 (2019). Figures 2a and 2b show the idealized moment curvature and moment rotation of the FRP confined plastic hinge region.
The FRP jacket required for the plastic hinge rotation and splice clamping pressure was extended to a length not less than the diameter of the column. A secondary FRP jacket was specified below the plastic hinge jacket to prevent the potential for local failure outside the main plastic hinge region. The secondary FRP jacket was designed to produce half the stiffness of primary plastic hinge jacket and extended half the length of the primary jacket. In this project, FRP layers outside the plastic hinge region were gradually reduced to avoid a sudden change in the confinement pressure. Finally, to avoid brittle shear failure of the enhanced columns and ensure that ductile flexural yield will be achieved, an additional FRP jacket was designed for the lower portion of the column to increase the shear strength of the columns as stipulated in equation 3.
Where 𝜙 is the resistance factor for shear considered as 1.0, Vn is the nominal shear capacity of the column, Vc , Vs , and VFRP are shear strength of concrete, steel and FRP jacket, and VP is plastic shear corresponding to the idealized plastic moment capacity of the designed M-𝜙 curve. It is worth noting that due to the expected cracking in the plastic hinge region, during a seismic event using the calculated concrete shear contribution was reduced, and the FRP jacket was checked to ensure it can provide the additional shear strength to cover the concrete shear strength losses. Figures 3a and 3b show a typical FRP layout along the column height and installation of the FRP jacket on a bridge column.
The FRP jacket solution was presented to both WSP engineers and CALTRANS technical staff. After addressing technical comments related to FRP detailing, the proposed FRP jacket solution was approved to be utilized to improve the seismic performance of the non-ductile concrete bridge columns.
FRP Jacket Installation
The existing columns extend 4-feet below grade; therefore, excavation was required to access the below grade portion of the columns and the FRP installation process ended up being broken into two phases. In Phase 1, the FRP jacket was installed on the upper portion of columns ahead of the excavation, and in Phase 2, the FRP jacket was installed on the column segment below grade. The following steps were used by the FRP contractor to install the FRP jacket:
- Concrete surface preparation was achieved using mechanical grinding to open up the concrete pores and grind down existing concrete form lines.. The target surface profile was CSP-3 as in ICRI Guideline 310.2R-2013.
- Applying epoxy primer coat to the prepared substrate and then using epoxy putty to smooth the surface and fill bug holes.
- Impregnating the dry FRP sheets with epoxy using mechanical saturators to control the amount of epoxy applied to the fiber sheets and avoid under or over-saturation of the sheets which can lead to dry fiber or sagging.
- Installation of FRP sheets.
- Using rib rollers to remove air bubbles after the installation of each FRP layer.
- After cure of the FRP jacket, applying the topcoat for long-term protection of the jacket.
Post-Construction Inspection
As with every project, it was essential to develop a quality control program to verify the process of the FRP installation and ensure that it can achieve the design properties and performance intent.
Multiple FRP panels were constructed onsite during FRP installation and cured on-site daily. A few of the panels were randomly selected and sent to a third-party laboratory for tensile testing per ASTM 3039 to evaluate the properties of the final FRP composite product installed on the columns. Visual inspections were used to verify the surface preparation prior to FRP installation. Acoustic sounding tests were performed on the installed FRP jackets to verify bond and check for any air pockets or gaps behind the cured FRP or between FRP layers. In addition, the bonding of the FRP jacket to the concrete substrate was verified by pull-off tests performed per ASTM D7522.
Innovative Improvements
The use of conventional construction materials or procedures does not always produce the most technically viable or cost-effective solutions. Utilizing FRP jackets on the US-50 Multimodal Corridor Enhancement and Rehabilitation Project illustrates how advanced FRP composites can be utilized to improve the seismic performance of concrete structures and provided a cost-effective alternative to conventional solutions. Table 1 provides a comparison of a few parameters for the FRP jackets versus steel jackets based on the current project that highlights the benefits of using FRP jacketing.
Seismic retrofit of older bridges and buildings is not the only area where advanced FRP composites can be utilized. FRP composites have been successfully used by the project team to address construction and/or design defects in newly constructed structures, as well as to increase their capacity for change in use, additional gravity loads, or to address conditions resulting from structural modifications such as the installation of openings and penetrations. Lastly, it is very important that FRP installers receive the proper training and undergo 2–5 years of experience before becoming qualified to work on complex projects.
By selecting a favorable installation sequence consisting of 90% of the upper portion of the columns before the excavation to expose the footing, Pullman was able to gain full access and availability of all columns at once by not having to rely on limitations and hazards related to excavation. Pullman, with the technical expertise of Structural Technologies, was able to work collaboratively with all stakeholders to explain the proof of concept and initial design of FRP for CALTRANS’ approval. The design and construction teams provided input for FRP optimization and specifications and supplied and installed a carbon fiber strengthening system that met the needs of the US-50 Multimodal Corridor’s structural needs. The project was completed on schedule and the final project cost was greatly improved through the value-engineering efforts of the project team.■
References
Priestley, M. J. N., Calvi, G.M., and Kowaisky, M.J., (2007) “Displacement-Based Seismic Design of Structures” IUSS Press, Pavia, Italy.
Seible, E, Priestley, M. J. N., and Innamorato, D. (1995). “Earthquake retrofit of bridge columns with continuous fiber jackets,” Vol. II, Des. Guidelines, Advanced Compos. Technol. Transfer Consortium, Rep. No. ACTT-95/08, Univ. of Calif., San Diego, La Jolla, CA.
California Department of Transportation, (2019), CALTRANS Seismic Design Criteria, Version 2.0, State of California.