In the early 1900s, reinforced concrete arch rib bridges with spandrel columns were designed mainly to carry vertical loads. These bridges were not designed or detailed to resist large seismic forces or accommodate large displacements induced by earthquake ground motion. The stability of a bridge structure in seismically active zones is dependent on the ability of its structural elements to resist the induced seismic forces and accommodate the large displacements of the superstructure without sustaining severe damage or collapse. The most common type of reinforcement used at the time was a series of longitudinal bars at the top and bottom of the section following the curve of the arch rib.
Today, most of these bridges are historic structures listed or eligible for listing in the National Register of Historic Places. Seismic analysis and retrofit of these arch bridges involves a thorough understanding of their structural behavior. Engineers must identify the load path the seismic forces take from the superstructure to the foundations as well as the deficient elements along that path. Retrofit strategies are then developed to strengthen the deficient bridge elements along the load path or modify the load path by strengthening only selected bridge elements. All retrofit strategies are to be developed bearing in mind the importance of minimizing environmental, aesthetic and cost effects. The seismic vulnerability of Fourth Street Bridge has been investigated.
An L.A. scene
Fourth Street Bridge over Lorena Street in Los Angeles, was constructed in 1928. It has been declared a city of Los Angeles historic monument and is also eligible for listing in the National Register of Historic Places.
The bridge consists of three main arch spans, two west approach spans and one east approach span. The bridge is approximately 389 ft long and 71 ft wide. Each arch span consists of four arch ribs connected transversely by concrete struts. Each arch rib has six spandrel columns supporting the deck floor beams. The west approach is a two-span 71.75-ft-long frame supported on a four-column bent and spread footings. The east approach span is 49 ft long, simply supported at the east abutment at one end and four columns at the east arch abutment at the other. The superstructure approach spans are cast-in-place concrete T-beams. The east and west abutments are high-cantilever type, approximately 24 ft high. The west abutment is supported on a spread footing, while the east abutment is supported on concrete piles.
Excluding the concrete deck, all other bridge elements above the ground are considered historic, especially the character-defining features of the bridge. These features are: the open arch spandrels; arched window railing; pylons; decorative light posts; and the color and surface texture of the cast-in-place concrete. The surface texture, known as the board finish, resulted from using the old wood forms. Any proposed retrofit should have minimum effect on the above character-defining features of the bridge and must conform to the secretary of interior’s standards for historic preservation projects. In-kind replacement of existing members of the bridge also is acceptable as long as it conforms to the secretary’s standards.
Vertical acceleration must be considered in the seismic analysis by combining it with the longitudinal and transverse acceleration. This will yield three load cases for the analysis as per the Caltrans Draft Guidelines for Seismic Analysis of Arch Bridges (1995). Load Case 1 consists of 100% longitudinal ground acceleration, 30% transverse acceleration and 30% vertical acceleration. Load Case 2 consists of 30% longitudinal ground acceleration, 100% transverse acceleration and 30% vertical acceleration. Load Case 3 consists of 30% longitudinal ground acceleration, 30% transverse acceleration and 100% vertical acceleration. A seismic as-built analysis was conducted for Fourth Street Bridge and the results indicated that the bridge is unstable under the Maximum Credible Earthquake (MCE) event, with a magnitude of 7.0 and peak bedrock acceleration of 0.75 g.
The bridge as-built deficiencies, which are typical for historic concrete arch bridges are as follows:
- Insufficient moment and shear capacities of the arch ribs, mainly at the base of the arch ribs;
- Insufficient ductility, moment and shear capacities of the spandrel columns, pier columns and transverse pier struts; and
- Instability of arch ribs after rib strut hinging (or failure).
Extent of the damage
The seismic performance criterion for retrofitting bridges should be preventing a collapse, either globally or locally, during an MCE event. Some bridge owners may specify a certain level of repairable damage after an MCE event depending on the importance of the bridge. The following is an acceptable level of damage for individual bridge components:
Arch ribs: Since the arch ribs are the main load-carrying element, they must remain “essentially elastic” under seismic loads. The ribs can undergo only limited inelastic deformations, particularly under cyclic loading, without losing significant load-carrying capacity due to reinforcing bar buckling or shear-strength degradation. The displacement ductility based on a combined biaxial flexure, shear and torsion seismic load must be less than two, provided that the ribs have adequate lateral reinforcement (confining steel). For arch ribs with no lateral reinforcement, the displacement ductility ratio should be limited to one.
Pier columns and spandrels: Inelastic deformation (plastic hinging) is acceptable in the pier columns, spandrel columns and struts, provided they have sufficient ductility and their axial and shear load-carrying capacities can be maintained while undergoing relative lateral deformations. Additional confinement will be required at plastic hinge location.
Arch rib struts: Plastic hinging, flexural failure or a combination of the two is acceptable in the parallel horizontal struts between the ribs, provided that the arch stability can be maintained at all load and deformation states.
Fourth’s fix
Retrofit strategies must be evaluated based on maintaining aesthetics and preserving the historic character-defining features of these bridges with minimum adverse effects. The retrofit strategies will be evaluated by the state’s historic preservation office. Below are some acceptable retrofits for historic arch bridges. Keep in mind that all new added concrete must match the color and texture of the historic concrete.
- Provide a 6-in. concrete jacket around the arch ribs, longitudinal reinforcement and lateral ties to increase the ductility, flexural and shear capacities of the ribs;
- Replace or provide a concrete jacket around the spandrel columns to increase the ductility, flexural and shear capacities;
- Provide a concrete jacket, replace or post-tension the main columns or piers; and
- Modify the load path by converting the bridge deck into a continuous diaphragm. This is accomplished by eliminating the deck expansion joints. More seismic loads will be transferred to stiffer bridge elements such as abutments and columns or piers, and lower seismic loads will be delivered to spandrels and arch ribs.
The retrofit strategy for Fourth Street Bridge has been developed to prevent the bridge from collapsing under the MCE event. The retrofit strategy was developed based on the above seismic performance criteria and the acceptable retrofits for historic bridges. It is detailed as follows:
Arch rib strengthening: A 6-in. concrete jacket will be constructed around the entire length of all arch ribs. Longitudinal reinforcing steel and lateral ties also will be provided. The longitudinal steel will be added by drilling holes in the arch piers and arch abutments and bonding the reinforcing steel. The provided lateral ties will be lap welded in order to provide the necessary cross-section confinement. The added longitudinal reinforcement will increase the flexural capacity of the arch ribs, while the added lateral ties will increase the ductility of the ribs.
Spandrel column replacement: The spandrel columns will be replaced in-kind. The new spandrels will have identical dimensions as the existing ones, but will have much higher moment, shear and displacement capacities.
Arch pier exterior column partial replacement: The exterior columns at the arch piers consist of two parts: decorative facia part carrying the sidewalk and railing and a main rectangular part carrying the deck floor beams. The facia part is lightly reinforced and will not be replaced, while the rectangular part will be removed and replaced with a new column. The new column will have identical appearance as the old columns but will have higher moment, shear and displacement capacities. A portion of the deck and floor beams at the new column locations also will be replaced.
Arch abutment column replacement: The east and west arch abutment columns will be replaced in-kind. The new columns will have identical appearance as the old columns but will have higher moment, shear and displacement capacities. A portion of the deck and floor beams at the new column locations also will be replaced.
Concrete finished surface: A board concrete surface finish will be applied to all new concrete in order to match the historic concrete texture.
The seismic investigation of as-built historic concrete arch bridges, such as the Fourth Street Bridge, indicated that the bridge structural main members do not have adequate strength to resist the forces and displacements induced by the MCE event. Generally, failure will occur in the arch ribs, spandrel columns, arch pier columns and arch abutment columns. A retrofit strategy was developed, in which the main load-carrying members were strengthened or replaced with ductile members having larger moment and shear capacities, while having approximately the same exterior dimensions. The retrofit strategy has very minimum impact on the historic character-defining features of the bridge and also very minimum visual impact on the aesthetics of the bridge.
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