Seismic Retrofitting of Existing Buildings
Seismic retrofitting is the modification of existing structures to improve their resistance to earthquake ground shaking. Many buildings constructed before the adoption of modern seismic codes are vulnerable to damage or collapse during earthquakes. The retrofitting of these buildings is essential for reducing the seismic risk in earthquake-prone regions. The first step in seismic retrofitting is the evaluation of the existing structure to identify its deficiencies. The evaluation uses the ASCE 41 standard for seismic evaluation and retrofit of existing buildings, which provides systematic procedures for assessing the seismic performance of existing structures. The evaluation considers the structural system, the material conditions, the load paths, and the connection details. The seismic performance level is defined as the maximum damage state that the building can sustain for a given earthquake hazard level, ranging from operational level with minimal damage through collapse prevention level with extensive damage but no collapse.
Common deficiencies in existing buildings that require retrofitting include inadequate lateral force resistance, brittle failure modes, weak stories, torsional irregularities, and poor connections between structural elements. Non-ductile concrete frames with insufficient transverse reinforcement in columns and beam-column joints are particularly vulnerable because they cannot develop the ductility needed to survive strong ground shaking. Soft story conditions where one level of the building is significantly weaker or more flexible than the levels above are a common deficiency in buildings with open ground floors used for parking or commercial space. The addition of new structural elements such as shear walls, braced frames, or moment frames can address the lateral force resistance deficiency. The new elements must be connected to the existing structure to provide a continuous load path from the roof to the foundation. The foundation must be upgraded to resist the increased overturning forces from the new lateral force resisting system.
The strengthening of existing concrete columns by adding concrete jackets, steel jacketing, or fiber-reinforced polymer wrapping increases the column strength and ductility. The concrete jacket adds a layer of reinforced concrete around the existing column that increases the cross-sectional area and provides additional longitudinal and transverse reinforcement. The steel jacketing wraps the column with steel plates that provide confinement and increase shear capacity. The FRP wrapping uses carbon or glass fiber sheets bonded to the column with epoxy resin to provide confinement that improves ductility and shear strength. The strengthening of existing beams by adding FRP sheets to the tension face increases the flexural capacity. The strengthening of beam-column joints by adding FRP sheets or steel plates in the joint region improves the joint shear capacity and prevents brittle failure during earthquake loading. The retrofitting measures must be designed by a licensed structural engineer with experience in seismic retrofit design.
Earth Retaining Structure Design
Earth retaining structures are used to maintain a difference in ground elevation where the natural slope of the soil would be unstable. The design of earth retaining structures requires an understanding of the soil properties, the groundwater conditions, and the loading conditions that will act on the structure. The lateral earth pressure is the primary load on retaining structures, with the magnitude of the pressure depending on the soil type, the wall movement, and the drainage conditions. The Rankine theory and Coulomb theory provide the classical methods for calculating active and passive earth pressures. The selection of the design earth pressure depends on the allowable wall movement, with active pressure used for walls that can tolerate some movement and at-rest pressure used for walls where movement must be minimized. fiber reinforced polymer wrapping for seismic column strengthening. mechanically stabilized earth wall design for highway embankments. construction surveying control network accuracy requirements. The design must also consider surcharge loads from adjacent structures, traffic, and stored materials that increase the lateral pressure on the wall.
Gravity retaining walls rely on the weight of the wall to resist overturning and sliding. The wall is constructed of mass concrete, stone, or concrete masonry units with a cross-section that tapers from a wide base to a narrower top. The stability of gravity walls depends on the friction between the base and the foundation soil and the passive resistance of the soil in front of the wall. Cantilever retaining walls use a reinforced concrete stem and base slab to resist lateral forces, with the weight of the soil on the heel contributing to stability. The reinforcement in the stem resists the bending moment from the lateral earth pressure, with the main vertical reinforcement on the tension face. Counterfort retaining walls with thin concrete walls spaced at intervals connecting the stem to the base are used for walls exceeding 20 feet in height where cantilever walls become uneconomical.
Mechanically stabilized earth walls use soil reinforcement to create a composite structure that resists lateral forces through tension in the reinforcement. The reinforcing elements are placed horizontally within the soil mass at regular vertical intervals. The tensile forces in the reinforcement provide the lateral resistance that allows the reinforced soil mass to act as a gravity structure. The MSE wall facing of precast concrete panels or wrapped geosynthetic provides erosion protection and a finished appearance. The design of MSE walls considers the internal stability of the reinforced soil mass, the external stability against overturning and sliding, and the overall stability of the retained soil and foundation. The reinforcement length is typically 60 to 80 percent of the wall height for adequate internal and external stability. MSE walls can be constructed to heights exceeding 50 feet and are economical for a wide range of applications including highway embankments and bridge abutments.
Construction Surveying and Layout
Construction surveying establishes the precise locations of structures, roads, and utilities on the ground according to the design plans. The surveying work begins with the establishment of horizontal and vertical control networks that provide reference points for all subsequent layout work. The horizontal control uses a network of points with known coordinates that are tied to the project coordinate system. The vertical control uses benchmarks with known elevations that are referenced to a vertical datum. The control network must be established with accuracy sufficient to meet the project tolerances, typically 1 part in 10,000 for horizontal control and 0.01 feet for vertical control. The control points must be located outside the construction area where they will not be disturbed by construction activities and must be protected and maintained for the duration of the project.
The layout of building foundations requires the precise positioning of the foundation corners and the establishment of offset lines that allow construction to proceed without disturbing the survey control points. The surveyor sets batter boards at each corner of the building that mark the foundation lines and provide reference points for excavation and form placement. The elevation of the foundation top is established from the vertical control network and marked on the batter boards or nearby benchmarks. As construction progresses, the surveyor checks the alignment and elevation of the work at each stage to verify compliance with the plans. The as-built survey documents the actual locations and elevations of the constructed work for record purposes and for verification that the work conforms to the design requirements.
The monitoring of construction includes the measurement of settlements, lateral movements, and vibrations that could affect adjacent structures or the construction itself. Settlement monitoring uses precise leveling surveys to measure the vertical movement of reference points on the structure or ground surface. Inclinometers measure lateral movements in excavations and slopes. The monitoring of vibrations from blasting, pile driving, and heavy construction equipment uses seismographs that record the particle velocity and frequency of the ground vibrations. The monitoring data is compared with established criteria to verify that the construction activities are not causing damage to adjacent structures. If the monitoring data exceeds the allowable limits, construction methods may need to be modified to reduce the impacts. The monitoring program should be designed by a geotechnical engineer or structural engineer with experience in construction monitoring.