Why Buildings Need Retrofitting
Building retrofitting is the process of strengthening existing structures to meet current code requirements and improve resistance to earthquakes, wind, and other loads. Many buildings constructed before modern seismic codes are vulnerable to damage or collapse during earthquakes. The 1994 Northridge earthquake in California caused over 40 billion dollars in damage, much of it to buildings that had not been retrofitted. Older concrete buildings with non-ductile reinforcement are particularly vulnerable because their reinforcement details do not provide the ductility needed to survive seismic shaking without brittle failure.
The assessment of existing buildings begins with a thorough evaluation of the structural system, material conditions, and load paths. Non-destructive testing methods including ground-penetrating radar, ultrasonic testing, and core sampling determine the existing reinforcement configuration and material strengths. The evaluation compares the existing capacity with the demands from current code-level loads. Buildings found to have deficiencies are candidates for retrofitting to bring them up to acceptable performance levels.
Common Retrofitting Techniques
Steel bracing is one of the most effective retrofitting methods for concrete and steel frame buildings. Concentric braced frames add diagonal steel members that resist lateral forces through axial tension and compression. Eccentric braced frames provide ductility through a specially designed link beam that yields during earthquakes. Buckling-restrained braces overcome the limitation of conventional braces by preventing compression buckling, providing symmetric tension and compression behavior. fiber reinforced polymer wrapping for structural strengthening. lime stabilization for clay soil improvement. kitchen cabinet construction quality and materials. The installation of steel bracing typically requires foundation upgrades to resist the increased overturning forces at the brace connections.
Fiber-reinforced polymer wrapping has become a popular retrofitting technique because it adds strength without significantly increasing the structural weight. Carbon fiber sheets are epoxy-bonded to concrete columns, beams, and walls to increase flexural and shear capacity. The FRP wrapping also provides confinement that improves concrete ductility and compressive strength. The lightweight nature of FRP materials eliminates the need for foundation upgrades in many cases. The installation can be completed with minimal disruption to building occupants.
Concrete jacketting adds a layer of reinforced concrete around existing columns to increase their strength and stiffness. The jacket is typically 4 to 6 inches thick with longitudinal and transverse reinforcement tied to the existing column through drilled dowels. The added concrete increases the column cross-section, improving both axial and flexural capacity. Shear walls added to existing frames provide additional lateral load resistance and reduce drift demands on the existing frame.
Base Isolation Systems
Base isolation is the most advanced retrofitting technology, decoupling the building from ground motion during earthquakes. Lead rubber bearings installed between the building foundation and superstructure allow the building to move laterally during an earthquake while the ground moves beneath it. The isolation system shifts the building natural period away from the dominant frequencies of earthquake ground motion, significantly reducing the forces transmitted to the structure. Base isolation has been used successfully on historic buildings, hospitals, and critical facilities where continued operation after an earthquake is essential.
The installation of base isolators requires jacking the entire building to create space for the bearings. This complex operation must be carefully planned and executed to avoid damaging the structure. The building must be stiff enough to be lifted without excessive differential movement between columns. Once installed, the isolators require access for inspection and maintenance throughout the building life. The cost premium for base isolation is typically 5 to 15 percent of the building value for new construction but can be higher for retrofit projects.
Design Standards and Building Code Requirements
All construction work must comply with the applicable building codes and industry standards that establish minimum requirements for structural safety, fire protection, accessibility, and energy efficiency. The International Building Code provides the comprehensive framework for building design and construction in most jurisdictions. The code requirements for each building element depend on the occupancy type, the building height, the type of construction, and the seismic design category. The designer must review all applicable code provisions during the design phase to ensure that the design complies with every requirement. The permit review by the building department verifies that the design documents demonstrate compliance with the applicable codes before construction begins.
The material standards published by ASTM International, the American Concrete Institute, the American Institute of Steel Construction, and other organizations provide the specifications for material properties, testing methods, and quality control procedures. These standards ensure that the materials used in construction meet the minimum quality requirements for the application. The reference standards are incorporated into the building codes by reference, making them legally enforceable requirements. The contractor must verify that all materials meet the applicable standards through mill certifications, test reports, and product labeling. The quality control testing during construction verifies that the installed materials achieve the specified properties.
Construction Methods and Installation Procedures
The proper installation of construction materials and systems requires adherence to the manufacturer’s instructions and industry best practices. The installation procedures for each product are developed through testing and field experience to achieve the specified performance. The contractor must ensure that the installation crew is properly trained and qualified for the work. The quality of the installation is verified through inspections at each stage of the work. Any deviations from the specified procedures must be approved by the designer before proceeding. The documentation of the installation process provides the record of compliance for future reference.
The sequencing of construction activities affects the quality and efficiency of the work. The work must be planned so that each activity is performed in the correct order and with adequate time for preparation and curing. The protection of completed work from damage by subsequent activities is essential for maintaining quality. The coordination between different trades working in the same area requires careful scheduling and communication. The site conditions including weather, temperature, and humidity affect the installation procedures and must be considered in the planning. The contingency plans for adverse conditions ensure that the work can proceed safely and efficiently under varying conditions.
Quality Control and Inspection Requirements
The quality control program for construction includes the inspection of materials upon delivery, the observation of work in progress, and the testing of completed work. The inspector must verify that the materials meet the specifications and are properly stored. The observation of the work identifies any deficiencies that must be corrected before the work is concealed. The testing of the completed work verifies that the installed materials achieve the specified performance. The documentation of the inspection and testing results provides the quality record for the project. The non-conformance report documents any deficiencies and tracks the corrective action to completion.
The special inspections required by the building code for seismic and wind resistance must be performed by qualified inspectors. The special inspection program identifies the elements and systems that require continuous or periodic inspection during construction. The inspector must document the results of each inspection and report any non-compliance to the building official. The structural observations by the licensed design professional verify that the construction conforms to the design intent. The completion of all required inspections and tests is documented in the certificate of occupancy application.
Long-Term Performance and Maintenance
The long-term performance of construction materials depends on the quality of the initial installation and the maintenance provided throughout the service life. The exposure to weather, environmental conditions, and usage gradually degrades materials over time. The regular inspection of the building systems identifies deterioration or damage that requires repair. The preventive maintenance program schedules cleaning, lubrication, and component replacement at regular intervals. The life cycle cost analysis considers the initial construction cost and the ongoing maintenance costs over the building life. The selection of durable materials with appropriate maintenance requirements reduces the total cost of ownership.
The service life of building components varies widely depending on the material type, the environmental exposure, and the quality of maintenance. The roof coverings typically last 15 to 30 years depending on the material. The HVAC equipment has a service life of 15 to 25 years. The exterior finishes require repainting or refinishing at intervals of 5 to 15 years. The structural elements can last the full building life of 50 to 100 years or more with proper maintenance. The renovation and replacement of building systems at the end of their service life is a normal part of building ownership that must be planned and budgeted for.