Design Considerations in Repair of Structures
The rehabilitated structure may be required to provide equivalent safety to that of a new structure. The rehabilitated structure is brought to current building code standards. The design codes only provide guidance, and the engineer must apply basic principles of structural mechanics and must have an understanding of material behavior to evaluate and design a structural repair, strengthening procedure, or both. Following are some important considerations for the design of repairs:
Current load distribution:
In a deteriorated state, a structural member or system distributes dead and live loads differently than first assumed values when the structure was new. Cracking, deteriorated concrete and corroded reinforcement changes the load path and changes in shear, moment, and axial load distribution takes place. With the removal / replacement of concrete and reinforcement before and during the repair the force effects are further modified. To get the load distribution close to the original pattern, relieving of the load by jacking or other means may be required.
Compatibility of materials:
A difference in physical properties of the repair and the original material of construction increases the effects of thermal changes, vibrations, and long-term creep and shrinkage effects. Hence, either the repair material must have the same properties as the original material or the structure must be designed for the extra movements.
Creep, shrinkage, or both:
High creep or shrinkage of repair materials compared with original construction results in loss of stiffness of the repair, redistributed forces, and increased deformations.
When the placed repair material is in a plastic state or until adequate strength has been developed, vibration of a structure during repair of other parts can result in reduced bonding of the repair material.
Water and vapor migration:
Water or vapor migration through a concrete structure can degrade a repair and its control is essential for success of the repair.
In general, the contractor is responsible for construction safety but the major safety measures like the need and extent of shoring and bracing must be decided by the engineer.
Material behavior characteristics:
Sometimes the repair material may have totally different properties than the original one. The damaged steel reinforcement may be replaced by carbon fiber-reinforced polymer (CFRP) applied to the external bottom face of the beam. FRP is stronger than steel, but has a more elastic and brittle behavior. The provisions of reinforced concrete design codes are not applicable for the repaired part. The repair is carried out considering the properties of these materials to provide at least an equivalent level of safety to the original design.