Design Considerations in Repair of Structures
When rehabilitating a structure, it is important to ensure that it meets the same safety standards as a new structure. This requires bringing the structure up to current building code standards. However, design codes only provide guidance, and the engineer must also apply basic principles of structural mechanics and have an understanding of material behavior to evaluate and design the repairs or strengthening procedures needed.
One important consideration for the design of repairs is the current load distribution of the structure. In a deteriorated state, a structural member or system may distribute dead and live loads differently than originally assumed when the structure was new. Cracking, deteriorated concrete, and corroded reinforcement can change the load path and result in changes in shear, moment, and axial load distribution. Additionally, during the removal/replacement of concrete and reinforcement in the repair process, the force effects may be further modified. To restore the load distribution close to the original pattern, relieving of the load by jacking or other means may be required.
Compatibility of materials is an important factor to consider during repairs of structures. If there is a difference in physical properties between the repair material and the original construction material, it can increase the effects of thermal changes, vibrations, creep, and shrinkage over time. To mitigate this, the repair material should either have similar properties to the original material, or the structure must be designed to accommodate the extra movements caused by the differences in properties.
Creep and shrinkage are also important considerations during repairs. If the repair material has high creep or shrinkage compared to the original construction material, it can result in loss of stiffness in the repair, redistribution of forces, and increased deformations. Vibration during the repair process can also affect the bonding of the repair material, especially if it is in a plastic state or has not yet developed adequate strength.
Water and vapor migration through a concrete structure can degrade a repair, so controlling water and vapor movement is essential for the success of the repair. Safety is also a crucial factor, with the contractor generally responsible for construction safety. However, the engineer must decide on major safety measures such as the need and extent of shoring and bracing during the repair process.
Another important consideration is the behavior characteristics of the repair material. Sometimes, the repair material may have significantly different properties compared to the original material. For example, damaged steel reinforcement may be replaced with carbon fiber-reinforced polymer (CFRP) applied to the external bottom face of a beam. CFRP is stronger than steel but has a more elastic and brittle behavior. In such cases, the provisions of reinforced concrete design codes may not be applicable to the repaired part, and the repair must be carried out considering the properties of these materials to ensure at least an equivalent level of safety as the original design.