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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 aial load

distribution takes place. !ith 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 #acking or other means may be required.

Compatibility of materials:

$ 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.

&ence, either the repair material must have the same properties as the original material or the

structure must be designed for the etra movements.

Creep, shrinkage, or both:

&igh creep or shrinkage of repair materials compared with original construction results in loss of

stiffness of the repair, redistributed forces, and increased deformations.

Vibration:

!hen 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:

!ater or vapor migration through a concrete structure can degrade a repair and its control is

essential for success of the repair.

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Safety:

In general, the contractor is responsible for construction safety but the ma#or safety measures

like the need and etent of shoring and bracing must be decided by the engineer.

Material behavior characteristics:

'ometimes the repair material may have totally different properties than the original one. The

damaged steel reinforcement may be replaced by carbon fiber%reinforced polymer (CF)*+ applied

to the eternal bottom face of the beam. F)* is stronger than steel, but has a more elastic and

brittle behavior. The provisions of reinforced concrete designcodes 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.