Retrofitting techniques; why it is important

Techniques of retrofitting

Retrofitting is the strengthening or modification of an existing building so that it can resist earthquake forces better than it was originally designed to. It improves strength, stiffness, ductility and load path of the existing structure without rebuilding it.

1. Local repair (member-level)

Fill cracks by epoxy or cement grout injection, replace crushed concrete or broken bricks, and replace corroded bars.

2. Jacketing of members

• RCC jacket: extra concrete (min. 75-100 mm) with new longitudinal bars and closely spaced ties is cast around a column or beam.
• Steel jacket: steel plates or angles with battens are wrapped around the member and the gap is grouted.
• FRP wrapping: carbon or glass fibre sheets are bonded to give confinement.

3. Adding shear walls or bracing

New RC shear walls, infill walls or steel X-bracing are added in frames to increase lateral stiffness and strength.

4. Strengthening of foundation

Widening of footings, adding tie beams or underpinning if foundation is weak.

5. Strengthening of masonry

Ferrocement or wire mesh plastering on both faces, tying with cross-links; inserting RC bands at plinth, lintel and roof levels; vertical bars at corners and jambs; grouting of cracks; stitching of cracks.

6. Improving connections and diaphragm

Anchoring roof/floor to walls, tying walls with steel straps, converting flexible floors to rigid diaphragms by an RC topping.

7. Base isolation and energy dissipation

Isolators or dampers are placed to reduce earthquake force transfer (costly; used for important buildings).

8. Reducing weight or irregularity

Removing heavy parapets, water tanks, or upper storeys; adding walls to remove soft storey.

Importance of retrofitting

• Many older buildings in Nepal were built without engineering design or seismic code (NBC 105), so they can collapse in a moderate earthquake (as in Gorkha 2015).
• Retrofitting saves lives and property at a cost much lower than demolition and reconstruction (typically 10-30% of new construction cost).
• Seismic codes are revised; buildings built to old codes become deficient.
• Change of use, added floors, or deterioration (corrosion, cracks, aging) reduces capacity.
• Heritage and important buildings (hospitals, schools, temples) must be preserved.
• Damage from a past earthquake must be repaired and the building made safe against the next one.