Earthquake protection & Retrofitting in building

Earthquake protection of buildings

It means planning, designing and constructing a building so that it resists earthquake shaking without collapse, protects life, and limits damage. Complete prevention of damage in a strong earthquake is uneconomical, so the aim is: no damage in minor shaking, repairable damage in moderate shaking, and no collapse (life safety) in a major one.

How it is achieved

1. Site selection: avoid unstable slopes, soft or liquefiable soil, fault zones and landslide areas.
2. Seismic zoning and code compliance: design for the zone factor, soil type and importance of the building as per NBC 105 (seismic design) and NBC 201/202 (mandatory rules of thumb for small buildings).
3. Proper configuration: simple, symmetrical and regular shape, low height.
4. Strong, ductile structural system: RC frames with shear walls, ductile detailing as per IS 13920 / NBC 205.
5. Quality materials and workmanship.
6. Special measures: bands, vertical bars, bracing, base isolation or dampers for important buildings.
7. Retrofitting of existing weak buildings.

Principles adopted in earthquake resistant construction

1. Simple and symmetrical plan to avoid torsion; L, T and U shapes are separated by seismic joints.
2. Light weight of the building, because inertia force = mass x acceleration; the centre of gravity should be low.
3. Regularity in elevation: no sudden change of stiffness or mass (avoid soft storey).
4. Strength, stiffness and ductility: members must be able to bend and yield without sudden brittle failure (closely spaced stirrups, strong-column weak-beam).
5. Continuous load path: all parts tied together by bands/diaphragms and continuous columns and walls from foundation to roof.
6. Strong connections between beams-columns, walls-roof and walls-walls.
7. Rigid floors and roof diaphragm to distribute lateral force to vertical elements.
8. Good foundation: same type and depth for all footings, tied by plinth beams, resting on firm soil.
9. Small openings symmetrically placed, away from corners.
10. Seismic gap between adjoining buildings to avoid pounding.
11. Non-structural elements (parapets, partitions, water tanks, chimneys) properly anchored.

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.

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.

Techniques of seismic retrofitting

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.

Choice of technique depends on the type of building (masonry or RCC), condition survey results (visual and NDT), the expected earthquake level, cost, and working conditions (occupied building or not).

Configuration means the overall size, shape and arrangement of the structural and non-structural elements of a building. A good configuration is the cheapest way to improve seismic safety.

1. Simple, symmetrical plan

Square or rectangular plans resist earthquake best. Irregular shapes (L, T, U, H) twist and concentrate stress at re-entrant corners; if needed, separate them into rectangular blocks by seismic joints.

2. Plan aspect ratio

Length : width should not exceed about 3. Long buildings get different ground motion at the two ends and develop large diaphragm stresses.

3. Symmetry of mass and stiffness

The centre of mass and centre of rigidity should coincide to avoid torsion. Place stiff walls/cores symmetrically.

4. Regular elevation and avoiding soft storey

Avoid sudden changes in stiffness, mass or strength along the height (open ground storey, set-backs, heavy top floor). Walls and columns should be continuous from foundation to roof.

5. Light weight and low height

Earthquake force is proportional to mass, so use light roof/floor, keep the centre of gravity low, and limit the number of storeys.

6. Openings in walls

Openings should be small and centrally placed, away from corners; total opening not more than about 50% of wall length in a storey, and not closer than 0.5 m (or one-fourth of wall height) to a corner.

7. Seismic gap

Adjacent buildings should have a gap (about 20-40 mm per storey height as per code) to avoid pounding.

8. Strong foundation and continuous load path

Same foundation type and depth throughout, with the structure tied by plinth band/tie beams; a rigid diaphragm to transfer loads to walls.

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.

Jacketing

Jacketing is the most common retrofit method for RCC members. A new layer of material is placed around an existing column, beam or wall, to increase strength, stiffness and ductility, and confinement.

RCC jacketing

A new concrete layer with additional steel is cast around an existing column or beam.

1. Remove loose concrete and roughen the surface; clean the bars.
2. Provide dowels (12-16 mm bars) drilled and epoxy-grouted into the footing and slab.
3. Place new longitudinal bars and closely spaced closed ties (6-8 mm at 75-100 mm c/c).
4. Apply bonding agent and cast with high-slump, small-aggregate (10-12 mm) concrete or micro-concrete; jacket thickness 75-100 mm.
5. Cure for at least 14 days.

   Before          After (RCC jacket)
  +------+       +--------------+
  |      |       | +----------+ |
  | old  |       | | old col  | |
  | col  |       | +----------+ |
  +------+       +--------------+
                  new bars + ties

Steel jacketing

Steel plates (or angles with battens/straps) are placed around the column, the gap (about 25 mm) is filled with non-shrink cement grout, and the plates are welded or bolted. It adds confinement and shear strength with little increase in size and fast work, but needs fire protection and corrosion protection.

   Steel plates      Angles + battens
  +==========+       L----------L
  | grout    |       | batten   |
  |  column  |       |  column  |
  +==========+       L----------L

Advantages of jacketing: increases axial, flexural and shear capacity and ductility of columns/beams. Limitation: section size grows (RCC) and workmanship at the old-new interface is critical.

Principles for masonry

• Use simple, symmetrical plan; avoid long walls (keep length between cross-walls not more than 4-6 m).
• Keep building low (limit storeys as per NBC 202 / IS 4326).
• Use good mortar (cement-sand 1:4 to 1:6, not mud) and fill all joints; use bricks of crushed strength not less than 3.5 N/mm^2 (class per code).
• Keep openings small and well placed (see below).

Measures to make masonry earthquake resistant

1. Horizontal bands: RC plinth, lintel and roof bands tying all walls (see bands).
2. Vertical reinforcement: steel bars at corners, T-junctions and jambs of doors/windows, anchored in the foundation and the roof band. Typical: 1 bar 10 mm (1-storey) to 12 mm (higher) in each corner.
3. Openings: total width of openings not more than 50% of wall length in single storey; piers between openings not less than 340 mm; distance of an opening from an inside corner not less than 600 mm (or per NBC 202 table).
4. Cross walls and buttresses: provide cross-walls at spacing limits and buttresses at long walls to prevent out-of-plane failure.
5. Good bonding at corners: use toothing or stitching (and dowel bars) so that walls are not separate.
6. Light roof, anchored well: use light roofing; anchor trusses or rafters to the roof band; avoid heavy mud roofs and unanchored parapets, chimneys and gable ends.
7. Strong foundation: firm, uniform soil with plinth band; avoid mixed foundations.
8. Seismic gaps and separation: for L or T plans.
9. Quality control: good workmanship and curing.

 Wall elevation with bands and vertical bars
  |==================| roof band
  | |  []     []  |  |  vertical bar at jamb
  |==================| lintel band
  |                  |
  |==================| plinth band
  ######## footing

Factors to be considered for seismic safety (configuration)

• Simple rectangular plan with symmetric walls; length : breadth not more than 3.
• Regular elevation with continuous walls; low height and light roof.
• Limited openings, well placed.
• Cross-walls at regular spacing; good corner bonding.
• Uniform foundation on firm soil, with plinth band.
• Seismic gap between different blocks.
• Good quality bricks, mortar and workmanship.

Principles for masonry

• Use simple, symmetrical plan; avoid long walls (keep length between cross-walls not more than 4-6 m).
• Keep building low (limit storeys as per NBC 202 / IS 4326).
• Use good mortar (cement-sand 1:4 to 1:6, not mud) and fill all joints; use bricks of crushed strength not less than 3.5 N/mm^2 (class per code).
• Keep openings small and well placed (see below).

Measures to make masonry earthquake resistant

1. Horizontal bands: RC plinth, lintel and roof bands tying all walls (see bands).
2. Vertical reinforcement: steel bars at corners, T-junctions and jambs of doors/windows, anchored in the foundation and the roof band. Typical: 1 bar 10 mm (1-storey) to 12 mm (higher) in each corner.
3. Openings: total width of openings not more than 50% of wall length in single storey; piers between openings not less than 340 mm; distance of an opening from an inside corner not less than 600 mm (or per NBC 202 table).
4. Cross walls and buttresses: provide cross-walls at spacing limits and buttresses at long walls to prevent out-of-plane failure.
5. Good bonding at corners: use toothing or stitching (and dowel bars) so that walls are not separate.
6. Light roof, anchored well: use light roofing; anchor trusses or rafters to the roof band; avoid heavy mud roofs and unanchored parapets, chimneys and gable ends.
7. Strong foundation: firm, uniform soil with plinth band; avoid mixed foundations.
8. Seismic gaps and separation: for L or T plans.
9. Quality control: good workmanship and curing.

 Wall elevation with bands and vertical bars
  |==================| roof band
  | |  []     []  |  |  vertical bar at jamb
  |==================| lintel band
  |                  |
  |==================| plinth band
  ######## footing

Bands (ring beams) in masonry

A band is a continuous horizontal reinforced element (RC, reinforced brick or timber) running around all walls of a building at a given level. It ties walls together so they act as a box, and reduces bending of walls out of plane.

Types of bands (NBC 202 / IS 4326)

1. Plinth band: at plinth level, over the foundation wall. Needed when soil is soft or likely to settle unevenly; it also acts as a damp-proof course base and reduces differential settlement.
2. Lintel band: at lintel level of doors/windows; most important band. It ties walls together and carries lintel loads. Lintels can be merged in this band.
3. Roof band (eaves band): at the top of walls under the roof. Needed where roof is of sloping/flexible type (sheeting, timber, trusses) to anchor the roof.
4. Gable band: a band running along the top of gable-end triangular walls to hold the gable.
5. Sill band (optional): at window sill level, for extra stiffness in tall wall areas.

Details: thickness of RC band at least 75 mm; width equal to wall thickness; concrete M15 or richer; two bars of 8-12 mm diameter depending on the span (about 8 mm for spans up to 5 m, larger for longer spans) with 6 mm stirrups at 150 mm c/c. Corners and junctions must have continuous reinforcement lap, and the band must be continuous around the building.

   ____ roof band ______
  |  []   []   []   | <- lintel band (ring)
  |__|__|__|__|__|__|
  |                  | <- plinth band
  ======== foundation

Building shape strongly influences how the structure behaves in an earthquake, so shape is decided at the planning stage.

Shape in plan

• Best: square, rectangle or circle - symmetric, load paths are direct, no stress concentration.
• Poor: L, T, U, H, Y, cross shapes - the wings vibrate differently and stress concentrates at re-entrant (inside) corners, causing torsion and damage there.
• Remedy: divide complex plans into simple rectangular blocks separated by a seismic joint (gap), or add strong ties/diaphragm at junctions.
• Aspect ratio: length/width preferably not more than 3 (2 is better).
• Projections such as cantilever balconies should be small (not more than about 1.2 m), and centre of mass and rigidity should be close.

 Good plans              Poor plans
 +-------+   (o)         +--+   +--+
 |       |               |  |   |  |
 +-------+               |  +---+  |   U-shape
                         +---------+

Shape in elevation

• Best: uniform, regular, tall-thin or box shape with a gradual change.
• Poor: sudden changes - set-backs, large overhangs, open (soft) ground storey, heavy top floors, floating columns, short columns on a slope.
• Principles: mass, stiffness and strength should be uniform or reduce gradually with height; walls and columns continuous from foundation; avoid a very tall slender building on a small base (overturning).
• A soft storey (open ground floor for parking) concentrates damage on that floor; add shear walls or bracing.

 Good           Poor              Poor
 +--+          +--+               +----+
 |  |          |  |              |    |
 |  |         +----+ (heavy top)  |    | 
 |  |         |    |              +-+--+
 +--+         +----+              | |  soft storey

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.

It is done after a structural assessment (condition survey, NDT, calculation of the capacity versus demand) that identifies weak points. The common methods are jacketing, adding shear walls/bracing, strengthening of connections and foundation, wall plastering with mesh, and use of FRP.

Why it is necessary

• 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.

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.

Retrofitting a brick masonry building

Steps: (1) condition survey and NDT of the walls, mortar and foundation; (2) selection of method; (3) execution; (4) quality check.

1. Crack repair: inject cement or epoxy grout into cracks; stitch large cracks with steel dowels or staples.
2. Replace damaged parts: remove crushed bricks and re-lay in rich mortar (1:4).
3. Re-pointing: replace weak mud mortar with cement mortar to a depth of 20 mm.
4. Ferrocement / mesh jacketing: weld mesh or chicken mesh fixed on both faces of walls with through-wall ties at 450-600 mm, covered with 20-30 mm cement plaster (1:3). Improves in-plane strength and out-of-plane stability.
5. Adding RC bands: insert plinth, lintel and roof-level bands (in lengths) or provide external steel bands.
6. Vertical steel at corners and jambs: drill holes through walls, insert bars, and grout.
7. Strengthening openings: put RC frames or steel angles around large openings; close unnecessary openings.
8. Tying of floors and roof: anchor roof and floor to walls; provide diagonal bracing in flexible roofs; add an RC topping to make a rigid diaphragm.
9. Adding buttresses or shear walls at weak walls; replace heavy mud roof with a light roof.
10. Improving foundation: underpin or widen the foundation, add a plinth beam.

   mesh + plaster on both faces
   |#|   wall   |#|
   |#|----------|#|   <- through tie every 450-600 mm
   |#|          |#|

Retrofit versus repair

Jacketing of columns

Column jacketing increases the strength, ductility and confinement of weak columns by adding a layer of material around them.

• RCC jacket: after roughening the column surface, new vertical bars and closely spaced ties (6-8 mm at 75-100 mm c/c) are placed, dowelled to the footing and slabs, and concrete (min. 75-100 mm thick) is cast with a bonding agent.
• Steel jacket: steel plates or angles with battens are placed around the column and the gap is grouted.
• FRP wrap: carbon/glass fibre wrapped around the column.

 +-----------+
 | +-------+ |
 | |column | |  <- new bars + ties
 | +-------+ |
 +-----------+

It increases axial and shear capacity, but increases the column size.

Repair versus retrofit

Configuration factors for seismic safety

1. Simple, symmetrical plan

Square or rectangular plans resist earthquake best. Irregular shapes (L, T, U, H) twist and concentrate stress at re-entrant corners; if needed, separate them into rectangular blocks by seismic joints.

2. Plan aspect ratio

Length : width should not exceed about 3. Long buildings get different ground motion at the two ends and develop large diaphragm stresses.

3. Symmetry of mass and stiffness

The centre of mass and centre of rigidity should coincide to avoid torsion. Place stiff walls/cores symmetrically.

4. Regular elevation and avoiding soft storey

Avoid sudden changes in stiffness, mass or strength along the height (open ground storey, set-backs, heavy top floor). Walls and columns should be continuous from foundation to roof.

5. Light weight and low height

Earthquake force is proportional to mass, so use light roof/floor, keep the centre of gravity low, and limit the number of storeys.

6. Openings in walls

Openings should be small and centrally placed, away from corners; total opening not more than about 50% of wall length in a storey, and not closer than 0.5 m (or one-fourth of wall height) to a corner.

7. Seismic gap

Adjacent buildings should have a gap (about 20-40 mm per storey height as per code) to avoid pounding.

8. Strong foundation and continuous load path

Same foundation type and depth throughout, with the structure tied by plinth band/tie beams; a rigid diaphragm to transfer loads to walls.

Conventional strengthening methods

1. RCC jacketing of columns, beams, and walls to increase section and confinement.
2. Steel jacketing/plates/angles with grout; steel bracing (X or V) in frames.
3. Adding RC shear walls or infill walls to give lateral stiffness.
4. Wire-mesh/ferrocement plastering on masonry walls.
5. Strengthening foundations: enlarging footing, underpinning, adding tie beams, grouting soil.
6. Strengthening connections and anchoring roofs; adding bands and vertical bars to masonry.
7. Epoxy/cement grouting for cracks and honeycombs.
8. Reducing seismic demand: remove upper storeys, heavy parapets and tanks.

Disadvantage: adds mass, enlarges sections, is slow, and disturbs occupants.

Retrofit using innovative materials

1. FRP (fibre reinforced polymer) wraps/laminates: carbon (CFRP), glass (GFRP) or aramid sheets bonded with epoxy to columns (confinement), beams (flexure/shear), walls and slabs. Light, high strength, corrosion-free, quick, no section increase, but costly and weak against fire and needs skilled work.
2. Fibre reinforced concrete / shotcrete and ECC (engineered cementitious composites) for jackets and overlays.
3. Steel fibre or polypropylene mesh/ geosynthetic and GFRP grids in plaster overlays for masonry (e.g., polymer mesh used in Nepal after 2015).
4. Shape memory alloys and prestressed straps: active confinement and crack closing.
5. Base isolators and dampers (rubber bearings, viscous dampers): reduce the force reaching the structure; used in important buildings.
6. Textile-reinforced mortar (TRM) and high-performance mortar overlays.

These materials give high strength with little added weight, making them suitable for retrofitting without major disturbance.

Tests on existing buildings are carried out to assess the actual strength and condition of materials and members before deciding repair or retrofit, check quality of construction, find hidden defects (voids, corrosion, cracks), and decide whether the structure is safe.

• Destructive tests (core cutting, rebar sampling, load test to failure on samples, brick/mortar samples) give direct strength values but damage the member.
• Non-destructive tests (NDT) (rebound hammer, ultrasonic pulse velocity, cover meter, half-cell potential) assess the strength and quality without damage and can be done at many locations.

Both are often combined: NDT for wide screening and a few cores for calibration, so retrofitting is designed on real data.

Non-destructive testing (NDT) assesses the strength, quality and condition of a building's materials and members without damaging them, so that the structure can continue in service. It is used for quality control, damage and age assessment, and before retrofitting.

Common NDT methods

1. Rebound (Schmidt) hammer test (IS 13311 Part 2): a spring-loaded plunger hits the concrete surface; rebound number relates to surface hardness and approximate compressive strength. Quick and cheap; affected by surface moisture, carbonation and smoothness; gives only surface strength.
2. Ultrasonic pulse velocity (UPV, IS 13311 Part 1): measures the time of ultrasonic pulse through concrete; velocity > 4.5 km/s means excellent, 3.5-4.5 good, 3.0-3.5 doubtful, < 3.0 poor. Detects voids, cracks and honeycombing.
3. Cover meter (profometer): electromagnetic device to locate rebars, measure cover and bar spacing.
4. Half-cell potential test: detects probability of rebar corrosion by measuring electrical potential.
5. Carbonation test: phenolphthalein spray shows depth of carbonated concrete (pink = alkaline, colourless = carbonated).
6. Penetration (Windsor probe), pull-out test: semi-destructive, estimate strength.
7. Radiography / ground penetrating radar, infrared thermography: detect voids, delaminations, moisture, and rebar layout.
8. Core test (semi-destructive) is used to calibrate the above.

Results of different methods are combined for reliable assessment.