2079 Bhadra — Regular

1. Elastic rebound theory

Draw the four stages: (a) original unstrained state, with a fence or road crossing the fault straight; (b) strain build-up over decades — the fence bends; (c) rupture when the strength is exceeded — the fence snaps; (d) rebound — the two halves are straight again but permanently offset. The cycle then restarts.

2. Origin of earthquakes in the Himalayas

The Himalaya is the classic continent–continent convergent margin:

1. About 225 million years ago the Indian plate separated from Gondwanaland and drifted northwards at roughly 10 cm/yr, the intervening Tethys oceanic crust subducting beneath Eurasia.
2. Around 55 million years ago the Tethys closed and India collided with Asia. Because both are continental crust of the same low density, neither could subduct beneath the other.
3. The collision pressure could only be relieved by crustal thickening and thrusting upward, raising the Himalayan range. This is why the Himalaya has great earthquakes but no volcanoes.
4. Convergence continues today; about 2 cm/yr of the ~4–5 cm/yr India–Eurasia convergence is absorbed across the Himalaya, so Nepal shortens by roughly 2 m per century.
5. The shortening is taken up on a system of north-dipping thrusts that all sole into one gently north-dipping detachment, the Main Himalayan Thrust (MHT): MFT (Main Frontal Thrust, southernmost and active), MBT (Main Boundary Thrust), MCT (Main Central Thrust) and STDS (South Tibetan Detachment System, a normal fault to the north).
6. The shallow part of the MHT is locked by friction between earthquakes. Strain accumulates for decades to centuries and is then released suddenly as a great earthquake — exactly the elastic rebound mechanism. The 2015 Gorkha earthquake (M_w 7.8) was a rupture of the locked MHT that propagated eastward from Barpak, Gorkha, at a focal depth of about 15 km.
7. Seismic gap: no great earthquake has ruptured the western Nepal segment of the MHT since 1505, so more than five centuries of strain is stored there. The 2015 event did not release it.

Draw a simplified north–south section: Indian plate underthrusting, the Himalayan wedge, the MHT with the MFT / MBT / MCT splays branching off it, and the typical focus at 10–20 km depth.

1. Magnitude

Magnitude is a quantitative, instrumental measure of the size of an earthquake — of the total energy released at the source. It is a single number for a given earthquake, independent of where it is measured.

• Richter scale (local magnitude M_L), 1935: based on the logarithm (base 10) of the maximum trace amplitude recorded by a standard Wood–Anderson seismograph, corrected to a standard epicentral distance of 100 km.
• An increase of one unit of magnitude = 10 times larger wave amplitude and about 31.6 (≈32) times more energy. So M7.7 releases ~31 times the energy of M6.7 and ~1000 times that of M5.7.
• Open-ended, can be negative, but it saturates above about M7.
• Moment magnitude M_w is derived from the seismic moment M₀ = µAD; it does not saturate and is now used for all large earthquakes (Gorkha 2015 = M_w 7.8).

2. Intensity

Intensity is a qualitative, observational measure of the actual effects of an earthquake at a particular place — on people, objects, buildings and the ground. Because the effects change from place to place, one earthquake has many intensity values but only one magnitude. It is highest near the epicentre and decreases outward; lines joining points of equal intensity are isoseismals, used to locate the epicentre of historic (pre-instrumental) earthquakes and to prepare seismic hazard maps. The scale used is the Modified Mercalli Intensity (MMI) scale with 12 grades, I to XII (others: MSK-64, JMA).

3. Comparison — Richter vs MMI (this table carries most of the marks)

Close with the approximate correlation: M 5–5.9 → MMI V–VI; M 6–6.9 → VI–VII; M 7–7.9 → VII–IX; M 8–8.9 → IX–X.

These are the three structural systems in which masonry is used. The distinguishing features are the load path and whether the wall is a structural element.

(A) Load-bearing masonry

• The masonry walls themselves are the main supporting elements — there is no separate frame.
• Load path: roof / floor slab → main (load-bearing) walls → plinth → foundation → soil. Both gravity and lateral loads travel through the walls.
• Walls parallel to the shaking act as shear walls (in-plane); walls perpendicular are loaded out-of-plane.
• Economical for 1–3 storeys; thickness increases rapidly with height. Unreinforced load-bearing masonry is brittle, so earthquake resistance must be supplied by bands, vertical bars, corner bonding and box action.

(B) Confined masonry

• Tie-columns (“practical columns”) are of much smaller cross-section than RC frame columns. They are not a moment frame — the masonry, not the ties, carries the load.
• The confining members are effective in: (1) enhancing the stability and integrity of the wall for in-plane and out-of-plane earthquake loads — they contain the damaged masonry; (2) enhancing the lateral strength; and (3) reducing the brittleness, thereby improving earthquake performance.
• Components: masonry walls, confining elements, floor/roof slabs acting as diaphragms, plinth band and foundation.
• It has emerged over the last 100 years as an economical alternative between unreinforced masonry and RC frame construction.

(C) Infill masonry

• Masonry panels enclosed by RC or steel frame members on all four sides. The frame, not the wall, is the designed structure.
• Load path: slab → beam → column → foundation. The infill is nominally non-structural.
• In reality it greatly increases the lateral stiffness of the frame, and under lateral load acts as a diagonal compression strut, so the infilled frame can be analysed as a truss.
• Dangers: a sudden loss of stiffness when the infill cracks can create a soft-storey mechanism; irregular distribution in elevation (the open ground storey) concentrates drift in one storey; partial-height infill produces the short-column effect.
• Conversely, continuous well-distributed infill has been shown to reduce the vulnerability of an RC structure.

Comparison table

Part 1 — In-plane vs out-of-plane behaviour

Part 2 — Failure modes

In-plane (four modes):

Out-of-plane (four mechanisms): simple overturning; composed overturning (dragging a wedge of the return walls); vertical flexure (hinge at mid-height); horizontal flexure (vertical crack at mid-span between cross walls).

Other typical damage: corner separation, delamination of multi-leaf walls, failure of piers between openings, gable and parapet collapse, roof separation, pounding.

Sketch to draw: one building isometric showing the in-plane wall with X cracks, the out-of-plane wall overturning, the diaphragm, and corner separation at the junction — all labelled.

1. Data collection and condition survey. Collect the as-built drawings; survey the building from outside and inside; record the structural system, materials, wall thicknesses, openings, roof type and the site conditions; note existing cracks and past interventions.
2. Seismic vulnerability assessment. Rapid visual screening first (e.g. FEMA 154 / NBC type checklists), then a detailed evaluation — material testing (NDT), damage grade classification (EMS G1–G5), and analysis of the existing capacity.
3. Identify the deficiencies. List exactly what is wrong: no bands, no through-stones, large or badly placed openings, weak mortar, heavy roof, soft storey, torsional irregularity, poor corner bonding, inadequate diaphragm.
4. Define the performance objective. Life safety, immediate occupancy or collapse prevention, and the corresponding target earthquake level.
5. Select the retrofitting strategy. Choose between increasing strength/stiffness (jacketing, new shear walls, bands), increasing ductility and integrity (vertical bars, corner stitching, confinement), reducing demand (lightening the roof, removing storeys, base isolation), or a combination.
6. Analyse and design the intervention, checking that the retrofitted structure meets the performance objective and that the new elements are compatible with the old.
7. Cost–benefit and feasibility check against the option of demolition and reconstruction; consider heritage value, disruption to occupants and available skills.
8. Prepare drawings, specifications and a construction sequence (shoring first, then integrity, then strength, then demand reduction).
9. Execute with strict quality control and supervision, followed by inspection and monitoring.

• Why: brickwork and blockwork have high compressive strength but very low tensile resistance, so steel is introduced to carry the tension — exactly as in reinforced concrete.
• Where the steel goes: vertical bars grouted into the voids created by particular bonds or into the hollow cores of blocks, and horizontal bed-joint reinforcement (ladder or truss mesh) laid in the mortar. Bed-joint reinforcement enhances the capacity of walls to resist lateral loading.
• Contrast with URM: unreinforced masonry is designed assuming the flexural tensile stress is resisted by the masonry, and any reinforcement present is neglected; the design is in the linear elastic range. So “unreinforced masonry” may actually contain steel — the term describes the design assumption, not the absence of bars.
• Benefits: real flexural and shear capacity, genuine ductility and energy dissipation, much better out-of-plane performance, taller walls and larger openings possible.
• Applications: shear walls, retaining walls, tanks, footbridges, reinforced masonry columns, beams and bands. Prestressed (post-tensioned) masonry adds precompression through vertical tendons.
• History: used in India and Japan since after the First World War, in America since after the Second.