Seismic Behavior of Masonry Structures

3.1  Basic Concept of Seismic Effect in Masonry Structures; Failure Mechanism

3.1.1  How do masonry houses behave during earthquakes?

Masonry buildings are brittle structures and are one of the most vulnerable of the entire building stock under strong earthquake shaking. The very large number of human fatalities in such buildings in past Nepali earthquakes is the direct consequence. Improving their seismic behaviour is therefore the central purpose of this course.

1. Ground vibration during an earthquake causes inertia forces at the locations of mass in the building — principally at the roof and floor levels and in the walls themselves.
2. These forces travel through the roof and walls to the foundation. The whole emphasis of earthquake-resistant design is to ensure that they reach the ground without causing major damage or collapse.
3. Of the three components of a masonry building — roof, wall and foundation — the walls are the most vulnerable to the horizontal force.
4. A wall topples easily if pushed horizontally at the top in a direction perpendicular to its plane (its weak direction) but offers much greater resistance if pushed along its length (its strong direction).
5. The ground shakes simultaneously in the vertical (Z) and two horizontal (X, Y) directions, but the horizontal vibrations are the most damaging to normal masonry buildings.
6. If all the walls are not tied together like a box, the walls loaded in their weak direction simply topple. If they are, the weak-direction walls can take advantage of the good lateral resistance offered by the walls loaded in their strong direction. The walls must also be tied to the roof and to the foundation to preserve overall integrity.

3.1.2  Shear walls and cross walls

• Shear walls are predominantly subjected to in-plane shear stresses and in-plane bending stresses. The in-plane bending stresses act normal to the bed joints.
• The in-plane shear stresses are responsible for the typical X-type cracking in shear walls; the in-plane bending stresses tend to cause separation of cross walls and shear walls at their junction.
• Although severe cracking may occur, a shear wall will not usually collapse outright unless a component of the ground motion acts normal to it — which, of course, it does a moment later.
• Stress concentration near openings in shear walls adds substantially to the vulnerability.

3.1.3  Concepts for earthquake resistant masonry

The basic principles of design and detailing in the codes are intentionally simple and easy to adopt. They focus on just two things:

The concept of “acceptable risk”

Certain critical structures — hospitals, power generating units, communication centres — must be designed to remain operational during and after an earthquake.

Unreinforced masonry buildings are very common in rural and semi-urban Nepal, built from adobe, stone, burnt brick, concrete block and stabilised mud block, with mud, lime, cement or composite mortar. Normally they are designed — if at all — only for vertical loads, and because masonry has adequate compressive strength the structure behaves well as long as the loads remain vertical. It is the horizontal load that destroys it.

3.1.4  The three basic failure mechanisms of a masonry wall

Magenes and Calvi (1997) reduced the in-plane seismic behaviour of a masonry wall to three principal mechanisms. Every in-plane failure observed in the field is one of these, or a combination:

3.2  Historical Buildings with Seismic Resisting Features

Long before any code existed, builders in seismic regions arrived empirically at features that improve performance. Recognising them matters because (a) they explain why some historic buildings survived 1934 and 2015, and (b) they are the ancestors of the modern band-and-tie system.

3.3  Lateral Force Resisting Systems

3.3.1  The load distribution system

A building is subjected to two families of load:

• Vertical loads — dead load of the materials of construction plus live load due to occupancy;
• Lateral loads — seismic forces and wind loads.

All walls in general can carry vertical load, but the ability of a wall to carry lateral load depends entirely on its disposition relative to the direction of that load. The chain is:

The strength and stiffness of the floors as horizontal girders is vital; roofs and floors of lightweight construction must therefore be used with care. A structure must have adequate stability in the direction of both principal axes — so-called “cross-wall” construction may have very little lateral resistance in the longitudinal direction. In multi-storey buildings a cellular or box-type layout is desirable for both stability and economy.

The size, shape and location of openings in the external walls has considerable influence on stability and on the magnitude of the stresses due to lateral load. If the openings in the longitudinal walls are placed so that portions of those walls act as flanges to the cross walls, the strength of the cross walls is considerably increased and the structure becomes much more stable — the flange-and-web action confirmed by shaking-table tests, in which walls A act as flanges to wall B acting as the web.

Ordinarily a load-bearing masonry structure is designed for permissible compressive and shear stresses (with no tension) as a vertical cantilever. No moment transfer is allowed for at the floor-to-wall connections, and the lateral forces are assumed to be resisted by diaphragm action of the floor/roof slabs, which, acting as horizontal beams, transmit the lateral force to the cross walls in proportion to their relative stiffness (moment of inertia).

3.3.2  Design loads and load combinations

3.3.3  Lateral load distribution — rigid and flexible diaphragms

Lateral loads from earthquake or wind are generally considered to act along the principal axes of the building. How they are shared out among the walls depends on the rigidity of the horizontal diaphragm and of the walls themselves.

3.3.4  Elements of the lateral load resisting system

The elements that actually resist lateral load in a masonry building are:

• Shear walls — the primary in-plane resisting element
• Floor and roof diaphragms — collect and distribute the force
• Anchorage between slabs and walls, and anchorage of the roof to the walls
• Tie beams and tie columns (confined masonry)
• Plinth band, sill band, lintel band, roof band and gable band
• Vertical containment reinforcement at corners, junctions and opening jambs
• Toothed or stepped vertical joints at wall intersections

3.4  In-plane and Out-of-plane Behaviour of Walls 2079 Bhadra, Q2b

3.5  Box Integrity Action of Masonry Structures

3.5.1  Why it is needed — the four-wall argument

Consider the four-wall system of a single-storey masonry building, with walls A1, A2 (say the long walls) and B1, B2 (the short walls). For shaking in the direction shown:

1. Inertia forces act in the strong direction of some walls and the weak direction of others.
2. Walls shaken in their weak direction seek support from the other walls — B1 and B2 seek support from A1 and A2.
3. More precisely, wall B1 pulls walls A1 and A2, while wall B2 pushes against them.
4. At the next instant the shaking direction reverses, and a moment later it acts along the perpendicular horizontal axis — now A and B swap roles: B1 and B2 become the strong walls and A1 and A2 the weak ones.
5. Thus walls transfer load to each other at their junctions, and through the lintel bands and the roof. Hence the masonry courses of the walls meeting at a corner must have good interlocking, and openings near wall corners are detrimental to good seismic performance.

3.5.2  Requirements for good box action

3.5.3  Configuration rules that support box action

• Break up complex plans. A building with horizontal projections in plan — shapes such as L, T, E and Y — should be separated into (almost) simple rectangular blocks, each of which has simple and good earthquake behaviour.
• Provide a seismic gap. Separated blocks oscillate independently and will hammer each other if they are too close; the codes give minimum seismic separations. Such separation may not be necessary if the horizontal projections are small — say up to about 15–20% of the length of the building in that direction.
• Watch the staircase. An integrally connected inclined staircase slab acts like a cross-brace between floors and transfers large horizontal forces at roof and lower levels — a classic area of damage. The remedy is to separate the staircase completely and build it on its own RC structure, with an adequate gap so the two do not pound.
• Keep the mass low and the plan compact. Large, tall, long and unsymmetrical buildings perform poorly.

3.6  Modes of Failure of Masonry Structures 2075 Chaitra2076 Chaitra2079 Bhadra2080 Bhadra

3.6.1  The standard seven-fold classification

The failure pattern of masonry structures during an earthquake is classified as:

1. Out-of-plane flexural and/or out-of-plane shear failure
2. In-plane shear and/or in-plane flexural failure
3. Separation of walls at junctions (corner separation)
4. Failure of masonry piers between openings
5. Local failures (parapets, gables, chimneys, cornices, projections)
6. Buckling / delamination of walls and piers
7. Separation of the roof from the walls

3.6.2  Failure under static (gravity) load 2075 Chaitra, Q2

The 2075 paper asked for failure modes under both static and earthquake load, so keep the two lists distinct. Under pure vertical (static) load a masonry wall or pier can fail by:

3.6.3  In-plane failure modes under earthquake load

Four modes, distinguished by the aspect ratio of the pier, the vertical precompression and the relative strength of unit and mortar:

Progressive damage sequence of a squat pier (laboratory observation)

For a relatively short wall with L/h_eff of about 1.7, in which bed-joint sliding and toe-crushing capacities are similar, the moderately ductile sequence is:

1. Flexural cracking at the heel of the wall.
2. Diagonally oriented cracks at the toe, typically with spalling and crushing of the units.
3. Sliding along a horizontal bed joint near the base, sometimes with stair-stepped bed-joint sliding higher up.
4. With repeated cycles the diagonal cracks grow, and finally crushing of the toes or excessive sliding causes failure.

For stockier walls with L/h_eff > 1.25 the sequence is: flexural cracking at the base that does not propagate all the way across (giving a series of horizontal cracks above the heel) → sliding on bed joints in the central portion of the pier → diagonal cracks at the toe → large cracks at the upper corners.

3.6.4  Out-of-plane failure modes (collapse mechanisms)

Out-of-plane failure occurs because of the lack of adequate wall ties, bands or cross walls. One common form is a rigid-body rocking motion occurring at three locations — at the top of the wall, at the bottom, and at mid-height. The important variables are the vertical stress on the wall and its height-to-thickness ratio; therefore walls at the top of buildings and slender walls are the most likely to suffer damage. In long walls without cross walls the failure mode is out-of-plane bending in the horizontal direction.

3.7  Typical Damages to Masonry Buildings in Earthquakes 2078 Bhadra2080 Bhadra2081 Bhadra

3.7.1  Corner separation

• Separation of orthogonal walls due to combined in-plane and out-of-plane stresses at the corners is one of the most common damage patterns in masonry buildings — damage typically starts at the corners.
• Separation on both sides of a wall leaves it unsupported at its two ends and produces an unstable condition leading to out-of-plane failure.
• The cause is the lack of lateral support at the two ends of the wall during out-of-plane loading, i.e. loss of box action.
• If all the corners separate, the lateral load carrying system of the whole building is significantly reduced.

3.7.2  Out-of-plane failure and gable collapse

3.7.3  Delamination of multi-leaf walls

Delamination — separation of the two (or three) wythes of a masonry wall through its thickness — is another major damage type. The outer leaf buckles and peels away, leaving the wall with a fraction of its original thickness and no capacity. It is the characteristic failure of random rubble stone walls with a weak inner core and no through-stones, and was the dominant mechanism in the 2080 Jajarkot earthquake.

3.7.4  Roof failure and other damage

3.7.5  Damage grade classification (EMS)

Post-earthquake assessment uses the five damage grades of the European Macroseismic Scale. The basic steps of a detailed evaluation are: survey the building from outside → identify the cracks → examine the site for geotechnical hazard → inspect from inside → inspect at critical locations → interpret.

3.7.6  Mitigation and Remedial Measures 2076 Chaitra, Q3a2080 Bhadra, Q1b

Two papers asked for the mitigation measures explicitly. Present them keyed to the failure they prevent — that structure alone earns marks:

3.8  Seismic Performance due to Various Factors

The syllabus lists nine factors explicitly. Each is a potential 2–4 mark sub-question.

(a) Geometrical and mass irregularities

• Plan irregularity — L, T, E, Y and U shapes have re-entrant corners where stress concentrates and where the two wings vibrate out of phase. Remedy: separate into simple rectangular blocks with a seismic gap, or tie the corner strongly. Projections up to ~15–20% of the plan dimension are tolerable.
• Vertical irregularity — setbacks, a storey much taller than the others, or walls discontinued at a lower level (vertical discontinuity in the load path). Every vertical lateral-load-resisting element must be continuous from the roof to the foundation.
• Mass irregularity — there should be no change in effective mass of more than about 80% from one storey to the next. A heavy water tank, a mud overlay on the roof, or a heavily loaded storage floor raise the inertia force locally.
• Soft / weak storey — the stiffness of any storey should not be less than 60% of that of the adjacent storey nor less than 70% of the average of the three storeys above; the strength should not be less than 70% of that of the adjacent storey. The open ground storey for shops is the classic offender.
• Slenderness of the building as a whole — tall, long and unsymmetric buildings perform poorly.

(b) Opening size and location

• Openings are a functional necessity, but their size and location decide the performance of the building.
• Openings too close to wall corners hamper the flow of forces from one wall to another — precisely where the pull/push transfer of §3.5.1 must happen.
• Large openings weaken the wall in carrying inertia force in its own plane: the wall becomes a row of narrow piers coupled by spandrels, and the piers fail in shear.
• Rule: keep all openings as small as possible and as far from the corners as possible; align them vertically from storey to storey; provide lintel and sill bands and vertical bars at the jambs.
• Codal guidance: distance from the inside corner ≥ 600 mm (or one-quarter of the storey height); total length of openings not more than about 50% of the wall length for a single-storey building; pier width between openings ≥ 600 mm.

(c) Eccentrically acting gravity load

• A slab or beam bearing on only part of the wall thickness, a wall offset from the storey below, or a cantilevered balcony all put the vertical load off the centroidal axis.
• The resulting moment reduces the permissible stress through k_s (Chapter 2, §2.6.6) and, when e > t/6, causes flexural tension and horizontal cracking on one face.
• Under earthquake this pre-existing eccentricity adds to the seismic bending, so the wall cracks and loses its precompression much earlier.

(d) Eccentrically acting lateral force — torsion

• Torsion arises from the eccentricity between the centre of mass (CM) and the centre of rigidity (CR) in plan. The building not only translates but also twists, and the walls farthest from the CR receive a large additional shear.
• Codal limits: the distance between the storey centre of mass and the storey centre of stiffness should be less than 30% of the building dimension at right angles to the direction of the force; a minimum (accidental) eccentricity of at least 5% of that dimension must always be assumed.
• Cause: unsymmetrical wall layout — e.g. a building with the ground floor open on one side, or a heavy stair/lift core placed at one end.
• Only a rigid diaphragm can transmit torsion; with a flexible diaphragm the torsional distribution simply does not develop, but the walls are then unsupported out-of-plane instead.
• The calculation of direct and torsional shear in shear walls is the subject of Chapter 4.

(e) Rigid and flexible floor diaphragm

See §3.3.3 and Fig. 3.4. In summary:

(f) Slender walls

• A high height-to-thickness ratio is the single most important variable in out-of-plane vulnerability; walls at the top of a building (low precompression) and slender walls are the most likely to be damaged.
• Slender walls also buckle under vertical load, which is why IS 1905 imposes limiting slenderness ratios (27 / 20 / 13 / 30 / 10 — Chapter 2, §2.6.5).
• In long walls without cross walls the failure mode becomes horizontal out-of-plane bending; the remedy is cross walls, buttresses or pilasters at regular spacing.

(g) Horizontal and vertical bands

• Horizontal bands (plinth, sill, lintel, roof, gable) give the walls horizontal bending strength against out-of-plane inertia, tie orthogonal walls together, stop cracks propagating over the full height, and turn the building into a box. The lintel band is the most important and must be provided in all storeys and all walls.
• Vertical bars (containment reinforcement) at corners, wall junctions and opening jambs resist the tension caused by overturning and rocking, and distribute the damage into many fine cracks instead of one collapse hinge (Fig. 3.18).
• Together they convert unreinforced masonry into something approaching confined masonry — the single most cost-effective seismic improvement available.

(h) Material properties

• Mortar: mud mortar has almost no bond or tensile strength and loses what it has when wet — the primary reason stone-in-mud houses collapse. Cement or cement–lime mortar of the correct grade transforms the behaviour.
• Units: rounded river boulders and irregular rubble give poor bond and thick joints; regular dressed stone or good burnt brick gives far better performance.
• Relative strength of unit and mortar decides whether the crack steps through the joints or runs through the units (§3.6.3).
• Wall density (total shear wall area per floor area) largely determines whether a building survives at all.
• Ductility: masonry is brittle; reinforcement, bands and confinement are the only way to introduce any.

(i) Construction quality

• The strength of brittle materials such as masonry and concrete is a minimum assured value and is highly sensitive to the quality of construction material, the workmanship, the supervision and the construction methods.
• Common defects: unfilled head (vertical) joints; uneven and excessively thick bed joints; no soaking of bricks; no curing; poor corner interlocking; building too fast so green mortar is overloaded; bands not continuous or not properly lapped; through-stones omitted.
• A perfectly designed building constructed badly will still collapse — which is why the codes prescribe detailing rules that a mason can follow without calculation.

(j) Pounding between adjacent buildings

Adjacent buildings of different height, mass or period vibrate out of phase and hammer each other. The damage is concentrated at floor level of the shorter building, where its slab strikes the taller building's column or wall mid-height. The remedy is an adequate seismic separation gap.

3.9  Seismic Performance of Masonry Buildings during Past Earthquakes 2078 Bhadra2081 Bhadra

3.9.1  2015 Gorkha earthquake (M_w 7.8) 2078 Bhadra, Q1b

The major failure mechanisms observed in masonry buildings during the Gorkha earthquake were:

1. Out-of-plane collapse of walls — by far the dominant mechanism, caused by unanchored flexible timber floors and roofs, the absence of bands and slender walls in the upper storeys.
2. Corner separation and vertical cracking at wall junctions, from poor interlocking of the courses at the corners.
3. Delamination and collapse of multi-wythe walls — both burnt-brick-in-cement and un-coursed random rubble walls with corrugated galvanised iron roofs, because of the missing through-stones.
4. In-plane diagonal (X) shear cracking of piers between openings, and vertical cracking at the corners of openings.
5. Failure of gable walls, which pulled down the top of the end walls and the roof.
6. Separation of the roof from the walls, followed by collapse of heavy tiled roofs on the occupants.
7. Failure of the soft/open ground storey in shop-front buildings and of the short columns created by mezzanines.
8. Total collapse of unreinforced stone-in-mud houses in the rural districts around the epicentre, while buildings with even a single RC lintel band frequently survived with repairable damage — the clearest field evidence available for the value of banding.
9. Widespread failure of temples, monuments and monasteries, several of which had already been weakened by earlier earthquakes, plus lifeline failures such as the collapse of a penstock in Sindhupalchowk.

3.9.2  2080 BS (2023) Jajarkot earthquake (M 6.4) 2081 Bhadra, Q2a

Although only a moderate M 6.4 event, it destroyed a very large number of houses because the building stock in Jajarkot and Rukum West is almost entirely random rubble stone masonry in mud mortar. The failure mechanisms specific to that construction are:

3.9.3  Other reference earthquakes

End of Chapter 3 – Seismic Behavior of Masonry Structures