Masonry Structures

2.1  History of Masonry Structures in Ancient and Modern Times

Masonry is the oldest structural material still in everyday use. It has been employed for at least 10,000 years in houses, private and public buildings and historical monuments. The masonry of ancient times involved two major materials: brick manufactured from sun-dried mud or burned clay and shale, and natural stone.

2.1.1  Ancient masonry

• The first masonry structures were unreinforced and intended to carry mainly gravity loads. Their sheer weight, and the resulting friction, is what stabilised them against lateral loads from wind and earthquakes.
• Bricks were first fired around 3500 BC in Mesopotamia (present-day Iraq), itself a high seismic risk area. The ziggurat temples at Eridu have withstood not only earthquakes but wars and invasions.
• Design was entirely empirical — by “rules of thumb” handed down by master masons.
• Landmark structures: the Egyptian pyramids, the Great Wall of China, Roman aqueducts and the Pantheon dome, the domes and arches of Islamic architecture, and Gothic cathedrals with their flying buttresses.

2.1.2  Masonry in Nepal

• The Kathmandu Valley has an unbroken tradition of brick-and-timber Newari architecture — multi-tiered pagoda temples, palace complexes and the courtyard bahal.
• Characteristic features are thick three-leaf (three-wythe) walls with a rubble-and-mud core, ma appa and the wedge-shaped facing brick dachi appa laid in mud mortar, and timber floors, joists, struts and elaborately carved windows.
• Outside the Valley, rural Nepal is dominated by random rubble stone masonry in mud mortar and by adobe, which is precisely the building stock that collapsed in 1934, 1988, 2015 and 2080 BS.
• Dharahara (1832, rebuilt 1934 and 2015) is the best-known Nepali masonry monument.

2.1.3  Modern masonry

2.2  Nature of Masonry Structures 2081 Bhadra, Q4a

2.2.1  What is masonry?

Unreinforced masonry (URM) is construction made of masonry units set in mortar without any reinforcement. It has no tension-resisting elements and relies on gravity load for its lateral resistance. It is this building type — not engineered masonry — that is responsible for the great majority of earthquake deaths worldwide.

2.2.2  Nature (essential character) of masonry — the exam answer

2.2.3  Where masonry is used

Masonry is normally used for components subjected to compressive loading: (1) walls, (2) columns, (3) arches, (4) domes. Masonry walls also have a limited capacity to carry horizontal loads and bending moments. Masonry is used not only for pure masonry buildings but also, very successfully, in mixed structures — for example masonry infill in RC or steel frames.

2.2.4  Advantages and Disadvantages of Masonry 2078 Bhadra, Q2a

2.3  Mechanical and Physical Properties of Masonry; Masonry Materials 2076 Chaitra2078 Bhadra2080 Bhadra2081 Bhadra

2.3.1  Masonry Units

A masonry unit is a brick, tile, stone, glass block or concrete block conforming to the requirements of the relevant code. The main families are:

Physical properties of bricks 2081 Bhadra, Q3a

Mechanical properties

2.3.2  Mortar

Requirements of a good mortar

• Strength — adequate crushing strength together with adequate tensile and shear strength; it must gain strength within a reasonable period so the masonry can take load early.
• Workability — must set early enough to let work proceed at a reasonable pace, while remaining plastic long enough to lay the units to line and level.
• Water retentivity — must not lose its water immediately to an absorbent brick and stiffen. Good water retentivity is what allows the mortar to develop a good bond and to fill the voids, giving resistance to rain penetration.
• Low drying shrinkage — to avoid cracking of the joints.
• Durability — resistance to sulphate and other chemical attack, and to frost.
• Accommodation of movement and of irregularities in unit size; good appearance; cost effective.

Types of mortar

Mortar grades (NBC 101 / IS 1905)

Mortars are classified for design convenience as H (high strength), M (medium) and L (low). The mortar type must be matched to the strength of the masonry unit:

Notes. A, B and C denote eminently hydraulic, semi-hydraulic and fat lime respectively. Mortar strength varies appreciably with the angularity, grading and fineness of the sand, so the sand content may be reduced where necessary. With plain cement–sand mortars a plasticiser should be added to improve workability. 1 N/mm² = 1 MPa = 10.2 kg/cm². For first-class brickwork the mortar should be H1 or H2.

2.3.3  Bonds and Brick-laying Terminology

A bond is the pattern in which units are laid. In a single-thickness wall the bond is largely decorative, but in double-thickness walls the bond is structural — it ties the leaves together and prevents continuous vertical joints, which would otherwise split the wall.

2.3.4  Strength of Masonry — Unit–Mortar Interaction 2078 Bhadra, Q2b2080 Bhadra, Q2a

The compressive strength of masonry is not the strength of the brick, and it is not the strength of the mortar. It is governed by the interaction between them:

Empirical relations

2.3.5  Factors Affecting the Compressive Strength of Brick Masonry 2076 Chaitra, Q1a2078 Bhadra2080 Bhadra

This is the most frequently asked theory question of the chapter. Organise the answer in four groups:

2.4  Types of Masonry Structures and Types of Walls 2076 Chaitra2078 Bhadra2079 Bhadra

2.4.1  Classification by Material 2078 Bhadra, Q2a

2.4.2  Classification by Structural System 2079 Bhadra, Q2a

This is the key classification for the exam. The distinguishing feature is the load path and whether the wall is a structural element or not.

(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 to it are loaded out-of-plane.
• Economical for low-rise buildings (up to 3–4 storeys); thickness increases rapidly with height.
• Unreinforced load-bearing masonry is brittle; earthquake resistance must be supplied by horizontal bands, vertical reinforcement, good corner bonding and box action.

(B) Confined masonry 2076 Chaitra, Q4b(ii)

• Tie-columns (also called practical columns) resemble RC columns but are of much smaller cross-section; tie-beams resemble RC beams. They are not a moment frame — the masonry, not the ties, carries the load.
• Load path: roof / floor → masonry walls (confined by the ties) → plinth band → foundation. Box integrity is provided by the confining elements.
• The confining members are effective in:
  1. enhancing the stability and integrity of the wall for both in-plane and out-of-plane earthquake loading — they contain the damaged masonry and stop it from disintegrating;
  2. enhancing the lateral strength of the wall; and
  3. reducing the brittleness of the wall, thereby improving its earthquake performance.
• It has emerged over the last 100 years as an economical alternative that lies between unreinforced masonry and RC frame construction, and is now the recommended system for small buildings in much of Latin America and Asia.

Structural components of a confined masonry building: masonry walls (carry gravity load down and act as bracing panels against horizontal force); confining elements (restrain the walls, prevent complete disintegration and ensure vertical stability); floor and roof slabs (transmit gravity and lateral load to the walls, acting as horizontal diaphragms); plinth band (transmits wall loads to the foundation and protects ground-floor walls from differential settlement in soft soil); and the foundation.

(C) Infill masonry

• Infill walls are 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, and in routine design it is treated only as a load.
• In reality the infill has a major effect:
  • It greatly increases the lateral stiffness of the frame, shortening the natural period and attracting more force. This stiffness must be accounted for when distributing the applied loads.
  • Under lateral load the panel acts as a diagonal compression strut, so an infilled frame can be analysed as a truss.
  • Infill should still be designed to resist the in-plane and out-of-plane loads acting on it, and any vertical load transferred to it by the frame.
• The dangers. A sudden loss of stiffness when the infill cracks can create a soft-storey mechanism at any floor level. An irregular distribution of infill in elevation (the classic open ground storey for shops or parking) concentrates all the drift in one storey. Partial-height infill produces the short-column effect. Reported earthquake damage studies consistently show a high level of damage in RC frames with masonry infill.
• Conversely, continuous and well-distributed infill has been shown to reduce the vulnerability of an RC structure — in a full-scale test of a three-storey flat-plate structure the addition of infill walls prevented the punching-shear slab collapse seen in the bare-frame test and increased both the stiffness and the strength of the structure (measured drift capacity 1.5%).

(D) Unreinforced vs reinforced masonry

Reinforced brickwork has been used in India and Japan since after the First World War and in America since after the Second. Bars are placed in the voids created by particular bonds and/or in the bed joints; bed-joint reinforcement in particular enhances the capacity of walls to resist lateral loading. Post-tensioning is also used for retaining walls, tanks and footbridges. Reinforced masonry is the subject of Chapter 5.

Comparison of the three systems

2.4.3  Types of Walls

2.5  Lateral Force Resisting Elements (Walls, Piers and Arches)

2.5.1  The Three Primary Elements of a URM Building

Any masonry building resisting an earthquake can be reduced to three primary elements. Which role a wall plays depends entirely on the direction of shaking at that instant — and because shaking reverses and acts in both horizontal directions, every wall plays both roles in turn.

2.5.2  Piers and Spandrels

• A pier is the length of wall between two openings in the same storey. It is the vertical element that actually carries the in-plane shear, so a perforated wall behaves as a set of piers coupled by spandrels rather than as a solid wall.
• A spandrel is the masonry above and below the openings, spanning horizontally between piers. It couples the piers and transfers shear between them.
• Whether the piers or the spandrels are the weaker element decides the failure mechanism: weak-pier/strong-spandrel gives a soft-storey-like shear failure of the piers (dangerous), while strong-pier/weak-spandrel gives cracking of the spandrels first (more ductile and preferable).
• The number, size and location of openings therefore controls the seismic behaviour — openings must be small, centrally placed, aligned vertically, and kept away from corners.
• Piers also attract an increase in axial load due to overturning, which is checked in Chapter 4.

2.5.3  Arches

• An arch spans an opening by converting the vertical load into compression along its curve, so it can be built from a material that has no tensile strength.
• The price is a horizontal thrust at the springing, which must be resisted by heavy abutments, buttresses or a tie rod. Under lateral shaking the thrust changes and the arch can spread and collapse — a very common failure of historic monuments and pati/sattal.
• Types by shape: flat/jack, segmental, semicircular, pointed (Gothic), parabolic, corbelled. The traditional Nepali equivalent is the corbelled opening built by stepping successive courses inwards.
• An arch is stable so long as the line of thrust stays within the middle third of the arch ring; hinges form where it touches the edge, and four hinges turn the arch into a mechanism.
• Lintels are the modern alternative; a reinforced brick or RC lintel avoids the thrust problem and is preferred in seismic regions. Arching action in walls over openings (rigid and gapped arching) is dealt with in Chapter 4.

2.5.4  Bands, Ties and Box Action

Individually strong walls are not enough — they must be tied together so the building behaves as a single closed box. The elements that provide this box integrity are:

Note. If the wall height up to eave level is 2.5 m or less, the lintel-level band may be omitted and the lintel integrated with the eave-level band.

2.6  Structural System of Historical Masonry Structures; Review on Design of Masonry Walls for Static Force 2075 Chaitra2076 Chaitra2080 Bhadra

2.6.1  Structural System of Historical Masonry Structures

2.6.2  Basic Principle of Design in Compression

IS 1905:1987 uses the working stress method — the actual stress produced by the loads must stay within a permissible limit:

What the permissible compressive stress depends on

1. Type and strength of the masonry units
2. Strength (grade) of the mortar
3. Slenderness ratio
4. Eccentricity of loading
5. Shape and size of the units
6. Cross-sectional area of the masonry

2.6.3  Behaviour under Axial and Eccentric Load

• If a pure axial load could be applied, the mode of failure would depend on the slenderness ratio: short stocky members fail by crushing of the material; long thin members fail by lateral instability (buckling).
• The failure stress at zero slenderness ratio depends on the strength of the units and mortar, and varies between about 7.0 and 24 N/mm².
• In practice it is virtually impossible to apply a truly axial load — that would need a perfect member with no fabrication error. The vertical load is generally eccentric to the centroidal axis and therefore produces a bending moment. This can be allowed for in two ways:
  1. add the stresses due to the equivalent axial load and the bending moment: σ = WA ± MZ where A and Z are the area and section modulus of the cross-section; or
  2. reduce the axial load-carrying capacity of the wall or column by a suitable factor — which is what IS 1905 does through k_s.

2.6.4  Effective Height, Effective Length and Effective Thickness

(a) Effective height h_eff

The effective height relates to the degree of restraint imposed by the floors and beams that frame into the wall or column. If the ends of a strut were free, pinned or fully fixed the effective height could be computed from Euler buckling theory; IS 1905 tabulates practical values instead. Typical values are:

Openings in walls. When openings occur such that the masonry between them acts as a pier/column:

(b) Effective length l_eff

The effective length depends on the end conditions along the length of the wall. The four conditions that occur, in various combinations, are:

• Free end
• Continuity of the wall (the wall carries on past the point considered)
• Support from a cross wall, pier or buttress
• Openings

Various combinations of these determine the effective length; the coefficients are given in Table 4 of IS 1905 (typically 0.8 L for a wall continuous or supported at both ends, 1.0 L for one end supported, 1.5–2.0 L where one end is free).

(c) Effective thickness t_eff

Table 6 — Stiffening coefficients for walls stiffened by piers, buttresses or cross walls

where S_p = centre-to-centre spacing of the pier or cross wall; t_p = thickness of the pier; t_w = actual thickness of the wall proper; W_p = width of the pier in the direction of the wall, or the actual thickness of the cross wall. Linear interpolation is permitted, extrapolation is not. If the thickness of a cross wall acting as a pier is not given, take t_p = 3 t_w. Strength is increased by providing piers, buttresses and cross walls, and this stiffening effect is what the coefficient represents.

2.6.5  Slenderness Ratio (SR) 2075 Chaitra, Q3(i)2080 Bhadra, Q2b

Influence of slenderness ratio on the strength of a wall

• SR is a measure of the wall's tendency to buckle. As SR increases, the mode of failure changes from crushing to lateral instability, and the failure stress falls (Fig. 2.16).
• For SR ≤ 6 the buckling effect is not appreciable and the basic compressive stress is not reduced (k_s = 1.0 when e = 0).
• For SR > 6 a reduction factor for slenderness (the stress reduction factor k_s) given in the code must be applied, and the permissible stress falls steadily — from 1.00 at SR 6 to about 0.43 at SR 27 for axial load.
• The buckling effect may be ignored within one-eighth of the height of the member above or below the position of lateral support — the allowable stress is not reduced there.
• Practical consequences: a taller storey, a thinner wall, or the removal of a cross wall/pier all raise SR and therefore reduce the permissible stress. Adding piers, buttresses or cross walls raises t_eff and lowers SR, increasing the capacity.

Limiting (maximum permissible) slenderness ratio

2.6.6  Eccentricity of Loading

Effect of eccentricity on the allowable stress (IS 1905, clause 5.4.1.4)

Note also that where the eccentricity ratio lies between 1/3 and 1/2 of the thickness, the stress reduction factor varies linearly between unity and 0.20 for slenderness ratios of 6 and 20 respectively.

2.6.7  The Three Modification Factors 2075 Chaitra2076 Chaitra

(a) Basic compressive stress f_b — IS 1905 Table 8

The values in Table 8 take into account the crushing strength of the masonry unit and the grade of mortar. They are valid for SR not exceeding 6, zero eccentricity, and a unit height-to-width ratio (as laid) of 0.75 or less. Alternatively the basic compressive stress may be based on a prism test (Appendix B of the code) on masonry made from the units and mortar actually to be used on the job.

A compact extract of the most-used part of the table:

(b) Stress reduction factor k_s — IS 1905 Table 9 2076 Chaitra, Q4b(i)

(c) Area reduction factor k_a — IS 1905 clause 5.4.1.2

Reason: a small section has little redundancy, so a single weak brick or badly filled joint has a proportionately larger effect on the capacity.

(d) Shape modification factor k_p — IS 1905 Table 10 2075 Chaitra, Q3(ii)

Influence of the shape modification factor on the strength of a wall

• A taller unit (larger height-to-width ratio as laid) means fewer bed joints per metre of wall height, hence less soft mortar, hence less lateral tension induced in the unit — so the masonry is stronger. This is the same mechanism as Fig. 2.8.
• The gain can be substantial: k_p = 1.8 for a unit of ratio 2.0–4.0 made of 5 N/mm² material — an 80% increase in permissible stress for exactly the same material.
• The benefit diminishes as the unit gets stronger (only 1.2 at 15 N/mm²) because with a strong unit the mortar joint is relatively less critical — and above 15 N/mm² the factor does not apply at all.
• A common brick laid flat has a ratio of about 57/115 ≈ 0.5, i.e. k_p = 1.0 — which is why the factor is described as being for “units other than common bricks” (concrete blocks, stone blocks, hollow blocks laid on edge, etc.).

2.6.8  Design Procedure for a Wall under Uniformly Distributed Axial Load

2.6.9  Design Criteria for Walls Subjected to Transverse Loading 2076 Chaitra, Q2a

A wall carrying wind or out-of-plane seismic pressure in addition to vertical load must satisfy the following:

1. Convert the transverse load into an equivalent eccentricity. The lateral pressure produces a moment M; combined with the vertical load W this is expressed as W acting at e = M/W, and the wall is then treated exactly as an eccentrically loaded wall (§2.6.6).
2. Establish the span direction. A wall supported top and bottom spans vertically; a wall supported by cross walls at its ends spans horizontally; a wall supported on all four sides develops two-way bending. The critical bending moment follows from the support condition.
3. Check the flexural tensile stress against the very low permissible value. Since masonry has almost no tensile strength, the usual criterion is that the resultant must remain within the middle third, i.e. no tension develops: e ≤ t/6.
4. Take advantage of the precompression. The vertical dead load produces a compressive stress that must first be overcome before tension can occur; this is why an upper-storey wall with little load on it is far more vulnerable out-of-plane than a heavily loaded ground-floor wall.
5. Check the combined stress: σ = W/A ± M/Z must be within the permissible compressive stress (with the 25% increase allowed for eccentric/lateral load where applicable), and the tensile side must not exceed the permissible flexural tension.
6. Check stability and the slenderness limit — free-standing walls and parapets have a limiting SR of only 10.
7. Provide positive anchorage to the roof/floor with metal anchors so the assumed support condition actually exists, and provide bands and cross walls to reduce the span.

2.7  Emerging Materials and Technologies in Masonry Structures

End of Chapter 2 – Masonry Structures