Masonry structures

In short

Masonry is a composite of units (burnt clay bricks, stone, concrete or AAC blocks) bedded in mortar, carrying load mainly in compression through walls, piers and arches. It is still the dominant walling system in India — load-bearing for one to four storeys, and infill in RCC frames almost everywhere else — so every civil engineer sizes, specifies or inspects it. Design in India follows the working-stress code IS 1905:1987 (with SP 20 as handbook) plus NBC 2016 Part 6 Section 4, and seismic detailing from IS 4326 and IS 13828. Examined in GATE and university papers through permissible-stress calculations, slenderness and eccentricity factors, bonds and material tests; asked in interviews by builders, PWD/CPWD, housing boards, L&T, Tata Projects and structural consultancies.

Explained

Units, mortar and why masonry is weaker than its units

• Burnt clay bricks (IS 1077): classified by minimum average compressive strength in N/mm² — 3.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 25, 30, 35. Modular size 190 × 90 × 90 mm (nominal 200 × 100 × 100 with 10 mm joints); the traditional non-modular brick is about 230 × 110 × 70 mm.
• Acceptance tests (IS 3495 Parts 1–4): compressive strength (frog filled, 24 h immersion), water absorption (not more than 20% by mass up to class 12.5, 15% above), efflorescence (nil/slight/moderate/heavy/serious), warpage. Dimensional tolerance is checked on a stack of 20 bricks, not one.
• Other units: hollow and solid concrete blocks (IS 2185 Part 1, strength grades from about 3.5 to 15 N/mm²), AAC blocks (IS 2185 Part 3; density roughly 550–650 kg/m³, strength 3–4 N/mm², light and insulating), fly-ash–lime bricks (IS 12894), burnt-clay fly-ash bricks (IS 13757), and stone (IS 1597 Parts 1 and 2).
• Mortar (IS 1905 Table 1 / IS 2250): graded by compressive strength — H1 ≈ 10, H2 ≈ 6, M1 ≈ 3, M2 ≈ 2, M3 ≈ 1.5, L1 ≈ 0.7, L2 ≈ 0.5 N/mm². Cement–lime–sand proportions are tabulated against each grade (read the exact proportions from Table 1; cement-only mixes of 1:3 to 1:6 and composite cement–lime mixes both appear).
• Mortar must not be stronger than needed: a weak, workable, lime-bearing mortar accommodates movement and distributes stress; an over-rich mortar cracks through the units. Rule of thumb — mortar strength comparable to, not far above, the masonry strength required.
• Why masonry strength ≈ 25–50% of unit strength only: mortar is softer with a higher Poisson's ratio, so under load it tries to squeeze out laterally, putting lateral tension in the units; vertical splitting of units, not crushing of mortar, is the usual failure. Hence thin joints (about 10 mm) are stronger than thick joints, and unit strength matters more than mortar strength (codes express strength as f ∝ f_unit^a · f_mortar^b with a > b).

Masonry types, bonds and construction terms

• By unit and build: brick masonry; stone masonry — rubble (random rubble un-coursed or built-to-course, square rubble, polygonal, dry rubble) and ashlar (fine, rough-tooled, rock-faced, block-in-course, chamfered); block masonry; composite/faced masonry; cavity walls (two leaves with a 50–100 mm gap, ties, weep holes — cuts rain penetration and heat gain); veneer walls (non-structural facing).
• Terms: header (unit laid across the wall), stretcher (along the wall), quoin (corner), bed/perpend joints, hearting (core filling), bond stone/through stone (ties the two faces of a thick stone wall — mandatory at about 1.5 m intervals), spalls, course, toothing, racking back.
• Brick bonds: stretcher bond (half-brick walls), header bond (one-brick curved work), English bond (alternate header and stretcher courses, queen closer next to the quoin header — strongest, standard for structural walls), Flemish bond (header and stretcher alternately in every course — better appearance, slightly weaker), garden-wall, raking/herringbone, rat-trap bond (bricks on edge forming cavities — saves roughly 20–25% of bricks and mortar, popular in low-cost and Laurie Baker–style housing), and stack bond (no overlap — avoid; no load path across joints).
• Golden rules of bonding: vertical joints must not continue through successive courses (minimum lap of a quarter brick), courses must be level, units of uniform size, and the wall raised uniformly — not more than about 1.0–1.5 m height per day for cement mortar.
• Supporting elements: DPC at plinth (a weak plane in shear — detail it with care), lintels over openings (minimum bearing usually taken as 100 mm or span/10), relieving arches, sills, plinth beams, bands, pilasters/piers that stiffen a long wall.
• Load dispersion: a concentrated load (beam bearing) spreads downward in the wall at about 30° to the vertical (IS 1905); the effective bearing area at any level follows from that spread.

Behaviour under load and failure modes

• Masonry is strong in compression, weak and unreliable in tension and flexure; design normally assumes no tensile strength across joints.
• Axial compression: stress = P/A; capacity reduced by slenderness (buckling) and by eccentricity of the load.
• Eccentric load / out-of-plane bending: combined stress σ = (P/bt)(1 ± 6e/t). For no tension anywhere, the resultant must lie in the middle third, i.e. e ≤ t/6. Beyond that the section partly separates and a triangular stress block is used.
• In-plane (shear-wall) action: lateral load is resisted by walls parallel to it; failure as diagonal (X) cracking through joints, sliding along a bed joint or DPC, rocking/toe crushing of slender piers.
• Out-of-plane failure of tall, thin, unrestrained walls (parapets, gables, long free-standing boundary walls) is the commonest earthquake and windstorm failure.
• Serviceability cracking: differential settlement (diagonal cracks widening upward or downward), thermal and moisture movement (clay bricks expand on wetting, concrete blocks shrink — hence control joints in block walls), sulphate attack, corrosion of embedded ties, vegetation and dampness.

Design of unreinforced masonry to IS 1905 (working stress)

Step-by-step for a load-bearing wall:

1. Loads: dead load of slabs, walls and finishes plus imposed load (IS 875 Parts 1 and 2); wind (Part 3) or seismic (IS 1893 Part 1) where they govern. Masonry self-weight is taken as about 19–20 kN/m³ for brickwork; use the IS 875 Part 1 value for the actual material.
2. Effective height H_e (Table 2): about 0.75 H when the wall is laterally supported *and* restrained at top and bottom (RCC slab bearing over at least about half to full wall thickness), 1.0 H for simple lateral support without restraint, and 1.5 H or more for a wall free at the top; column cases are listed separately.
3. Effective length L_e (Table 3): roughly 0.8 L continuous *and* supported by cross walls at each end, 0.9 L continuous at one end and supported at the other, 1.0 L supported at both ends, 1.5 L continuous at one end and free at the other, 2.0 L free at one end.
4. Effective thickness t_e: actual t for a solid wall; t × stiffening coefficient (Table 5, about 1.0–1.5) for a wall stiffened by piers or cross walls; two-thirds of the sum of the two leaf thicknesses for a cavity wall (with the loaded-leaf rule applied).
5. Slenderness ratio SR = smaller of H_e/t_e and L_e/t_e. Code limits (Table 4) are of the order of 27 for cement-based mortars and lower (about 20, and less for taller buildings) for weak lime mortars — confirm against the table and number of storeys.
6. Basic compressive stress f_b (Table 8): read against unit crushing strength and mortar grade; for common Indian brickwork it falls in the range of roughly 0.25–2.0 N/mm², broadly one-tenth of the unit strength.
7. Multiply by the three modifiers:
8. Stress reduction factor k_s (Table 9): function of SR and e/t — 1.00 at SR 6 falling to about 0.43 at SR 27 for zero eccentricity.
9. Area reduction factor k_a = 0.7 + 1.5A (A in m²), applied only when the loaded cross-sectional area A < 0.2 m² — i.e. for small piers and columns.
10. Shape modification factor k_p (Table 10): > 1 for units whose height-to-width ratio is large (tall blocks), used only with units of crushing strength up to about 15 N/mm².
11. Check actual stress ≤ permissible stress, and the SR limit. A 25% increase in permissible stress is allowed in defined cases (certain concentrated-load and wind/seismic combinations) — verify the clause before using it.
12. Shear check: permissible shear stress is small, of the order of 0.1 N/mm² plus a fraction of the dead-load compressive stress, capped at about 0.5 N/mm² (IS 1905 Cl. 5.4.3; the exact expression should be taken from the code edition in force).
13. Minimum practical thicknesses: 230 mm for load-bearing brick walls (115 mm only for partitions and small single-storey work), 150–200 mm for load-bearing blockwork; thicker at lower storeys of multi-storey load-bearing buildings.

Reinforced, confined and seismic masonry

• IS 1905 covers unreinforced masonry only. Reinforced masonry (bars grouted into hollow-block cores or bed joints, reinforced lintel and parapet bands) is addressed by NBC 2016 Part 6 Section 4 and project specifications; there is no separate full-fledged Indian reinforced-masonry design code in wide use.
• Confined masonry: masonry panels built first and then confined by thin RC tie-columns and tie-beams cast against them. It is not a frame with infill — the panel carries gravity load and the ties provide confinement and ductility. Widely recommended for low-rise housing in high seismic zones.
• IS 4326 (earthquake-resistant construction of buildings): horizontal seismic bands — plinth, lintel (the most important), roof/eave, and gable bands — plus vertical reinforcement at wall corners, T-junctions and the jambs of large openings, and reinforcement around openings.
• Opening restrictions (IS 4326 Table 4): total opening length limited to roughly 50% of wall length for single storey, 42% for two storeys and 33% for three storeys in weak (mud/lime-sand) mortar, with higher limits (about 60/50/42%) in cement or cement-lime mortar; openings kept at least about 450 mm (or half the opening height) from an inside corner in three- to four-storey work, with piers between openings of at least about 340 mm for one storey rising to 560 mm for three or four.
• IS 13828 gives the same philosophy for low-strength masonry (random rubble, mud-mortar brickwork): storey height limits, wall-length-to-thickness limits, a maximum number of storeys, through stones and bands.
• Unreinforced masonry buildings attract the lowest response reduction factor R in IS 1893 Part 1:2016 (R = 1.5), which is why bands, confinement and shear walls matter; the detailed seismic treatment belongs to the earthquake-resistant design note.

Comparison and limits of masonry as a structural system

• Advantages: cheap local materials, no formwork or skilled steel fixing, excellent fire resistance and durability, good thermal and acoustic mass, walls serve simultaneously as structure, enclosure and finish, easy to alter and repair.
• Limitations: heavy (large seismic mass), negligible tensile and flexural strength, brittle failure, slow labour-intensive construction with quality depending entirely on workmanship, inefficient for long spans and tall buildings, thick walls consume carpet area at lower floors.
• Load-bearing masonry vs RCC frame: masonry is normally economical up to about three or four storeys with regular wall layouts and small spans; beyond that an RCC frame (with masonry only as infill) wins. Infill walls are usually treated as non-structural in design but do stiffen the frame — ignoring them causes short-column and soft-storey problems.

Key points

• Masonry - units bonded with mortar, designed essentially as a compression member.
• IS 1905:1987 - the Indian code for *unreinforced* masonry; working-stress (permissible-stress) method, not limit state.
• Brick class - the designation number *is* the minimum average compressive strength in N/mm² (IS 1077).
• Water absorption limit - 20% by mass up to class 12.5, 15% for higher classes.
• Mortar grades - H1, H2, M1, M2, M3, L1, L2 in descending strength (about 10 down to 0.5 N/mm²).
• Joint thickness - about 10 mm; thicker joints reduce masonry strength appreciably.
• Masonry strength - only about a quarter to a half of the unit strength, because the soft mortar induces lateral tension in the units.
• Slenderness ratio - lesser of H_e/t_e and L_e/t_e; limited to about 27 for cement mortars.
• Permissible stress - f_c = k_s · k_a · k_p · f_b.
• k_a = 0.7 + 1.5A - applies only when the loaded area is under 0.2 m².
• Middle-third rule - e ≤ t/6 keeps the whole section in compression.
• English bond - strongest common brick bond; stack bond has no lap and is structurally unsound.
• Through/bond stones - essential in thick stone walls so the two faces act together.
• Lintel band - the single most effective seismic measure in a masonry building (IS 4326).
• Confined masonry - masonry panel built first, then confined by RC tie-columns and tie-beams; far better seismic behaviour than plain masonry.
• Load dispersion - a bearing load spreads at about 30° to the vertical through the wall.
• R = 1.5 - response reduction factor for unreinforced masonry in IS 1893 Part 1:2016, the lowest of all systems.
• Common failures - out-of-plane collapse of unrestrained walls, diagonal shear cracking, sliding at DPC, settlement cracks.

Formulas and examples

Stress reduction factor k_s for zero eccentricity (IS 1905 Table 9, e/t = 0):

(Values fall further as e/t rises to 1/24, 1/12, 1/8, 1/6 — interpolate from the full table.)

Example 1 — capacity of a 230 mm load-bearing brick wall. Clear storey height 3.2 m, RCC slabs above and below (laterally restrained), wall continuous past cross walls 4.0 m apart (the 0.8 L case), solid modular bricks; take f_b = 0.96 N/mm² for the chosen unit–mortar pair (read from Table 8).

• H_e = 0.75 × 3200 = 2400 mm → SR = 2400/230 = 10.4; L_e = 0.8 × 4000 = 3200 mm → SR = 13.9. Governing SR = 10.4.
• k_s: between 0.89 (SR 10) and 0.84 (SR 12) → 0.89 − (0.4/2)(0.05) = 0.88.
• A = 0.23 × 4.0 = 0.92 m² > 0.2 → k_a = 1.0; standard bricks → k_p = 1.0.
• f_c = 0.88 × 1.0 × 1.0 × 0.96 = 0.845 N/mm².
• Capacity = 0.845 × 230 × 1000 = 194,350 N/m ≈ 194 kN per metre run.

Example 2 — load on that wall in a two-storey house (check). One-way slabs span 3.6 m onto the wall (tributary width 1.8 m), slab 125 mm thick, concrete 25 kN/m³.

• Roof: 0.125 × 25 = 3.13 + finishes 1.5 + LL 1.5 = 6.13 kN/m² → 6.13 × 1.8 = 11.03 kN/m.
• Floor: 3.13 + 1.0 + LL 2.0 = 6.13 kN/m² → 11.03 kN/m.
• Wall self-weight (20 kN/m³): 0.23 × 3.2 × 20 = 14.72 kN/m per storey × 2 = 29.44 kN/m.
• Plaster 12 mm both faces ≈ 0.49 kN/m² × 3.2 m × 2 = 3.13 kN/m.
• Total at plinth = 11.03 + 11.03 + 29.44 + 3.13 = 54.6 kN/m → stress = 54,600/(230 × 1000) = 0.237 N/mm² against 0.845 permissible. Safe, about 28% utilised — typical, because slenderness and not strength governs low-rise masonry.

Example 3 — small brick pier (area reduction bites). Pier 345 × 345 mm, 3.0 m high, no lateral restraint at mid-height, same f_b = 0.96 N/mm².

• H_e = 1.0 × 3000 = 3000 mm → SR = 3000/345 = 8.7 → k_s = 0.95 − (0.7/2)(0.06) = 0.93.
• A = 0.345² = 0.119 m² < 0.2 → k_a = 0.7 + 1.5 × 0.119 = 0.879.
• f_c = 0.93 × 0.879 × 0.96 = 0.785 N/mm²; area = 119,025 mm² → capacity = 93.4 kN.
• Note the small pier loses about 12% of its stress capacity purely because of its size.

Example 4 — eccentric bearing on a 230 mm wall. P = 100 kN/m, load eccentric by e = 60 mm from the centre line.

• t/6 = 230/6 = 38.3 mm; e = 60 mm > 38.3 mm → the middle-third rule is violated, tension would be implied.
• Elastic check (if tension were possible): σ = (100,000 / 230,000)(1 ± 6 × 60/230) = 0.435 × (1 ± 1.565) → σ_max = 1.12, σ_min = −0.25 N/mm² (impossible in masonry).
• No-tension triangular block: bearing width = 3(t/2 − e) = 3(115 − 60) = 165 mm; σ_max = 2 × 100,000 / (1000 × 165) = 1.21 N/mm² > 0.845 permissible → fails.
• Remedies: reduce eccentricity (full bearing, bed plate or RCC band to centre the reaction), thicken the wall, or raise the masonry grade.

Example 5 — materials for 1 m³ of modular brickwork in 1:6 cement mortar.

• Nominal unit with 10 mm joints = 0.20 × 0.10 × 0.10 = 0.002 m³ → 500 bricks per m³ (50 bricks per m² of a 100 mm wall).
• Net brick volume = 500 × (0.19 × 0.09 × 0.09) = 500 × 0.001539 = 0.770 m³ → wet mortar = 0.230 m³.
• Dry volume = 0.230 × 1.33 = 0.307 m³ → cement = 0.307/7 = 0.0438 m³ = 0.0438 × 1440 = 63 kg ≈ 1.26 bags; sand = 6 × 0.0438 = 0.263 m³.
• At current market rates (which vary widely by state and season) material cost is typically a few thousand rupees per m³ — always price from the local schedule of rates (state PWD/CPWD DSR), never from memory.

Where it is used

• Low-rise housing and institutional buildings: load-bearing brick or block walls for PMAY houses, schools, PHCs, staff quarters, built by PWD, CPWD, housing boards and private builders.
• High-rise construction: AAC-block and concrete-block infill in RCC frames by L&T, Tata Projects, Shapoorji Pallonji and Godrej Properties — chosen to reduce dead load and plaster thickness.
• Infrastructure and civil works: stone and brick retaining walls, abutment wing walls, canal lining, inspection chambers, boundary and compound walls, railway platform walls.
• Heritage and conservation: lime-mortar masonry in ASI monuments, forts, temples and colonial buildings — repair must use compatible lime mortars, not cement.
• Job roles: site/execution engineer (bond, joint thickness, curing, verticality), QA-QC engineer (brick testing to IS 3495, mortar cubes), quantity surveyor (brickwork measurement per IS 1200, DSR rate analysis), structural designer (load-bearing wall and seismic band design), safety/retrofit engineer in seismic strengthening programmes.
• Rural and sustainable construction: rat-trap bond, compressed stabilised earth blocks and confined masonry in NGO, HUDCO and state rural-housing schemes.

Interview questions

• Q: Why is masonry much weaker than the bricks it is built from? A: The mortar is softer and more deformable than the units, so it tries to expand laterally under load and puts lateral tension into the units, splitting them well before their own crushing strength is reached.
• Q: What is the slenderness ratio of a wall and why does it matter? A: The lesser of effective height/effective thickness and effective length/effective thickness; it measures buckling vulnerability and sets the stress reduction factor k_s, with a code limit of about 27 for cement mortars.
• Q: State the middle-third rule. A: If the resultant load stays within the middle third of the section (e ≤ t/6), no tension develops anywhere — essential for a material with negligible tensile strength.
• Q: Which brick bond is strongest and why? A: English bond — alternate header and stretcher courses give a good lap in both directions and no continuous vertical joint; a queen closer beside the quoin header maintains the lap.
• Q: Why should mortar not be made as strong as possible? A: An over-rich mortar is stiff and shrinks, so cracks pass through the units instead of the joints and movement cannot be accommodated; a slightly weaker, more workable mortar gives better overall masonry.
• Q: What is the area reduction factor and when does it apply? A: k_a = 0.7 + 1.5A, applied only when the loaded cross-sectional area is less than 0.2 m², to account for the greater effect of a single defective unit in a small pier.
• Q: What are seismic bands and which one matters most? A: Continuous reinforced concrete bands at plinth, sill, lintel, eave and gable levels that tie the walls into a box; the lintel band is the most important as it ties the walls at the weakest level, above the openings.
• Q: How does confined masonry differ from an RCC frame with brick infill? A: In confined masonry the wall is built first and the thin tie-columns and tie-beams are cast against it so the wall carries gravity and lateral load; in a frame the concrete is cast first and the infill is treated as non-structural.
• Q: Why are through stones used in rubble masonry? A: They run the full thickness of the wall at roughly 1.5 m intervals and tie the two faces together, preventing the wall from splitting into two independent thin leaves.
• Q: Which Indian code governs masonry design, and what is its design philosophy? A: IS 1905:1987, *Structural use of unreinforced masonry* — a working-stress code with permissible stresses, supported by SP 20 and NBC 2016 Part 6 Section 4; there is no limit-state masonry code in general Indian use.
• Q: A 230 mm wall is 4 m high with no lateral support at the top. What is the problem? A: The effective height becomes about 1.5 times the actual height, so SR = 1.5 × 4000/230 ≈ 26 — right at the limit of 27 for cement mortar, and well past it for weak mortar or more than two storeys; the wall is also vulnerable to out-of-plane collapse. It needs piers, a band or a restraint at the top.
• Q: Why do unreinforced masonry buildings get R = 1.5 in IS 1893? A: They are brittle, with almost no ductility or energy dissipation, so the code allows almost no reduction in design seismic force.

Common mistakes

• Using the greater of H_e/t_e and L_e/t_e for the slenderness ratio — the code requires the lesser.
• Applying the area reduction factor to full-length walls; it applies only when A < 0.2 m².
• Taking the basic compressive stress as the brick's crushing strength, or forgetting to apply all three factors k_s, k_a and k_p.
• Using the actual height and thickness instead of the effective values, or ignoring the stiffening coefficient for piers.
• Allowing tension in design calculations — reporting a negative stress as if masonry could carry it, instead of recomputing with the no-tension triangular block.
• Mixing units: stress in N/mm² with length in metres, giving answers 1,000 times out; always carry mm and N together.
• On site: thick and uneven mortar joints, no soaking of bricks, raising a wall more than about 1.0–1.5 m a day, using stack bond for structural walls, or breaking the lintel band at a corner or opening.

Related topics

Reinforced concrete design (IS 456) · Earthquake-resistant design · Tall buildings, shear walls and lateral loads · Repair and rehabilitation of structures · Strength of materials: stress and strain · Building materials and construction practice