Introduction to masonry structures

Masonry is the construction of units (clay brick, stone, concrete or hollow block, compressed earth block) bound by mortar. It is the oldest and the most widely used building system in Nepal and South Asia, especially for low and medium-rise buildings.

Uses

1. Load-bearing walls in houses, schools and hospitals (up to 2 to 3 storeys in Nepal, NBC 202 and NBC 109), which carry floor and roof loads to the foundation.
2. Infill and partition walls in RC frames, for enclosure, privacy, sound and fire separation.
3. Foundations, plinth and retaining walls, boundary walls, parapets and compound walls.
4. Special structures: arches, domes, chimneys, culverts, bridges' piers and abutments, dams, water tanks and monuments.
5. Cladding and finishing: exposed brick facades and flooring.

Importance and advantages

• Locally available and cheap materials, labour with simple tools, easy to learn.
• Good fire resistance, thermal and sound insulation, and durability.
• Walls serve as structure, enclosure, and insulation at once.
• Good compressive strength, aesthetics, and low maintenance.
• Reuse of materials and a lower carbon footprint than concrete.

Limitations

Low tensile and shear strength, heavy self-weight, brittle failure in earthquakes (the 2015 Gorkha earthquake showed the damage of un-reinforced masonry in mud mortar). Hence reinforced and confined masonry with bands (NBC 105 and NBC 109), and sound design to IS 1905 are used.

   ======== roof slab ========
      |   |   |   |   |      load path:
      | brick wall    |      roof -> wall
      |   |   |   |   |      -> footing -> soil
   ---+---+---+---+---+---
     [    footing    ]
   /////////////////////

Masonry infill is brick or block masonry built within an RC (or steel) frame to form partitions and enclosure. It is usually considered non-structural and ignored in design, but it strongly changes the behaviour of the frame under lateral loads.

Role and effect on frame behaviour

• Under lateral load the infill acts as a diagonal compression strut between the frame corners, so stiffness and lateral strength rise, and storey drift is reduced.
• Initial stiffness is 5 to 10 times that of a bare frame; the fundamental period is reduced, so seismic force attracted rises.
• Cracking and crushing of infill dissipates energy.

  +------------------+
  |\  .             |    infill acts as a
  |  \   .          |    diagonal strut
  |    \    .       |    (equivalent width w)
  |      \    .     |
  +------------------+
  frame + infill under lateral load

Equivalent strut width (Mainstone): $w \approx 0.175\,\alpha_h^{-0.4} D$ with $D$ the diagonal length.

Advantages

1. Increases stiffness, strength and energy dissipation of the frame.
2. Reduces drift and damage to non-structural items at low intensity shaking.
3. Gives thermal and sound insulation, fire resistance, enclosure, and low cost with local materials.

Disadvantages

1. Irregular or non-uniform placement (open ground storey) creates soft storey and torsion.
2. Partial height infill leads to short column effect and shear failure of columns.
3. Brittle, and prone to out-of-plane failure if not tied to the frame (needs bands, ties, and mesh as per NBC 105 and IS 4326).
4. Adds mass and so increases seismic force; heavy walls need strong frames.
5. Diagonal strut action may overload the column and joint.

Masonry is the construction of units (clay brick, stone, concrete or hollow block, compressed earth block) bound by mortar. It is the oldest and the most widely used building system in Nepal and South Asia, especially for low and medium-rise buildings.

Uses

1. Load-bearing walls in houses, schools and hospitals (up to 2 to 3 storeys in Nepal, NBC 202 and NBC 109), which carry floor and roof loads to the foundation.
2. Infill and partition walls in RC frames, for enclosure, privacy, sound and fire separation.
3. Foundations, plinth and retaining walls, boundary walls, parapets and compound walls.
4. Special structures: arches, domes, chimneys, culverts, bridges' piers and abutments, dams, water tanks and monuments.
5. Cladding and finishing: exposed brick facades and flooring.

Types of bond in brick masonry

Bond is the arrangement of bricks in courses so that vertical joints do not come over each other and the wall acts as one unit. The main types are English, Flemish (single and double), stretcher, header, garden wall, rat-trap bond and raking bond.

English bond

English bond has alternate courses of stretchers and headers. A stretcher is a brick laid along the length of the wall (long face visible); a header is laid across the wall (short face visible). It is the strongest bond for walls thicker than one brick.

Rules

• Each header course is placed directly over the stretcher course, and each vertical joint (perpend) of one course is a quarter-brick (half-brick lap for stretchers) away from the joint in the next course.
• A queen closer (a brick cut lengthwise to half its width) is placed next to the quoin header at the end of the heading course to maintain the lap.
• Walls thicker than one brick have headers inside to tie the faces; the bond is very strong.
• Joints are about 10 mm thick.

 Elevation (English bond, 1 brick thick)
 +-------+-------+-------+-------+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-+---+---+---+---+---+---+---+-+
 |Q| H | H | H | H | H | H | H |Q|   header course
 +-+---+---+---+---+---+---+---+-+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-------+-------+-------+-------+
 S = stretcher, H = header, Q = queen closer

Flemish bond

In Flemish bond each course has headers and stretchers alternating, and a header of the next course is centred over a stretcher below. The face looks neat with a regular chequered pattern. A queen closer (or a brickbat) is used near the quoin.

Features

• Gives a good appearance, so it is used for facing brickwork of thin walls (the bond is slightly weaker than English bond, as there are fewer through-headers).
• For one-brick wall each course shows alternate headers and stretchers on both faces.
• It needs greater skill and care in laying, and the bricks must be of uniform size.
• It uses fewer full bricks for the facing, and economical where brickwork is face-pointed.

 Elevation (Flemish bond, 1 brick thick)
 +-------+---+-------+---+-------+
 |   S   | H |   S   | H |   S   |   course 1
 +-+---+-------+---+-------+---+-+
 |Q| H |   S   | H |   S   | H |Q|   course 2
 +-------+---+-------+---+-------+
 |   S   | H |   S   | H |   S   |   course 3
 +-------+---+-------+---+-------+

Other bonds

• Stretcher bond: all stretchers; only for half-brick walls and partitions.
• Header bond: all headers; for curved work and 1-brick thick circular walls.
• Rat-trap bond: bricks laid on edge with hollow space inside, a light, cheaper bond using about 25% fewer bricks; used in Nepal for low-cost houses.
• Garden wall bond: Flemish with several stretchers per header.

English bond

English bond has alternate courses of stretchers and headers. A stretcher is a brick laid along the length of the wall (long face visible); a header is laid across the wall (short face visible). It is the strongest bond for walls thicker than one brick.

Rules

• Each header course is placed directly over the stretcher course, and each vertical joint (perpend) of one course is a quarter-brick (half-brick lap for stretchers) away from the joint in the next course.
• A queen closer (a brick cut lengthwise to half its width) is placed next to the quoin header at the end of the heading course to maintain the lap.
• Walls thicker than one brick have headers inside to tie the faces; the bond is very strong.
• Joints are about 10 mm thick.

 Elevation (English bond, 1 brick thick)
 +-------+-------+-------+-------+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-+---+---+---+---+---+---+---+-+
 |Q| H | H | H | H | H | H | H |Q|   header course
 +-+---+---+---+---+---+---+---+-+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-------+-------+-------+-------+
 S = stretcher, H = header, Q = queen closer

Flemish bond

In Flemish bond each course has headers and stretchers alternating, and a header of the next course is centred over a stretcher below. The face looks neat with a regular chequered pattern. A queen closer (or a brickbat) is used near the quoin.

Features

• Gives a good appearance, so it is used for facing brickwork of thin walls (the bond is slightly weaker than English bond, as there are fewer through-headers).
• For one-brick wall each course shows alternate headers and stretchers on both faces.
• It needs greater skill and care in laying, and the bricks must be of uniform size.
• It uses fewer full bricks for the facing, and economical where brickwork is face-pointed.

 Elevation (Flemish bond, 1 brick thick)
 +-------+---+-------+---+-------+
 |   S   | H |   S   | H |   S   |   course 1
 +-+---+-------+---+-------+---+-+
 |Q| H |   S   | H |   S   | H |Q|   course 2
 +-------+---+-------+---+-------+
 |   S   | H |   S   | H |   S   |   course 3
 +-------+---+-------+---+-------+

Data: brick 230 × 110 × 55 mm (typical Nepali brick) with 10 mm joints; wall one and a half brick = 345 mm thick. In English bond the courses alternate; in a 1½ brick wall each course has headers on one face and stretchers on the other, reversed in the next course, with a queen closer after the quoin header. S = stretcher, H = header, Q = queen closer.

Plan of odd and even courses

 Plan, odd course (1 1/2 brick = 345 mm), outer face at top
 +---------+---------+---------+---------+
 |    S    |    S    |    S    |    S    |  110 mm
 +--+----+----+----+----+----+----+----+--
 | Q| H  | H  | H  | H  | H  | H  | H  |  230 mm
 +--+----+----+----+----+----+----+----+--

 Plan, even course
 +-+----+----+----+----+----+----+----+---
 |Q| H  | H  | H  | H  | H  | H  | H  |    230 mm
 +-+----+----+----+----+----+----+----+---
 +---------+---------+---------+---------+
 |    S    |    S    |    S    |    S    |  110 mm
 +---------+---------+---------+---------+

For the two perpendicular walls (corner): in the odd course the long wall runs through to the corner and its end bricks are laid as header at the quoin, while the cross wall butts into it with stretchers; in the even course the arrangement is reversed, the cross wall runs through and the long wall butts against it. Thus the corner is interlocked in alternate courses and there is no continuous vertical joint.

Elevation (face of the wall)

 Elevation (English bond, 1 brick thick)
 +-------+-------+-------+-------+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-+---+---+---+---+---+---+---+-+
 |Q| H | H | H | H | H | H | H |Q|   header course
 +-+---+---+---+---+---+---+---+-+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-------+-------+-------+-------+
 S = stretcher, H = header, Q = queen closer

In the face elevation the stretcher courses and header courses alternate. A queen closer is next to the quoin header, every perpend lies a quarter-brick from that in the course below, and the back face repeats the same pattern with courses reversed.

Masonry is the construction of units (clay brick, stone, concrete or hollow block, compressed earth block) bound by mortar. It is the oldest and the most widely used building system in Nepal and South Asia, especially for low and medium-rise buildings.

Uses

1. Load-bearing walls in houses, schools and hospitals (up to 2 to 3 storeys in Nepal, NBC 202 and NBC 109), which carry floor and roof loads to the foundation.
2. Infill and partition walls in RC frames, for enclosure, privacy, sound and fire separation.
3. Foundations, plinth and retaining walls, boundary walls, parapets and compound walls.
4. Special structures: arches, domes, chimneys, culverts, bridges' piers and abutments, dams, water tanks and monuments.
5. Cladding and finishing: exposed brick facades and flooring.

English bond

English bond has alternate courses of stretchers and headers. A stretcher is a brick laid along the length of the wall (long face visible); a header is laid across the wall (short face visible). It is the strongest bond for walls thicker than one brick.

Rules

• Each header course is placed directly over the stretcher course, and each vertical joint (perpend) of one course is a quarter-brick (half-brick lap for stretchers) away from the joint in the next course.
• A queen closer (a brick cut lengthwise to half its width) is placed next to the quoin header at the end of the heading course to maintain the lap.
• Walls thicker than one brick have headers inside to tie the faces; the bond is very strong.
• Joints are about 10 mm thick.

 Elevation (English bond, 1 brick thick)
 +-------+-------+-------+-------+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-+---+---+---+---+---+---+---+-+
 |Q| H | H | H | H | H | H | H |Q|   header course
 +-+---+---+---+---+---+---+---+-+
 |   S   |   S   |   S   |   S   |   stretcher course
 +-------+-------+-------+-------+
 S = stretcher, H = header, Q = queen closer

Flemish bond

In Flemish bond each course has headers and stretchers alternating, and a header of the next course is centred over a stretcher below. The face looks neat with a regular chequered pattern. A queen closer (or a brickbat) is used near the quoin.

Features

• Gives a good appearance, so it is used for facing brickwork of thin walls (the bond is slightly weaker than English bond, as there are fewer through-headers).
• For one-brick wall each course shows alternate headers and stretchers on both faces.
• It needs greater skill and care in laying, and the bricks must be of uniform size.
• It uses fewer full bricks for the facing, and economical where brickwork is face-pointed.

 Elevation (Flemish bond, 1 brick thick)
 +-------+---+-------+---+-------+
 |   S   | H |   S   | H |   S   |   course 1
 +-+---+-------+---+-------+---+-+
 |Q| H |   S   | H |   S   | H |Q|   course 2
 +-------+---+-------+---+-------+
 |   S   | H |   S   | H |   S   |   course 3
 +-------+---+-------+---+-------+

Masonry structure is a structure built from individual units (bricks, stone, concrete or hollow blocks, compressed earth blocks) laid in courses and bonded together with mortar. The walls and piers carry the loads mainly by compression.

Structural limitations

• Very low tensile and flexural strength; mortar joints are weak planes, so cracks open easily.
• Poor resistance to lateral loads (earthquake, wind) because the material is heavy, brittle and has little ductility.
• Limited height and span; slenderness ratio is restricted (IS 1905 Table 7: max 27 for cement mortar, 12 for columns).
• Openings must be small and well placed, otherwise wall strength falls sharply.
• Strength depends on workmanship, quality of units and mortar, and curing.
• Heavy self weight, so large foundation loads; walls are thick and reduce floor area.

Four advantages of masonry structures:

1. Economical and locally available: bricks, stone and blocks are made from local materials, and little skilled equipment is needed, so cost is low.
2. Durable and fire resistant: burnt clay brick and stone resist weather, fire and decay with little maintenance.
3. Good thermal and sound insulation: thick walls keep the interior cool in summer and warm in winter and reduce noise.
4. Easy to build and modify: units are small and handled by hand, no formwork is required, and walls can be built in any plan shape and repaired or extended easily.

(Also: good compressive strength, pleasing finish without plaster, and architectural flexibility.)

Good brick masonry depends on workmanship as much as on material. The main points to be followed are:

1. Quality of bricks: use well-burnt, uniform bricks of specified strength (not less than 3.5 N/mm$^2$ for load-bearing work). Soak bricks in clean water for at least 2 to 3 hours before laying so they do not absorb water from the mortar.
2. Mortar: proportion and mix as designed (e.g. 1:4 or 1:6 cement:sand); sand clean and well graded. Use mortar within 30 minutes of mixing (cement mortar); do not re-temper stiff mortar.
3. Bond: use English or Flemish bond so that vertical joints (perpends) of successive courses do not coincide; use closers at the end of each course; lay bricks with frog up and full header or stretcher faces.
4. Joints: bed and cross joints fully filled with mortar; joint thickness about 10 mm; joints struck, flush or raked for plastering; raking depth about 12 mm.
5. Line, level and plumb: check with plumb bob, spirit level and string line; each course horizontal. Corners are raised first.
6. Rate of rise: do not raise the work more than about 1.0 to 1.5 m in one day, so lower courses are not crushed or displaced. Where work stops, leave toothing (not a steep rake) for later joining.
7. Damp-proof course (DPC) at plinth level; lintels over openings with at least 200 mm bearing.
8. Curing: keep the wall wet for at least 7 days (10 days for cement mortar); protect from rain, sun and frost.
9. Scaffolding and safety: independent scaffolding; keep holes for scaffolds minimal and fill them properly.

Closers

A closer is a brick or part of a brick used to complete a course and to break the continuity of vertical joints (perpends) near the end of a wall, quoin, jamb or opening. Without it the bond cannot be completed.

• Queen closer: a brick cut lengthwise to half the width. Placed next to the quoin header, at corners and reveals, to start the bond of the stretcher course.
• King closer: a brick with one corner cut diagonally (half-width at one end), used at quoins or splayed ends and for square reveals.
• Bevelled closer: a brick bevelled lengthwise over its whole length (half of the face), used where the end of a wall must show a splayed face.
• Mitred closer: a brick cut at $45^\circ$, used at angles of walls and in splayed or obtuse corners.
• Bat: a brick cut across its length (half bat, three-quarter bat) used to fill the gap to set out perpends.

English bond: key points

• Alternate courses of headers and stretchers; each header course is centred over a stretcher course.
• A queen closer is placed after the first quoin header in each header course so that vertical joints in successive courses are staggered by quarter brick.
• Strongest brick bond for walls of 1 brick and thicker; heart is filled with headers and stretchers, so there is a good transverse bond.
• Perpends of header course line up with every alternate course only (vertical joints coincide in alternate courses).

Flemish bond: key points

• Each course has headers and stretchers alternately; the header is centred over a stretcher below.
• Closers (queen or a bat) are needed after the quoin header to start the bond.
• Better appearance than English bond, needs fewer whole bricks on the face, but slightly weaker for thick walls; needs more skilled labour. Used where the facework is exposed.

English bond (face)        Flemish bond (face)
 ____ ____ ____ ___         __ ____ __ ____ __
|____|____|____|___|       |__|____|__|____|__|
|_|_ _|_ _|_ _|_ _|_|      |____ ___ ____ ___ |
|____|____|____|___|       |__|____|__|____|__|
|_|_ _|_ _|_ _|_ _|_|

(Quoin headers, queen closer after the first header in each header course.)

English bond

Alternate courses of stretchers and headers. A queen closer is placed beside the quoin header in each header course to keep the vertical joints staggered. It gives a strong transverse and longitudinal bond and is the most common bond for load-bearing walls of 1 brick or more.

Elevation (1 brick wall)       Plan, course 1 / course 2
 ________ ________ ______     course 1 (stretcher)
|________|________|______|    |==========|==========|
|__|__ _|_ _ _|_ _ _|_ __|    course 2 (header)
|________|________|______|    |==|==|==|==|==|==|==|==|
|__|__ _|_ _ _|_ _ _|_ __|

Rat-trap bond

Bricks are laid on edge (rowlock) so that a hollow cavity is formed inside the wall. Stretchers on edge form the two faces and headers on edge tie them at intervals, so a cavity as wide as the brick thickness remains. The wall is usually 230 mm thick (brick $230\times110\times55$ mm or similar) and the cavity appears as a "rat trap".

Plan of alternate courses (wall thickness t)
Course 1:  |#####|     |#####|     |#####|   <- header on edge
           |#######################|
           |#######################|  face stretchers on edge
Course 2:  |#######################|
           |   cavity (hollow)     |
           |#######################|

Features: about 25 percent saving in bricks and 10 to 15 percent in mortar and cost, good thermal insulation and lighter wall. Cavity may be filled with lean concrete or insulating material.

Rat-trap bond

In rat-trap bond the bricks are laid on edge (rowlock) in a wall of about 230 mm, with stretchers on edge on both faces and headers on edge across the wall at regular spacing. This leaves a hollow cavity inside, equal to the thickness of the brick. The pattern of the face looks like Flemish bond but with bricks on edge.

Face (elevation)               Plan: alternate courses
 ____ ____ ____ ____          1: |##|  |##|  |##|  (headers)
|____|____|____|____|            |##############|
|_|__|_|__|_|__|_|__|           |##############|  cavity
|____|____|____|____|          2: |##############|
                                  |  cavity      |
                                  |##############|

Advantages over other bonds (English, Flemish)

1. About 25 percent fewer bricks and 10 to 15 percent less mortar, so cheaper.
2. Overall cost saving of about 10 to 25 percent.
3. Good thermal insulation: the air cavity keeps the building cooler in summer and warmer in winter.
4. Lighter wall: reduces dead load and foundation size.
5. Better resistance to dampness and rain penetration because the cavity breaks capillary movement.
6. Cavity can be used for electric conduit or filled with insulation; plaster is not essential (good appearance).
7. Environmentally friendly: less fuel (burnt brick) is needed.

Limitations: needs skilled masons, lower compressive strength than a solid wall of the same thickness, not suitable for walls with heavy concentrated loads unless the cavity is filled.

Unreinforced masonry (URM)

A masonry structure built of units and mortar only, with no steel bars in the walls. Strength comes from compression of the masonry; tensile and shear capacity is small. Load-bearing brick or stone buildings with RCC bands at most (as in IS 1905 / NBC 109) are of this type.

Reinforced masonry (RM)

Masonry in which steel bars (vertical bars in cores or pockets of blocks, horizontal bars in bed joints, or RCC bands and tie columns) are provided to carry tension, shear and flexure and to give ductility. It resists earthquake and wind better and allows larger openings and taller walls.

Rat-trap bond

Bricks are laid on edge with stretchers on the faces and headers across at regular intervals, leaving a cavity inside the wall (usually 230 mm thick wall with brick thickness cavity).

Plan, alternate courses        Elevation
1: |##|  |##|  |##|           ____ ____ ____ ____
   |##############|           |____|____|____|____|
   |##############|           |_|__|_|__|_|__|_|__|
2: |##############|           |____|____|____|____|
   |   cavity     |
   |##############|

Advantages

• Saves about 25 percent bricks and 10 to 15 percent mortar; economical.
• Good thermal insulation and dampness resistance because of the cavity.
• Lighter wall, smaller foundation load.
• Pleasing face; plaster not essential.

Types of brick bonds

Bond is the arrangement of bricks in courses so that vertical joints do not coincide, giving a homogeneous wall.

1. Stretcher bond: all bricks are stretchers; used for half-brick (partition) walls.
2. Header bond: all bricks are headers; used for curved walls and 1 brick thick walls with small radius.
3. English bond: alternate courses of headers and stretchers with a queen closer after the quoin header; the strongest and most common bond.
4. Flemish bond: headers and stretchers alternate in every course; good appearance.
5. Rat-trap bond: bricks on edge forming a cavity; economical.
6. Others: garden-wall bond, raking bond, facing bond.

Stretcher bond    Header bond      English bond      Flemish bond
 ___ ___ ___      _ _ _ _ _ _      ___ ___ ___       __ ___ __ ___
|___|___|___|    |_|_|_|_|_|_|    |___|___|___|     |__|___|__|___|
|_|___|___|_|    |_|_|_|_|_|_|    |_|_ _|_ _|_|     |___ __ ___ __|

Rat-trap bond: advantages and disadvantages

A bond is the arrangement of bricks or stones in a wall so that vertical joints of successive courses do not coincide, and the wall acts as a single unit. The common types are:

1. Stretcher bond: only stretchers (long faces) show; used for half-brick walls and partitions.
2. Header bond: only headers (short faces) show; used for curved work, e.g. 1 brick thick circular walls.
3. English bond: alternate courses of headers and stretchers; a queen closer after the first header of the header course staggers the joints. Strongest bond for walls.
4. Flemish bond: each course has headers and stretchers alternately; a header lies over the middle of a stretcher. Attractive but slightly weaker.
5. Rat-trap bond: bricks on edge with a cavity; saves material.
6. Garden-wall bond: three stretchers to one header in each course (or three courses of stretchers and one of headers); used for boundary walls.
7. Raking bond: bricks inclined at $45^\circ$ in some courses to give long walls a transverse strength.

English bond                 Flemish bond
 ____ ____ ____ ___           __ ____ __ ____ __
|____|____|____|___|         |__|____|__|____|__|
|_|_ _|_ _|_ _|_ _|_|        |____ ___ ____ ___ |

Rules of good bond: avoid continuous vertical joints, use closers at the end, lay headers across the thickness of the wall, and use full bricks wherever possible.

• Arrises: the sharp edges (straight lines) formed by the meeting of two adjacent faces of a brick. In good brickwork the arrises must be sharp and unbroken.
• Perpends: the vertical joints (cross joints) between bricks in a course. In a good bond the perpends of alternate courses lie in one vertical line, but never in two successive courses.
• Lap: the horizontal distance by which one brick overlaps the brick below it in the next course, measured along the length of the wall. For good bond the lap should be one-quarter of the brick length (quarter-brick lap, about 55 mm for a 230 mm brick) in English bond and not less than 1/4 brick in any bond.

 ______ ______ ______
|______|______|______|   perpend = vertical joint
 |__lap__|______|___    lap = overlap between courses

When a masonry infill wall sits inside an RC frame and the frame sways under lateral load, the frame and infill separate at the tension corners and the infill touches the frame only at two opposite corners. The wall then acts like a diagonal compression strut between these corners. This is the equivalent strut (equivalent diagonal) model.

  +------------+
  |\           |
  |  \  strut  |
  |    \       |      width w = 0.175 (alpha_h)^-0.4 d
  |      \     |      d = diagonal length of infill
  +------------+

The strut has thickness $t$ equal to the wall, width $w \approx 0.175\,\alpha_h^{-0.4}\,d$, where $\alpha_h = h\left[\dfrac{E_m t \sin 2\theta}{4E_f I_c h_m}\right]^{1/4}$ ($E_m$ masonry, $E_f$ frame modulus, $I_c$ column inertia, $h_m$ infill height). It is modelled as a pin-ended diagonal brace in frame analysis.

Is it advantageous? Yes, in general: the strut greatly increases the lateral stiffness and strength of the frame, reduces drift and dissipates energy, so partial damage can be tolerated. But the benefit depends on it being well built and uniformly distributed; if the infill is irregular it can cause soft storey, short column and torsion problems, and out-of-plane failure of the wall may occur. These effects must be considered in design.

Hollow concrete blocks are precast concrete units with one or more large voids (cores), usually 40 to 50 percent of the gross volume, made of cement, sand and fine aggregate.

Uses:

• Load-bearing and non-load-bearing walls of residential and industrial buildings, and as infill in RC frames.
• Partition walls, boundary walls and parapets.
• Reinforced masonry: cores can be filled with grout and steel bars for earthquake resistance.

Because they are large, light and uniform, they lay quickly, need less mortar, give good thermal and sound insulation and reduce dead load.

Hollow blocks (concrete or clay)

Uses: walls of buildings (load-bearing up to 2 or 3 storeys), partitions, infill, boundary walls and reinforced masonry (cores filled with grout and bars).

Advantages:

• Larger size than brick, so faster construction and less mortar.
• Lighter, so a lower dead load and smaller foundation.
• Good thermal and sound insulation from the voids.
• Uniform size gives plumb, straight walls; pipes and conduit can run through cores.
• Cores can be reinforced, giving good earthquake resistance.

Compressed earth blocks (CEB)

Blocks made by compressing a moist mix of soil, sand and a little stabiliser (cement or lime, 5 to 10 percent) in a manual or mechanical press, then curing but not burning.

Uses: load-bearing walls of low-rise rural houses, schools and health posts; infill and boundary walls.

Advantages:

• Made from local soil on site: low cost and low transport.
• No firing, so very low energy and no fuel; environmentally friendly.
• Accurate shape and size, strong and durable when stabilised; needs little mortar.
• Good thermal comfort (high thermal mass).
• Interlocking CEBs reduce mortar and skilled labour.

Limitations: must be protected from continuous moisture and erosion (good plinth, roof overhang, plaster).