external envelope and solid wall construction

2 main functions of external loadbearing walls

1. transfer loads from suspended floors and the roof to the foundation (except in framed structure)

2. provide environmental protection for the building

functional requirements of external walls

• strength and stability

• weather protection

• good thermal insulation

• good acoustic control

• fire protection

• durability

what method reduces lateral stresses on a strip foundation generated by the swelling and shrinking of soil?

void of trenches is backfilled with previously excavated soil

what factors determine the strength of a wall?

• its thickness (in relation to height of the wall - slenderness ratio)

• compressive strength of the material it’s made of

what is thy typical compressive strength of bricks/blocks used below ground level?

5-7 Nmm^-2

what does the stability of a wall relate to? if a wall is unstable, what might be witnessed?

• stability relates to the ratio of the thickness of the wall in comparison the height of the wall

  • slenderness ratio = thickness/height

• buckling - when overly slender

• overturning - due to application of lateral forces

what is the maxiumum slenderness ratio?

1/16 - for every one cm of thickness, 16 cm of height

what are the 3 main principles of weather exclusion?

1. the sponge principle

2. the impervious skin principle

3. the cavity principle

how do the 3 principles of weather exclusion link together?

• clay bricks readily absorb water - highly porous - like a sponge

• can take days for wall to dry when rain stops

• in this time, water may be absorbed and travel to interior envelope

• prevent by painting outer layer with impervious skin

  • OR introduce cavity to prevent moisture breaching

what is thermal insulation measured by

u-value (the higher the u value, the less effective the material is at preventing thermal transfer)

what is the approx u-value of common red brick?

0.6Wm^-1K^-1

what factors impact sound insulation/acoustic control?

• mass/density of material

• air-borne sound

• structurebourne or impact sound - physical breaks

3 main weathering effects of clay bricks - define them

1. spalling - frost action when clay bricks are saturated

2. efflorescence - crystallisation of soluble salts created when moisture evaporates

3. expansion of mortar when brickwork is wet for a prolonged period

how is fire resistance measured?

time taken for occupants of a building to vacate

• 30 mins for low rise domestic

• 60 mins for other - more complexity, more people etc

what types of structures does the building safety act focus on?

‘high risk’ buildings - 15m high or more

characteristics of common bricks

• acceptable performance

• cheaply produced

• appearance is poor - usually rendered to hide appearance of brickwork

• not used in modern house construction

strength and water absorption of common bricks

strength: 5-9N/mm²

water absorption: 12-26%

characteristics of facing bricks

• similar performance to common brick

• more expensive to produce than common bricks

• good appearance

• used for external walls of houses

characteristics of engineering bricks

• more expensive to produce

• appearance is not great

• enhanced strength & low porosity - ideal for below ground

• used in civil engineering structures

• high compressive strength capabilities

strength and moisture absorption of engineering bricks

strength: 9-25N/mm²

moisture absorption: 4.5-7%

standard brick size

characteristics of dense concrete blocks

• made of cement, sand, crushed gravel

• suitable for below ground foundation walls

• suitable for party walls and loadbearing partitions

• high density - good conductors of heat

• potential cheaper alternative to engineering bricks

characteristics and composition of lightweight/aerated concrete blocks

• made from cement, lime, sand, pulverised fuel ash, aluminium powder

• high levels of thermal insulation

• light weight, easy to cut and handle

• holds screws, fittings etc well

• used for internal leaf of modern cavity walls

• low density - poor sound insulation

standard block size

• height of block = 3 courses of brick + 2 strips of mortar

advantages of blocks rather than bricks

• lightweight - easily handled when constructing wall

• enables modular coordination

• comparatively cheap to produce

• better thermal resistance than brick

• accept nail and screw fittings easier than brickwork

• keyed/textured surface for application of plaster or render

characteristics of stretcher bond

• only stretcher face visible

• 1/2B thick

• quickest, most economical

• not suitable for loadbearing walls

header bond characteristics

• only header face visible

• one brick wall

• unattractive

• 1B thick - suitable as loadbearing wall

english bond characteristics

• alternating layers of stretchers & headers

• 1B thick - can form thicker walls

• requires quoins & queen closers on corners

flemish bond characteristics

• alternating bricks of headers and stretchers

• 1B thick - can be thicker

• more economical than english bond

• prettier - commonly used in house building?

english garden wall bond characteristics

• 3 stretcher courses to 1 header course

• quicker to build than english bond but not as strong

flemish garden wall bond

• 3 stretcher bricks to 1 header brick in each course

• quicker to build than flemish bond

• sometimes used on sides/back of house?

what are the building regulations on solid wall construction?

• minimum thickness 350mm - 1½ B thick +

  • for adequate resistance to moisture penetration

• minimum thickness 450mm - 2 B thick +

  • for adequate thermal insulation