Approved Document A – Structural Stability, Loading, Ground movement and Disproportionate collapse.
Basic stability of a small house of traditional masonry constriction is largely dependent on the provision of a braced roof structure which is adequately anchored to walls restrained laterally by buttressing walls, piers or chimneys. Fully boarded or hipped roofs will provide in-built resistance. However, this is not provided when extra wind bracing may be required.
Trussed rafter roofs have, in the past been susceptible to collapse during high winds. If this form of construction is used, then it should be braced in accordance with
- BS 5268 Structural use of timber, Part 3: 1985 code of practice for trussed rafter roofs.
Small buildings of masonry construction having walls designed in accordance with section 1B of ADA1/2 will be satisfactory about structural stability if the roof is braced as mentioned above.
- All buildings are required to be constructed so that dead, imposed and wind loads are sustained and transmitted by it the ground
- Structures are required to be strong enough, so they stand against winds and the weight of the house, including the inhabitants and their activities.
- Regard should to be taken to the imposed wind loads, on which any house is subjected to. This should be based on the environment and the structures intended use.
Dead and imposed loads may be assessed by reference to BS 6399 loading for buildings, Part 1:1984 Codes of practice for dead and imposed loads.
Similarly, imposed roof loads are covered in the same code, Part 3: 1998 Code of practice for imposed roof loads
Wind loads may be assessed by reference to CP 3: Chapter V: Part 2: 1972 wind loads.
A2, Ground Movement – Structures should be constructed so that ground movement caused by the following, does not impair stability in any part of the structure.
- Shrinkage/Freezing of subsoils
- Land slide/Subsidence
Structural safety depends on the successful interrelationship between design and construction, particularly about:
- Degree of loading
- Material used
- Design analysis
- Constructional details
- Safety factors
- Standards of workmanship
It is important that the numerical values of safety factors which are used are derived from a consideration of the above factors, since any change in one of these could disturb the structure.
Additionally, loads used in calculations should consider possible cases when dynamic concentrated or peak loads which may arise. Approved doc A1/2 is arranged into 4 sections which gives guidance that may be adopted.
A3: Disproportionate collapse
Approved document A3 contains guidance on measures designed to reduce the sensitivity of a building to disproportionate collapse in the event of an accident and which also may avoid or reduce hazards to which a building may be exposed.
Three approaches may be adopted depending on the extent to which it is possible to tie the structural members together
- Provide effective horizontal and vertical ties complying with:
- Clause 184.108.40.206 of BS 8110 structural use of concrete
- Clause 220.127.116.11 of BS 5950 Structural use of steelwork in a building
- Clause 37 of BS 5628 Code of practice for the use of masonry.
Compliance with these measures will require no further action to be taken into the structural design
Wall Ties – Should comply with BS 1243: 1978 Specification for metal ties for cavity wall construction, unless conditions of severe exposure occur (Severe exposure is defined in BS 5628: Part 3: 1985).