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Design for Structural Stability -
Structural alterations to traditional domestic low rise housing
1. Summary
This paper may be of interest to architectural designers tasked with preparing drawings/specifications for alteration/extension works to domestic property, for Surveyors carrying out building survey reports or structural engineers looking for some examples and comment on typical structural design issues to consider when confronted with proposed alteration work to domestic property. This paper forms part of a presentation prepared for CPD Conferences and considers a number of structural stability issues that may require considering when structural alteration is proposed to existing low rise domestic housing of traditional construction within England. Varying trends over time for domestic housing alteration work is discussed, structural issues that may require consideration and typical methods adopted for achieving structural stability are included. The paper provides some examples of common structural stability issues faced by structural engineers considering proposed alteration works and goes on to briefly discuss design guidance within The Building Regulations Approved Document A and recent published guidance provided by the Institution of Structural Engineers on the submission of calculations to Building Control. Risk associated with varying procurement methods adopted by homeowners for building alteration work is considered plus some discussion on requirement for structural assessment, Building Control Applications and finally how imminent changes to the CDM Regulations (CDM2015) will be relevant to structural alterations to domestic housing.
2. Introduction
Although uncontrolled structural collapse of low rise housing in England is very rare, some care is necessary when considering structural alteration to existing domestic properties. During the design and construction stage there is opportunity to identify structural issues that may require addressing to enable the design and construction of the proposed alteration works to effectively consider structural requirements. Homeowners and some designers may not appreciate the structural impact or practicality of the proposed alteration work. Varying methods of procurement are adopted by homeowners wishing to have alteration works carried out to their property each carrying varying risks in terms of cost, delay and adequately addressing structural requirements. Age, condition, form of construction, extent of previous alterations, ground conditions, foundations and below ground drainage are examples of factors that can vary between properties and require specific consideration on a project by project basis. Ideally structural design should be communicated and stored in such a way that it is available and of use to assist persons in the future that may need to consider and report on signs of structural defect, further alteration or demolition of the building. Structural alteration to domestic housing has been going on for many years as housing needs have changed and trends developed. Byelaws and Building Control requirements have emerged and developed over time, often in response to major events such as the great fire of London, poor living conditions during the industrial revolution, collapses such as Ronan Point, storm wind damage in 1987 and more recent requirements to increase thermal performance for example. Relatively recent revisions to the Building Regulations resulting in wider cavity widths and a need to reduce thermal bridging in conjunction with damp proofing requirements has had an impact on junction detailing between new and existing construction for alteration and extension works.
The following list provides a summary in rough chronological order of some trends in domestic house alteration work since the late nineteenth century:
• Installation of bathrooms and plumbing within upper floors to older properties.
• Removal of walls within rear projections of older properties to extend kitchen space into former externally accessed store/w.c.
• Lounge/Diner knock-through openings.
• Chimney breast removals.
• Conversion into flats.
• General extensions for increased bedrooms/living space.
• Garage conversion and knock-through works.
• Loft conversions/dormers.
• Internal knock-through openings to create open plan kitchen/dining space.
• Kitchen extensions with formed openings through original external walls.
• Bi-fold door openings.
Often a property may have been altered on several occasions over time and may contain varying forms of construction. Structural alterations have a tendency to concentrate loading, alter original structural load path and can have an impact on structural stability. For architectural designers it is worthwhile having an awareness of structural stability requirements contained within Approved Document A of the Building Regulations and to seek specialist advice when existing, proposed or alteration works fall outside the scope of the deemed to satisfy guidance.
3. Common structural issues when considering proposed structural alterations
The following structural issues may typically require some consideration when assessing structural requirements for proposed alteration works to domestic property:
• An understanding of the existing form of construction and structural load path down to ground.
• An understanding how stability of the existing building is achieved.
• An appreciation of the extent/severity of any existing movement or damage.
• Effects of any previous alterations.
• Structural requirements for the proposed arrangement.
• Checking of existing structure required to support additional or revised loading.
• Consideration of ground conditions and foundation requirements.
4. Normal methods of attaining structural stability for low rise domestic housing
Modern low rise housing of masonry construction usually relies on the following traditional arrangements for overall lateral stability within certain defined overall dimensional limits defined within the Building Regulations Approved Document A:
• Adequate corner return walls to ends of supporting walls.
• Intermediate buttressing walls or piers along lengths of walls.
• Floors and roof structure providing structural diaphragm action.
• Connections between walls, floor and roof to transfer lateral forces back to buttressing walls.
• Restriction of door and window opening sizes and locations.
• Adequate foundations to suit ground conditions.
In addition individual structural elements or wall panels may have connections, bracing or reinforcement to help achieve local stability:
• Connection between cut timber roof elements and attic ceiling joists to control roof spread via triangulation of forces across the roof space.
• Plan bracing to timber roof trusses to help control raking.
• Close boarded timbers over cut timber pitched rafters.
• Holding down straps.
• Bed-joint reinforcement to masonry wall panels or wind posts within external wall panels.
• Herringbone strutting/packing between suspended timber joists.
Other methods of achieving lateral stability exist and are sometimes encountered including, often engineer designed:
• Sway frames.
• Wind posts.
• Cantilever columns off fixed foundations.
• Vertical bracing.
• Remedial work buttressing/ties.
• Floor/ceiling/roof diaphragm action back to non-traditional buttressing arrangement.
• Non-traditional house construction, proprietary housing systems and houses constructed in timber, steel or concrete frame construction.
5. Some structural stability issues associated with alterations to low rise housing:
5.1 Roofs
• Roof spread associated with raised ceilings or inadequate tie action.
• Formation of dormers compromising existing roof triangulation and load path.
• Structural inadequacy of the original construction, past alterations and ability to accommodate the revised loading arrangement.
• Roof re-covering with a different roofing material of lesser or greater weight.
• Raking stability including bracing or sheathing requirements.
• Flat roof holding down straps against wind uplift where proposed formed openings may remove sections of existing wall.
• Additional weight associated with conversion of flat roofs to balconies and design and detailing of balustrade base fixings back to the main structure.
• Potential for a roof structure to be unexpectedly supported off the outer leaf of a cavity wall.
• Cantilever backspans.
5.2 Chimneys
• Provision of adequately stiff support to remaining sections of chimney breast to control support beam deflections to acceptable limits to help avoid remaining sections of chimney relying on corbel action.
• Deteriorated chimney brickwork within a roof space being covered from view during loft conversion works.
5.3 Walls
• Junctions between new and existing external walls where walls vary in overall thickness and form of construction.
• Formed vertical damp proof courses compromising existing buttress wall corner returns.
• Newly formed reveals within existing cavity construction with no vertically spaced cavity wall ties adjacent to reveals.
• Stiffness of lintels/beams over formed openings to help reduce deflections and cracking of brittle finishes over to within acceptable limits.
• Formed or modified openings compromising the ability of masonry to arch over openings/lintels at lower level.
• Formed openings compromising wall panel stability or affecting overall lateral stability.
• Loss of lateral restraint to walls back to floor diaphragms.
• Suitability of existing walls/lintels.
• Addition of loading onto slender internal walls/columns.
• Instability due to out of plumb, bulging or cracking to existing walls.
• Masonry found laid on edge either to internal walls or cavity construction.
• “Solid” walls containing timber studwork with masonry infill.
• Connections between masonry walls back to floors, ceilings and roof structure.
• Interaction and connection of wall with sway frames.
• Changes to external ground levels and retaining action.
5.4 Floors
• Formed openings in the floor compromising structural diaphragm action back to buttressing walls.
• Provision of strutting between joists.
• Deflection issues due to undersized joists or severe holes and notches for plumbing and electrical services.
• Cantilever backspans.
5.5 Sub-structure / Foundations
• Form of construction of existing foundations, ground conditions and suitability to support revised loading.
• Controlling differential foundation movement.
• Alterations to walls and loading arrangement over reinforced concrete ground beams between pile locations or raft foundations.
• Differential movement and damage due to subsidence and heave risk in clay soil.
• Location of existing or proposed service voids through substructure walls below ground level where lintels over such openings may be subject to concentrated additional loading.
• Identifying depth of existing or proposed buried services and specification of foundation depths to suit.
• Undermining existing foundations or neighbours foundations.
• Retaining changes in ground level.
• Identifying the existence and extent of a basement and means of support to structure above.
6. The Building Regulations Approved Document A (Structure)
The current Building Regulations 2010 with Approved Documents and subsequent amendments provides guidance on meeting the requirements of Building Regulations. Sections 2 to 4 of Approved Document A (Structure) provide guidance for architectural designers when designing new build of alterations to low rise properties of traditional construction with masonry walls. When a works are designed and built in accordance with Approved Document A and Approved Document 7 (Materials and Workmanship) the structural aspects of the work can be deemed to satisfy without further specialist advice. Proprietary structural items such as pre-stressed concrete beam and block floors or timber trusses for roofs would require design input from an engineer but a specialist manufacturer usually provides that function. Similarly, proprietary lintels may be specified by a specialist manufacture or otherwise selected from product manufacturers load/span tables by a competent person. Cut timber roof construction requires careful consideration and is outside the scope of Approved Document A with span load tables and design guidance published by TRADA. There is a requirement within Approved Document A to ensure structural stability is achieved. Approved Document A can be applicable to new build, extension and alteration work as long as the structure including the existing property generally meets the requirements for Approved Documents A and 7. When the building arrangement or form of construction of an existing building or proposed alteration/extension work is beyond the scope of or otherwise does not comply with Approved Document A then specialist advice may be necessary. The Building Act 1984 and Approved Documents initially came into force on the 11th November 1985 replacing the previous and first set of national Building Regulations 1965 which themselves had been subject to amendment over time. There have been a number of revisions to the current Building Regulations. The 1994 amendment included guidance on requirement for herringbone strutting between floor joists and updated guidance for buttressing and lateral stability of masonry walls, notably following the storm of 1987.
Other notable amendments to Approved Document A have included: Load/span tables for timber elements Up to 2004 load span tables for domestic floor joists, attic ceilings, ceiling binders, pitched rafters, purlins and flat roof joists was included within the Approved Document A. The load/span tables were omitted from Approved Document A in 2004 following concern the tables where being used with a lack of appreciation of requirements to consider connection details and overall stability of the structure. Foundation depths Approved Document A was amended in 2013 to include reference to the Eurocodes, updated disproportionate collapse and foundation depth requirements. Until 2013 Part A stated minimum foundation depth for traditional mass concrete footings in clay soil was to be 0.75m possibly increased to suit bearing into suitable ground. The 2013 revision has brought the guidance more in line with the NHBC Standards Chapter 4.2 where minimum foundation depth is related to the shrinkability of the clay subsoil with high shrinkability clay requiring a minimum foundation depth of 1.0m. Note greater depths may be required when building within the influence of trees/vegetation.
Approved Document A is broadly based on the British Standard BS 8103 which differs in scope to some extent. There is an anomaly between Diagram 14 of Approved Document A and BS 8103-2:2013 Figure 19 for minimum length for external corner returns to walls. BS 8103-2:2013 states the minimum external corner return dimension back to a door/window opening should be 780mm for cavity walls with a cavity greater than 75mm and 665mm for cavity walls not exceeding 75mm cavity width, whereas Part A does not distinguish between cavity widths and states a minimum 665mm return. Recent years has seen cavity widths increase beyond 75mm up to 100mm or more whereas when Approved Document A was originally released in 1985 cavity widths greater than 75mm within domestic housing were not common. Designers aware of this issue would now adopt the 780mm dimension although the 665mm dimension would still seem to be acceptable for those only referencing Approved Document A. Approved Document A is a relatively concise document and an understanding and appreciation of the scope of the guidance and requirements for structural stability contained within it would be of assistance to anyone assessing or reporting on domestic building structures. An awareness of requirements for stability within Sections 2 to 4 of Approved Document A aids identifying areas that may not comply and therefore may help identify areas requiring closer inspection or specialist advice.
7. Varying procurement methods for domestic structural alteration works
Some examples of varying procurement methods adopted by homeowners is considered below each with varying risk of structural issues being overlooked or ignored and varying risk of structural issues coming to light at a future date:
7.1 Self build / D.I.Y. without plans or structural design input and potential for no submission of a Building Regulations Application.
7.2 A rogue trader as first point of contact with potential for no Building Regulations application and no appointment of an architectural services consultant or structuraldesigner.
7.3 Budget cost local architectural services consultant as first point of contact, minimal architectural plans, possibly including remote structural design services excluding a site visit by the engineer and thereafter the owner relying on a builder to progress the works with varying risk.
7.4 A competent builder as a first point of contact leading to varying options for the appointment of a designer(s), submission of a Building Regulations Application and obtaining of a Building Regulations completion certificate.
7.5 Approaching a chartered structural engineer direct for advice and design information for a minor internal alteration prior to obtaining competitive builders estimates and submitting a Building Notice and progressing the works.
7.6 A kitchen/bathroom specialist or a replacement door and window company as a first point of contact with variable risk subject to the competency and skill of the individual/company approached and advice provided.
7.7 Appointment of a carefully selected architectural services consultant or architect based on referral and competitive written quotation with the consultant identifying and addressing structural issues with the appointment of a structural engineer if necessary and works carried out to the satisfaction of Building Control. At some stage in a building project consideration of structural stability and structural requirements is required. It is rare a structural engineer is the first point of contact and it therefore falls on the architectural designer or builder to carry out necessary assessment and advise the homeowner if specialist advice is necessary, ideally before construction work commences.
8. Structural Assessment
When beyond the scope of Sections 2 to 4 of Approved Document A then guidance from a structural engineer may be necessary. A structural engineer will generally require a site visit to inspect and familiarise themselves with the existing arrangement. Sometimes it is possible to agree a limited remote structural design service although this should be subject to defining responsibility for site assessment and accuracy of site information provided. Any structural design assumptions requiring verification prior to construction should be clearly communicated. Some degree of information gathering and assessment is necessary for all structural alteration design work and may include reference to one or more of the following depending on the complexity of the proposed works and arrangement of the existing property:
• Proposed architectural plans/sections.
• Existing architectural plan survey drawings.
• A site visit.
• Information available via enquiry with the building owner or local knowledge.
• Inspection of the existing property for obvious signs of structural distress, cracking, movement or deterioration in and around the area of proposed works.
• Detailed inspection with a builder to open up areas to view either to determine structural arrangement or otherwise locally verify record drawing information.
• Reference to Local Authority Planning and Building Control archive viewed at council offices subject to written approval from the homeowner.
• Reference to Building Control Approved Inspectors to similarly view information or otherwise obtain copies of relevant documents subject to necessary approvals.
• Reference to historical maps and local British Geological Survey maps.
• Trial holes to determine form of construction of foundations, foundation depth and subsoil conditions.
When assessing or reporting on properties that have been subject to alteration or extension care is required to inspect for any obvious signs of distortion or cracking that may be indicative of differential movement or structural distress in and around the area of work including consideration of the age and condition of finishes that may mask evidence of cracking/movement.
9. Structural Design Information for Building Regulations submission
Structural design information and details are generally required to form part of a Building Regulations application and for construction purposes. The Institution of Structural Engineers has recently published guidance on the submission of structural calculations to Building Control within the Structural Engineer journal volume 93 issue 2 dated February 2015. This is currently available to view online:
http://www.istructe.org/webtest/files/cf/cfd44f46-987b-4013-9f30-750e4a1...
In summary it is recommended that a design summary should be submitted with every submission including the following, if relevant:
• List of structural design codes used.
• A description of the basis of design sufficient to enable others to understand the structural principles used.
• Class of building for disproportionate collapse.
• Progressive collapse measures taken.
• Structural load path description down to ground and how stability is achieved.
• Name of person responsible for overall design responsibility as defined within design codes.
• Schedule of computer software used and confirmation its application and limitations are understood and results verified.
• Designer qualifications, whether checked and name and qualifications of the checker.
For a full description of the guidance including guidance on when to submit, extent of design required and acceptable codes of practice please refer to the original publication.
10. Building Regulations Applications
Building Regulations applications for domestic properties can take the form of either Full Plans or a Building Notice. Building Notices are best suited to work of a minor nature only and may be submitted a minimum 48 hours prior to commencing work whereas Full Plans applications require detailed drawings and specification of works to be prepared and time for Building Control to assess and provide Building Regulations Plans approval. For people purchasing domestic housing that has been altered or extended, (sometimes to near capacity) the availability of a Building Control Full Plans Approval certificate, a Building Regulations Completion certificate and knowledge of the existence and location of structural design calculations and drawings will be of value.
11. Construction (Design & Management) Regulations 2015
By the time of this presentation the current CDM Regulations 2007 may have been replaced with the newly proposed CDM 2015 Regulations. UK domestic clients under CDM2007 were exempt from a duty holder role under the CDM Regulations. That exemption will no longer apply under CDM2015 and the full requirements of the CDM Regulations will be applicable on all projects where there is more than one contractor on site.
http://www.hse.gov.uk/pubns/books/l153.htm
It is expected that the domestic homeowner’s legal responsibilities will transfer by default to the contractor (Principal Contractor) and designer (Principal Designer). One outcome of the revised CDM 2015 will be a legal requirement for a Health & Safety File to be prepared and handed to the owner on completion for work on projects where more than one contractor was on site. The Health & Safety File will require safe keeping and making available to view to persons considering future works to the property. The File will require updating over time and handing over to new owners on sale of the property. The Health & Safety File should include comment on residual structural hazards, key structural issues, means of structural stability and design loadings adopted that may be of assistance for future reference for persons considering alteration, extension or demolition work.
The Health & Safety File would not necessarily contain detailed structural calculations and drawings but a homeowner may wish to file such information together.
12. Conclusions
A homeowner relies heavily on the skill and competency of the people they appoint to safely design and construct their proposed structural alteration and extension works. Much building work is carried out successfully without issues but due to varying homeowner attitudes, varying standards of design and construction and limitations within the Building Control approval process there is scope for structural alteration works to result in latent defects or weakness that could result in structural issues over the longer term. Understanding requirements for structural stability can help architects and surveyors identify properties where structural stability issues may be more likely and therefore when more detailed inspection or specialist advice may be necessary. Domestic housing is increasingly being subjected to alteration and extension work and difficulties can arise when attempting to form structurally sound junctions between existing and new construction, when modifying roof structures or when the formation of new or modified openings can compromise means of support or stability of the property. The availability of original construction information including structural calculations and drawings for alteration, modification and extension works is likely to become increasingly important and of value to both homeowners and persons considering structural alterations to existing properties.
Andrew Haste BEng(hons) CEng MIStructE
Hastens Consulting Engineers Ltd