Structural survey

A structural survey is an inspection and appraisal of a building's structure, carried out by a structural engineer, to establish its arrangement, its condition and its capacity to carry load. It is undertaken where the structure itself is in question, rather than the general state of the building.

[edit] How it differs from other surveys

The term is often used loosely, and three distinct exercises are frequently confused.

A condition survey records the state of a building's fabric and elements, and is generally concerned with defects, deterioration and maintenance need.

A building survey is a broad inspection of a property's overall condition, usually undertaken by a surveyor for a purchaser or owner.

A structural survey is narrower and deeper. It is concerned with how the structure carries load, whether it is adequate for the loads it carries or is proposed to carry, and what would be required if it is not.

A structural survey may identify condition issues, and a condition survey may raise structural concerns, but they answer different questions and are not interchangeable.

[edit] When a structural survey is undertaken

Typical triggers include a proposed change of use or an increase in imposed load; the addition of new permanent load to an existing structure, such as rooftop plant or a solar photovoltaic array; visible signs of distress, such as cracking, deflection, corrosion or displacement; alterations that remove or modify load-bearing elements; the absence of reliable records, where a structure must be appraised from first principles; and a requirement from a funder, insurer or purchaser for independent confirmation of adequacy.

[edit] What the survey establishes

On site, a structural survey typically records the structural arrangement, meaning frame type, spans, member sizes, spacings and how load is transferred to the foundations; the condition of members and, critically, of their connections, which are frequently the governing element; evidence of previous alteration, repair or damage; any deviation between the structure as built and the structure as recorded on the drawings; and constraints on access, including any restrictions that limited the inspection.

Where elements are concealed, the extent of what could not be inspected is recorded as a limitation rather than assumed.

[edit] Appraisal of existing structures

Appraising an existing structure differs from designing a new one. The engineer is working with what is there, often without complete records, and must establish material properties, member capacities and load paths from a combination of records, measurement and inspection. Where information is missing, conservative assumptions are made and stated.

Guidance on the appraisal of existing structures is published by the Institution of Structural Engineers, and the structural Eurocodes provide the basis for the assessment of actions and resistances.

[edit] Where additional load is proposed

Where the trigger for the survey is new permanent load, the survey establishes the spare capacity available rather than simply confirming that the structure stands. For retrofit solar photovoltaic installations, BRE Digest 489, revised 2014, identifies that where the applied roof loading increases by more than 15%, Approved Document A requires the structural integrity of the roof structure and the supporting structure to be assessed. This applies to building works carried out in England, and in Wales in some circumstances, with equivalent provisions in Scotland and Northern Ireland. The Digest notes that an assessment remains appropriate below that threshold.

[edit] The report

A structural survey report would normally set out the purpose and scope of the survey and who it is prepared for; the date of inspection, the weather and access conditions, and the extent of what was inspected; the structural arrangement as found, supported by measurements and photographs; the assumptions made and the basis for them; the loading considered and the standards applied; the findings, and a clear conclusion on adequacy; where the structure is not adequate, the options, being revision of the proposal, strengthening, or further investigation; and the limitations of the survey, stated explicitly.

[edit] Reliance

A structural survey report is prepared for a named party for a stated purpose. Where a third party such as a funder, insurer or purchaser needs to rely on it, this is normally addressed by a letter of reliance, which extends a defined duty of care to that party on stated terms. A report should not be relied upon by a party it was not prepared for, or for a purpose it did not address.

[edit] Limitations

A structural survey is a point-in-time inspection. It cannot report on what was concealed, it does not constitute a warranty of future performance, and its conclusions are conditional on the assumptions recorded in it. Where a structure is critical or its history uncertain, opening up, testing or monitoring may be required in addition.

[edit] Related articles on Designing Buildings

Condition survey · Structural engineer · Defects in buildings · Structural assessment for rooftop solar PV · Eurocodes

[edit] External references

BS EN 1990, Eurocode: Basis of structural design.

Institution of Structural Engineers, Appraisal of Existing Structures.

BRE, Digest 489: Wind loads on roof-mounted photovoltaic and solar thermal systems, revised 2014.

Solar Surveys, structural surveys of commercial roofs for solar photovoltaic installations — an example of the survey scope and reporting described in this article.