This Digest gives guidance to professional engineers on the structural appraisal of existing buildings, including making a structural appraisal for a material change of use.

Part 2 introduces structural appraisal, and discusses the factors that might influence the outcome of an appraisal, and what might be involved in preparing for it. It looks at the many factors that can influence the structural behaviour and performance of buildings, including: through-life perspectives; cultural heritage issues; sources of hazard and risk; accidental loadings and actions; structural systems and materials; stability and resilience of buildings; loads and actions; verification criteria; safety considerations; material influences; defects, deterioration and damage mechanisms and sustainability issues.

designing a new building, where the flow of forces follows the choice of structural form and materials, and the procedures for structural analysis follow on. In design, the engineer can decide on appropriate means for satisfying issues of structural stability, load capacity, and serviceability. In appraisal, the engineer has to deal with an existing building or structure in which aspects of the structural form and the characteristics of the materials are established, but are generally much less well known. Although, notionally, these are definable qualities, depending upon the amount spent on the task, the appraising engineer must determine the condition of the existing building or structure and form an opinion on its suitability for future use in the envisaged circumstances.

STRUCTURAL APPRAISAL OF EXISTING BUILDINGS, INCLUDING FOR A MATERIAL CHANGE OF USE Part 2: Preparing for structural appraisal

Stuart Matthews

DIGEST DG 366 Part 2

Figure 1: A modern building complex

1 INTRODUCTION TO STRUCTURAL APPRAISAL

A structural appraisal is undertaken to check the adequacy of an existing structure with respect to a current or future use. Often the scope of these activities may extend to making a prognosis of future behaviour and safety. Structural appraisal is therefore a process of gathering and evaluating information about the form and current condition of a structure and its components, its service environment and general circumstances, so that its adequacy for future service can be established against specified performance requirements, such as loadings, actions, or durability.

The art and processes of appraising an existing structure are different from those associated with

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It has been said that a structure will explore all alternative load paths and try all means of remaining standing before it ‘gives up’ and falls down. Therefore, as an engineering discipline, key aspects of appraising an existing structure are understanding and exploring these alternatives, in order to appreciate what the mechanisms involved are, and how close a structure may be to ‘giving up’, particularly if it has experienced deterioration or damage. This involves seeking to establish the margin of safety that may exist, as well as how conservative, reliable and dependable the resulting appraisal may be.

An essential aspect of the science and art of appraising an existing structure is recognising what the structure can ‘communicate’ about its condition, the form that this may take and, critically, what it may not reveal. Surveying, testing and monitoring techniques provide tools to interrogate a structure, but the results have to be interpreted, and their underlying meaning understood.

Thus it is not an easy task. Advanced analytical procedures may help in establishing the boundaries of the ‘solution’, but engineering judgement forms an essential part of the process of forming a view on the structure’s safety – a view that the engineer must stand behind when advising the client, even though the limitations of the process need to be recognised.

Methods for evaluating the strength and serviceability of existing construction have evolved from previous structural appraisal experience. Typically, the process requires:• considering the level of safety appropriate to the use of

the building or structure• assessing the nature and magnitudes of the applied

loadings and actions• establishing a suitable method or methods for

evaluating structural behaviour• determining the structure’s components and

constituent materials, its strength now and in the future, plus any immediate need for remedial or strengthening works

• considering future maintenance, preventive or remedial works activities.

Structural appraisal for a material change of use of part or all of an existing building has to assess compliance with requirements A1, A2 and A3 of the Building regulations 2010[1]. The primary concerns are the structural safety, strength and stability of the building under normal loads and actions associated with requirements A1 and A2, as well as seeking to reduce its sensitivity to disproportionate collapse under accidental loads and actions, as addressed by requirement A3. It is likely to be more difficult to demonstrate compliance with requirement A3 for traditional buildings (ie those constructed using rules of thumb and experience for sizing structural members) than for those with structures designed, calculated and specified according to engineering principles. The task of reducing sensitivity to disproportionate collapse under accidental loads or actions might therefore pose a considerable burden in appraising such buildings. For this reason this matter is a particular focus of this Digest.

The Annex provides a definition of terms relating to the stability of the structure.

2 ESTABLISHING THE BRIEF FOR A STRUCTURAL APPRAISAL

A structural appraisal of an existing building may be required or become necessary for various reasons, including the circumstances associated with a material change of use of part or all of the building.

The brief for the engineer undertaking a structural appraisal needs to be in writing, agreed by both the client and the engineer, and prepared before any significant work is done on the structural appraisal. With lay clients it is particularly important to explain clearly at the outset what is to be done, and so the engineer may need to develop or clarify his or her brief for the client’s review and agreement. This may go beyond the direct contractual considerations to include related aspects, such as means of access (both inside and outside the structure) and agreed responsibilities for providing labour and scaffolding for opening-up and for making good afterwards.

A key aspect of the brief is the definition of associated performance requirements for the building’s intended use. This may include durability, serviceability, acoustic and thermal performance, as well as sustainability considerations, especially where these go beyond the minimum requirements. The engineer must recognise the responsibilities and legal duties arising from the appointment. Important ancillary issues are procedures for identifying hazards, and for communication between the engineer and the client on matters concerning risk. This includes establishing criteria for risk acceptance by the client or other parties.

Clearly, the client has obligations and activities as part of the appraisal process. It is helpful if the client can provide relevant background information, such as a summarised history of the building’s past and present use, plus details of the original construction and any subsequent alterations (which might be available from previous surveys, investigations or appraisals).

3 GENERAL CONSIDERATIONS AND INFLUENCING FACTORS

3.1 IntroductionThis section introduces some of the many factors that can influence the structural behaviour and performance of a building, and which therefore may need to be considered when undertaking a structural appraisal of an existing building.

3.2 Through-life perspectives: deterioration, repairs, alterations and historical insights

This Digest is concerned mainly with a material change of use of a building.

Most buildings provide satisfactory performance over an acceptably long service life, but deterioration affects all building materials to varying degrees; so through-life care and management are needed to maximise the service life achieved.

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Buildings may deteriorate for various reasons:• poor design, specification, detailing or execution

during construction• poor planning or implementation of maintenance

operations• lack of funds for routine maintenance• past underestimation of the role of proper and timely

maintenance• ageing processes or the effects of an aggressive

environment• other factors, such as damage caused by increased

loading• accidental events such as fires or explosions.

The chemical reactions associated with deterioration processes typically need moisture, and so buildings will usually deteriorate much more slowly when little moisture is available. Deterioration processes tend to reduce the achieved levels of functionality, safety and serviceability of buildings with time. If the service life of a building is to be extended significantly beyond that originally planned, intervention works may be needed to slow any such processes.

Existing buildings, particularly those that have been in service for many years, could have been repaired or altered at least once in their life. The appraising engineer therefore needs to be able to recognise what might plausibly be found in buildings of a particular age, and conversely what should not be found (or at least is unlikely to be), in order to establish whether a previous repair or alteration may have been made. It is generally advisable to investigate the reasons for, and implications of, previous repairs and alterations. The Institution of Structural Engineers’ guidance document Appraisal of Existing Structures[2] provides general information on how materials and forms of construction have been employed in the UK at various times through history.

Consider, for example, a late 19th Century building. Structural metalwork in this era was generally made from wrought or cast iron, although this was the period in which early steels were first used. As both wrought and cast iron are difficult to weld (even now), and as metal arc welding technology was not available at the time of construction, if welds are present it suggests that the metalwork might be a steel element introduced in alterations or repairs during the life of the building. In these circumstances careful investigation would be needed to confirm the material type and the associated permissible stresses. Also, details of the nature of the structural metalwork – ie whether it was in the form of rods, plates, beams or columns, and of rolled or riveted construction – would help in the process of identification.

If the building had dated from the mid-19th Century, the structural metalwork would very likely be made from wrought or cast iron. In this case the presence of welds would almost certainly indicate alterations or repairs during the building’s life. Thus the older the building, the more likely it is that the presence of welds indicates some form of later intervention, possibly on more than one occasion.

Details of the building’s through-life care and management over the years provide valuable insight into its conservation history, as well as an understanding of any conservation philosophy driving this, or resource constraints limiting it.

The through-life management and conservation of existing buildings are considered further in Section 4 of Part 3 of this Digest.

3.3 Cultural heritage issuesAdditional considerations apply to heritage buildings. The attitude and opinion of the authorities responsible for a heritage structure are critical. This may affect fundamental aspects of the structural appraisal process, including the scope and nature of any investigations, as well as the nature and character of any intervention works that may be deemed necessary or desirable. Early consultation with local heritage officers and bodies such as English Heritage or its equivalent is recommended, as they may be able to provide useful information on the forms of construction involved. The relevant authorities should also be able to help establish what activities can reasonably be carried out. Timely liaison could also save a lot of future delay and, potentially, costs if it is later found to be necessary to seek approvals for various activities.

BrE Digest 502[3] sets down principles for masonry conservation management, and gives valuable background information on the relevant legislative framework.

3.4 Sources of hazards and riskPotential sources of hazards to buildings and their occupants are wide ranging. They include:• fire within the building, especially during

refurbishment or remedial works• accidental internal or external gas explosions• accidental impact by vehicles, including road vehicles,

trains and aeroplanes• human errors during the building’s design and

construction, during its operation• lack of proper maintenance during the building’s

service life• structural modifications that are unauthorised, or

inadequately planned or designed• environmental effects such as exceptionally strong

wind or heavy snow – these are normal loading effects, which could have either a local or a global influence on behaviour

• misuse or abuse, such as overloading of a floor slab • ground movements associated with nearby deep

excavations, cuttings or changes in ground level• the effects of flooding (impact of flood water and flood

debris), and any associated scour or deposition of material

• malicious attacks, such as deliberately caused internal or external explosions.

Most such sources of potential hazard arise from accidental loads and actions, but some may also arise from extreme values of normal load effects (eg dead, wind, snow and imposed loads).

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Vandalism and other malicious acts or attacks are outside the scope of this Digest, as are explosions caused by detonations (eg explosions associated with high explosives or bombs).

3.5 Loads and actions – structural Eurocode approach

According to the structural Eurocode conventions and the terminology described in BS EN 1990[4], a building will be subject to various loads and actions while in-service. Any structural appraisal of an existing building must take these factors and their effects into account. The terms are defined as follows:• Loading/direct actions: A set of forces (loads) applied

to the structural elements of a building including the dead load due to the weight of the structure, imposed loads from various sources including wind, snow, machinery, etc.

• Indirect actions: A set of deformations, accelerations or movements imposed upon the structural elements of a building including those associated with temperature changes, fire or other thermal effects, moisture effects, seismic events, differential movements or settlements, etc.

The effect of the loads and the actions is to mobilise internal forces (such as axial forces, shear forces and moments) either within individual structural elements or in the structural system forming the whole building. In turn these are manifested as strains, deformations, deflections or rotations of individual elements or of the whole building.

The loads and actions may be static, or vary slowly with time, or be dynamic in nature. BS EN 1990 defines numerous terms to describe these effects, and procedures for assessing their combined effect. These terms are different from those used in previous UK national codes and standards for structural design. They include:• Permanent action, G: an action that is likely to

act throughout the given reference period (eg the design working life or other period used to assess the

variability of actions statistically) for which the variation in time is negligible or the variation of which is always in the same direction (monotonic) until the action attains a limit value.

• Variable action, Q: an action for which the variation in time is neither negligible nor monotonic.

A variable action has four representative values to be used for the appropriate design situations for the ultimate or serviceability limit state verifications. These are:• its characteristic value, Qk

• the combination value, y0Qk, where y0 is the combination factor ≤1

• the frequent value, y1Qk, where y1 is the frequent value factor ≤1

• the quasi-permanent value, y2Qk, where y2 is the quasi-permanent factor ≤1.

The combination value y0Qk, the frequent value y1Qk and the quasi-permanent value y2Qk) are shown in Figure 2.

For design, appraisal and risk management purposes, loadings and actions are commonly divided into two classes: normal loads and actions and accidental loads and actions.

Normal loads and actions: These are considered to be the effect of identifiable and foreseeable loads and actions that may act for varying periods of time and whose magnitude may vary with time, such as gravity and wind loads, including self-weight, imposed and snow loads, together with effects associated with imposed deformations or accelerations, such as thermally induced movements or differential ground displacements. Loads associated with building services should also be examined, to ensure that appropriate values are used in the appraisal – especially where the building services have been modified, renewed or upgraded during the life of the building (which is common).

BS EN 1991 comprises several parts, which include the respective National Annexes. Parts 1-1 to 1-7[5, 6, 7, 8, 9, 10, 11] are relevant to the types of building and circumstance considered

in this Digest, but Parts 2 to 4 generally are not. The various parts of BS EN 1991 should be read in conjunction with BS EN 1990[4].

Accidental loads and actions: BS EN 1990 defines these as actions, usually of short duration but of significant magnitude, that are unlikely to occur on a given structure during the design working life. Although BS EN 1990 notes that seismic actions and earthquakes are taken to be accidental actions, they are unlikely to be of particular concern in the UK, apart from some special types of building, such as those storing regulated hazardous substances.

Characteristic value Qk

Combination value ψoQk

Frequent value ψ1Qk

Quasi-permanent value ψ2Qk

Time

Instantaneous value of Q

∆t2 ∆t1 ∆t3

Figure 2: Diagrammatic portrayal of the representative values for variable actions

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The BS EN 1990 definition is qualified by the following two notes:• An accidental action can be expected in many cases

to cause severe consequences unless appropriate measures are taken.

• Impact, snow, wind and seismic actions may be variable or accidental actions, depending upon the available information on statistical distributions.

More detailed guidance is given in BS EN 1991-1-7[11], some aspects of which were discussed in Section 3.5 in Part 1 of this Digest.

3.6 Accidental loadings and actionsExisting buildings are susceptible to three principal forms of accidental loading: gaseous and other types of explosions, vehicle impacts and fires. These are discussed below. In the UK there are standard classification categories for structurally significant damage to buildings caused by gas explosions and by vehicle impacts: moderate, severe, and very severe[12, 13].These classifications are used below. BS EN 1991-1-7[11] provides approaches for considering the accidental actions associated with impacts arising from both road vehicles and derailed rail traffic.

Gas explosionsThere are two types of gas explosion: those that occur within buildings (ie internal ones) and happening outside buildings (ie external ones).• Internal gas explosions: The total number of

reported explosion incidents in the UK has declined considerably over the past two decades, but in recent years there has been a noticeable increase in the number of explosions causing significant structural damage that are attributed to cylinder gas and aerosol cans. The number of structurally significant explosions in all types of building remains very low, though: it is about 10 incidents per year within the entire UK building stock[14]. This includes buildings both with and without a piped gas supply. Most fatalities associated with explosions in buildings occur in circumstances where structural damage is minor.

• External gas explosions: Such explosions are, fortunately, very rare, but this form of incident can create a substantial overpressure, which could severely damage a building at a substantial distance, possibly hundreds of metres, from the seat of the explosion[14].

Vehicle impactsVehicle impacts with buildings may involve three main forms of transport:• Road vehicles: Although this is the most probable

type of vehicle impact involving buildings, historical data indicate that over 20 years no recorded road vehicle impacts caused severe or very severe damage to a UK building with a height of five storeys or over[14]. road vehicle impacts almost exclusively affect low-rise buildings adjacent to roads[13].

• Aircraft: Aircraft rarely hit buildings, but impacts by both small and large aircraft have occurred

around the world. Impacts by large aircraft, with the associated fires resulting from their large fuel load, have caused severe structural damage and, in a few cases outside the UK, complete collapse of some or all of a building. As the risks associated with the impact of aircraft with buildings are small, they may be regarded as being insignificant and adequately controlled[14].

• Trains: Statistically these appear to pose a very low risk of causing severe damage to buildings, but instances of such damage have occurred outside the UK[14].

FiresFires potentially pose the greatest hazard to existing buildings, because they are much more frequent than the other possible sources of hazard mentioned above. Structural damage is caused by severe thermal heating and expansion effects. The occurrence of fires in the UK is analysed and published every year in the Communities and Local Government Fire Statistics[15]. These statistics reveal that about 10% of fires in all UK buildings cause severe structural damage, and about 3% result in the destruction of the building or structure concerned.

3.7 Structural system and materialsDetails of the structural arrangements, member sizes, materials of construction, connections and joint arrangements may be available from existing drawings. If they are not, it will be necessary to conduct appropriate surveys, inspections, testing and opening-up (exposure) of the structure to obtain the necessary information. The as-built details may be significantly different from those portrayed on design drawings. Also, in some buildings the structure will have been altered after construction, or may have deteriorated while in service. So, even if design or as-built drawings are available, it is prudent to check selected items.

Installation, in-service modification or renewal of building services may have involved cutting or modifying parts of the structure, and checks should be made that such activities have not compromised the original structure. For example, the installation of plumbing has been known to result in the excessive notching of timber floor joists, which has severely compromised their structural load-carrying capacity.

For the materials of construction, it will be necessary to establish some strength values to be used in the appraisal. It may be possible to justify the structure for the loads and actions acting on it simply by making conservative (ie lower bound) estimates of the materials’ potential strength without conducting sampling or testing.

The materials in an existing building are generally stronger than anticipated during design. This can have both positive and negative influences on structural performance and behaviour. Higher material strength is generally advantageous for load capacity, but it may come with the downside of reduced ductility. It may also be a disadvantage in some conditions, such as for concrete structures subject to fire. In such circumstances a concrete strength that is higher than anticipated will typically

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reduce the concrete’s permeability, potentially making it more prone to surface spalling under the elevated temperatures and severe thermal gradients associated with a fire.

If the materials have deteriorated or been damaged, the above approach may not be feasible, and appropriate surveys, inspections, sampling and testing are likely to be needed. Thus it is also important to establish and report on the current condition of the building and the structural members, along with the environmental conditions influencing their potential durability. These matters are discussed further in Part 3 of this Digest.

When it is necessary to open up to expose structural details and take material samples – activities that damage finishes, disrupt building users and potentially incur significant reinstatement costs – the engineer will have to exercise judgement about how far to take the exercise. Often there has to be a trade-off between gaining a greater degree of understanding and certainty about the nature and condition of the existing structure and the practicality and cost of obtaining the desired information. If constraints prevent the engineer from obtaining all the information considered necessary, this should be discussed with the client and the situation be recorded in the appraisal report.

A brief inspection of adjacent similar buildings may be also helpful by providing additional insights, especially where common forms of deterioration or damage have been experienced.

Issues relating to materials are considered further in Section 2 of Part 4 of this Digest.

3.8 Stability and resilience of buildingsVarious factors can influence the structural stability and resilience of a building, and hence what may need to be taken into account when undertaking a structural appraisal of an existing building. There must be a clear path by which vertical and horizontal loads can be safely transmitted down to the foundations and into the ground. These considerations should include issues such as anticipated deformations as well as strength (limit state or ultimate capacity).

A building’s structural form and detailing will significantly affect its stability and its resilience – that is, its ability to withstand accidental or exceptional

loading without experiencing an undue degree of damage, such that progressive collapse or disproportionate damage occurs. Structural systems containing parallel load paths (ie redundant structural systems) provide multiple routes by which loads can be transmitted to the ground. Thus redundancy in the structural system is valuable in this regard. Effective means of redistributing loads often also require a satisfactory degree of ductility.

The concept of applying notional horizontal loads to structural systems underpins many contemporary design approaches to achieving robustness. However, in older buildings issues of resistance to horizontal forces and lateral stability were not generally given the degree of consideration required under modern codes for structural design and Building regulations.

Although most engineering design calculations do not accurately represent real structural behaviour, the resulting buildings are satisfactorily safe and serviceable. However, such calculations do not represent various types of structural behaviour, such as brittle behaviour, and generally they do not directly examine issues of structural stability. As a result, the safety factors incorporated in conventional design calculations are not relevant to these behaviours. Such types of failure, which may be sudden and therefore give little warning, would need special consideration in a structural appraisal.

In many masonry buildings the individual structural elements are held together predominantly by gravity forces (ie those mobilised through friction or adhesion effects). The associated lack of continuity leaves such structures vulnerable to damage by accident or exceptional events.

The following three topics are considered in more detail below:• terminology• actual performance of buildings and structures• ways of reducing sensitivity to disproportionate

collapse through structural robustness and collapse resistance, with the latter aligning with the BS EN 1991-1-7[11] philosophy of verifying that the structure can sustain the accidental action concerned.

TerminologyThe term ‘robustness’ is referred to but not defined within The Building regulations 2000, Approved Document A – Structure[16]. However, in the technical literature there are various definitions of this characteristic of a structure or building. Clause 1.5.14 of BS EN 1991-1-7[11] gives the following definition:

‘Robustness is the ability of a structure to withstand events like fire, explosions, impact or the consequences of human error, without being damaged to an extent disproportionate to the original cause.’

Clause 2.1 of BS EN 1990[4] provides a somewhat different form of words on this topic, noting that ‘A structure shall be designed and executed in a way that it will not be damaged by events such as explosion, impact, and the consequences of human errors, to an extent disproportionate to the original cause.’ It also notes

Figure 3: A residential building undergoing renovation works which includes upgrading the thermal performance of the external building envelope

7

that the events that are to be taken into account are those agreed for an individual project with the client and the relevant authority.

These forms of words are not entirely satisfactory, as ‘robustness’ should simply be a quality of the structural system concerned (eg its strength, form, or ductility), and so should be independent of the form of loading or action causing the damage to the structural system. To address this issue a slightly amended working definition of robustness is given in the Annex. This definition is supported by some qualifying observations.

Another approach to enhancing robustness postulated in the technical literature is to introduce ‘strong floors’ into a building to halt the vertical progression of any collapse. However, this requires that the structural elements supporting such strong floors (eg columns and walls) have sufficient strength to resist the large forces that are likely to be mobilised during any collapse.

Collapse resistance could be a useful concept in relation to the appraisal of some types of existing building. Although the term is not mentioned in Approved Document A, the Annex to this part of the Digest gives a working definition, indicating that it provides a measure of a structural system’s sensitivity to specified accidental loads or actions, and the structural system’s ability to survive them. Accordingly a building or structural system would be considered to be collapse resistant if it could sustain the specified accidental loads or actions without the initiation of a progressive collapse.

Actual performance of buildings and structuresExisting buildings usually behave better than anticipated in design calculations, thanks to hidden strengths and behaviours in the structural system. As a result, the incidence of structural collapse appears to be significantly lower than that anticipated in the structural design process. This beneficial outcome may perhaps be explained by the existence of load-sharing mechanisms, unaccounted for in design, that contribute to the realised strength of a building. For example, floor and wall slabs may be designed as one-way spanning for the ultimate limit state condition, but in reality can act as two-way spanning for most in-service situations because of the nature of the support or continuity provided for the slab.

Non-structural elements often have a significant influence upon the observed stiffness and strength both of individual structural members and of the overall structure under normal and accidental load situations, especially where damage has been caused and alternative load paths are mobilised. For example, heavy concrete claddings and masonry infill construction commonly add greatly to the lateral stiffness of a framed structure. Some types of internal partition can provide alternative secondary load paths in the event of damage to the main structure of a building.

Composite action between structural and non-structural components may also mean that the structural components appear to perform much better than if they were tested in isolation. Load sharing between individual structural components of a larger element can produce similar results: floors formed from individual precast

concrete beam or plank components provide a good example of this.

Another load-carrying mechanism generally unaccounted for in design is the high compressive membrane forces that can be generated in a floor slab by whole building behaviour effects (arching action within the floor slab). In certain circumstances this mechanism can significantly increase the load-carrying capacity of a floor slab to many times that anticipated by design calculations based upon flexural behaviour.

These are just a few examples of factors that can influence performance; there are a number of other possible ones.

Reducing sensitivity to disproportionate collapse: structural robustness and collapse resistanceThe three main methods of reducing sensitivity to disproportionate collapse are listed below. Such measures are relatively easy to implement in new buildings during construction, but they are significantly more difficult and costly to implement in an existing building:• Method 1: The provision of horizontal and vertical

ties, in accordance with the recommendations of the relevant material codes. This is a commonly used approach. The ties provide a minimum degree of strength and continuity, plus an associated but unquantified amount of ductility in the overall structure. Such provisions have historically provided a satisfactory degree of robustness in the general population of buildings.

• Method 2: Mobilising an alternative load path, such as catenary action, after the loss or notional removal of a loadbearing element. Some degree of local collapse is permitted, as long as it remains within the prescribed limits (see Box 2 and Section 3.5 in Part 1 of this Digest).

• Method 3: The provision of key elements designed to resist notional forces arising from a specified overpressure. For existing buildings, the overpressure values used are 17 kN/m2 in buildings without a piped gas supply to any part of the building, and 34 kN/m2 in buildings with a piped gas supply. Further details of the origin and use of these overpressure values are given by Ellis and Currie[12] and Matthews and reeves[14].

Method 1 is the most commonly used technique. The aim is to reduce sensitivity to disproportionate collapse by using either horizontal ties alone (eg to meet Building Class 2A requirements) or both horizontal and vertical ties acting in combination (eg to meet Building Class 2B requirements).

Ties enhance the structural system’s ability to mobilise alternative load paths around an area of local damage. This ability is typically a function of the degree of redundancy within the structural system, together with the strength and ductility of the elements and the joints between them. This approach is satisfactory when the extent of local damage is relatively small – perhaps involving one or two vertical loadbearing elements – but mobilising alternative load paths within the structural system tends to increase the overall extent of damage

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caused across the building, although typically the severity and consequences of that damage are reduced.

When the initial damage to the building might be more extensive, the overall extent of collapse and damage might be better controlled by other means, such as segmenting the building into zones by introducing joints or discontinuities to limit the propagation of collapse and damage across the building. A collapse in one such zone should not propagate across the boundary joints or discontinuities into the adjacent zones of the building.

On this basis, ‘robustness’ is not only related to the strength, form and ductility of the structural system, but may also be influenced by the division of a building into zones by joints or discontinuities. robustness is a quality of the structural system alone; it is independent of the cause of the damage or the probability of initial local failure.

The Institution of Structural Engineers has published practical guidance on ways of reducing sensitivity to disproportionate collapse in some classes of buildings, and for various forms of construction[17]. This guidance does not address buildings within Building Class 3, as defined in Table 11 of Approved Document A. See Table 1 and Section 3.2 of Part 1 of this Digest for further information about building classes.

3.9 Loadings and actions – existing, historical and future

Existing permanent loadings can be estimated by survey and measurements made on site, but it is often more difficult to establish appropriate values for variable actions, such as imposed loads. It may be possible to obtain an indication of historical loadings from archive records of use, or from evidence obtained on site. This may include talking to the current or previous occupants of the building, neighbours of long standing, and those with a professional involvement, such as facilities managers. The appraisal will usually be based on contemporary load values. Part 3 of this Digest gives further details of the above considerations.

Previous structural alterations may have resulted in changes in loading, and influenced other characteristics such as load paths, or the degree of fixity or continuity of structural members. Changes on adjacent plots, to nearby buildings or underground infrastructure may also be relevant, including demolition or construction of buildings. It may also be appropriate to consider other influences, such as the effect of rising groundwater level on the bearing capacity of foundations, and increased settlement or heave effects.

Intended future loadings should be defined via the client’s brief.

The various types of load and action that might need to be considered in a structural appraisal include:• dead loads• imposed loads• storage loads• dynamic loads• wind loads• snow and ice loads

• extreme events and accidents, including fire and internal or external gas explosions, flooding, vehicle impacts

• soil pressures and ground movement• loads arising from machinery, appliances and

equipment• strains and deformations induced by fabrication, lack

of fit during assembly and differential movements.

3.10 Verification criteriaVerification is the analytical process that examines whether the performance requirements for the building are satisfied. These requirements are typically concerned with the structural and durability aspects.

There are several different formats that can be used both in design and in structural appraisal as the basis for verifying the safety of a building or structure, and its compliance with other performance criteria or limit states being considered. Work over recent years has developed additional approaches to those recognised in previous codes and standards for design, resulting in an extended overall framework of possible verification formats. The new fib Model Code 2010[18] defines one such framework. The various formats that it defines for verifying safety and other parameters are briefly described below. The underlying philosophy is generally consistent with that employed in the recently introduced structural Eurocodes. These approaches can be applied to the structural appraisal of existing buildings. Selection of the approach to be used will be influenced by the objectives of the study and the complexity of the circumstances.

Probabilistic safety format: Sometimes also referred to as the fully probabilistic method, this allows explicit evaluation of the reliability requirements in terms of the reliability index (the β-value) and the reference period (in years) concerned. This is the most technically demanding and complex format. It can be used both for designing structures and for appraising existing structures, in situations where the effort involved in undertaking a structural appraisal of this type is economically justified. The probabilistic approach will rarely be used for the design of new structures, because of the lack of statistical data; it is better suited to the appraisal of existing structures, particularly for calculating residual service life.

Global resistance format: In this approach the structural resistance is considered at a global structural level, rather than by local verification of the adequacy of structural elements using partial safety factors. It is especially suitable for design based on non-linear analysis, where numerical simulation is used to verify limit states.

Partial safety factor format: This is the usual way in which the design of new structures (on an elemental basis) is verified for safety. It is a simplified concept based on past experience, and is calibrated so that the general reliability requirements are satisfied with a sufficient margin for a defined period of time. In the future it could also be used for verifying service life and durability-related parameters, provided adequate long-term experience has been gained, or sufficient data are available to undertake a calibration by the probabilistic method.

9STRUCTURAL APPRAISAL OF EXISTING BUILDINGS, INCLUDING FOR A MATERIAL CHANGE OF USE – DG 366 PArT 2

Deemed-to-satisfy approach: This involves selecting appropriate values for the parameters concerned from a set of predetermined alternatives given in a standard. This method is the usual way of verifying the service life or durability design of new structures, but it can potentially also be used in the first stage of an appraisal of the durability of existing buildings and structures.

Design by avoidance: This approach is used mainly for verifying the service life or durability design of new structures, but it can equally be used for these purposes in appraising existing buildings and structures.

Other potential verification approaches: These include design assisted by testing, which is an approach recognised in the recently introduced structural Eurocodes. Clearly, this concept can also be applied in the context of existing buildings as structural appraisal assisted by testing. In the context of procedures for establishing the strength or load capacity of structural elements, or more general aspects of structural behaviour, appropriate load testing of selected parts of the structure would be carried out. Generally this would be in conjunction with some form of analytical modelling to help in interpreting the results. Load testing might also be used to verify performance against serviceability criteria, such as deflections, if these were considered critical. Appropriate forms of durability testing can also be undertaken for making a prognosis about durability, usually be in conjunction with analytical modelling to help in interpreting the results, and in predicting future in-service performance.

3.11 Safety considerations: acceptable risk levels for existing structures

The choice of the target reliability level for a building should take into account the possible consequences of failure in terms of risk to life or injury, potential economic losses, the degree of societal inconvenience, and the potential environmental damage that might result. The choice should also recognise the expense and effort required to reduce the risk of failure, which will generally be significantly greater for an existing structure than for one that is being designed. The maximum acceptable probability of failure depends on the type of limit state being considered and the consequences of failure.

For practical design procedures, an acceptably low probability of structural failure is achieved through the use of partial safety factors applied to both the load effect and the resistance sides of the basic design equation employed for strength evaluation as follows:

where gf is the (combined) partial factor for loads (actions) and load (action) effects, and gm is the partial factor for material strength, etc.

This approach can be applied not only to the ultimate limit state, to meet structural safety requirements, but also to serviceability or other conditional limit-state situations. The values of the partial factors have to be adjusted appropriately for the particular situation.

The choice of target reliability levels, and the adoption of revised reliability index levels (β-values) (reduced compared with those used for design) and partial safety factors to be used in the appraisal of existing buildings, are discussed in Section 8.3 of Part 3 of this Digest.

Many existing buildings do not meet the requirements of contemporary or even recent design codes. This does not mean that they are unsafe, but the engineer making an appraisal will need to exercise judgement about the acceptability of the risks involved. An existing building structure designed using engineering principles to accommodate the envisaged flow of forces may give greater confidence than one constructed simply using rules of thumb and experience to select the structural form and for sizing members and connections.

3.12 Defects, deterioration and damage mechanisms: condition of the structural parts now and in the future

All buildings deteriorate with time, although the rate and extent of deterioration vary considerably, as the processes concerned are affected by many factors. Most buildings will provide satisfactory performance over many decades, but some may contain defects or experience premature deterioration, perhaps only in a single zone or of a particular type of component, which requires one or more remedial interventions during the building’s life. Defects and deterioration, which is most likely in buildings with an aggressive or demanding service environment, may affect functionality under normal working conditions, and potentially impact on the building’s performance and safety, as well as on its appearance. Such difficulties may be compounded by a lack of appropriate maintenance. Without timely remedial intervention, durability can be adversely affected.

To establish the current condition of a building or its component parts, especially those with a structural function, its ability to meet appropriate performance requirements related to its current use should be evaluated. This typically involves gathering and evaluating information about the current status of a building and its components, together with the effects of any active deterioration mechanisms. Consideration of future performance requires a prognosis of the future condition of the building and its components in the envisaged service environment, taking account of other relevant aspects of its general circumstances.

Issues relating to deterioration and damage mechanisms are considered further in Section 3 of Part 4 of this Digest. Further information on future performance and residual service life estimation is given in Annex B of Part 3 of this Digest.

3.13 Sustainability-related influences on decisions and option selection

So far the discussions have concentrated on the functional requirements of a building – the technical performance requirements. But there are wider economic, sociocultural and environmental factors – the non-technical factors or issues – that may also have an important bearing upon decisions associated with a building’s future use

gf (loadeffects)≤ (1)structural resistance (of the materials)gm

and management. These factors are commonly brought together under the heading of sustainability. Appraisal and management decisions need to be undertaken holistically, balancing the different considerations. This should be done within a suitably broad framework, such as that offered by the BrE Environmental Assessment Methodology (BrEEAM)[19]. These matters are discussed further in Section 4 of Part 4 of this Digest.

3.14 Malicious actions or attacks and vandalism

Measures to reduce the sensitivity of a building to disproportionate collapse in the event of an accident (requirement A3) are described in Approved Document A[16], but acts such as vandalism and malicious actions or attacks do not fall within the formal scope of Approved Document A or the supporting referenced standards (eg the structural Eurocodes). However, the brief agreed with the client for the structural appraisal might require consideration of such matters. Although such acts are outside the scope of this Digest, the principles discussed in relation to structural robustness and resilience might be applied to such circumstances, and to the potential hazards identified from any threat analysis undertaken.

In this context it should be noted that the Health & Safety at Work etc Act 1974[20], which is the principal UK Act regarding health and safety at work, requires designers to consider all reasonably foreseeable hazards, rather than requiring and building to be constructed so that in the event of an accident it will not suffer collapse to an extent disproportionate to the cause as is required by Approved Document A[16]. Harding and Carpenter[21] have drawn attention to and discussed the differences in these requirements, noting the need to consider accidental and malicious actions at any stage in a structure’s life where the Health & Safety at Work etc Act and associated workplace legislation applies.

REFERENCES1 Department for Communities and Local Government

(DCLG). The Building regulations 2010 (England and Wales). London, TSO, 2010.

2 IStructE. Appraisal of existing structures. London, Institution of Structural Engineers, 2010, 3rd edn.

3 De Vekey rC. Principles of masonry conservation management. BrE DG 502. Bracknell, IHS BrE Press, 2007.

4 BSI. Eurocode 0: Basis of structural design (including National Annex). BS EN 1990. London, British Standards Institution, 2002.

5 BSI. Eurocode 1: Actions on structures: Part 1-1: General actions – Densities, self-weight, imposed loads for buildings (including National Annex). BS EN 1991-1-1. London, British Standards Institution, 2002.

6 BSI. Eurocode 1: Actions on structures: Part 1-2: General actions – Actions on structures exposed to fire (including National Annex). BS EN 1991-1-2. London, British Standards Institution, 2002.

7 BSI. Eurocode 1: Actions on structures: Part 1-3: General actions – Snow loads (including National Annex). BS EN 1991-1-3. London, British Standards Institution, 2003.

8 BSI. Eurocode 1: Actions on structures: Part 1-4: General actions – Wind actions (including National Annex). BS EN 1991-1-4. London, British Standards Institution, 2005.

9 BSI. Eurocode 1: Actions on structures: Part 1-5: General actions – Thermal actions (including National Annex). BS EN 1991-1-5. London, British Standards Institution, 2003.

10 BSI. Eurocode 1: Actions on structures: Part 1-6: General actions – Actions during execution (including National Annex). BS EN 1991-1-6. London, British Standards Institution, 2005.

11 BSI. Eurocode 1: Actions on structures: Part 1-7: General actions – Accidental actions (including National Annex). BS EN 1991-1-7. London, British Standards Institution, 2006.

12 Ellis Br and Currie DM. Gas explosions in buildings in the UK: regulation and risk. The Structural Engineer, 1998, 76 (19) 373–380; associated discussion in The Structural Engineer, 1999, 77 (19) 29–30.

13 Ellis Br and Dillon PJ. road vehicle impacts on buildings in the UK: regulation and risk. The Structural Engineer, 2003, 81 (3) 36–40.

14 Matthews SL and reeves B: Handbook for the structural assessment of large panel system (LPS) dwelling blocks for accidental loading. BrE report Br 511. Bracknell, IHS BrE Press, 2012.

15 Department for Communities and Local Government (DCLG). Fire statistics United Kingdom. Available from the Communities and Local Government website: www.communities.gov.uk/publications/corporate/statistics

16 Department for Communities and Local Government (DCLG). The Building regulations 2000. Approved Document A: Structure. London, DCLG, 2004.

17 IStructE. Practical guide to structural robustness and disproportionate collapse in buildings. London, Institution of Structural Engineers, 2010.

18 fib. Bulletins 65 and 66: Model code 2010: revised final draft, volumes 1 and 2. Model code. Lausanne, Fédération Internationale du Béton (fib), 2012.

19 www.breeam.org. 20 The Health & Safety at Work etc Act 1974. London, HMSO,

1974.21 Harding G and Carpenter J. Disproportionate collapse of

‘Class 3’ buildings: the use of risk assessment. The Structural Engineer, 2009, 87 (15–16), 29–34.

All UrLs accessed 25 May 2012. The publisher accepts no responsibility for the persistence or accuracy of UrLs referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

10 STRUCTURAL APPRAISAL OF EXISTING BUILDINGS, INCLUDING FOR A MATERIAL CHANGE OF USE – DG 366 PArT 2

11STRUCTURAL APPRAISAL OF EXISTING BUILDINGS, INCLUDING FOR A MATERIAL CHANGE OF USE – DG 366 PArT 2

Annex: Definition of terms relating to the stability of the structure

Collapse resistance is a measure of the ability of a structural system to resist the effects of specified accidental loads or actions occurring at or below a defined threshold. It provides a measure of the sensitivity of a structural system to specified accidental loads or actions and the ability of the structural system to survive them. Collapse resistance is a combined property of the structural system and the loading/action configuration applied to it. It is influenced by numerous factors including the strength, form and ductility of the structural system; which have a bearing upon the possible causes of initial local failure. In circumstances where the accidental loads or actions result in impulsive loads being applied to the structure, collapse resistance will be greatly influenced by the strength of the elements involved and the ability of the structural system concerned to absorbed energy. Collapse resistance issues may apply at both local and global structural system levels.

Disproportionate damage arises in a collapse in which the failure front moves progressively away from the initial trigger point of local damage to envelope portions of the structure significantly larger than the part directly damaged by the initial incident such that the overall degree of damage is out of proportion with the magnitude of the initiating event.

Ductility is the ability of a structural material to accommodate a large degree of plastic deformation without breaking or experiencing a significant reduction in its toughness. Accordingly this property is the ability of a structural system to accommodate large displacements or rotations without catastrophic failure and/or to absorb or dissipate large amounts of energy whilst ensuring that specified overall displacements or rotations remain within acceptable specified bounds.

Fragile buildings or structures are ones that are vulnerable to progressive collapse and where there is an unacceptable probability that their collapse resistance might be exceeded.

Hazard is an occurrence which has the potential to cause deterioration, damage, harm or loss through the occurrence of an undesired event.

Malicious act or attack is a deliberate act, commonly premeditated, carried out with the intention of doing harm. The potential forms of malicious attack are wide ranging. The intended consequences of the act may be to cause significant loss of life and injuries and/or economic loss; eg causing severe damage and possible collapse of buildings or other structures, with or without a warning being given. Malicious attacks have included internal and external explosions that were deliberately caused, vehicle impacts, as well as direct mechanical damage to vulnerable parts of structures (eg bridge stay cables).

Progressive collapse is a collapse in which the failure front moves progressively away from the initial trigger point of local damage to envelope portions of the structure significantly larger than the part directly damaged by the initial incident. NB: The extent of a progressive collapse may not be disproportionate to the nature or magnitude of the initial incident.

Resilience is the ability of a building or structural system to withstand an accidental or exceptional loading incident without experiencing an undue degree of damage, such that progressive collapse or disproportionate damage occurs.

Robustness is the ability of a building or other structure subject to accidental or exceptional loading or other action(s) to sustain damage or local failure without experiencing a disproportionate degree of overall distress or collapse.

Qualifying observations: In buildings robustness is commonly taken to be the ability of the structural system to mobilise alternative load paths around an area of local damage, which is typically a function of the degree of redundancy within the structural system together with the strength and ductility of the elements and joints between them. Whilst this approach is satisfactory when the extent of local damage is relatively small, perhaps involving one or two vertical load bearing elements, mobilising alternative load paths within the structural system tends to have the effect of increasing the overall extent of damage caused across the building, although typically the severity and consequences of that damage is reduced.

However, when the initial damage could be more extensive than the local failure described above, the overall extent of collapse and damage may be better controlled by segmenting the building or structure into zones by the introduction of joints or discontinuities which are intended to limit the propagation of collapse and damage. Thus it would be intended that a collapse in one such zone should not propagate across the boundary joints or discontinuities into the adjacent zones of the building or structure.

Thus robustness is related not only to the strength, form and ductility of the structural system, but may also be influenced by the division of a building into zones by joints or discontinuities. robustness is a quality of the structural system alone and is independent of the cause of the damage and/or the probability of initial local failure.

Stability is the state of being stable, which implies that the performance or functionality of a structural component or structural system will remain largely unchanged with time and is unlikely to be disturbed or diminished by an anticipated external trigger event or set of circumstances/actions such that its ability to carry its intended load and to serve its intended structural function would be significantly diminished.

Structural diversity is the concept that there can be a number of alternative ways of achieving specified structural performance requirements which are not dependent upon the same form of structural behaviour. The term also acts as an indicator of the extent that this concept may be applicable to a particular structural system. For example, if a building has a high degree of structural diversity, this would indicate that its sensitivity to the effects of local damage is reduced, which accordingly would improve the resilience of the structural system. An example of this concept is the ability of a framed structure to develop catenary action following local damage which impairs/disables the original load carrying mechanisms.

Vulnerability is the susceptibility of a structural component or structural system to damage and potential collapse.

12 page header left – page header leaflet number

BRE is the UK’s leading centre of expertise on the built environment, construction, energy use in buildings, fire prevention and control, and risk management. BrE is a part of the BrE Group, a world leading research, consultancy, training, testing and certification organisation, delivering sustainability and innovation across the built environment and beyond. The BrE Group is wholly owned by the BrE Trust, a registered charity aiming to advance knowledge, innovation and communication in all matters concerning the built environment for the benefit of all. All BrE Group profits are passed to the BrE Trust to promote its charitable objectives.BrE is committed to providing impartial and authoritative information on all aspects of the built environment. We make every effort to ensure the accuracy and quality of information and guidance when it is published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain.BrE, Garston, Watford WD25 9XX Tel: 01923 664000, Email: enquiries@bre.co.uk, www.bre.co.uk

BRE Digests are authoritative summaries of the state-of-the-art on specific topics in construction design and technology. They draw on BrE’s expertise in these areas and provide essential support for all involved in design, specification, construction and maintenance. Digests, Information Papers, Good Building Guides and Good repair Guides are available on subscription in hard copy and online through BrE Connect. For more details call 01344 328038. BRE publications are available from www.brebookshop.com, orIHS BrE Press, Willoughby road, Bracknell rG12 8FB Tel: 01344 328038, Fax: 01344 328005, Email: brepress@ihs.comrequests to copy any part of this publication should be made to:IHS BrE Press, Garston, Watford WD25 9XX Tel: 01923 664761 Email: brepress@ihs.com www.brebookshop.com

DG 366, Part 2© BRE 2012

July 2012ISBN 978-1-84806-269-6 (Part 2)

ISBN 978-1-84806-267-2 (4-Part set)

Acknowledgements

The preparation and publication of this Digest was funded by BrE Trust.

12 STRUCTURAL APPRAISAL OF EXISTING BUILDINGS, INCLUDING FOR A MATERIAL CHANGE OF USE – DG 366 PArT 2

HANDBOOk FOR THE STRUCTURAL ASSESSMENT OF LARGE PANEL SySTEM (LPS) DwELLING BLOCkS FOR ACCIDENTAL LOADING

Get new guidance on the structural assessment and strengthening options for large panel system (LPS) dwelling blocks, focusing primarily on their resistance to accidental loading associated with gas explosions.

This book (ref. Br 511) is available in print and pdf format from: brepress@ihs.com, +44 (0)1344 328038, www.brebookshop.com.

IHS BRE Press

HANDBOOK FOR THE STRUCTURAL ASSESSMENT OF LARGE PANEL SYSTEM (LPS) DWELLING BLOCKS FOR ACCIDENTAL LOADING

Stuart Matthews and Barry Reeves

HANDBOOK FOR THE STRUCTURAL ASSESSMENT OF LARGE PANEL SYSTEM

(LPS) DW

ELLING BLOCKS FOR ACCIDENTAL LOADINGM

atthews and Reeves

RELATED TITLES FROM IHS BRE PRESSDESIGN OF DURABLE CONCRETE STRUCTURES FB xx, 2012

CONCRETE: CONSTRUCTION’S SUSTAINABLE OPTIONEP 92 (6-volume set), 2008

NON-TRADITIONAL HOUSES Identifying non-traditional houses in the UK 1918–75BR 469, 2004

CONCRETE REPAIRSEP 81 (2-volume set), 2007

HANDBOOK FOR THE STRUCTURAL ASSESSMENT OF LARGE PANEL SYSTEM (LPS) DWELLING BLOCKS FOR ACCIDENTAL LOADINGThis handbook presents new guidance on the structural assessment and strengthening options for large panel system (LPS) dwelling blocks, focusing primarily on their resistance to accidental loading associated with gas explosions, and supported by extensive background information.The progressive collapse of part of Ronan Point tower block in east London in 1968 was a significant event in structural engineering in relation to the understanding of disproportionate damage to structures. Extensive research and investigations since then, including full-scale structural load tests on a block in Liverpool, are taken fully into account. This handbook:• defines the requirements to be met and the criteria against which the results of a structural assessment of this particular class of building should be judged. These are seen to effectively supersede the previous guidance set down in the Ministry of Housing and Local Government Circulars dating back to 1968.• gives guidance on how to undertake the structural assessments which are required, drawing on previously unpublished technical information.• details the historic background to these requirements, with these being brought up to date and set in the contemporary philosophical context of the requirements of the recently introduced structural Eurocodes.• explains the risk environment which applies to this class of building.• provides an overview of durability assessment/intervention and strengthening options.

IHS BRE Press, Willoughby RoadBracknell, Berkshire RG12 8FB

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9 781848 062009

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Get an overview of the structural fire engineering design process and the techniques available to ensure the safe and economical fire design of concrete structures. This guide is the result of a collaborative research project funded by the UK government and the concrete industry. It will

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This report (ref. Br 490) is available in print and pdf format from: brepress@ihs.com, +44 (0)1344 328038, www.brebookshop.com.

CONCRETE STRUCTURES IN FIRE