1 Introduction. 1.1 Scope of this Manual. 1.2 Purpose

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1 1 Introduction 1.1 Scope of this Manual This Manual proposes a systematic risk assessment framework for the design of high-risk structures against disproportionate collapse. Risk assessment is a requirement for all structures, but for Class 1 and 2 buildings (BS EN Annex A 1.1 ) is usually addressed through good quality management and through adherence to recognised codes of practice. This Manual is concerned with systematic risk assessment as defined in BS EN Annex A 1.1, principally with regard to the design of Class 3 buildings. It complements the Institution of Structural Engineers publication Practical guide to structural robustness and disproportionate collapse in buildings 1.2, which provides a useful primer on the concepts of robustness and the (largely) qualitative methods used in the design of lower-risk buildings against disproportionate collapse. The Practical guide does not, however, give guidance on the design of high-risk (Class 3) buildings, which is the primary focus of this Manual. The approach set out for systematic risk assessment in this Manual is designed to be equally applicable to any high-risk structure. This may include structures and buildings that do not fit into the building risk classification system in BS EN , structures where there are particular risks associated with the design itself perhaps by virtue of it being novel or unusual or where there are particular risks to which the structure may be exposed, such as malicious risks including terrorism. It may also include structures where there is exposure or vulnerability in a temporary state, and the design of the extension, alteration or change of use of existing structures where design constraints may limit the level of robustness than can practically be achieved. In addition to consideration of risks to life safety, the same methodology may equally well be applied for the consideration of risk unrelated to safety, such as where a higher level of performance is necessary in the structure to support some critical function, purpose or use. Boxed examples are given throughout the text to highlight either lessons from past failures, examples that would constitute a poor design, or examples of good design practice. This chapter includes the legal background, while in Chapter 2 some of the key concepts are introduced together with a discussion the Eurocode requirements. Chapter 3 presents the proposed framework for a systematic risk assessment and is the core of the document, and is followed by discussion of each part of the risk assessment process and the factors that should be considered in its development. Chapter 4 presents a commentary on the spectrum of risk reduction measures that could be employed, both hard measures (changes to the design) and soft measures (quality assurance, use of peer review and so on). Chapter 5 gives a worked example for a typical Class 3 building. The reader is referred to three related Institution publications: Safety in tall buildings and other buildings of large occupancy 1.3 provides many recommendations for designing tall buildings to be intrinsically safe and is a direct complement to this Manual. Risk in structural engineering 1.4 provides a foundation in the principles of risk management and how to manage risk in design. Appraisal of existing structures 1.5 has an equally important relationship to this Manual, which in Chapter 6 outlines how systematic risk assessment can be applied to designing for robustness in the retrofit of existing structures, where the existing nature of the building is likely to place constraints on the design feasibility. 1.2 Purpose The risk-based approach outlined in this Manual may be used to consider the full spectrum of natural, accidental and malicious hazards. It is intended to provide an approach through which the Eurocode requirement (BS EN Annex A 1.1 )to undertake a systematic risk assessment for Class 3 buildings can be met, together with the recommendations of the Building Regulations Approved Document A 1.6 and the equivalent documents in the devolved administrations of the United Kingdom for Class 3 buildings. The Manual is written for the professionally qualified structural engineer who has a certain level of experience in safety engineering and the principles of risk assessment. It is intended to aid the practising structural engineer in the design of robustness in a high-risk building, though the structural engineer tasked with preparing the robustness design has a responsibility to ensure that they are suitably qualified and experienced (SQEP) to do so. No formal register exists designating individuals with suitable qualification and experience for the systematic risk assessment of high-risk buildings, but it will normally be expected that the systematic risk assessment will be authored by a senior structural engineer with a track record of designing similar buildings, while past experience of designing against extreme hazards will be advantageous. While intended to aid the structural engineer in the process, the Manual is not intended to be the sole means by which the engineer will gain the skills and knowledge required and hence become competent in the preparation of a systematic risk assessment. In addition to providing guidance to the practising structural engineer, the Manual highlights the duty of care owed by the structural engineer to deliver a robust design, and the responsibilities held by the client in the decisions made on the basis of the assessment findings. It is also written to provide guidance to the building control body in evaluating a building control submission for a Class 3 building. The Institution of Structural Engineers Manual for the systematic risk assessment of high-risk structures against disproportionate collapse 1

2 Development of a systematic risk assessment 3.9 upon the construction sequence, controls to ensure adequate quality management through the supply chain and during construction, inspection and maintenance regimes to be implemented during the operation of the building, and so on. The financial cost associated with the risk mitigation measures should also now be available. What follows in the remaining steps is largely to do with testing the sensitivity of the risk assessment to the underlying assumptions, which may modify the findings but is unlikely to cause wholesale changes, and the review and acceptance of the residual risks by others. ultimate force to which it might be subjected, and yet still be governed by a brittle failure mode such as shear but be designed in accordance with the codes of practice. This would completely overlook the importance of the element to the robustness of the structure. If an element is shown to be critical to the design of the structure, a sensitivity study should be undertaken to ensure that there is only a gradual change in the response of the structure at a load of up to, say, 40% above the design (i.e. factored) load. Box 3.10 Pipers Row car park, Step 8: Review the residual risk In this step the residual risks that remain in the design following the application of the risk reduction measures should be reviewed with the client and any other risk stakeholders who will become the risk owners for the residual risks in the design. Cognisance must be given to any designers who are not appointed such as temporary works designers, and limits defined on their subsequent design be properly documented such that they can be effectively communicated upon their appointment. Both the client and other risk stakeholders must agree with and accept the level of residual risk inherent in the design as they will be the eventual risk owners who will be responsible for controlling the residual risks Step 9: Check the sensitivity of the risk assessment Common areas of sensitivity In this step the findings of the risk assessment are checked in respect of: cliff edge effects low likelihood/high consequence hazards combined hazards. The sudden failure of the car park at Pipers Row, Wolverhampton was due to loss of punching shear capacity. Deterioration of the upper surface of the concrete slab due to frost, water and crystallisation of de-icing salts caused loss of the cover to the top layer of rebar, fissuring and cracking of the concrete matrix, carbonation of the concrete and corrosion of the reinforcing bars. The friable degraded material extended to a depth of approximately 100mm, well below the layer of the top reinforcement. The punching shear capacity rapidly reduced with greater depths of erosion/ disruption of the concrete, leading to a sudden collapse. The car park was designed using the contemporary code of practice CP , which has since been shown to give an over-prediction of unconservative shear strengths in reinforced concrete. The concrete was highly variable: some with low cement contents, poor mixing and compaction, so that localised strengths were sometimes substantially less than the specified 20.5N/mm 2. Reinforcement in both faces of the slab was set too low, leading to premature corrosion of the bottom flange and a reduction in the shear strength of the slab. Surfacing was also thinner than the design value which, combined with the poor concrete compaction, reduced the dead load from 6.2kN/m 2 to 5.4kN/m 2. Poor tolerances in the setting-out of the slab led to an imbalance in the load distribution and increase in the effective shear stresses around some columns by some 40% The investigation found that, if properly maintained, the structure should have had a reasonable factor of safety in relation to the in-service loads. However, deterioration of the concrete led to the sudden punching shear failure of the concrete slab and the car park was subsequently demolished Cliff edge effects For each hazard, check through structural analysis whether small changes in the assumptions will lead to a gross change in the consequences of the hazard (Boxes ). If so, consider the beyond-design basis event in which the action is slightly worse than assumed in the risk assessment, and eliminate the cliff edge from the response or ensure that it is made sufficiently distant from the design basis to reduce the sensitivity of the design to the underlying assumptions. Consider the potential error in the assumptions: if the hazard can be accurately characterised, consider 5% or 10% changes in the assumptions; but if the hazard is difficult to evaluate, consider the effect of a 20% or even a 40% change. It is acceptable for the structure to sustain damage under such circumstances so lower partial factors can be used, provided the deformation is controlled and sudden failure is avoided. An element which is shown in a scenarioindependent alternative load path analysis to be critical to the structure could be designed for the Box 3.11 Shear failure of a reinforced concrete transfer slab Transfer slabs are commonly designed without shear links for the sake of ease of construction, cost and programme. Shear transfer relies on the shear strength of the concrete alone, and a brittle mode of failure is exhibited. Shear links may add marginal costs to the construction of a transfer slab, but given the importance of a transfer element in the structural system the inclusion of links are probably not a disproportionate risk reduction measure. The Institution of Structural Engineers Manual for the systematic risk assessment of high-risk structures against disproportionate collapse 31

3 3.11 Development of a systematic risk assessment Box 3.12 Brittle failure of a steel-framed structure A steel-framed structure, though ductile as a material and usually ductile on an element level using plastic methods of design, can exhibit brittle failure modes either at a local or a global level. Brittle failure can occur due to the failure of connections as well as the more obvious modes of brittle failure such as fatigue and corrosion. Partial-strength connections, which will comprise the majority of connections in a typical structure, can exhibit sudden failure due to applied forces being larger than designed, or by the connection being loaded in a manner such as rotation for which it was not designed. Both actions will often result under a progressive collapse scenario, and the failure mode of the structure may be unexpectedly brittle if connections have insufficient rotational ductility or are unable to develop the forces and moments necessary to arrest a progressive collapse. Even moment connections can fail in a brittle manner if not properly detailed unexpected brittle fracture of steel moment connections was a key issue in the Northridge and Kobe earthquakes Low likelihood/high consequence hazards Low likelihood/high consequence hazards are particularly sensitive to small changes in the underlying assumptions and, as described in Section 3.2.4, have the potential to significantly affect the overall risk profile. A specific review of the assumptions made in the derivation of the risk associated with such hazards is therefore warranted. As with cliff edge effects, a sensitivity study should be conducted to determine whether the risk to occupants could be made significantly worse if the hazard is slightly larger than assumed, or the structural response slightly worse than calculated. Additionally, if the risk of a low likelihood/high consequence event is significant, consider whether additional mitigation is necessary. Consider how the consequences would be viewed if the hazard did materialise would the consequences really be considered tolerable? As discussed in Section 3.4, this decision is often put into sharp focus by considering the conditional probability: given that the event may occur, what would be the tolerable level of damage? Combined hazards Likelihood Frequent/common Likely Consequence Minimal Minor Significant Serious Substantial Severe Catastrophic The risk associated with combined hazards has a higher level of sensitivity than single hazards because it is sensitive to changes in the likelihood in two or more underlying events (such as the risk of fire following a vehicle impact). Consider the hazard from two angles when determining whether the level of damage is tolerable (Box 3.5): the likelihood of each event separately and the level of damage considered intolerable, and given that the first hazard may occur, how much damage is considered proportionate to the risk of the second hazard? Unlikely Rare Improbable 3.11 Step 10: Review the overall level of risk Negligible Figure 3.8a Risk assessment indicative of a design which is sensitive to the risk reduction measures Consequence Likelihood Frequent/common Likely Unlikely Rare Improbable Negligible Minimal Minor Significant Serious Substantial Severe Catastrophic Figure 3.8b Risk assessment indicative of a design which is sensitive to the assumptions made in the risk assessment Review the overall level of risk in the design, by considering the aggregated totals of both the unmitigated and the mitigated risks. Unmitigated risks may be high, but highly-effective mitigation measures may have been identified and incorporated. In this instance a great deal of importance is being placed on the mitigation measures, which renders them safety-critical (Figure 3.8a). Equally, the overall level of risk may be high but not intolerable, but it may not have been possible to make much impact through the mitigation measures. In this instance the mitigation measures are not particularly important but there remains a significant risk to occupants (Figure 3.8b), and the design is sensitive to the assumptions made in the risk assessment. In either case, the design of the structure against disproportionate collapse is of particular importance and an independent peer review of the systematic risk assessment by a suitably qualified and experienced person is warranted. This may be a peer review by another team within the same firm or a peer review by a third party (known as Category 2 and Category 3 checks respectively 3.54 ), depending on the level of this sensitivity. The peer review should 32 The Institution of Structural Engineers Manual for the systematic risk assessment of high-risk structures against disproportionate collapse

4 4 Approaches to design against disproportionate collapse 4.1 Introduction This chapter outlines some of the approaches to designing for robustness, with more detailed information available in the references given at the end of the chapter. Both the structural mechanisms of resistance for resisting collapse and some procedural measures which can be at least equally as important in reducing risk are described. 4.2 Minimum requirements As a minimum, a Class 3 structure should be at least as robust as a Class 2B structure. The designer should be able to pursue an alternative approach to meeting the Class 2B requirements in which explicit demonstration is given that an alternative solution is preferable. In this case the designer should demonstrate that the design exhibits a level of robustness at least equal to the intent described by the Class 2B requirements. This is particularly relevant for special structures such as sculptures, fairground rides, observation wheels, observation decks and masts, where the requirements for Class 2B structures may have no practical application. 4.3 General design recommendations Robustness is generally enhanced and risks reduced by application of the following principles: (1) Ductile design and the ability to dissipate energy is an overarching principle of a robust design 4.1, 4.2. (2) Systematic procedures should be employed to identify weaknesses in the structural form. This will also lead to the identification of the critical elements in a structure. (3) The structural design should provide alternative load paths, with explicit checks undertaken of their ability to carry loads redistributed from the loss of a member. (4) The horizontal and vertical load paths should be separated, such that horizontal actions will not cause failure of the vertical load path. (5) The designer should ensure compatibility between the strength-based assumptions made in developing resistance against progressive collapse and the necessary ductility to support those assumptions, and ensure the ductility provided is adequate throughout the design. (6) Vertical loadbearing elements should be designed such that failure is produced in the adjoining slab/beam rather than in the column ( strong column/weak beam ) to limit the extents of damage. (7) Continuity through improved connection detailing generally enhances the robustness of structures. In some structural forms this will exacerbate the collapse because the structure is incapable of carrying the redistributed loads. In these cases, discontinuities in the form of expansion joints or structural fuses can be beneficial in limiting the extents of damage 4.3. (8) Design for robustness should consider the consequences of loss of or damage to elements of the stability system, not just elements forming the vertical load path. (9) For particularly severe hazards local to a specific part of the building such as a screening area or loading bay, compartmentalisation can be beneficial by providing a secondary sacrificial structure, or a structural discontinuity such that the damage does not spread to the adjoining structure. (10) The local absorption of energy is important in containing the damage and is a key role of the structural connections. Brittle connections should not be used. (11) Large spacing between columns or supports significantly increases the extents of the potential damage. Reducing the spans so that redistribution of load becomes possible should be considered, but if the large spans are necessary the columns or supports become critical. Attention should then move to eliminating the hazards and minimising the risks that could impair the columns or supports (refer to Section 4.11). 4.4 General procedural recommendations Procedural measures employed to reduce risks can be structured around the following principles: (1) Single point of responsibility as with the overall coordination of the structural design, the lead structural engineer should be directly responsible for the overall control of the design with respect to all issues relating to stability and robustness , and the contracts should be written to permit this. (2) Design team interfaces should be clearly identified and controlled such that there is explicit agreement between the parties on either side of each interface as to where the responsibilities lie, and how the design interface is to be managed. (3) Design information (e.g. loads, loadcase combinations, connection forces) should be clearly and unambiguously communicated and steps taken to ensure each designer has a clear understanding of them. (4) Design change should be clearly managed and controlled. (5) Good management processes with adequate checks to eliminate errors should be in place for both the design and the construction. (6) Risks should be kept under regular review during the design process. The initial risk assessment 36 The Institution of Structural Engineers Manual for the systematic risk assessment of high-risk structures against disproportionate collapse

5 Approaches to design against disproportionate collapse 4.5 should be undertaken at the earliest stages of the design and should keep pace with the design through periodic review and further development of the detail as the design progresses to take account of both the increasing level of detail and of design changes. The assessment should be updated as the design is finalised, and updated again to reflect the as-built design once construction is completed. (7) Robust quality management procedures should be designed and enforced to manage the flow of information and document control such that the risk of failures in the communication of information is minimised. (8) The structural engineer should develop an appropriate inspection and maintenance regime for the structure as a core part of the design activities, and not merely as an after-thought shaped to fit the design after it has been finalised. This should be agreed with the client during the development of the design and must be properly communicated by the designer before the design can be said to be complete. (9) The required level of competence to construct the building to the quality required should be determined, together with identification of the means by which this will be evaluated and controlled. (10) Rigorous measures should be implemented for quality management in the design process, which should be commensurate with the level of risk on the project. Design measures that are associated with large reductions in risks or where a high level of residual risk remains in the design are indicative of where additional measures are warranted in the checking and review of the design, up to and including an independent peer review of the design Site testing should be specified for those aspects of the construction where the design intent depends upon the quality of workmanship, in order to ensure the structure will behave as intended. (11) Rigorous measures should be implemented for quality assurance both through the supply chain and in the construction itself, proportional to the level of risk in the design. Examples indicative of where particularly close supervision and testing of the construction is warranted include elements: known to be particularly important to the robustness of the structure in which there is a high risk of mistakes or errors being made where supervision is known to be difficult during construction where a high degree of reliability is necessary because subsequent inspection and maintenance will be difficult. 4.5 Design loadcases and performance criteria Eurocode 0 (BS EN 1990: ) defines the combination of actions for accidental loading. A typical building with imposed office floor loading and wind as accompanying variable actions gives the two loadcases shown in Equations 4.1 and 4.2. The partial (g-) factors and combination (c-) factors are taken from the UK National Annex Equation G k þ (1.0 or 0.0) 0.5Q k þ W k þ 1.0A k Equation G k þ (1.0 or 0.0) 0.3Q k þ W k þ 1.0A k where: G k is the dead load Q k is the imposed load (partial factor depends on whether action is adverse or beneficial) W k is the wind load is the accidental load. A k In determining the accidental load A k, the dynamic effects of the load must be considered. This will comprise both the sudden redistribution of load through alternative load paths due to the failure of a structural element, and dynamic debris impact e.g. of a structural slab onto the floor below 4.13, Structures with little ductile capacity must be designed to remain broadly elastic. Structures designed and detailed to develop significant ductility post-yield may adopt less onerous performance criteria. Guidance on appropriate performance criteria is given in UFC Design approaches Approaches to design against disproportionate collapse generally fall into one of two types (Figure 4.1): Load Redistribution (LR), in which the ability of the structure to redistribute the loads resulting from the loss/damage of the local structural elements is enhanced. Local Protection (LP), in which measures are taken to guard against the loss of elements occurring in the first place. In typical multi-storey buildings LR methods will generally be preferable to LP methods, because LR methods will usually have a beneficial effect on both the extents and the severity of the collapse against events of unspecified cause and of increasing magnitude. These methods are Figure 4.1 Effective tying Alternative load path analysis Load redistribution (LR) methods Local protection of critical structural element Compartmentalisation Local protection (LP) methods Categorisation of approaches to design against disproportionate collapse The Institution of Structural Engineers Manual for the systematic risk assessment of high-risk structures against disproportionate collapse 37