Joint meeting between the Institution of Civil Engineers and the Institution of Structural Engineers held at 11 Upper Belgrave St. on 13 February 2012
The paper by Harding and Carpenter [1] was written in part to encourage further debate and discussion on the subject of ‘systematic risk assessments’ applied to Class 3 buildings. The joint evening meeting held on 13 February 2012 originated from that suggestion and was designed also to assist in the drafting of a guide on this subject [2].
The meeting was well attended with over a hundred on the waiting list. Four short presentations were given, and the attendees then broke into discussion groups before coming together to be informed of the key points arising.
Approved Document A and BS EN1991-1-7 make explicit reference to ‘systematic risk assessment’ in connection with the prevention of disproportionate collapse of Class 3 buildings. However, it is recognised that guidance is required to assist industry in delivering this in a value adding and effective manner.
John Carpenter, who instigated and chaired the meeting, began by providing some background to the process noting that safety legislation was the governing requirement (as this had wider implications than Approved Document A or the code) and this required consideration, as appropriate to the building, of malicious and accidental actions, over the whole life of the structure. A suggested hazard grouping was proposed [3]:
P1, P2, P3 relate to the ‘3Ps’ of People, Process and Product, promulgated by SCOSS [4]. It was noted that most failures are due to lack of quality management or gross error which fall outside reliability considerations and hence the scope of the partial factors of safety. This, together with a general absence of statistical data on extreme events, means that much of this analysis will be based on engineering judgement.
Jeremy Wells (WSP- Hibernian FC stadium), Paul Officer (Mott MacDonald- Mbombela stadium) and Steve McKechnie (Arup- Pinnacle building and Kings Mill Hospital) then each gave short presentations on the named Class 3 structures, where a risk assessment had been implemented. There was much commonality of approach with thorough reviews of hazard scenarios and analyses assuming loss, or protection of selected members and other preventative measures against defined events. The particular importance of joint design was evident. Examples were used which tackled disproportionate collapse practically and proportionately. The examples shown all resulted in documented outputs illustrating the procedures adopted. It was notable that this work required client involvement to agree the hazard scenarios.
Q and A
In the discussion following the presentations queries were raised regarding the avoidance and identification of conceptual flaws and ‘common mode failures’ i.e. a fault occurring in a member or detail which occurs a number of times within the structure. Some contributors, using nuclear industry experience, also mentioned the need to consider ‘cliff-edge’ situations, i.e. where a slight increase in, say, load creates a disproportionate reduction in resistance capability. It was noted that multiple or sequential hazard scenarios should be at least considered as part of the assessment (in this regard it will be necessary to consider whether they are linked or statistically independent).
Breakout Groups
The breakout groups, and subsequent feedback, raised a significant number of points; of these the key issues are summarised below. The views expressed are recorded as given; there was not time to assess the degree of support.
Whole-life consideration was necessary (including construction), including geotechnical aspects or influences. Safeguarding records of the assessment over this period was also viewed as being a key issue.
It was considered that other parties would have a keen interest in the production of ‘ground rules’ for risk assessment e.g. Building Control and the Insurance industry. Examples were given where there may be conflict with the client over the cost of preventative measures considered necessary by the designer. The need for client involvement generally in agreeing levels of risk was noted.
The undertaking of the risk assessment process will necessitate suitable competency. In parallel with this it was thought that having one person responsible for the robustness of the structure was vital. (This was explicitly stated in the withdrawn BS codes, but is not explicit in the Eurocodes although it can be inferred. The IStructE guide on robustness [5]) strongly supports the concept of a single point of responsibility).
The need for good connections, carefully detailed, was recognised, as was ductility and redundancy.
Feedback on the risk assessment process itself included a suggestion that although prescriptive lists should be avoided on their own, in conjunction with open procedures the combination might be beneficial. However, informal risk assessment requires good engineering judgement. Multiple events should be included and dynamic analysis may be required for some structures. The problems of dealing with low probability/high consequence events, or short duration severe events were raised.
It was not possible in the time available to debate all these issues. However it was apparent from the meeting that there was a keen interest in this topic and the need for proportionate and practical guidance.
Reference
| 1 |
Harding G and Carpenter J, Disproportionate Collapse of Class 3 buildings; the use of risk assessment
The Structural Engineer August 2009 |
| 2 |
Systematic risk assessment of Class3 buildings. Institution of Structural Engineers (tbp) |
| 3 |
Hazard groupings taken from Carpenter J, The systematic risk assessment of class 3 structures in respect of disproportionate collapse. (Submitted for publication) |
| 4 |
Standing Committee on Structural Safety (SCOSS) 15th Biennial Report |
| 5 |
Practical guide to structural robustness and disproportionate collapse. Institution of Structural Engineers October 2010 |