CROSS Safety Report
Concern about design principles for tall buildings
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A reporter raises questions about very tall and complex reinforced concrete structures that incorporate combined transfer systems some of which may be supported on corner columns.
Key Learning Outcomes
For civil and structural design engineers:
Be aware that codes and guidance that advise on robustness draw attention to the special considerations that must be given to transfer structures
It is good practice to have a suitably qualified and experienced engineer in overall charge of both stability and robustness during the design of structures and not least when multiple structural disciplines are involved as in hybrid structures
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A reporter raised questions about very tall and complex reinforced concrete structures that incorporate combined transfer systems (Figure 1) some of which may be supported on corner columns. The reporter’s question highlighted a common concern which is that in the case of accidental damage to, or removal of, any key structural element, such structures may not have adequate redundancy to redistribute the loads without collapse.
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Very tall buildings require high standards of design and construction and the careful consideration of aspects which may not affect smaller structures. Architects are creating elaborate and complex building shapes to which engineers must match structures, some of which demand innovative solutions. There is a generation, worldwide, of striking buildings, sometimes described as ‘innovative, complex and unusual’ (ICU) buildings. Providing the processes are well thought out in terms of structural engineering, operation and maintenance, then all will be well.
Combined transfer structures may be taken to mean horizontal members whose supports then transmit loads onto other horizontal members whose ends, in turn, are eventually supported on corner (or other) columns. Such transfer beams need particular attention because of the difficulty in ensuring that they could span without one of their end supports. This was a significant reason for the extensive damage caused by the bomb attack on the Murrah building in Oklahoma as reported by FEMA (Federal Emergency Management Agency). A single member should not be carrying a disproportionate percentage of the structure.
Class 3 structures
In the UK buildings of this type would be Class 3 structures so a risk assessed approach would be taken. Part A of the Building Regulations for England says:
‘For Class 3 buildings – A systematic risk assessment of the building should be undertaken taking into account all the normal hazards that may reasonably be foreseen, together with any abnormal hazards. Critical situations for design should be selected that reflect the conditions that can reasonably be foreseen as possible during the life of the building.’
As a minimum Class 3 buildings should meet the principles of Class 2 buildings. Eurocode requirements are similar (see BS EN 1990 and BS EN 1991-1-7).
The three approaches commonly used to resist disproportionate collapse are:
Tie force design methods
Alternative load paths
Key elements design
Codes and guidance that advise on robustness draw attention to the special considerations that must be given to transfer structures.
Robustness and disproportionate collapse
The Institution of Structural Engineers report on robustness: “Practical guide to structural robustness and disproportionate collapse in buildings” published in October 2010 whilst not dealing with Class 3 structures makes the following very important point. “A key presumption is that for any one building, there should be one engineer in overall charge of both stability and robustness and not least when multiple structural disciplines are involved as in hybrid structures.” The subject is also being studied by NIST in the USA in the programme: Prevention of disproportionate collapse.
In view of the complexities involved the question can also be raised as to whether buildings like these should have a mandatory check and, separately, a review, as part of Regulatory Control procedures. Within UK Safety legislation there are requirements for competency to be exercised by all concerned in a project but it is not thought that is often either appreciated or enforced. Going further should there be a new process for checking on the competency of organisations and individuals to be responsible for such designs?
CROSS would be interested in receiving other views on this important topic.