The importance of building structures has increased in the modern times due to the increasing complexity in the buildings due to increased demands in terms of functional requirement, aesthetics and height. This project requirement as well as many other external factors has increased the risk and chances of accidents.
This has also increased the importance of engaging structural consultants from start to end of the project and then evaluating and maintaining the building for its safe performance.
Unsafe practices and ignorance of structural design and good construction practices has resulted in many building collapses in the past as well as in recent times. Some of these are disproportionate collapses which resulted in large number of causalities and loss.
What is disproportionate collapse?
Collapse to an extent more than the cause of collapse is called a disproportion-ate collapse. The collapse of even a small, some times a non structural element like a wall may trigger larger collapses and this may end up in a progressive collapse in which a large or even the entire building comes down like a pack of cards.
Why should we attend to disproportionate collapse?
Disproportionate collapses results in severe causalities and economic losses which are much more in magnitude than the real reason of the accident which is small. Therefore it is very sensible and essential to control disproportionate collapses by appropriate structural design and detailing.
Is disproportionate collapse and its control Relevant in India?
Some of the recent failures of buildings should prompt us to look in to the history and provisions of disproportionate collapse in India and across the world.
IS-456-2000, the Indian standard for plain and reinforced concrete does not provide any guidelines. This does not mean that disproportionate collapse can be ignored. There have been many cases of disproportionate collapses in India in the near past.
Clause 0.3.3 of IS 875-Part II (Code of practice for design loads) states that “The buildings and structural systems shall provide such structural integrity that the hazards associated with progressive collapse such as that due to local failure caused by severe overloads or abnormal loads not specifically covered therein are reduced to a level consistent with good engineering practice.’’ This clause is misinterpreted and it is often stated that good detailing practices takes care of the progressive collapse. This interpretation could be true for smaller less complicated buildings. However, as per this clause the designer has the responsibility to reduce accidental load to a lesser magnitude so that good detailing practice takes care of accidents. Reduction in load so that the detailing takes care the effects of progressive collapse is possible only by a careful scheming of structure by providing adequate number and sizes for columns and beams. The main point is to ensure multiple load path in the structure incase of failure of a member so that a local failure (say of a beam) do not progress from the local area near the failed beam to areas away from the failure location thus avoiding the disproportionate collapse. Also the latest code of practice for General construction in steel IS 800‐2007‐ cl‐3.1.1 & 5.1 gives guidelines for progressive collapse. These guidelines are similar to that available in the British standards.
History of UK regulations for Disproportionate collapse.
The partial collapse of a building –Ronan Point in 1968 initiated the discussion in the UK for the provision of accidental load clauses. In 1968, an explosion in the kitchen on 18th floor of this apartment building blew out some walls. The walls fell on the slab and the 18th floor slab collapsed on to lower level slab and triggered a progressive collapse though the explosion was not major and was in 18th floor.
Provisions for disproportionate collapse in BS-8110
BS-8110-Part 1 & 2, UK Building regulations -Approved document A3 and National House Building Council Technical guidelines to A3 can be referred for guidelines for disproportionate collapse control.
BS categorizes buildings in to class 1, 2A, 2B and 3 (increasing order of importance)
For class 1 building (Houses up to 4 floors, agricultural shed etc.), following code of practice strictly and good detailing practices is recommended.
Logic: Less complicated buildings will resist a local failure if it is detailed properly since the forces due to a local collapse will be small.
This is true in Indian conditions as well provided even smaller buildings are designed for seismic and wind forces, detailed and execution inspected by a specialist structural engineer. Also the seismic design provides additional robustness for a local failure due to accident load.
For class 2A buildings (Houses up to 5 floors, 1 floor education building etc), Horizontal and Vertical ties are recommended. Peripheral and internal ties are recommended. Also the building has to be designed for a minimum/notional horizontal load. It shall be noted that even if the building is designed for a lateral wind load, notional horizontal load governs the designs if wind load is lesser.
Logic: The ties provide alternate load path should a local failure occur.
Minimum horizontal load is recommended since UK is not in a seismic zone. In Indian conditions, this minimum load case may not arise if we design buildings for seismic forces.
For class 2B buildings (residential up to 15 floors, buildings with area between 2000 to 5000m2 at each floor etc.), ties shall be provided similar to class 2A buildings. In addition, checks can be made that upon the notional removal of supporting elements the building remains stable and that the risk of collapse does not exceed 15% of the floor area of that storey or 70m2, whichever is smaller. If this exceeds the values, members has to be designed as a key element capable of sustaining an accidental design loading of 34 kN/m2 applied in the horizontal and vertical directions (in one direction at a time)
Logic: For more important buildings, tying may not be sufficient to provide alternate load path should an accidental failure occur. Analyzing by removing certain elements is nothing but checking the bridging capacity of the structure if that particular member fails due to accident. If bridging also fails, then the additional design force of 34kN/m2 will absorb a certain amount of accident.
This should work for Indian conditions as well since seismic design provides certain robustness to absorb a gravity load failure. However it is important to remember that the figure of 34kN/m2 is derived from the Ronan Point apartment collapse in UK which was triggered by a cooking gas explosion. It may be wiser to derive a more realistic value for Indian conditions for various kinds of structures.
For class 3 buildings (Buildings more important that 2B) - A systematic risk assessment of the building should be undertaken taken into account all the normal hazards that may be foreseen together with any abnormal hazards.
The structural form and concept and protective measures should be chosen and the detailed design of the structure and its elements undertaken in accordance with the recommendations given in the Codes and Standards.
Logic: It may not be economic to design for all kind of risks. Therefore a risk analysis is recommended. Also some risks can be mitigated by a non structural solution mostly by avoiding the risk by taking measures to avoid the accident.
Conclusion: Buildings designed for disproportionate collapse has performed well during accidents and therefore it is sensible to cater for it during the conceptual stage itself. Since buildings in India are any ways to be designed for seismic forces, the additional cost for taking care of accidental loads may not be huge. Though it may add a little additional cost, it is sensible to mitigate risks. A careful analysis of risks and mitigating it first by non structural means and then catering for the non avoidable risks structurally shall be made mandatory for reducing accidents. Also this additional robustness, some times provides the builder/Architect/engineers flexibility to accommodate unforeseen changes.
Robustness also increases the blast resistance of the structure. When all this is possible with no much extra cost, it is sensible to look in to all these basic principles of risk mitigation