CROSS Safety Report
Shear failure during demolition
This report is over 2 years old
Overview
A reporter raises concerns about how a number of buildings being demolished ‘top-down’ have suffered from partial collapse during the process.
Key Learning Outcomes
For civil and structural design engineers:
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The reporter's firm are concerned that shear failures may be a significant risk during top-down demolition particularly for flat slab and ribbed slab (hollow pot) floors
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Tests are usually carried out to establish the bending capacity of slabs, but it may be prudent to test to determine the shear capacity as well
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A bending failure usually leaves some residual capacity in a slab whereas a shear failure often results in a complete loss of capacity and hence collapse
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Be aware that structures may have been susceptible to poor detailing and construction, as well as degradation over time which may need to be accounted for prior to demolition
For demolition contractors:
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Stockpiling of demolition material and access ramps formed from crushed concrete should be adequately controlled to ensure the structure is not overloaded
Full Report
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A number of buildings being demolished ‘top-down’ have suffered from partial collapse during demolition, some well publicised, others less so. The process of top-down demolition involves using excavators and other plant sitting on the partially demolished structure and using this as a working platform for the demolition of the remaining building.
The technique is described in the National Federation of Demolition Contractors (NFDC) document Deconstruction of Tower Blocks Floor by Floor Guidance Notes. This usually results in the building carrying larger loads than originally designed. The established procedure was to assess the capacity of the existing building by investigation and analysis and then either use plant that could work without overloading the structure without propping, or to provide props to distribute the loads. The assumption is that the building is generally in the condition it was constructed.
The risk of shear failure
The reporter's firm have seen areas with significant degradation of the condition of concrete, reinforcement, and structural steel. It has become common practice to load test buildings to establish that larger plant can be used than can be justified by back analysis. However, the nature of testing has generally been to establish moment capacity at mid-span rather than shear capacity and check the shear capacity by calculation.
The reporter's firm are concerned that shear failures may be a significant risk during top-down demolition of flat slab and ribbed slab (hollow pot) floors. A bending failure usually leaves some residual capacity in a slab whereas a shear failure often results in a complete loss of capacity and hence collapse.
The reporter's firm are concerned that shear failures may be a significant risk during top-down demolition of flat slab and ribbed slab (hollow pot) floors
Overloading issues during demolition
The reporter is concerned that the sources of loading during demolition are inadequately controlled and can result in significant overloading. Stockpiling of demolition material and access ramps formed from crushed concrete are two sources of this overload. In addition, whilst in the firm's experience concrete buildings seem to be far more robust than calculations suggest, they have been susceptible to poor detailing and construction, as well as degradation over time.
These issues can mean that there are local areas of weakness that are not identified during testing or analysis. Although the analysis approach almost always gives a lower capacity than testing, it will not necessarily overcome construction defects.
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Structural failures are more common in buildings undergoing alteration and demolition, and part of the reason for this may be because of the uncertainties with the condition of the structure. In structures designed for overload conditions such as in seismic design a standard requirement is to aim for failure by bending before failure by shear. This is partly because a shear failure is a brittle failure whereas bending failure is ductile. Bending usually gives warning and the structure can accommodate more loading under gross deflection states (say by catenary action). This is not so in the case of shear failures which can be sudden and catastrophic.
In structures designed for overload conditions such as in seismic design a standard requirement is to aim for failure by bending before failure by shear. This is partly because a shear failure is a brittle failure whereas bending failure is ductile
There have been concerns that some early examples of flat slabs had weaknesses associated with shear around columns and potential problems may have been exacerbated by water leakage and ageing. It should be noted that for concrete without shear reinforcement the approach to shear has become more conservative as codes have been developed, indicating that previous codes may not have the level of reserve expected. If such slabs are recognised, e.g. flat slabs and some rib slabs, then, as the reporter says, care must be taken with modifications or with demolition.
A good example of shear failure is to be found in the Pipers Row Car Park Collapse investigation. If flat slab construction is recognised then, as the reporter says, caution is needed.