CROSS Safety Report
Do current regulations apply when repairing older buildings?
This report is over 2 years old
Overview
A 19th century three story mill was subject to a fire which resulted in the sudden collapse of part of the interior structure.
It was recommended to the owner that the whole building should be upgraded to comply with the building regulations in respect of provisions against progressive collapse.
Key Learning Outcomes
For clients and the design team:
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If there is a significant change in use, then the current building regulations may apply to the whole building
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It is good practice to discuss the changes with the local authority building control at an early stage
Full Report
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The building in question is a mid-19th century mill with cast iron frames spanning 8.0m at 3.3m centres and has jack arch floors. It has three storeys and a roof. The footprint of the building is 24m wide by 67m long. It was subject to a fire which resulted in the sudden collapse of part of the interior structure (Figure 1 & 2).
The reporter describes this as progressive collapse due to the failure of cast iron beams and shearing of the lower level cast iron beams and the collapse of the flooring. There was considerable damage generally and specifically to two large masonry piers.
Preventing progressive collapse
As the engineer for carrying out the remediation works, the reporter notified building control authorities. Following consultations with them, it was recommended to the owner that the whole building should be upgraded to comply with the building regulations in respect of provisions against progressive collapse.
He proposes to do this by introducing a new grid of beams to provide horizontal ties in each direction and considers that this is best practice.
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The issue of repairs to old buildings is a wide topic and some general views are offered here. However, common sense and the application of good conservation practice is often the way forward. The forthcoming Institution of Structural Engineers (IStructE) Guide on Robustness (due to be published in November 2010) recognises this dilemma and offers suggestions.
Historic Mill Buildings
The risk of progressive collapse in old mill buildings is real and historically it is documented that this happened even when they were under construction. Many of these building have been demolished because, usually without adequate maintenance, they became dangerous structures.
The issue of robustness of mill buildings is difficult. Some were built only to house machinery, where the load from the weaving machine was carried on the main beams, which fitted the drive belt arrangements for power from the steam engines in the boiler house.
The brick arch floors were a later improvement to reduce fire risk from flammable materials. These floors typically had minimal load carrying capacity. One hundred and twenty to one hundred and eighty years later, when rain and frost damage have weakened the structure, the risks are potentially high and difficult to assess.
Regulatory aspects to consider
The building regulations provide baseline standards, as do British Standards and Eurocodes and repairs may be considered in this light. There is no definition of repair in the regulations. However, a there is an old definition from London Buildings Acts which says that if anything more than 25% of an element is replaced, it is considered a new element.
There is no definition of repair in the regulations. However, a there is an old definition from London Buildings Acts which says that if anything more than 25% of an element is replaced, it is considered a new element
It is not reported in this case that there is any material change of use or a material alteration. Therefore the repair work must be no less satisfactory than the original; even if the original works did not previously meet the requirements (see Building Regulations 3.(2)).
As the structure did not originally comply with disproportionate collapse requirements then the repaired building is not required to comply. This argument derived from Regulation 3(2) b is that the building is no worse than before.
Preventing disproportionate collapse
However, a responsible engineer, and indeed client, could well believe that the repairs should include measures to prevent disproportionate collapse. This could be the introduction of horizontal ties as a matter of good stewardship and good practice.
However, a responsible engineer, and indeed client, could well believe that the repairs should include measures to prevent disproportionate collapse. This could be the introduction of horizontal ties as a matter of good stewardship and good practice
Other Duties for Engineers
The engineer also has to consider duty of care and compliance with statutory duty, both of which point towards designing a safe structure as is currently understood. This would mean giving explicit consideration to robustness. Under CDM 2015 regulations this is a duty on the designer. If the building is a workplace then additionally: ‘…the building shall have the stability and solidity appropriate to the nature of the use of the workplace’.