CROSS Safety Report
Overloading of existing steelwork frame
A reporter said that damage occurred to a flat in a multi-storey block, due to inadequate structural investigations and inadequate design of alteration works.
Key Learning Outcomes
For owners and clients:
- Ensure engineers are competent for the project in hand before appointing
- Time and money spent on site investigations is well spent
For structural design engineers:
- Detailed desk top and physical surveys are essential such that the existing structure is understood
- The effect of any revised loading conditions (temporary or permanent) must be assessed
- Engineers should undertake full design assessments of existing structural elements when considering alterations
- The effect upon foundations must be assessed
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A reporter said that vertical cracking, displacement and damage occurred to a flat in a multi-storey block due to inadequate structural investigations and inadequate design of alteration works.
Framing details of the original structure were not available. There was also minimal opening up to provide details of the existing steelwork sections.
Design calculations for the alteration works assumed a significant increase in the allowable stress for existing steel sections with no justification. In addition, there was no substantiation of column sections.
These inadequacies led to overstressing of the existing steelwork and damage to the fabric of the building.
The reporter highlighted the following:
- When considering alterations to existing buildings, structural engineers should be in possession of all relevant facts concerning the building.
- Foundation assessments are required.
- Where existing structural steelwork is concerned, assessment to verify both the strength and chemical composition of the metal are required.
- The local authority may well be aware of any local building acts and bylaws, that would have applied at the time of construction.
Expert Panel Comments
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This report amply demonstrates that modifications to existing metal frames (both steel and iron) is a very skilled task that requires the engineer to intimately understand the existing framework (layout, sizes, materials, grades, age, condition, deformations and loadings) and how the framework interacts with the materials around it. The engineer should also understand how the framework was intended to perform, how structural form may have changed over time and how the structure performs at present, before assessing how any changes will impact the whole structure. Research should be carried out to establish the relevant regulations that were in force at the time of construction along with other published guidance.
Engineers should normally assume, unless investigations show otherwise, that the load bearing capacity of existing structures is fully utilised. Alteration (adding load or changing the basic structure) can only be made if a clear demonstration of reserves is forthcoming. This is especially important when the consequences of introducing a weakness are potentially disproportionate as in the case of alterations to one flat in a block. Only by knowing the structural form and condition can an engineer begin to understand the effects of changes to that structure.
Engineers engaged on a project within a block of flats, should be particularly mindful that other works in the block may have already caused unexpected shedding of loads, or even overloading in their project area; local or even disproportionate collapse could result from poor design of alterations.
Knowledge, skill base and competence
With decarbonisation of construction gathering momentum, reuse of structures is going to increase, and as with all design, the knowledge and skill base of the engineer must be appropriate to the job in hand. Engineers must have a good depth and breadth of experience to be competent for alteration projects of this type. This applies to all building forms including those buildings using loadbearing masonry, steel or concrete beams/frames and various forms of flooring.
This case illustrates that cutting corners on site investigations and design (through resourcing constraints or incompetent design) will invariably lead to one or more of safety, reputational or financial loss for designers and/or owners.
cutting corners on site investigations and design will invariably lead to one or more of safety, reputational or financial loss
Clients and designers should be aware of their liabilities under the Construction Design and Management Regulations 2015 (CDM 2015). The skills, knowledge, experience and organisational capability of all appointed to construction work should be examined as explained in the Health and Safety Executive CDM 2015 guidance.
There is significant guidance upon building reuse, refurbishment and alteration to help engineers and contractors.
Records are a key asset
Ensuring that all buildings have a record of their original structure (preferably as built) will reduce resources required for future repurposing. Where records are not available and investigations are carried out these should be included in the building’s health and safety file and made available as required.
The Institution of Structural Engineers has published a paper desk research of existing buildings which provides insights to uncovering information about buildings.
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