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CROSS Safety Report

Cladding subcontractor change of connection strategy led to failure

Report ID: 1189 Published: 21 August 2023 Region: CROSS-UK


Overview

As part of their final design submission, a cladding subcontractor changed the connection strategy for their cladding system. During their assessment of this design submission, the project structural engineer did not identify the design change. When the cladding was installed, the cladding loading resulted in excessive deflection and some twisting of the primary perimeter steel.

Key Learning Outcomes

For the client and design team:

  • Be aware that one engineer acting as overall coordinator of the structure is best practice
  • Ensure design responsibilities for packages of work are clear and coordinated between packages
  • Ensure project change control processes are used by the whole project team

For all designers:

  • Principal designers and designers have responsibilities under the Construction (Design and Management) Regulations 2015 to communicate, cooperate and coordinate
  • Ensure all design changes are appropriately communicated across the design team

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A cladding subcontractor changed the connection strategy for their cladding system within their final design submission but failed to inform the project structural engineer of the change. The project structural engineer did not identify the design change during their assessment of the design submission. The reporter explains that, when the cladding was being installed, the unexpected loading resulted in excessive deflection, and torsion induced twisting of the primary perimeter steel which required significant remedial works to resolve.

 

A value engineering exercise changed the concrete floor slab to a slimmer concrete system

The reporter states the structural design originally included cladding brackets that sat on top of a concrete floor slab on top of perimeter steelwork. However, a value engineering exercise changed the concrete floor slab to a slimmer concrete system. A further change made later was the omission of the floor screed for a timber deck option. This final configuration did not allow space for the originally conceived cladding brackets. The changes required the cladding brackets to be relocated to the bottom flange of the steel perimeter beams. These beams had, however, not been designed for this connection strategy and loading.  During installation, the perimeter beams started to twist and deflect, manifesting itself as the closing up of cladding joints above and below the deflecting perimeter beams.

The reporter adds that the deflections were only discovered (and then deemed to be a fault) when the cladding joints had fully closed and investigations as to the cause were made. The discovery was made more difficult because the cladding masked the perimeter beams and the deflections were not readily visible from inside the building. The perimeter beam twisting was found to result from the altered load path with the cladding acting on an unrestrained lower flange.

It was concluded the changes in design to implement the value engineering savings had led to the problem. The value engineering savings had not been fully checked by the design team and, in particular, by the structural engineer so structural capacity checks had not been undertaken before the revised structure was built.  The cladding had to be removed over multiple floors and the steelwork straightened and reinforced in situ. Some of the distorted steel could not be fully straightened so new steelwork was required at a significant cost.

The changes in design to implement the value engineering savings led to the problem

The reporter argues the specialist subcontractors should have undertaken checks on the revised structural arrangements or at least clearly notified the project structural engineer of the changes they proposed. However, in this case, reliance was placed on the structural engineer identifying the design changes from drawings submitted to them (that they had previously reviewed) without any warning that changes to the design were being proposed. The reporter also highlights the structural engineer had a limited checking scope as the design and build contractor placed reliance on their subcontractors.

The reporter concludes that, under a design and build contract, when a specialist subcontractor proposes changes these should be approved by the project structural engineer.

Expert Panel Comments

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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.

Design interfaces, in this case between steelwork designer and cladding designer, can promote fragmentation of design responsibilities. Where design is fragmented, risk can thrive. This report highlights the need for the responsibilities of each designer to be clearly defined. Care should be taken to ensure contractual arrangements, such as a design and build arrangement, do not prejudice coherent design interfaces. As so well illustrated by this report, gaps and inconsistencies between designers can lead to safety issues.

 

This report illustrates the importance of someone taking overall responsibility for all structural matters

This report also illustrates the importance of someone taking overall responsibility for all structural matters, regardless of the contractual framework in place. The engineer responsible for the overall stability and robustness of the structure could ensure the compatibility of the design and details of parts and components, even where some or all the design and details of those parts and components are not made by this engineer. This engineer should verify the form of any connections adopted at the interfaces between other designers, and verify that the load paths are acceptable. The Institution of Structural Engineers (IStructE) Manual for the design of steelwork building structures to Eurocode 3 makes these points very clear as do other IStructE design manuals. The need for one engineer to take overall responsibility for all structural matters has featured in CROSS Safety Reports on a number of occasions, including CROSS-UK report 1172 – Cladding failure in strong winds, published in 2023.

Designers have responsibilities under the Construction (Design and Management) Regulations 2015 to communicate, cooperate and coordinate

A designer's decisions can affect the health and safety of all those involved throughout the lifespan of a building, including its construction. Designers have responsibilities under the Construction (Design and Management) Regulations 2015 to communicate, cooperate and coordinate with any other designers (including the principal designer) so that all designs are compatible and ensure health and safety, both during construction and beyond.  

A lack of effective communication and coordination between designers appears to be at the heart of the reported failure

Principal designers have responsibilities that include ensuring that everyone involved in the pre-construction phase communicates and cooperates, coordinating their work wherever required. A lack of effective communication and coordination between designers appears to be at the heart of the reported failure. CROSS-UK report 1128 - Unsafe design of retrofit cantilever balconies, published in 2022, considered designers and principal designers responsibilities under the Construction (Design and Management) Regulations 2015.

Managing design changes

The need for stronger change control processes was an issue highlighted in the review led by Dame Judith Hackitt following the Grenfell Tower tragedy. Management of design changes can be a very significant problem for design teams. It is poor practice not to flag up changes to all other design team members since the implications of proposed changes, across contractual and design boundaries, may not be grasped by those making the change, as appears to be the case in this report.

It can be quite surprising how, under effective change management, a proposed change gets flagged as a potential problem by a discipline quite remote from the change. Value engineering processes will likely propose changes that must be managed. Project specific change control processes should detail how all proposed changes are communicated and validated across the design team. Problems caused by ineffective change control can apply across all territories as illustrated by the CROSS-AUS report 822 - Managing changes to design, published in 2019.

This report illustrates the need to fully describe what revisions have been made when issuing revised designs and drawings. In this case, simply noting what had changed on the drawings would have quite likely uncovered the unacceptable proposals before they were enacted. It is not at all helpful just to adjust the document revision reference.

 

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