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CROSS Safety Report

Responsibilities for hybrid concrete construction

Report ID: 574 Published: 1 October 2016 Region: CROSS-UK

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Overview

Concerns are raised after a substantial precast element, weighing over 10t, had been placed in position and used as part of the shuttering for an in-situ pour without it being designed to act as a shutter.

Key Learning Outcomes

For civil and structural design engineers:

  • During the development of the design, be clear about who is responsible for what aspects of the design and for what phase of the asset’s lifecycle

  • While responsibility for the detailing of the elements may be clear, the responsibility for ensuring that the elements are able to fulfil temporary works, temporary condition and permanent works roles also needs to be clear

  • If design is for the permanent condition and temporary loads have not been considered, this should be clearly conveyed on the design drawings and documents to the contractor

For construction professionals:

  • Consider appointing a competent temporary works coordinator (TWC) on site who should be able to ensure all temporary works are carefully considered and planned

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There has been a report about a problem during the construction of a hybrid concrete over-bridge. A substantial precast element, weighing over 10t, had been placed in position and used as part of the shuttering for an in-situ pour. During the pour the element was pushed out of alignment by the pressure of wet concrete and there was a substantial spillage onto an operating area below. This caused considerable disruption, delay, and additional cost.

It was subsequently established that the designer and construction team did not adequately review the proposed design to agree upon the construction sequence, including any limitations of the proposed permanent works design to resist temporary loads.

It was subsequently established that the designer and construction team did not adequately review the proposed design to agree upon the construction sequence, including any limitations of the proposed permanent works design to resist temporary loads

The contractor assumed the temporary load case would have been considered by the permanent works designer. In turn there was an assumption by the permanent works designer that the temporary loading would be checked as a matter of course by the contractor.

At no point was the absence of dowels or other form of fixing queried by the site team. The site team assumed the unit had been designed to resist construction loading from wet concrete. A lesson learned is that the force which the riser units could have resisted should have been stated on the drawing rather than relying on management of the construction sequence.

Expert Panel Comments

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If the intention was always to use the pre-cast element as shuttering in the temporary condition, the design engineer should have recognised that the force from wet concrete would affect stability. There should have been anchorage for the precast units, or clear instructions on drawings about limiting pour sizes.

CDM 2015 requirements for designers

CDM Regulations 2015 (CDM 2015) requirements mean that designers are obliged to consider the steps within the construction process. This is to ensure that their structure in its incremental state of construction will be inherently safe. So, if a designer chooses a particular method of construction, such as the integration of precast concrete with wet concrete, then the designer must understand the interim steps of the construction. This is to ensure that it knows what the final stress state will be in the structure (and thereby knowing what capacity is available for environmental and live loading).

The designer should be aware of any safety issues that it should be flagging to the contractor in advance. It is only when the construction sequence can have no effect on the final state of the structure and that it remains stable throughout the process that the designer can afford to sit back. Most designs will require the designer to consider interim steps that the design will go through on its route to completion of the construction process.

The designer should be aware of any safety issues that it should be flagging to the contractor in advance

Are the various stages of construction being considered?

Once again, this report suggests that engineers are not being taught to properly consider the implications of the various stages of construction. Report 529 described a similar incident and the main points from this are repeated here:

  • In developing the design, be clear about who is responsible for what aspects of the design and for what phase of the asset’s lifecycle

  • While responsibility for the detailing of the elements may be clear enough, the responsibility for ensuring that the elements are able to fulfil temporary works, temporary condition and permanent works roles also needs to be clear

  • Sub-let contractors design portions need coordination, and both the principal contractor and the lead designer (under CDM2015, the principal designer) have responsibilities

  • Principal contractors should refresh their memories that they are expected to coordinate all temporary works and construction methodology to ensure the safety and welfare of all on and adjacent the site

  • The role of the temporary works coordinator is clear: it is to coordinate the work of all who have an influence on the temporary works irrespective of commercial boundaries. The involvement of sub-contractors does not detract from this duty; if anything the involvement of sub-contractors enhances it.

  • Notwithstanding explicit design responsibilities, design management processes need to involve cross checking (is what we are assuming to be happening actually happening?) and double checking (is what I am told to be correct, actually correct?)

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