CROSS Safety Report
Design responsibility for temporary stability of steel frame building
This report is over 2 years old
A reporter raises concerns about the division of responsibility for the temporary stability of a four-storey steel frame structure with precast concrete planks and a structural topping.
They feel that splitting design responsibility for temporary works inevitability provides opportunity for confusion.
Key Learning Outcomes
For construction professionals:
- Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design and prevent the removal of bracing members as highlighted in this report
- Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
- Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly
- The Temporary Works forum (TWf) is a source of useful information on this topic, and published Information Sheet 3 - CDM 2015 - Principal Designer: Guidance on Temporary Works in 2017
For civil and structural design engineers:
This report highlights the potential value of visits by the design team who may (as on this occasion) identify a problem. If you are unable to attend site, consider asking the contractor for site photos of the installation of critical structural elements such as bracing members.
Specific to steel framed building, the British Constructional Steelwork Association (BCSA) Guide to the Erection of Multi-Storey Buildings provides advice on maintaining stability during construction
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
This event concerns the temporary stability of a four-storey steel frame structure with precast concrete planks and a structural topping. A reporter says that during the erection of the structure, the contractor had provided temporary steel bracing to a number of the bays. This was to stabilise the structure and prevent it from swaying. The bracing was in the form of flat steel plates arranged diagonally.
Unbolted bracing member
Whilst attending site, the visiting structural engineer found one of the bracing members unbolted at the base of a column. The column where the bracing should have been fixed to was located at the perimeter of the building where the outside ground level was lower than the internal slab level. To protect operatives from falls, edge protection had been provided. But when installing the edge protection, the bracing had been unbolted.
Several temporary works design concerns
The situation was plainly unsafe and indicated both a disregard for safety by the operative who unbolted the bracing and a lack of control, supervision and oversight from the main contractor. For the same project, several concerns were found about the temporary works design for the temporary stability of the frame.
How the division of responsibility can lead to confusion
During the project, the engineer had communicated that until the concrete planks were grouted together, the diaphragm for distributing the lateral loads should be assumed to be incomplete. The implication of this was that the stability of the structure was the responsibility of the contractor until the diaphragm was complete.
The steel frame subcontractor was responsible for both the erection of the steel frame and the landing of the precast concrete planks. Whilst they were willing to take on design responsibility for the temporary stability of the steel frame without planks, they passed on design responsibility for the steel frame when the precast planks had been landed to the main contractor.
This division of responsibility was unexpected and led to confusion between the two temporary works designers. The steel frame subcontractor used moment fixity from the beam end-plate connections and the main contractor assumed perfectly pinned beam-column connections that required cross-bracing. The latter approach eventually proved to be very conservative and had to be revised to be cost effective.
This division of responsibility was unexpected and led to confusion between the two temporary works designers. The steel frame subcontractor used moment fixity from the beam end-plate connections and the main contractor assumed perfectly pinned beam-column connections that required cross-bracing
Lack of accompanying connection designs raises concerns
When the main contractor began the temporary stability design for the steel frame with concrete planks, they requested to see the calculations that had been completed by the steel frame subcontractor for the frame without planks.
Stability was justified based on the moment capacity of the end-plate connections, . Whilst this principle was sensible, there were no accompanying calculations. The structural engineer deemed this was insufficient to demonstrate that the frame would be stable during construction.
There are, says the reporter, two lessons learned from the above experiences. The first is that temporary works designers for main contractors may not have adequate experience to undertake the temporary works design for the stability of a steel frame. Given the size of the structure, the lack of calculation initially provided by the steel subcontractor was also of concern and may be indicative of a more widespread problem within the industry.
The second lesson is that splitting design responsibility for temporary works inevitability provides opportunity for confusion, but thankfully did not endanger safety in this case.
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This CROSS report was raised as a concern typical of those encountered in the steel fabrication and erection industry by the British Constructional Steel Association (BCSA) and steelwork contractors. The question was put to the BIM 4 Health & Safety Working Group - how can BIM make a difference and improve safety in these cases?
At a meeting in December 2019, the Working Group considered this case, and others, where for example additional steel, unplanned for at (contractor) tender stage, is required to brace a structure in its temporary state during erection. Who should be responsible for identifying the need for and carrying out the design of the bracing and for planning the insertion, management and often more challenging, the removal of this bracing safely?
The Working Group considers that 3D visualisation of erection sequences is both practical and valuable in informing duty holders of the risks to be treated, and safe systems of work which should be employed. Good practice examples already include method statements which incorporate drawings of sequences of erection, using suitable software. The use of 4D sequencing goes a stage further because it requires planners to consider, and then model, the steps between the temporary state and the permanent state of the structure.
More fundamentally, BIM facilitates collaboration between the client, design teams and contractors. The Principal Designer (PD) on a project has an explicit responsibility under CDM 2015 to identify foreseeable risks, and to liaise with the Principal Contractor (PC) in order manage these according to a hierarchy of control. Issues which affect stability of steel structures in the temporary state will always be significant. Part of the PD role is to evaluate steelwork design to ensure that stability can be properly maintained during both construction and when in operation. The software and digital design tools used in BIM enable more detailed assessment of the design to be carried out earlier in the project, and enable more effective scrutiny by the PD.
In considering the role of BIM, and the guidance contained in PAS 1192-6, the Working Group offers these recommendations of good practice:
1. Clients must set out arrangements, in partnership with the PD, to ensure that the PD, design teams and contractors collaborate from the outset and specify which safety critical sequences require 4D models or visualisations for planning and assessment purposes.
2. Designers should provide information and instruction (CDM 2015 Reg.8(6)) to ensure stability of temporary states to the PC (CDM 2015 Reg.19), especially the need for additional elements of design, through notation or links attached to models. The Client, advised by the PD, should specify the detail required in 3D or 4D models. The PC (CDM 2015 Reg.19) must ensure that they are content with the level of design, sequencing and method information provided by the design team to ensure stability at all stages. The PC must take up any concerns or suggested changes/improvements with the PD to ensure that deviation from the arrangements proposed by the designer is analysed and agreed at an early stage.
3. Cat II or Cat III temporary works as defined in BS 5975 and any other “safety critical” temporary works should be fully designed as part of the permanent works design with the Client, advised by the PD, specifying the 4D sequencing required. In order to achieve this, early contractor involvement will be essential.
4. Less complex temporary works should be considered to a proportionate level during the permanent works design, with sufficient information communicated to assist later stage temporary works designers, taking into account the capability and experience of contractors, as specified by the Client.
5. All temporary works should be subject to specific design and planning reviews by the PD and PC.
6. As constructed records shall record agreed sequencing to help facilitate future de-construction as appropriate.
For CROSS report 789, adopting the collaborative approach set in PAS 1192-6 and utilising 3D or 4D models would have provided those involved with the opportunity to actively communicate the need for the design and use of temporary bracing during erection, and therefore significantly reduce the risks and subsequent consequences that occurred.
I agree the CDM Regulations place a duty on the Principal Contractor. I consider that, in addition to this, the Principal Designer's duties extend to temporary works designs, even though this is during the construction phase. The Principal Designer should ensure that all designers comply with their duties and ensure cooperation between designers. This applies not just to permanent works designers, but to temporary work designers too.
Expert Panel Comments
Expert Panels 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-UK Expert Panels page.
CROSS has always advocated that there should always be one designer with overall responsibility for stability. Whilst this normally applies to preserving stability of the finished structure, the principle ought equally to apply during construction when arguably the risk of an instability failure is highest.
Principal contractor duties under CDM 2015
Regulation 13 of CDM Regulations 2015 (CDM 2015) ultimately places the duty on the principal contractor to plan, manage and monitor the construction phase and coordinate matters relating to health and safety during the construction phase to ensure that, so far as is reasonably practicable, construction work is carried out without risks to health or safety.
In all situations, the principal contractor’s Temporary Works Coordinator (TWC) (or contractor’s if a small job) should have oversight of maintaining stability. However, the designer should be involved in a collaborative manner to ensure stability.
The value of site visits by the design team
This report also highlights the potential value of visits by the design team who may (as on this occasion) identify a problem. A current trend to minimise site attendance by the design team is most undesirable; a matter that was brought up in the Edinburgh Schools Inquiry and the Independent Review of Building Regulations and Fire Safety: final report.
Reference should also be made to the following;
BS 5975, Code of practice for temporary works procedures and the permissible stress design of falsework
PAS 8811, Temporary works - Major infrastructure client procedures - Code of practice
Temporary Works Forum website for guidance on the management of structures in temporary conditions.
Specific to steel framed building, the British Constructional Steelwork Association (BCSA) Guide to the Erection of Multi-Storey Buildings provides advice on maintaining stability during construction.