CDM 2015 Designer Duties – Considering temporary works in permanent works design
CDM 2015 Regulation 9 requires Designers to eliminate, reduce or control the risks through design. Where it is not possible to eliminate risks, they should provide information on the significant residual risks that could not be designed out. As part of this, permanent works designers are expected to understand how the structure can be constructed and the requirement for temporary works.
Permanent works designers may not be aware of the range of guidance available to help them consider temporary works in their designs. To address that, this article by Mike Webster provides an overview of the issues and signposts the guidance available to civil and structural engineers on contemporary industry practice. Much of this has been prepared by the Temporary Works forum and its members, and is free to download.
Much guidance already exists on temporary works. However, in the past, it tended to concentrate on Contractors and the safe construction and use of temporary works – little was available for permanent works designers (PWD). In recent years, more guidance on contemporary industry practice has become available to enable permanent works designers to discharge their CDM 2015 Designer duties. This article summarises the key issues and provides links to that guidance.
Although this article is aimed primarily at permanent works designers, it will also be of use to Principal Designers and Clients by informing them of what to expect of permanent works designers on their projects.
If anyone reading this article is aware of other relevant guidance, please contact me and I will update the article.
Why is this important?
Most construction work requires some form of temporary works (e.g. to support excavations, to support concrete whilst it gains strength, to support tower cranes, etc.) However, temporary works can represent a significant part of the construction process and have a significant impact on cost, time, quality and safety(2.1).
Whilst the focus of the permanent works designer may be on producing the permanent works design, attention to detail and decisions taken during that design process can have a significant impact on how easy and safe a structure is to build.
Searching the CROSS database(2.2) for reports involving ‘temporary works’ reveals a range of near misses, many of which highlight the value of input from permanent works designers.
2.1 Temporary Works forum: Considering temporary works early in a project: Reducing cost, improving buildability and reducing risk, TWf Information Sheet No. 14, October 2021 (https://www.twforum.org.uk/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=86c8b620-5ca4-5e89-aa12-6ef251398219&forceDialog=0)
2.2 Confidential Reporting on Structural Safety (CROSS): Database search on ‘temporary works’ (https://www.structural-safety.org/search-results/?search=temporary+works&classification=0)
What does CDM 2015 require of Permanent Works Designers?
The requirements of Regulation 9 of CDM 2015(3.1) mean that Designers must eliminate, reduce or control the risks through design. Where it is not possible to eliminate risks, Designers should provide information on the significant residual risks that could not be designed out to the Principal Designer, and to the Contractors and other Designers who could be affected by those residual risks.
The Institution of Civil Engineers suggests in their Design Risk Management Guidance(3.2) that if Designers apply the following Three Principlesthey should be doing sufficient to satisfy CDM 2015 Regulation 9:
- Designs shall be safe to construct, commission, operate, maintain and demolish – the Designer may assume that these activities will be undertaken by capable persons who will be able to manage the ‘normal’ day to day construction / operational / repair tasks and their associated risks, arising from the design;
- Designs shall comply with contemporary good industry practice as regards health and safety – unless there is good reason for not doing so (the references listed in this article are potential sources of such contemporary practice);
- Information on significant residual risks shall be communicated to those who need to know – these are the significant risks that are not likely to be obvious to a capable Designer or Contractor, or are unusual or are likely to be difficult to manage effectively.
To discharge these duties, Carpenter(3.3) and the Temporary Works forum(3.4) suggest that permanent works designers need to:
- Understand how the structure can be constructed, and temporary works erected, used and dismantled safely;
- Determine if, by altering or supplementing the permanent works design in some way (so far as is reasonably practicable), temporary works risks arising from construction, use or dismantling of temporary works can be eliminated or reduced; and
- Consider what useful information should be passed on to the Contractor (via the Pre-Construction Information).
A simple way for designers to remember these duties is the Eliminate-Reduce-Inform-Control (ERIC) framework(3.5).
It is not the role of the permanent works designer to design the temporary works, just to consider them as suggested in the three bullet points above.
There may be a number of ways in which a structure could be constructed, and the temporary works erected, used and dismantled safely. However, the permanent works designer needs to have identiﬁed at least one likely and feasible method, and understand the manner in which the permanent works design impinges on this, or vice versa. Information on construction methodology is provided by McBride(3.6).
A summary of the key temporary works issues for Permanent Works Designers is provided by Webster(3.7).
There are many ways in which temporary works risk may be eliminated or reduced by alterations to the permanent works design. Carpenter(3.3) suggests that the following options:
- Consider ways of facilitating lifting
- Provide a moment connection (even if not needed in the ﬁnal structure) to allow for a predictable temporary situation
- Make provision for predictable temporary works cast-in elements
- Add bracing or reinforcement to accommodate predictable interim conditions
- Enhance the strength of a member to accommodate predictable temporary loading
- Ensure adequate space is provided for predictable temporary works items
- Consider issues associated with demolition-related temporary works
Passing useful information to the contractor is an important aspect of a project. As a minimum, the suggested construction sequence should be illustrated (generally as an option) unless it is obvious to a capable contractor. The purpose of this is not to restrict the Contractor, but to explain the permanent works designer’s thinking and provide information.
As Carpenter(3.3) notes, it’s worth asking the question: If I was the Contractor or the temporary works designer, what would I reasonably expect, or want to know, so that I could proceed safely, but also economically?
3.1 Health and Safety Executive: Managing Health and Safety in Construction, Guidance on CDM 2015 Regulations, Publication L153, HSE Books, March 2015 (http://www.hse.gov.uk/pUbns/priced/l153.pdf)
3.2 Institution of Civil Engineers: Design Risk Management Guidance, September 2018 (https://www.ice.org.uk/ICEDevelopmentWebPortal/media/Documents/Disciplines%20and%20Resources/Best%20Practice/drm-guide-version-1-3.pdf)
3.3 Carpenter, J: ‘CDM 2015 and the responsibilities of permanent works designers with regard to temporary works’, Temporary Works Toolkit, Part 2, The Structural Engineer, November 2016, pp 34-36 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2016/246c7928-9e2f-4ba4-a82c-c62bf6a31763.pdf)
3.4 Temporary Works forum: The role and responsibilities of permanent works designers with regards to temporary works, July 2016 (https://www.twforum.org.uk/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=4ab50ef0-b136-aea9-9a0a-9a960f08d22d&forceDialog=0)
3.5 Carpenter, J: ‘Risk management with ERIC’, The Structural Engineer,88 (7), 7 April 2010.
3.6 McBride, D: ‘The importance of understanding construction methodology’, Temporary Works Toolkit, Part 13, The Structural Engineer, July 2017, pp 32-36 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2017/568bd8c9-c1d1-463f-8ec0-fbca6008c149.pdf)
3.7. Webster, M P: ‘Temporary Works for permanent works designers’, Temporary Works forum Yearbook 2021/22, Page 47 (https://7764074f.flowpaper.com/TWFCroppedReduced/#page=48)
What does CDM 2015 require of Principal Designers?
The requirements of Regulation 11 of CDM 2015(4.1) mean that the Principal Designer must plan, manage, monitor and coordinate health and safety in the pre-construction (design) phase of a project. This includes working with the Designers in the design team to identify foreseeable risks and ensuring that the design team has complied with its duties to eliminate, reduce or control risks through design and pass on information on significant residual risks.
The Principal Designer’s duties do not require them to undertake, check or approve designs or undertake risk assessments to inform the design – those are the responsibility of the Designers. Their role requires them to ensure (so far as reasonably practicable) that the identified risks have been addressed by the Designers in the design team.
The Principal Designer has two roles in relation to temporary works: one with the permanent works designers who are considering temporary works as part of their permanent works design; and the other in relation to the temporary works designers. Both roles are addressed in References 4.2 and 4.3.
Both references highlight the need for Principal Designers to check if permanent works designers have followed the general principles of prevention, by avoiding or reducing (so far as is reasonably practicable) the need for temporary works within their permanent design (including their documented suggested construction sequence).
The Principal Designer is involved during the pre-construction phase. With temporary works, a lot of that pre-construction phase takes place during the construction phase as design work is being undertaken then.
4.1 Health and Safety Executive: Managing Health and Safety in Construction, Guidance on CDM 2015 Regulations, Publication L153, HSE Books, March 2015 (http://www.hse.gov.uk/pUbns/priced/l153.pdf)
4.2 Banks, J: ‘The role of the principal designer with reference to temporary works’, Temporary Works Toolkit, Part 9, The Structural Engineer, March 2017, pp 16-19 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2017/aab85b3b-07af-4532-9a21-67f6919ba7fa.pdf)
4.3 Temporary Works forum: Principal Designer: Guidance on Temporary Works, TWf Information Sheet 3, January 2017 (https://www.twforum.org.uk/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=01c474f7-b924-8c48-d4c5-732972ed7ae4&forceDialog=0)
What does BS 5975 require?
BS 5975: 2019(5.1) (the code of practice for temporary works procedures) gives recommendations and guidance on the procedural controls to be applied to all aspects of temporary works in the construction industry. BS 5975 recognises that temporary works need to be managed within the context of CDM 2015.
The Temporary Works forum has published a free guide on the requirements of BS 5975: 2019 in a format that is more useful to SMEs(5.2). Pallett(5.3) has produced a brief summary of the key changes from the 2011 to 2019 versions of BS 5975. Webster(5.4) has produced a simplified guide to managing temporary works.
In relation to permanent works designers, BS 5975 states that: ‘permanent works designers should address the buildability of the permanent works and identify and make provision for any temporary works required by their design and their assumed method of construction.’
BS 5975 outlines a series of expectations of the permanent works designer including the need to:
- Take full account of the construction process, giving particular attention to new or unfamiliar processes;
- Consider the stability of partially erected / demolished structures and, where this is not immediately obvious, providing information to show how temporary stability could be achieved;
- Identify where standard industry details are not suitable, and where detailed structural design will be carried out by others;
- Consider the effect of the proposed work on the integrity of adjacent / existing structures, particularly during refurbishment;
- Ensure that the overall design takes account of temporary works which might be needed, no matter who is to develop those works; and
- Ensure that consideration has been given to the availability of sufficient space and adequate ground support required to construct or maintain the structure.
It also notes that where the permanent works designers are appointed by the Client under a contract separate from that of the Principal Contractor, the Designer should liaise with the Principal Designer to provide all necessary information relevant to any temporary works or temporary conditions to the Principal Contractor through the Client and / or Principal Designer.
5.1 BSI: Code of practice for temporary works procedures and the permissible stress design of falsework, BS 5975, 2019
5.2 Temporary Works forum: The safe management of temporary works: The basics for small and medium-sized enterprises (SMEs), TWf Information Sheet 6, June 2019 (https://www.twforum.org.uk/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=71ca9edc-0bfe-3e86-7771-61b152f52238&forceDialog=0)
5.3 Pallett, P: ‘A viewpoint on temporary works procedures’, Concrete, February 2019, pp 34-36 (http://www.temporaryworks.info/CONCRETE_Feb_19_Overview_TW_Procedures.pdf)
5.4. Webster, M P: Managing Temporary Works: An overview, CIOB Technical Information Sheet 3, July 2021 (https://www.ciobacademy.org/product/managing-temporary-works/)
Specific design issues
Guidance is readily available, much of it free, on the following design issues:
- Concrete buildings
- Propping of reinforced concrete flat slabs
- Reinforcement cage stability
- Concrete bridges
- Steel buildings
- Steel bridges
- Rail infrastructure
- Integrating permanent and temporary works
This guidance is summarised in the following sections.
There are a range of areas where, by considering temporary works at the design stage, the permanent works designer can have a beneficial impact on the temporary works. Carpenter(7.1) addresses these in the document Designing for Safer Concrete Structures. Specific points include:
- Cold weather concreting – this will delay strength gain and must be allowed for in stripping and early loading times and hence the overall contract period;
- Edge protection – where there is an anticipated need for edge protection, consideration should be given to making provision for this in the permanent works design;
- Fixings – check the permanent works for predictable attachment loads from temporary works and that the permanent works are detailed to receive them with minimum difficulties
- Ground-bearing capacity – does the ground have sufficient capacity to support temporary works, mobile cranes, etc?;
- Interim stability – is there a feature of the permanent works that creates an interim stability problem that should be taken into account by the temporary works design? If so, this should be pointed out;
- Lateral loads – temporary works for falsework require lateral stability, specifically at forkhead level. Have the columns or other features of the permanent works been designed to accommodate this predictable loading? If not, has this been indicated on the drawings?;
- Slab propping – can the slab be designed to avoid propping? If not, is the specification clear as to stripping times and concrete strengths, and the means by which concrete maturity is to be measured? Can the permanent works accommodate predictable propping loads? Is method of depropping set out clearly? (this subject is covered in more depth in the next section);
- Space – is there sufficient space for the anticipated temporary works, cranage, deliveries, etc? This is often critical for inner-city projects;
- Tower cranes – if the location of the tower cranes has been established, has a review been made of their impact upon the permanent works; for example: space, foundations, fixing brackets into the structure? For compact sites the design may need to ‘build in’ the crane position as temporary works; and
- Variations in dimension – where it is reasonable to do so, wall thicknesses should be kept constant to avoid having to adjust falsework systems and hence negate the associated need for work at height.
7.1 Carpenter, J: Designing for Safer Concrete Structures – Outlining how designers can make a difference, The Concrete Centre, CCIP-043, November 2011
Propping of reinforced concrete flat slabs
When concrete slabs are poured, the weight of the wet concrete, formwork and construction loads will have to be carried by the floors below. However, most modern buildings are designed for imposed loads that are relatively small in relation to the self-weight of the slab itself (around 30% to 40%) – this leaves little ‘spare capacity’. Therefore, the self-weight of the slab to be constructed cannot be taken on the recently completed slab directly below. The loads will need to be distributed to lower, already completed, ﬂoor slabs to avoid overloading the slab directly below. This transfer of load is known as ‘backpropping’.
Pallett(8.1) notes that: ‘Backpropping during construction creates a totally foreseeable load on the structure. Hence both constructors and designers have to consider backpropping and understand the mechanics of load transfer during construction of the designed structure.’ The Concrete Centre(8.2) suggests that: ‘The temporary works designer and permanent works designer should work together to assess whether a higher design load should be used to cater for the construction load conditions.’
Pallett provides an introduction to backpropping(8.1) and worked examples that illustrate how the loads are distributed to lower slabs(8.3). Flat slabs tend to be more slender now than when the methods for estimating loads in backprops was undertaken in the mid-1990s. To address this, Pallett(8.4, 8.5) has established more realistic criteria for load transfer, particularly for those slender structures with span to depth ratios exceeding 30.
8.1 Pallett, P: ‘An introduction to backpropping of ﬂat slabs’, Temporary Works Toolkit, Part 4, The Structural Engineer, December 2016, pp 38-41 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2016/e8d94849-fb84-4368-8ab6-289d5b6b4cd6.pdf)
8.2 The Concrete Centre: Early age construction loading St George Wharf Case Study, TCC/03/04, 2004 (https://www.construct.org.uk/wp-content/uploads/2017/01/Construction-Loading.pdf)
8.3 Pallett, P: ‘Backpropping of ﬂat slabs – design issues and worked examples’, Temporary Works Toolkit, Part 6, The Structural Engineer, January 2017, pp 30-32 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2017/4b2b1445-b08b-48b9-830f-d9d4ac3a01d5.pdf)
8.4 Pallett, P: Backpropping flat slabs, PFP/136F, 8th May 2018 (http://www.temporaryworks.info/PFP_136F_Backpropping_Flat_Slabs.pdf)
8.5 Pallett, P and Szembeck, D: ‘Backpropping’, Chapter 28, Temporary Works, 2nd Edition, ICE Publishing, 2018 pp 403-420
Reinforcement cage stability
Reinforcement cages are temporary structures until cast into hardened concrete. There have been collapses of reinforcement cages(9.1, 9.2, 9.3, 9.4) during construction before concreting leading to fatal and major injuries. These collapses have included wall cages, column cages and deep beam / slab cages. Guidance was relatively limited until the Temporary Works forum started publishing initial guidance in 2013.
The Temporary Works forum has now published detailed Management Guidance(9.5) and Technical Guidance(9.6) – these documents are available for free download from the Temporary Works forum web site.
Filip and Tyler(9.2)and the Temporary Works forum(9.1) have identified the changes in construction process contributing to collapses of reinforcement cages as:
- Design refinements – leading to the amount of reinforcement being rationalised leading to smaller diameter, more slender reinforcement that reduces cage stability; and
- Changes in site practice – reinforcement used to be ﬁxed in situ, tied back to one side of propped formwork with scaffolding used for access (and restraint), but now it is more common for cages to be prefabricated offsite and lifted into place, with mobile elevated working platforms (MEWPs) being used for access rather than scaffolding.
The Temporary Works forum(9.1) recommends that any designer(s) of reinforcement cages in the temporary condition – whether a permanent works designer or a temporary works designer – should consider the different failure modes. These include:
- Foundation, slab and beam elements – support of the top mat, racking, lateral instability;
- Column and wall elements – discontinuity at lap positions, bending failure of bars, axial buckling, in-plane racking (side sway); and
- Cages that are lifted – individual bars may drop from the cage, or the cage may be unstable and/or break up as a result of the lifting forces imposed.
The Temporary Works forum(9.1) also recommends that permanent works designers should give consideration to the risks associated with:
- Cages that involve man-entry;
- Tall wall and column cages, particularly if above the following heights for each bar diameter 2.4m (12mm rebar), 3m (16mm), 3.5m (20mm), 4.5m (25mm) and 5m (32mm and above); and
- Large or heavy elements at the top of cages, e.g. projecting slab starters or corbel reinforcement.
The Temporary Works forum(9.5) has provided free access to detailed guidance on the management of the temporary stability of reinforcement cages prior to concreting. Design technical guidance and worked example calculations are provided in Part 2 of this guidance(9.6).
9.1 Temporary Works forum: Stability of Reinforcing Prior to Concreting, TWf Safety Bulletin, TW2015.116, October 2015 (https://twforum.connectedcommunity.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=b45d4394-42f8-a4fa-7563-7554fc27f9e6&forceDialog=0)
9.2 Filip, R and Tyler, M: ‘Rebar stability’, Chapter 32, Temporary Works, 2nd Edition, ICE Publishing, 2018 pp 465-477
9.3 Temporary Works forum: Stability of Reinforcing Cages Prior to Concreting, TWf2013:01, 2013 (https://www.twforum.org.uk/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=09acf678-f560-baa2-775a-a708344a8d40) (Addendum 2014 https://www.twforum.org.uk/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=19be0434-5c61-e7e9-90b4-7b5050f47c9b&forceDialog=0)
9.4 Highways England: Stability of reinforcement cages prior to concreting, Guidance, March 2016 (https://www.gov.uk/government/publications/highways-agency-safety-alerts/stability-of-reinforcement-cages-prior-to-concreting)
9.5. Temporary Works forum: Temporary condition of reinforcement cages prior to concreting – Part 1 Management guidance, October 2020 (https://www.twforum.org.uk/viewdocument/rebar-guide-part-one-management)
9.6. Temporary Works forum: Temporary condition of reinforcement cages prior to concreting – Part 2 Technical guidance, November 2022 (https://www.twforum.org.uk/viewdocument/rebar-guide-part-two-technical)
Temporary works form a significant portion of the total cost of a bridge construction project. It is usually the responsibility of the Contractor to design the temporary works for a project. However, initial decisions taken by the designer regarding the various alternative arrangements of temporary works can significantly improve the buildability of bridges.
Ray, Barr and Clark(10.1) cover a range of temporary works issues of relevance to the permanent works designer under three main heading: conceptual issues of buildability, interconnected specific issues of buildability and specific buildability issues. The issues covered include:
Conceptual issues of buildability
- Health and safety
- Safe clearances and access for constructing the temporary works especially if required in central reservations
- Vehicular collision loads to be carried by and/or stiffness requirements of the temporary works
- Requirements of thorough soils investigation for planning of temporary works in the ground
- Aesthetical considerations of using temporary works as permanent works
- Considerations at the design stage of the likely abandonment of temporary works left in place in the ground
- Method of construction of temporary works near live rail or road traffic
- Constraints imposed by existing structures and easements required for temporary works
- Tolerances in the girder spacings and the use of permanent formwork
- Adequate working space for installation and removal of temporary works
Interconnected specific issues of buildability
- Standardisation of top flange splice plate and connections to allow seating of precast concrete or GRP etc. permanent formwork
- Allowance of cast-in sockets or insert plates or drilled holes for resin anchor bolts for fixing temporary works to precast concrete elements
- Location (and standardisation if possible) of drilled holes in steel deck girders for fixing temporary works
- Standardisation of shear connector size and spacing to suit precast concrete permanent formwork
- Cladding of temporary works when used as permanent works
Specific buildability issues
- Temporary works used as permanent works
- Standardisation of solutions
- Temporary works supported from permanent works
- Permanent works used as temporary works
- Temporary diversions
CIRIA also provide guidance on how to address the associated detailing issues for concrete bridges(10.2).
Precast concrete bridge beams
Bennion(10.3) has produced a guide on temporary works associated with precast concrete bridge beam construction that is primarily aimed at the permanent works designer. In this guide, Bennion notes that: ‘By designing out the necessity for extensive temporary works, the permanent works designer can save signiﬁcant sums for the client. In so doing, they can also minimise working at height and eliminate other associated hazards at source.’
Bennion(10.3) focuses on the contribution that permanent works designers can make to eliminating or reducing the use of temporary support trestles at the piers of multi-span continuous precast concrete bridges by:
- For deck continuity, arranging post-ﬁxed reinforcement in the spaces between the webs of U beams to enable composite action to take place between the precast and in situ concrete elements. The beams would be landed on previously cast portions of the permanent works, designed to include the temporary loading condition;
- Incorporating high-level corbels in the permanent supporting structure, on which temporary support beams could be subsequently mounted for initial erection and subsequent bearing replacement;
- Utilising rectangular voids at a high level in bridge piers, through which temporary support beams could be installed. These voids would either remain as a feature or be wholly or partially inﬁlled following removal of the temporary works;
- Adopting tapered elevations to bridge piers, preferably square section in plan, to facilitate use of a wedged temporary support device ﬁxed at a high level; and
- Introducing a permanent shelf at a high level, on which precast bridge beams are landed and the in-situ diaphragm cast.
10.1 Ray, S; Barr, J and Clark, L: Bridges – design for improved buildability, CIRIA, Report 155, 1996
10.2 Soubry, M: Bridge detailing guide, CIRIA, C543, 2001
10.3 Bennion, C: ‘Temporary works associated with precast concrete bridge beam construction’, Temporary Works Toolkit, Part 11, The Structural Engineer, May 2017, pp 37-41 (https://www.istructe.org/journal/volumes/volume-95-(2017)/issue-5/temporary-works-toolkit-part-11-temporary-works-a/)
The available guidance on steel buildings(11.1) focuses more on the impact that decisions taken by permanent works designers have on buildability rather than on temporary works. It provides information on the impact of a range of design decisions on buildability.
The guidance notes permanent works designers’ obligation to consider erection under the CDM Regulations, and notes that information to be passed on to the site team must include:
- The method of erection the designer assumed;
- Requirements for temporary bracing or propping, and conditions for their removal; and
- Features which would create a hazard during erection.
Further guidance is available on the steel construction.info web site(11.2) which summarise the key issues and provides signposting to the relevant BCSA publications.
11.1 Steel Construction Institute: Design for Construction, SCI Publication 178, 1997 (https://www.steelconstruction.info/images/2/24/SCI_P178.pdf)
11.2 SteelConstruction.info: Health and safety, (https://www.steelconstruction.info/Health_and_safety)
The Steel Bridge Group has produced a comprehensive set of guidance notes(12.1) that cover a range of issues that will influence design decisions.
Guidance Note 4.04 suggests that the Designer should describe the anticipated construction method and related design assumptions in the contract documents. The information given should include:
- A diagram of the construction sequence
- Information from the risk assessment
- Information concerning the stability of the steelwork during construction (bracing requirements, limits on loading, etc.)
- Ideally, the envelope of permanent load effects permitted should be stated
The guidance note also notes that: ‘However, it is not the Designer’s responsibility to prescribe the construction method that must be used. While it is good practice to choose a method / sequence that is less likely to preclude alternatives, when choosing a method, advice should be sought from fabricators or erectors.’
CIRIA also provide guidance on how to address the associated detailing issues for steel bridges(12.2). Further guidance is available on the steel construction.info web site(12.3) which summarise the key issues and provides signposting to the relevant BCSA publications.
12.1 Steel Bridge Group: Guidance notes on best practice in steel bridge construction, SCI Publication 185, 6th Issue, 2015 (https://www.steelconstruction.info/images/1/10/SCI_P185.pdf)
12.2 Soubry, M: Bridge detailing guide, CIRIA, C543, 2001
12.3 SteelConstruction.info: Health and safety, (https://www.steelconstruction.info/Health_and_safety)
Network Rail has published a series of guides in its Safe by Design series. It’s Briefing Note 01(13.1) sets down Network Rail’s expectations of the significant hazards to be communicated in design as being those that are:
- Not likely to be obvious to a competent contractor or designer – such as potential asset instability, contact with materials with a health risk e.g. fragile roofs, asbestos or lead paint;
- Difficult to manage – these may be common hazards but be in awkward situations such as working at height, working near electrified railway lines, transporting or manoeuvring fabricated structural components on site; and
- Unusual – these may be ‘common’ hazards but occurring in unusual circumstances, or unusual because of the nature of the construction or site, including structural stability attained through arching action, or obstructing signal sighting.
This Briefing Note(13.1) also identifies the information to be communicated as:
- Notes on drawings – this is preferred, since the notes will then be immediately available to those carrying out the work;
- Written information provided with the design – this should be project specific, and only include particular (not generic) hazards which will be useful to those constructing or maintaining the asset; and
- Suggested construction sequences – this should be project specific, and should show, where it is not obvious, how the design could be erected safely.
Network Rail also provides guidance(13.2) on constructability and temporary works. This contains a range of examples of where design decisions have led to collapses of temporary works. One of the key recommendations is in Section 18 where it states that ‘Designers should undertake Constructability reviews at various stages of a project.‘ Recommendations for when constructability reviews should be undertaken, and a suggested agenda are also included.
13.1 Network Rail: Safe by Design – Design Risk Assessment, Briefing Note: No. 01, July 2012 (https://safety.networkrail.co.uk/wp-content/uploads/2015/09/SbD-Briefing-Note-July-2012_revised2.pdf)
13.2 Network Rail: Safe by Design – Early Focus on Constructability and Temporary Works, Guidance Note, Version 3.0, May 2019 (https://safety.networkrail.co.uk/wp-content/uploads/2016/04/Safe-by-Design-Early-Focus-on-Constructability-and-Temporary-Works-Guidance-V3-15.5.19.pdf)
The free CIRIA guidance(14.1) provides a series of case studies and toolbox talks highlighting what can go wrong during smaller refurbishment projects. Many of the issues relate to (the lack of) temporary works and will provide the permanent works designer with an indication of the risks that need to be managed.
Filip(14.2) provides guidance on propping and needling, and notes that temporary vertical propping is likely to be required to:
- Provide support when existing supports are removed or are replaced / strengthened because the supports are no longer capable of supporting the existing or new loads;
- Provide temporary support until any new structure becomes self-supporting (e.g. concrete curing);
- Limit deﬂection which may cause damage;
- Gain access for work;
- Provide temporary support beneath a slab or ﬂoor, to allow for construction plant, labour, materials or demolition rubble to prevent overloading (backpropping subsequent levels below may then also be necessary in order to carry these loads through a structure to the foundations or to a more solid part of the structure)
14.1 CIRIA: Guidance on structural stability during refurbishment, C740, 2017 (https://www.ciria.org/ItemDetail?iProductCode=C740F&Category=FREEPUBS)
14.2 Filip, R: ‘Propping and needling’, Temporary Works Toolkit, Part 10, The Structural Engineer, April 2017, pp 30-35 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2017/ca6f9c33-d4e6-4f29-8706-fae4a5ba1c7e.pdf)
Thomas, Richardson and Fletcher(15.1) suggest that when designing any structure where deep excavation is required (e.g. shafts, tunnels, deep basements, tanks and pipework infrastructure, etc.) it is essential that design engineers understand the signiﬁcance of the risks associated with groundwater. They suggest an early-stage assessment of groundwater risks by permanent works designers would give the following benefits:
- A preliminary groundwater control strategy can be put in place at pre-tender stage, so that tendering main contractors can include for the same groundwater control measures required, on a competitively equal basis;
- By combining the temporary and permanent works groundwater solutions, signiﬁcant cost savings to the project can be made, thus avoiding duplication of effort and resources; and
- Permanent works designers are able to meet their CDM buildability obligations, and signiﬁcantly reduce the project’s exposure to risk.
The authors also provide a checklist to identify and assess groundwater risk and provide summaries of the methods of controlling groundwater (designed sump pumping, wellpoint dewatering, shallow-well suction dewatering, deep-well borehole pump dewatering and ejector dewatering) and the risks associated with groundwater pumping
15.1 Thomas, S; Richardson, G and Fletcher, D: ‘Dealing with risks from groundwater – guidance for design engineers’, Temporary Works Toolkit, Part 12, The Structural Engineer, June 2017, pp 22-26 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2017/4df6dfe8-4805-48c7-ba0d-cd06a5e7af38.pdf)
Integrating permanent and temporary works
Temporary works can be expensive, time-consuming to install and remove, and can be considered a waste as they as they do not form part of the permanent works. Therefore, it makes sense to reduce the amount and complexity of the temporary works by using elements of the permanent works to do the job of the temporary works. Markham(16.1) has provided guidance on three areas where permanent works designers can do this and have a major inﬂuence on the temporary works by:
- Designing multi-storey reinforced concrete framed structures for construction loads – to reduce or eliminate backpropping;
- Making assumptions about the layout of proﬁled-steel permanent falsework and showing it on the construction drawings along with potential deflections when the concrete is placed – the supplier of the steel decking can then add them to the nominal thickness of the slab when specifying the steel decking; and
- Using permanent works as temporary works in retaining wall construction – by adding additional reinforcement (to minimise the need for propping) or designing capping beams to act as walers (to avoid using additional temporary steel waler beams).
16.1 Markham, P: ‘Integration of permanent and temporary works’, Temporary Works Toolkit, Part 3, The Structural Engineer, November 2016, pp 42-45 (https://www.istructe.org/IStructE/media/Public/TSE-Archive/2016/19e3c48f-bfae-427c-b692-6557e98a12ba.pdf)
Earlier in this article, the concept of contemporary good industry practice was introduced in relation to CDM 2015 and the Institution of Civil Engineers Guidance on Design Risk Management. The references provided throughout this article indicate that such guidance is readily available (and free).
Further information on CDM 2015
A brief summary of eight key changes to the CDM 2015 Regulations from CDM 2007 and what those changes mean in practice is provided in this post.
A summary of published answers to frequently asked questions relating to the CDM 2015 Regulations is provided in this post.
Detailed clause-by-clause comparisons of the changes between the CDM 2015 Regulations and CDM 2007 are available on the MPW R&R web site for Clients, Designers and Principal Contractors / Contractors.
I gave a 25-minute presentation at the Institution of Structural Engineers Small Practitioners Conference on 26 June 2018 on understanding the CDM 2015 Designer Duties as they relate to civil and structural engineers. The video of my presentation and the accompanying slides are available in this post.
An overview of the issues involved in CDM 2015 Designer Duties – Designing for maintenance are discussed in this post and the guidance available to civil and structural engineers on contemporary industry practice is highlighted.
A summary of published discussions on key issues relating to the CDM 2015 Principal Designer duties are provided in this post.
This post discusses the impact of deterioration on the safety of concrete structures and suggests steps that designers can take to minimise the risks of deterioration to safety and discharge their duties under the CDM 2015 Regulations.
The HSE web site contains a range of information on CDM 2015 including the guidance on the regulations, a short guide for clients and a guide on construction phase plans.
The CITB web site contains industry guidance for each of the duty holders.
About the author:
Dr Mike Webster is a chartered civil and structural engineer (FICE, FIStructE) with over 30 years’ experience. He specialises in construction and structural safety, CDM and risk, and founded MPW R&R to provide Consulting, Forensic and Expert Witness services in those areas.
Mike has worked on the design, appraisal and site supervision of building and bridge structures. He has developed guidance for assessing the safety of existing structures. Mike led an independent review of CDM 1994 and the independent evaluation of CDM 2007. In addition , he also led the review of the use of CDM 2007 in the construction of London 2012.
Mike has been instructed as an expert witness by both defence and prosecution teams in cases involving allegations of gross negligence manslaughter, breaches of the Health and Safety at Work Act and the CDM Regulations and the appeal of enforcement notices. Several of these cases have involved temporary works.
Mike is the author of around 20 published reports and papers on construction health and safety and the CDM Regulations. He is also the author of a range of articles on CDM 2015. He is a member of the Temporary Works forum, Structural-Safety and the Institution of Structural Engineers Health and Safety Panel.
For more information email Mike at firstname.lastname@example.org or give him a call on 07969 957471.