This article explains some of the key technical issues around PAS 8812 – the current guide on temporary works design to European standards.
Temporary works are very essential components in the construction process. These structures, which include scaffolding, shoring, formwork, and temporary bridges, provide the necessary support and stability during various construction phases. They are designed for short-term use, temporary works prioritize flexibility, ease of assembly and disassembly, and cost-effectiveness. Temporary work contrast from permanent work in the sense, permanent works are built for long-term use, while temporary works are for short term use.
Historically, engineers in the UK designed temporary works according to British Standards, primarily using permissible stress design approaches, with BS 5975 being a key standard. The complete adoption of the Eurocodes in 2010 led to the withdrawal of conflicting British Standards. Consequently, structural design for temporary works now mandates the use of a limit state design approach. This shift sparked apprehension within the industry about adapting temporary works designs to the new European standards. Why? This is explained in the next section.
Concerns in the Application of European Standards to Temporary Works Design
The Eurocodes outline principles and requirements for structural safety, serviceability, and durability for both permanent and temporary works. Designers must recognize that the challenges and risks in designing temporary works differ significantly from those for permanent works. These differences necessitate careful consideration and application of Eurocodes in temporary works design to maintain the reliability and safety standards previously established in the UK.
European product and execution standards, specifically for temporary works and supplementing the Eurocodes, offer simplified approaches. These approaches tend to result in more conservative designs compared to those using Eurocodes alone. While these simplified methods are generally suitable for most straightforward applications, they may not be appropriate for more complex scenarios, especially those involving interactions with permanent works or geotechnical design.
For more detailed information on applying European standards to temporary works design, refer to the guidance published by the Temporary Works Forum (TWf)2.
Need for a Guidance Document
Designers who are more familiar with the design of temporary work to BS 5975 can tell that is offers explicit design guidance to establish clear procedures for managing risks associated with designing, executing, and dismantling temporary works systems. This guidance remains fully valid when applying European limit state-based design standards. However, the UK, High Speed Two (HS2) recognized the necessity for a consistent approach to designing and procuring temporary works according to the new European standards. Consequently, HS2 provided funding to the British Standards Institution (BSI) to develop two Publicly Available Specifications (PAS) specific to temporary works, co-sponsored by HS2 and the Temporary Works Forum (TWf):
- PAS 8811 Temporary works. Major infrastructure client procedures. Code of practice: This provides recommendations for UK infrastructure client procedures regarding temporary works within construction projects.
- PAS 8812 Temporary works. Application of European Standards in design. Guide: This offers complementary guidance on applying European standards to the design of temporary works in the UK. WSP was appointed to draft PAS 8812.
Thus, this article aims to provides an overview of the second document: PAS 8812.
Overview of PAS 8812
While preparing PAS 8812, the steering group remained aware of the significant improvements in industry practice following several major temporary works failures in the 1970s, which led to the Bragg Report and the creation of BS 5975. Consequently, the group aimed to provide high-level guidance to assist designers accustomed to previous permissible stress-based codes and the use of equipment and materials rated with a safe working load. This guidance helps them transition to the current suite of limit state design standards.
Purpose and Scope
Recognising that there’s a need to ensure consistency in the design approach of temporary design is the purpose of PAS 8812. This document aims to achieve:
- High-level guidance on applying European standards: This includes recommendations on suitable partial factors for actions and materials, combinations of actions and analysis approaches, structural stability considerations, and reuse of equipment, especially for proprietary systems and components.
- Clarification of the relationship between Eurocodes and other European standards: This focuses specifically on temporary works.
- Clarification of design requirements: This target identified groups of temporary works.
PAS 8812 is intended for use by qualified and experienced practitioners in the structural and geotechnical design of temporary works. Alongside the companion document PAS 8811, PAS 8812 aims to maintain good practice during the transition to European standards by recommending:
- Application of appropriate procedural controls: These are introduced in BS 5975 and recommended in PAS 8811.
- Transfer of key data and information: This ensures continuity during the lifecycle of temporary works, including design, installation, use, and removal.
- Establishment of guidance on appropriate partial factors: This ensures the overall safety of temporary works.
- Recommendation of a notional lateral destabilising load: This ensures stability, as advised by BS 5975.
Structure
PAS 8812 offers guidance on six groups of temporary works (Box 2), organized into six sections and three annexes. Table 1 provides a brief overview of each part.
[supsystic-tables id=68]
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Temporary Works Design to European Standards: Key Aspects to Consider
Designing temporary works to European standards demands careful attention to specific topics to ensure safety levels match past UK practices and align with European practices. The following paragraphs outline key issues.
Eurocodes Terminology and Design Philosophy
Eurocodes introduce terms that may differ from traditional UK practice, requiring clarification to prevent misinterpretation. Annex B of PAS 8812 clarifies these terms and definitions, aiding users in understanding Eurocodes better.
Supported Construction
Supported construction refers to permanent works elements supported by temporary works. According to BS EN 1991-1-6 and BS EN 12812, supported construction is a variable action. However, the load it imposes on temporary works typically remains stable once in place, as noted by BS EN 12812. Thus, special attention is needed for supported construction across different limit states. PAS 8812 addresses this by recommending values for ψ factors and partial factors γQ.
Safe working load (SWL)
SWL, or Safe Working Load, defines the maximum unfactored load that proprietary equipment can safely withstand. While absent from the Eurocode suite, it’s crucial to differentiate SWL from the characteristic values of actions and resistances. PAS 8812 guides on this distinction. Furthermore, when utilizing proprietary equipment with an SWL rating from its manufacturer, ensuring adherence to the manufacturer’s intended usage is vital to maintaining appropriate safety standards.
Wind
Temporary structures typically face reduced exposure to characteristic wind, based on a 50-year return period in BS EN 1991-1-4. Adjustments using probability and seasonal factors are recommended to accommodate shorter usage periods. However, any reduction in wind action from its 50-year return period value must be carefully evaluated. PAS 8812 offers guidance on managing this aspect.
Equivalent Horizontal Force
Equivalent horizontal forces are lateral loads applied on structures to account for imperfections. The Bragg Report emphasizes the necessity of applying a minimum horizontal force for ensuring lateral stability. PAS 8812 recognizes this critical aspect and suggests incorporating an equivalent horizontal force (referred to as ‘notional lateral force’ in BS 5975).
This force is commonly expressed as a percentage of the vertical actions exerted on the structure (See: article)
Imperfections
Temporary works are often susceptible to initial imperfections such as fitting issues or eccentricities. The influence of both global and local imperfections should be carefully accounted for in relevant analyses. Eurocodes offer recommendations on managing imperfections specific to each material, with additional guidance provided in standards like BS EN 12810 to 12812.
Reuse of Proprietary Equipment
When proprietary equipment undergoes multiple uses, it’s essential to consider a minimum partial material factor to accommodate potential imperfections like bowing, eccentricity, and lack of redundancy. PAS 8812 provides detailed guidance on addressing these concerns.
Falsework
Falsework represents one of the few categories of temporary works covered by a specific European standard, BS EN 12812. This standard supplements Eurocode provisions by offering structural design information. However, clarity is needed on BS EN 12812’s application scope across various classes of falsework systems. PAS 8812 offers comprehensive guidance on navigating this issue.
Conclusion
Designing temporary works according to European standards necessitates careful consideration of unique features and risks, distinct from those encountered in permanent works. Currently, there remains a scarcity of comprehensive guidance tailored to the design of temporary works under European standards. It is crucial to meticulously evaluate available standards to uphold the established levels of reliability and safety in temporary works design.
In response to these challenges, the development of the BSI guide PAS 8812 addresses this gap by offering coherent guidance. PAS 8812 aims to foster consistency in the design approach for temporary works, providing clarity amidst the diversity of existing standards and guidance documents. Together with PAS 8811, this guide supports the continuity of best practices while offering specific insights into key technical considerations highlighted in this article.
Also See: Temporary Works and the Structural Engineer
Sources & Citation
- Temporary Works Forum (2014) TWf2014:01: The use of European Standards for Temporary Works design [Online] Available at: www.twforum.org.uk/media/54386/twf2014.01_use_of_european_standards_for_tw_design_24_november_2014_final.pdf (Accessed: August 2017)
- British Standards Institution (2017) PAS 8811:2017 Temporary work. Major infrastructure client procedures. Code of practice, London: BSI E4)
- British Standards Institution (2016) PAS 8812:2016 Temporary work. Application of European Standards in design. Guide, London: BSI
- Institution of Structural Engineers (2017) ‘Temporary Works Toolkit: Designing Temporary work to European Standards – an overview of PAS 8812.’ The Structural Engineer, Professional Guidance Notes 88(9)
