This article explores the integration of temporary and permanent works, focusing on technical strategies that optimize construction efficiency.
Temporary works play a crucial role in construction, providing structural support during different phases of the building process. However, they are ultimately removed, contributing to project costs without adding lasting value. As a result, engineers must consider ways to integrate temporary and permanent works to improve efficiency, reduce costs, and streamline construction.
Permanent works designers (PWDs) have significant control over how temporary works are used. By making strategic design choices, they can minimize the need for excessive temporary structures. This approach enhances buildability, ensures safety, and accelerates project timelines.
This article explores the integration of temporary and permanent works, focusing on technical strategies that optimize construction efficiency. By understanding load transfer, material behavior, and construction sequencing, engineers can develop designs that reduce reliance on temporary works while maintaining structural integrity.
The Role of Temporary Works
Temporary works refer to structures or systems that support construction activities. These include falsework, formwork, propping, scaffolding, and excavation supports. Despite their importance in ensuring stability, they introduce additional costs and time constraints.
Temporary works facilitate critical construction operations such as concrete casting, structural assembly, and excavation support. They provide stability until permanent elements gain sufficient strength. Without them, many construction processes would be unsafe or impractical.
However, extensive temporary works can complicate logistics, requiring additional labor and prolonging project durations. Reducing their use by leveraging permanent structures is a key strategy in optimizing construction.
Permanent works can be designed to assume temporary load-bearing functions, eliminating unnecessary supports. This requires a deep understanding of load distribution, material properties, and early-age strength development.
Optimizing Reinforced Concrete Slabs to Minimize Back-propping
Multistory reinforced concrete (RC) buildings require temporary supports for suspended slabs. The newly cast slab transfers loads to the floor below, which may not be designed to carry such loads. If this occurs, back propping is required to distribute the weight further down the structure.
Load Analysis for Temporary Slab Support
A typical 300 mm thick RC slab has a self-weight of approximately 7.5 kPa. Additional construction loads, including formwork and live loads, contribute further:
- Falsework load: 0.5 kPa
- Live load from workers and materials: 1.5 kPa
- Total characteristic load: 9.5 kPa
Full-scale tests indicate that when two levels of backpropping are used, at least 50% of the new slab’s load is carried by the slab immediately below, while only 30% is distributed further down.
Strengthening Slabs for Early-Age Load Bearing
To reduce reliance on back propping, slabs can be designed with additional reinforcement to withstand construction loads earlier. Increasing the concrete strength class or using admixtures for early-age strength gain allows slabs to carry imposed loads sooner.
A slab should be capable of carrying at least 50% of the construction load from the slab above, which equates to a characteristic load of 4.75 kPa. Designing for this requirement minimizes propping, reducing material waste and construction delays.
Steel Decking as an Alternative to Traditional Falsework
Steel decking provides an efficient alternative to traditional timber falsework. It functions as both a formwork and a structural component, allowing concrete to be poured directly onto the decking. After curing, the decking remains in place as part of the composite system.
Technical Considerations in Steel Decking Design
Steel decking is typically designed to act compositely with the slab, improving load distribution and reducing deflection. However, improper assumptions about support conditions can lead to unexpected failures.
A common issue arises when steel decking suppliers assume rigid beam supports, while in reality, steel frames exhibit flexibility. Deflections of up to 60 mm can occur, leading to:
- Increased slab thickness due to excessive concrete settlement.
- Unexpected redistribution of dead loads, causing additional stress in beams.
- Compromised serviceability due to excessive deflections.
Preventing Excessive Deflections in Composite Decking
PWDs should consider the following design adjustments to prevent excessive deflection:
- Pre-deflection Analysis – Anticipate beam deflections and incorporate them into slab thickness calculations.
- Temporary Propping – Introduce minimal temporary supports at mid-span where deflections exceed allowable limits.
- Optimized Deck Span Layouts – Use two-span or three-span decking configurations to improve load sharing.
Retaining Walls: Integrating Temporary and Permanent Functions
Excavation for basements and deep foundations often requires temporary retaining walls to prevent soil collapse. Conventional methods involve separate temporary walls, followed by permanent RC retaining walls. This duplication increases costs and complicates construction.
A more efficient approach involves designing a single embedded retaining wall that serves both temporary and permanent purposes. Secant, contiguous, or sheet pile walls can be designed to provide immediate excavation support while also functioning as part of the finished structure.
Retaining walls experience high lateral earth pressures during excavation. If the wall is designed with minimal reinforcement, temporary props or bracing systems become necessary. However, by strengthening the wall to withstand these temporary forces, additional bracing can be eliminated.
In some cases, the capping beam can serve as a waling beam, reducing the need for separate steel walers. This requires minor reinforcement adjustments but significantly simplifies construction.
Construction Sequencing and Load Transfer Considerations
Integrating temporary and permanent works requires careful planning of load transfer mechanisms. Misalignment between construction methods and design assumptions can lead to structural deficiencies or unnecessary costs.
During construction, the load path of a structure changes as new elements are introduced. Engineers must analyze the sequence of loading to ensure that all structural elements remain within safe limits at every stage.
For example, a basement retaining wall designed for final loading conditions may still experience excessive deflections if temporary conditions are not adequately considered. Adjusting reinforcement detailing can mitigate this risk.
Contractors often prefer flexibility in propping arrangements. By designing uniform reinforcement distributions in slabs and beams, engineers allow for greater adaptability in positioning temporary supports. This reduces conflicts between design intent and site execution.
Maximizing Cost Savings Through Efficient Design
Reducing temporary works directly impacts project costs. Every additional prop, falsework frame, or retaining structure increases expenses. Engineers must balance structural integrity with cost-effectiveness to achieve optimal results.
Increasing reinforcement or using higher-strength concrete may raise initial material costs, but these expenses are often offset by:
- Reduced labor costs from faster construction.
- Lower material wastage from minimized temporary works.
- Improved project scheduling with fewer disruptions.
A holistic cost-benefit analysis ensures that design decisions result in overall savings without compromising safety.
Temporary works contribute to significant material waste, especially in formwork and falsework systems. By integrating temporary and permanent structures, engineers can develop more sustainable construction practices. Optimizing steel usage, reducing timber falsework, and incorporating modular design strategies all contribute to greener construction.
Conclusion
Integrating permanent and temporary works improves construction efficiency, reduces costs, and enhances safety. By optimizing slab design for early-age strength, using steel decking for formwork elimination, and designing retaining walls for both temporary and permanent functions, engineers can streamline construction processes.
Understanding construction sequencing and load transfer mechanisms further ensures that structures remain stable throughout all phases. Engineers must adopt a proactive approach, balancing material efficiency with structural safety to minimize reliance on temporary works.
Also See: Understanding the Root Causes of Temporary Works Failure
Sources & Citations
- Markham, P. (2016). Temporary Works Toolkit: Part 3 – Integration of Permanent and Temporary Works. The Structural Engineer. [Source Document]
- Rackham, J. W., Couchman, G. H., & Hicks, S. J. (2009). Composite Slabs and Beams Using Steel Decking: Best Practice for Design and Construction. Metal Cladding and Roofing Manufacturers Association.
- Gaba, A. R., Simpson, B., Powrie, W., & Beadman, D. R. (2003). Embedded Retaining Walls – Guidance for Economic Design. CIRIA Report C580.
- British Constructional Steel Association (2010). Manual Handling Survey. Available at: www.steelconstruction.info
