Construction projects often require the design, construction and use of temporary structures to provide access or protection to workers or the public, to protect adjacent structures or to support temporary loads during construction. Examples of temporary structures include excavation support systems, underpinning, scaffolds, shoring, formwork, falsework, roadway decking, roof protection and on-site contractor facilities. Ordinarily, project owners, developers and their design teams, do not need to be particularly concerned with temporary structures. They are viewed as merely the contractor’s “means and methods” and are the sole responsibility of the contractor. However, the owner and developer are stakeholders in the implementation of temporary systems on their project and should be concerned about the risks and rewards associated with temporary structures.
Typically, a temporary structure is designed by or for the contractor that uses it, based on the needs of the project and design standards that may include a combination of local or national codes, project specification, the contractor’s preferred means and methods and project specifications. Effective selection and implementation of temporary structures can be the difference between winning or losing a bid or between a project being completed at a profit or a loss. The contractor, therefore, has to balance cost, performance, safety, schedule implications and reliability of their temporary structures.
Temporary structures are fundamentally different from permanent structures. Obviously, they typical serve different and generally more specific functions than permanent work. They also have a shorter service life. Consequently, temporary structures are subjected to different types of loads that have different levels of uncertainty than those to which permanent structures are subjected. There is disagreement among engineers as to whether loads on temporary structures are more or less certain than loads on permanent structures. Compared to permanent work, temporary structures are more likely to be built with used materials or assemblies. They are typically constructed to much looser tolerances and may be subject to secondary forces that are not accounted for in their design. While literature exists to provide guidance for all sorts of temporary structures, there are few authoritative standards and applying the available literature to a particular situation may be a challenge.
The qualifications of designers of temporary structures are as varied as these systems themselves. On one extreme are world-class structural engineering consulting firms and construction engineering specialists. On the other extreme are individuals like the chemical engineer preparing underpinning plans on his kitchen table, who I once heard a New York City building official describe encountering. Far more common are employees of contractors and temporary system suppliers and “geostructural” engineering consultants, who are typically civil engineers with geotechnical design experience. There is nothing wrong per se with a construction engineer or geotechnical engineer designing temporary structures, but problems can be encountered when designers do not recognize failure modes resulting from unusual or complex structural behaviors or do not adequately understand codes and standards that are developed for other purposes to apply them to temporary structures.
For example, a lot of temporary structure designers are not proficient in structural stability, since this is typically taught to graduate level structural engineering students. There are a number of case studies involving stability failures of temporary structures (such as the excavation support failure shown above) and I have reviewed designs that have not considered stability appropriately. Like any other engineering specialty, designing temporary structures requires education and experience, and being qualified to design one type of temporary structure does not qualify someone to design all types in all situations.
Failure or nonperformance of temporary structures is relatively common. In most cases, the consequences are limited, since the temporary structure is, in effect, a tool of the contractor. Therefore, many in the construction industry discount the importance of temporary structures more generally and as a result, they are subject to less oversight. Temporary work is considered to be the contractor’s problem. On most projects, the structural engineer of record (SER) will not review the design of temporary structures unless they are concerned about specific impacts on permanent work or some other interest of the client. There are some good reasons for this: first, structural engineers who design buildings typically do not have expertise in designing temporary structures; second, absent of specific contract provisions, there may be no basis for SER’s review, turning the exercise into a free peer review for the contractor; and third, reviewing the design could result in liability being transferred to the SER and the owner.
While the design team may understand the risks of reviewing temporary structure designs, they may not understand that everyone involved with the project is exposed to varying degrees of risk from the contractor’s temporary structures. This risk varies according to the specifics of the project, the site conditions and local laws. One of the major sources of risk from temporary structures are systems that are used to protect the public and abutters, such as excavation support systems, underpinning and roof protection. Damage claims and stop work orders for nonperformance of these systems are not rare and can cause costly project delays. While the contractor and the contractor’s engineer are primarily responsible for the performance of a temporary system, there are scenarios in which the owner can be liable for the contractor’s behavior. In addition, if a failure occurs that causes injury or significant property damage, everyone involved in the project will likely be entangled in the resulting lawsuits. Therefore, it can be advantageous to consider what temporary systems will be required to construct a project and look for ways to mitigate the risk involved during the design process. Abutters should consider retaining professionals to review temporary structures used to protect their property.
On some projects, it is advantageous to incorporate temporary structures into permanent work or allow part of the permanent structure to resist loads during construction, before completion of the structure. An example of this is the use of secant pile wall or slurry wall to support a multi-level basement excavation. These excavation support systems are subject to temporary and permanent soil and water pressures, usually with temporary and permanent supports being at different locations. They are also subjected to permanent gravity loads and perhaps in-plane lateral loads. For these types of systems, the temporary and permanent work must be well coordinated. A temporary load can cause damage or problematic deflections and in some cases, temporary forces can be locked into the permanent structure, reducing capacity. The SER, therefore, has to determine what part of the design, if any, should be delegated to the contractor. Sometimes, the design by the contractor is very specifically defined, so as to comply with the SER’s analysis of the permanent system. Alternatively, the SER might provide permanent loads and required the contractor to develop a turn-key system.
Another scenario in which the project team should take interest in temporary structures is when it becomes obvious that the demands for temporary systems are disproportional to the scope of permanent work. This can help to level bids, but most importantly it helps ensure that bidders are aware of the complexity of engineering required to build the project. If the contractor understands the complexity of the project when bidding, the risk of claims may be reduced. In addition, some jurisdictions, such as the City of New York and the District of Columbia, as well as some owners require that designs for certain types of temporary structures be included in the construction documents. In both of these cases, a mechanism should provide in the specifications to allow the contractor to modify or redesign temporary structures to better suit their needs or reduce costs. This process should also consider to what extent and for what purpose the revised design will be reviewed by the design team and how the benefits and risks will be shared by the contractor and owner.
Since the temporary structures used on a project presents both risks and opportunities, it is not in the interest of the owner, developer and the design team to ignore them. Instead, an appropriate level of consideration should be given to temporary systems throughout the design phase to manage the associated costs and risks. The architect’s and structural engineer’s conceptual design should be sensitive to the means and methods that might be required. The geotechnical report should provide a site-specific and project-specific discussion of construction considerations and include sufficient information for the design of below ground temporary systems. In some cases, the complexity of the project or site will be such that it may be advantageous to involve a construction consultant and/or a foundation engineering specialist to advise the design team on constructability, opportunities to improve construction efficiency through design and likely impacts on adjacent facilities. While there is a cost to considering temporary structures during design, it may be negligible compared to the potential for construction cost savings and project risk reduction.
The information and statements in this document are for information purposes only and do not comprise the professional advice of the author or create a professional relationship between reader and author.
- Robert T. Ratay. “Temporary Structures in Construction – USA Practices”, Structural Engineering International, April 2004, pp 292-295.
- Temporary Structures in Construction, Third Edition. Robert T. Ratay, Ed. New York: McGraw-Hill: 2012.
- OSHA. Investigation of November 19, 1990 Excavation Collapse at 14th and H Streets, N.W. Washington, D.C. (Photo)