Temporary structures such as scaffoldings, platforms and formworks are typically assembled on construction sites in a short period of time to provide an elevated workplace for workers or to support temporary loads. Such structural systems are common on construction sites owing to their versatility and quick assembly that save time for the constructors.
Although using temporary modular systems accelerate the construction process, temporary structures are more prone to collapse in comparison with other types of structures.
There is no unique standard to determine the likely loads applied on all temporary structures given their vast variety. As such, design procedure of such structures is less regulated and some critical assumptions are typically left to the judgment of the constructor.
Ben Daee, Senior Consultant, Property & Construction, Rimkus Consulting Group, Inc.
Furthermore, human error and omission in the assembly of these structures may directly disrupt the load path, potentially resulting in injuries and collapse.
The Infrastructure Health & Safety Association of Ontario reports that scaffold accidents are one of the most serious safety issues leading to casualties on construction sites. This is primarily as a result of the innate vulnerability of scaffolds stemming from their simple geometry, prescriptive design and installation.
While potential construction hazards can be mitigated by following strict safety regimes in conjunction with comprehensive training programs for workers, the safety of temporary structures predominantly depends on structural integrity.
As such, it is critically important that insurers not only understand the types and inherent risks associated with design and erection of temporary structures, but also employ proper pre-emptive measures to minimize the risk exposure on construction sites.
WHAT ARE THE TYPES OF TEMPORARY STRUCTURES?
From a structural prospective, temporary structures can be classified in three different categories in terms of the load bearing mechanism:
- Group (i) structural systems that hold lateral pressures of material (for example, concrete formwork or a trench box);
- Group (ii) structural systems that support an existing building during remediation or alteration (for example, a bracing system or shoring); and
- Group (iii) temporary systems that form an independent and full structure, including all fundamental components (for example, falsework or a platform).
Design and erection of each group is associated with various uncertainties, such as the applied loads, connections and geometry. While some insurers may view the groups identically, underlying risk rises from (i) to (iii) as a result of more complexity.
Consider that for Group (i), a primary input for design of a lateral supporting system, such as a trench box, is the soil condition. Underwriters are likely to request a complete geotechnical report, including soil conditions in various locations and depths.
Careful review of the plans may reveal the earthwork slope and distance from any adjacent buildings, utilities or access roads, all of which should be taken into account when designing a soil supporting system.
A heavy rainfall may saturate the soil and overstress a supporting system. That being the case, duration and timeline of the construction work is a substantial input for the risk evaluation of the group.
For Group (ii), the risk associated with the design and installation of structural components that stabilize a portion of an existing building (for example, a bracing system), is considerable since the designer must make some assumptions with respect to the load-bearing capacity of the existing elements, some of which might not be known at the time.
Quantification of a building’s conditions during the design phase is a common source of error that may impose additional risk on the project.
Typically, the design documents of the existing building, in conjunction with the alteration plan, should be obtained, reviewed and understood by the engineer of record prior to any remedial work being done.
And for Group (iii), falseworks, scaffoldings, platforms and cranes are independent structures that should be designed similarly to a normal structure and according to their specific standards.
Self-stabilization is a key in design and erection of these structures, necessitating that provisions be made to ensure that lateral stability is properly provided. As such, calculation notes and design drawings signed and sealed by professional engineers are imperative for safety assurance.
WHAT ABOUT SAFETY?
The safety of structures closely correlates with the quality of design, erection and communication between the office and site engineering team. A high-rise building is typically designed by a team of experienced engineers employing analytical approaches, while temporary structures oftentimes are installed on site based on either no or simplified calculations.
An inaccurate mindset exists that temporary structures are pre-designed and pre-examined components, and hence, safe. As such, some constructors do not update the analyses and template drawings based on actual site conditions.
For instance, design of a concrete formwork is primarily based on load tables, erection procedures and recommendations provided by the supplier. Unknown site conditions, on-site alterations of initial plans, and a simplified design approach escalate the vulnerability of temporary structures.
WHAT CONSTITUTES A FAILURE?
Unlike temporary structures, failure or injuries on sites for permanent construction are not necessarily the result of structural deficiencies; external factors such as equipment failure, fire and extreme weather conditions are more likely to trigger an incident.
For temporary structures, though, the probability of root causes of failure is typically different and can be ordered in a hierarchy as follows:
1 Construction deficiencies: Statistical studies, including the Study of Recent Building Failures in the United States released in 2003, note that failure of temporary structures show that design and erection deficiencies are the most common factors that enable a failure. Improper lateral bracing, shoring, foundation or connection components, as well as poor workmanship and maintenance of the elements, are the most recorded deficiencies.
That said, however, most such deficiencies can be prevented through supervisory measures.
2 Procedural causes: Inadequate interaction between the design and installation team, on-site alteration and hit-or-miss site inspection are procedural causes that may contribute to an incident. The presence of a design team representative as a site supervisor may resolve possible discrepancy between the plan and build, such as overloading.
3 External causes: Vibration, impact and environmental events, including wind gusts, freezing temperature, flood and heavy rainfall beyond the degree prescribed in the design standard, are probable external causes. Adverse wea-
ther conditions are inevitable, but, typically, can be anticipated and managed in advance. For example, removing the tarps off scaffolding once the wind speed is expected to exceed the plausible standard threshold elevates the structural safety.
WHAT ARE THE PREVENTIVE MEASURES?
Failure of construction, even partial, may result in an economic disaster and, in some events, catastrophic injuries. With regard to temporary structures, a simplistic design viewpoint, overlooked safety concerns and innate vulnerability of the proprietary systems because of a lack of structural redundancy raises the associated risk of the construction. However, this susceptibility to damage can be managed and mitigated through additional quality assurance steps.
Prior to erecting a large temporary structure such as a bridge falsework, an insurer can request technical documents, including updated design plans, calculation notes, site inspector qualifications and duties of the engineer of record to clarify the responsibilities of the involved parties at each stage. For high-profile or high-risk projects, a third engineering party can be involved to peer-review the plans, identify potential human errors, inspect the construction site and monitor the erection.
Offering discounts on premiums can persuade an insured to take extra precautions on a construction site, especially for facultative policies.
The insurer may also elect to retain an engineering consultant to occasionally monitor the practice of contractors who are regularly engaged in erecting temporary elements on construction sites for the purpose of reviewing the technical and procedural documents. An insurance policy can mandate the presence of a supervisor engineer on sites during installation and removal of the structural components.
Intense competition in the insurance market to underwrite a policy and attract potential clients is, indeed, a challenge for insurers, particularly once competitors take additional risks by offering price breaks on premiums and asking for less paper work from their clients.
Nonetheless, the higher potential risk exposure of temporary structural systems and the number of related incidents necessitate a dynamic risk mitigation plan since a large loss may force an insurer out of the market.
If a bridge falsework under construction collapses, it may result in work injuries, project delays, environmental catastrophe and enormous litigation expenses, perhaps leading to a seven-digit insurance claims.
Therefore, higher premiums are not a remedy; rather implementing supervisory measures and assigning qualified individuals before and during the installation of a temporary structure will help lower the potential risks and bring an added value for insurers.
Ben Daee, Senior Consultant, Property & Construction, Rimkus Consulting Group, Inc.