Large Event Sign Collapse Expert Article

This case involves a scaffold structure used for signage at an outdoor event that toppled over during reasonably foreseeable wind conditions and injured several people. Robson Forensic was retained to perform a structural analysis to determine why the structure collapsed and who was responsible.

Expert Investigation of Event Sign Collapse

The Incident Sign (Structure)

The sign in this incident was constructed with tubular scaffolding wrapped in vinyl banners. The design consisted of two towers, joined by a horizontal assembly at the top, creating an entry/exit portal under which invited guests passed.

Assembled, the structure stood approximately 15 feet tall by 31 feet wide and 5 feet deep. The original design called for three soil anchors at each tower to stabilize the structure.

Temporary Structures

The incident sign in this case is considered a temporary structure. Building codes and standards organizations, such as the International Code Council (ICC) and American Society of Civil Engineers (ASCE) make specific reference to temporary structures because of risks associated with their temporary nature, and to highlight the importance of a proper design for those temporary structures.

• Wind Loads – Temporary structures need to be designed to safely resist reasonably foreseeable wind loads. The process engineers use for determining the design allowable wind speed considers environmental factors of the site as well as the duration for which the structure will be erected.
• Loss of Capacity – Temporary structures that are repeatedly assembled and disassembled are subject to degradation of both material and connections. Engineers need to consider this loss of capacity when designing these structures.

Load Paths

All forces travel through structures in a predictable route, eventually making their way down to the supporting soils; in engineering, this is referred to as the load path. Once the anticipated loads have been established, structures must be designed and constructed to withstand loads across the load path. 

For the temporary structure in this case, it is essential that all structural elements along the load path, from the top components down to the anchorage system, are designed to withstand the anticipated forces. Failures along the load path often result in failure of the system.

Determining Causation and Liability

As mentioned, the design of the incident structure called for a total of six soil anchors to secure the structure and resist wind loads. This anchorage plan was later changed on site to accommodate the structure being installed on a paved surface.

Rather than consulting with an engineer on this design change, the erector pieced together a solution based on other installations they’d done in the past.

In place of soil anchors, the installer utilized concrete deadman anchors (“jersey barriers”) attached to the scaffold through tube and clamp members. It was ultimately the tube and clamp members that failed, allowing the structure to be toppled by a reasonably foreseeable wind event.

Expert Findings

An engineering analysis was performed by our expert and it was determined that, based on the structure, the site conditions, the intended lifespan of the structure, and applicable codes and standards, the minimum wind speed that the sign was required to safely resist was 70mph.  

Further, our expert found that the structure, as assembled and braced, would be subjected to instability and overturning at a reasonably foreseeable wind speed of 47 mph.

A meteorological study for the day of the incident showed that the actual wind speeds at that event were between 48-50 mph.

The revised scaffolding design, based on anecdotal experience rather than engineering rigor, created an unreasonably dangerous condition for invited guests to that event.

The contractor, who was responsible for the design, installation, and removal of the temporary structure was ultimately found responsible for the structural failure and related injuries.

Investigating Structural Failures and Collapse Incidents

Forensic structural engineers are uniquely qualified to investigate structural failures due to their intimate knowledge of how structures are designed, constructed, maintained, and how they are intended to safely perform. The experts in the Structural Engineering practice at Robson Forensic are experienced in investigating incidents of structural failure and collapse.

Submit an inquiry or call us at 800.813.6736 for more information.

Featured Expert

Mark Duckett, P.E., S.I.

Structural Engineer, Special Inspector & Construction Expert

Mark Duckett is an expert in structural engineering, including building design and inspection, professional liability, value engineering, and building defects. He is also an expert in wind… read more.