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CROSS Safety Report

Channel slab roof deck collapse during reroofing

Report ID: 970 Published: 10 May 2021 Region: CROSS-US


Overview

This report highlights the structural risk caused by machinery during reroof a project.

It describes the partial collapse of a roof (consisting of precast concrete ‘channel slabs’) of a 45 year old manufacturing building that occurred during reroofing.

Key Learning Outcomes

For structural design engineers and specifiers of reroof projects:

  • Consider reviewing guidance documents before assessing the condition of existing structures for reroofing since it can be challenging, especially in older structures that have deteriorated. See suggested guidance documents referenced in the Panel’s comments below

  • Specifications for reroof work that contain project-specific requirements will help avoid the contractor’s overloading or damaging the structure

For contractors:

  • Conduct a thorough survey of existing conditions prior to commencing work

  • Use care to not overload or damage the structure during reroofing operations

For building owners and managers:

  • Consider retaining a qualified independent professional to monitor reroofing work

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In 2020 an existing roof deck experienced a partial collapse during a reroofing project. The subject facility was a manufacturing plant, originally built in the mid-1950s. The existing roof deck consisted of conventionally reinforced, conventional structural lightweight precast channel slabs (not autoclaved-aerated concrete). Each channel slab is approximately 2 ft (0.61 m) wide and spans approximately 8 ft (2.4 m) between supports. The channel slabs were reinforced with a single reinforcing bar in each flange (two total per channel slab) and a wire mesh within the web of the channel. A non-composite lightweight insulated topping slab was installed atop the precast channels slabs and provided support for two separate roofing systems, including various layers of insulation and roofing membranes. The total thickness of the roof assembly varied from 14 to 18 in (36 to 46 cm) atop the precast channel slabs. The roof was flat with no slope. Long term roof leaks had led to the deterioration of some precast channel slabs. This deterioration generally included cracking and spalling over reinforcing steel at these locations. This cracking and spalling was generally isolated and extended from approximately 1 to 3 in (2.5 to 7.6 cm) along the length of the precast channel slabs.

Prior to the reroofing project an engineering study was undertaken, which included limited observations of the precast channel slabs, chemical and petrographic analysis of the existing concrete material, and infrared scanning of the roof to determine the approximate extent of the roof leaks. There was no indication that the reinforcing steel within the precast channel slabs had debonded from the channel slabs as the distress was limited in length. It was determined by the engineer that, absent any deterioration, the structural roof deck consisting of the precast channel slabs was generally capable of supporting the new roof in accordance with Section 705.2 of the International Existing Building Code (IEBC). During the reroofing operation, debris containment netting was provided over select portions of the facility to catch small portions of spalling concrete and other construction debris that might fall through the joints between precast channels slabs during the reroofing project.

During tear-off of the existing roof, a channel slab collapsed and fell to the ground. No one was injured in the collapse. Workers atop the roof were able to get off the failing panel as it began to collapse. No occupants were in the portion of the building at the time of the collapse. Reports from the workers atop the roof at the time of the collapse indicated that the channel slab ‘suddenly snapped in half’ while a worker was sweeping debris off a portion of the roof in preparation for the new roof installation.  Later investigation revealed that the roofing contractor was using concrete demolition hammers (jackhammers) to remove portions of lightweight insulated concrete topping which was installed above the precast channel slabs.

Reports from the workers atop the roof at the time of the collapse indicated that the channel slab ‘suddenly snapped in half’ while a worker was sweeping debris off a portion of the roof in preparation for the new roof installation

A review of the debris including the broken panel revealed that reinforcing bars had debonded and separated from the precast channel slab, leading the channel slab to act as an unreinforced concrete section causing a flexural failure under service loading. No corrosion or rust-staining was present on either the reinforcing steel or the concrete debris. It was determined that the collapse was likely the result of inappropriate equipment use during roof tear-off.

Expert Panel Comments

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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.

This case highlights two important topics (1) the challenges of assessing the capacity of an existing structure in building repair and maintenance projects and (2) a contractor’s obligation to not overload or damage the existing structure.

It was difficult for the engineer specifying the reroof project to assess the existing conditions. The building was old, and the roof planks exhibited some, but apparently limited, distress. The extent of any potential debonding of the planks’ reinforcement could not be directly determined. Care must be used in extrapolating ‘limited’ observations and probes to an overall assessment. The post collapse observations of the absence of rust or staining on reinforcing steel or concrete debris suggest the engineer made a reasonable judgement in assessing existing conditions. The lack of redundancy in the planks’ structural capacity caused by simple spans with no reinforcement at the plank ends is not uncommon in roof decks of various materials, but it meant that a midspan flexure failure would result in sudden collapse with little warning. One potential avenue in the preconstruction assessment of capacity may have been to load test a representative sample of the planks. ASCE’s Guideline for Structural Condition Assessment of Existing Buildings (SEI/ASCE 11-99) offers additional guidance.

Care must be used in extrapolating ‘limited’ observations and probes to an overall assessment

Contractors must use care not to overload or damage the structure during their operations. The reroofing provisions of the International Building Code (IBC 2018), Section 1511.2 Structural and construction loads, requires, ‘Structural roof components shall be capable of supporting the roof-covering assembly and the material and equipment loads that will be encountered during the installation of the system.’ The NRCA Roofing Manual: Membrane Roof Systems – 2019, Chapter 9 – Reroofing states, ‘The structural integrity of the roof assembly must be maintained during reroofing operations, including loading on the roof attributable to workers and material being present during this special time.’ It is common in reroofing projects for the project specifications to include specific provisions to avoid overloading and structural damage.

There are similarities between this case and failures of reinforced autoclaved aerated concrete (RAAC) planks reported in a May 2019 SCOSS Alert, although the planks of this case were not AAC.

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