During a concrete placement on the second story of a building under construction, the cold-formed steel beams supporting the decking and concrete collapsed. Four workers were injured, one fracturing his hip. Approximately two-thirds of the deck had been placed. The project structural engineer had been at the site earlier but had left before the collapse.
The collapse occurred while concrete was being placed onto steel decking on the second floor of the structure. The steel decking was supported by 203-mm (8-in.) deep cold-formed 1.21-mm (0.0478-in., 18 gauge) steel beams without shoring. Some of the workers raised concerns about the safety of the structure with the project structural engineer. He assured the contractor and workers that shoring was unnecessary and that the beams were rated for more than enough capacity to support the concrete.
The testimony of the workers and the photographs available indicated that good construction practices were followed with respect to placing and finishing the concrete. The project structural engineer contended that the failure occurred because workers allowed the concrete to form a pile on the formwork, thus increasing the loading.
Immediately before the collapse, concrete was being placed from a pump onto the decking. An experienced worker was using the pump nozzle to spread the concrete. The workers started at one end, moving toward the other end of the second floor. One worker claimed that the deck was vibrating during the concrete placement.
When approximately two-thirds of the concrete had been placed, the decking on the longer 4-m (13-ft) span gave way suddenly, and five of the workers fell. Two workers were able to grab wire mesh and avoid falling the entire distance. The others fell onto the first floor. One fell onto a plumbing fixture pipe and broke his pelvis. Photographs taken immediately after the collapse showed that the beams were bent at the interior wall support and at about the midpoint of the longer 4-m (13-ft) span. The damaged beams hung downward from the interior wall.
The structural integrity of the beams and decking was questioned, but the structural engineer provided assurances that they were adequate. No supporting documents were available.
The structural engineer contended that the collapse occurred because of poor construction practices, leading to concrete piling up and causing unbalanced loading. However, the testimony of the workers indicated that the concrete placement was carried out in accordance with good practice. There was no testimony from the workers or observers present that the workers allowed the concrete to pile up at any point on the decking. In fact, this would have made screeding and finishing the concrete much more difficult.
It is well known that the structural integrity of formwork for concrete is important. Hanna writes, “Partial or total failure of concrete formwork is a major contributor to deaths, injuries, and property damages within the construction industry” (Hanna 1999, p. 6).
The investigation was hampered because it was not possible to access the site and because the failed structure had been removed before the investigation started. Unfortunately, material samples had not been retained. Therefore, it was necessary to analyze the failure solely from the available documents and records.
There was only one engineer qualified by training, experience, and professional licensure on the project. The structural engineer should have consulted the proper references and performed the necessary structural calculations to ensure that the structure would be safe against collapse, under the load combinations prescribed by building codes. He should have analyzed the beam under an unbalanced load of concrete and live load and compared the calculated moments to the section capacities provided in the AISI manual (AISI 1996a).
There are important differences between design procedures for hot-rolled structural steel, which are taught in most civil engineering undergraduate programs, and those for cold-formed steel. Designing with cold-formed steel requires a knowledge of failure modes, such as local buckling, that can often be safely ignored with hot-rolled steel. Engineers designing with this material should take care to obtain the proper codes and design documents.
This case study is discussed in Chapter 6 of the book Beyond Failure: Forensic Case Studies for Civil Engineers, Delatte, Norbert J., ASCE Press.
References:
American Iron and Steel Institute. (AISI). (1996a). Cold-Formed Steel Design Manual, AISI, Washington, D.C. with errata.
American Iron and Steel Institute. (AISI). (1996b). Specification for the Design of Cold-Formed Steel Structural Members, Washington, D.C.
Delatte, N. J. (2005). “‘Failure of Cold-Formed Steel Beams during Concrete Placement,’ technical note.” J. Perf. of Constr. Fac., 19(2), 178-181.
Hanna, A. (1999). Concrete Formwork Systems, Marcel Dekker, Inc., New York.
Illustration from Chapter 6 of the book Beyond Failure: Forensic Case Studies for Civil Engineers, Delatte, Norbert J., ASCE Press.
