CROSS Safety Report
Inappropriate material storage causes collapse of ceiling of a truck dock mezzanine
This report describes the collapse of an overloaded framed ceiling.
It relates that the ceiling above a mezzanine at a truck access dock was inappropriately used for material storage and caused it to collapse.
Key Learning Outcomes
Consider discussing the use of the structure with the building’s owner, especially when the intended use of a space may be unclear or subject to misinterpretation
Design strategies for avoiding overloading of structures include (1) functional design that precludes access to the space, (2) direct design for worst-case loads, and (3) posting load limits
For structural design engineers:
Consider, within reason, any foreseeable conditions that may exceed the code-prescribed minimum requirements
The structural engineer, however, is not obligated to design for misuse of the structure
It is good practice to follow design documents and not to improvise
For building owners and managers:
It is good practice to monitor use and loading of your structures
Consult with qualified design professionals before modifying or adding to the structure
For building authorities:
It is good practice to look for ways in which the structure may be subject to overload when reviewing plans and during inspections
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The subject facility is a parts storage building associated with an automobile dealership. A storage mezzanine, which is used to store extra parts, is located adjacent to the exterior wall. A portion of this storage mezzanine is located over the parts receiving garage. According to onsite workers, this receiving garage is normally left open overnight to receive overnight deliveries without allowing access to the rest of the parts storage facility. During the original construction, a wood and gypsum panel ceiling was framed approximately 5 ft (1.5 m) above the mezzanine elevation for the purpose of air-sealing the parts storage building when the receiving garage is open. The ceiling is located above the mezzanine level to permit a delivery truck access into the receiving garage and is constructed using 2x4s (4x9 cm) ceiling joists at 2 ft (61 cm) on center supporting a plywood top and a gypsum board bottom. This framing was nailed using wood nails to the exterior wall. Over time, this ceiling adjacent to the storage mezzanine was utilized for additional parts storage.
Eventually this ceiling collapsed, separating from the exterior wall and causing the ceiling and the parts stored above the ceiling to fall to the floor below. Unfortunately, the collapse led to the hospitalizations of two workers, who were located under the ceiling at the time of the collapse.
Eventually this ceiling collapsed, separating from the exterior wall and causing the ceiling and the parts stored above the ceiling to fall to the floor below.
Designers need to be cognizant of how spaces will be used and especially careful when heavy live loads are involved. In this case, extending the wall up to underside of the roof structure would have prevented the top of this ceiling from being used as storage at a minimal additional cost.
Expert Panel Comments
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It is unclear from the report whether a structural engineer or other design professional specified the subject ceiling. Regardless there are useful lessons here for design professionals, code officials/inspectors, and building owner/operators.
Designs must meet the minimum requirements of the applicable code. While the International Building Code (IBC 2018) has specific live load requirements for ‘attics’ of residential structures, there are no similar provisions for storage warehouses, as typical warehouses do not have attics. Building codes specify minimum requirements and, where necessary, it is incumbent on the design professional to consider reasonably foreseeable conditions that may exceed the code-prescribed minimums. (ASCE/SEI 7-16, Para. 4.3.1 states, ‘The live loads used in the design of buildings and other structures shall be the minimum loads expected by the intended use or occupancy but in no case be less than the minimum uniformly distributed loads required by Table 4.3-1.’ This begs the question of what conditions are reasonably foreseeable and what are not. The structural engineer is not obligated to design for abject misuse of the structure. A 2017 Structure magazine article Failure of Imagination discusses the dilemma of misuse and design for unforeseen conditions.
The structural engineer is not obligated to design for abject misuse of the structure
Where the intended use of a space may be unclear or subject to misinterpretation, it is advisable for the design professional to discuss the use of the structure with the building’s owner. In this case the inclusion of plywood atop the ceiling joists may have suggested to a user that the ceiling was intended for storage. Safeguards can include:
Arranging the structure to preclude overloading. In this case that might have included walling off the space above the ceiling as suggested by the reporter.
Designing for a maximum likely foreseeable load under the intended use.
Posting signs prohibiting unintended loading or stating a maximum design capacity. OSHA 1926.250(a)(2) requires, ‘Maximum safe load limits of floors within buildings and structures, in pound per square foot, shall be conspicuously posted in all storage areas, except for floor or slab on grade. Maximum safe loads shall not be exceeded.’ A risk with sign postings is that they might be removed or ignored.
Where the intended use of a space may be unclear or subject to misinterpretation, it is advisable for the design professional to discuss the use of the structure with the building’s owner
Problems may occur if owner agreements and understandings made at the design stage are not passed on to the building’s managers and operators. Code officials and inspectors should be on alert for possible misunderstandings as well. Regardless of the above, building manager/operators should monitor use and loading of their structures.
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