A CASE STUDY ON A PARTIAL COLLAPSE OF A BUILDING WITH LIGHT GAGE STEEL FRAMING SYSTEM

Ibrahim Erdem, Ph.D., P.E., S.E., M.ASCE,1 and David B. Peraza, P.E., M.ASCE,2

1 Senior Engineer, Exponent, Inc., 420 Lexington Avenue, Suite 1740, New York, New York 10170; Telephone: (212) 895-8112; e-mail: ierdem@exponent.com

2 Principal Engineer, Exponent, Inc., 420 Lexington Avenue, Suite 1740, New York, New York 10170; Telephone: (212) 895-8103; e-mail: dperaza@exponent.com

Abstract

This paper presents a partial collapse of a building that occurred during construction. The subject building was a five-story residential building measuring approximately 48 feet by 80 feet in plan. The structure of the building consisted of concrete floors on metal deck supported by exterior light gage steel stud walls and interior steel beams and columns. The collapse occurred when the concrete placement for the fourth floor was being performed. Approximately half of each of third, fourth and fifth floor collapsed, causing a fatality. The authors were retained immediately after the collapse on behalf of the owner and the general contractor to investigate the cause of the collapse.

This paper presents the extent of the collapse and the deficiencies in the design and the construction of the light gage steel stud wall system. This paper also provides recommendations to the owners, engineers and contractors to prevent the occurrence of similar building collapses.

Introduction

Light gage, cold-formed, steel has been a popular construction material for several decades for construction of both residential and industrial structures due to its light weight, ease of construction, various shapes and suitability for versatile applications. The most commonly used light gage steel products include corrugated metal deck, walls and columns consisting of studs, floor/roof joists and beams. When light gage steel studs are used in construction of exterior walls, they can be multi-purpose, a bearing wall for the floor, a part of the lateral system and support for building cladding.

The design of light gage steel products in the US is done according to the design standards and specifications prepared by American Iron and Steel Institute (AISI), which are also used in the formation of the design standards used in the other countries. If light gage steel members are not designed using one of the design standards, or if light gage steel members are not properly constructed, significant structural issues may arise, which then may result in partial or total collapse of the structure.

This paper presents a partial collapse of a building with light gage steel framing that occurred during construction, causing a fatality. The authors of this study were retained immediately after the collapse on behalf of the owner and the general contractor to investigate the cause of the collapse. This paper presents the extent of the collapse and the deficiencies in the design and the

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construction of the light gage steel stud wall system. This paper also provides recommendations to the owners, engineers and contractors to prevent the occurrence of similar building collapses.

Building and Collapse

The subject building was a five-story residential building, rectangle in plan measuring approximately 48 feet by 80 feet. The structure of the building included concrete floors on corrugated metal deck, supported by light gage steel joists that were supported by exterior light gage steel stud walls and interior hot rolled steel beams and columns (Figure 1). The first floor exterior walls were concrete masonry unit (CMU) walls. The exterior light gage steel framing were supported on hot rolled steel beams on the second floor and light gage header beams on the upper floors. The header beams consisted of two C-shape light gage steel joists oriented to form a box shape. The exterior light gage steel frames included box shaped posts, formed by pairs of studs, along the wall openings and individual studs in between. All connections in the light gage steel framing were made by steel screws and powder-actuated fasteners.

Figure 1. Typical structure of the building

Prior to the collapse, the steel framing of all floors, except for the roof framing had been completed, the CMU walls had been installed and the first and second floor concrete slab had been installed. The plan was to install concrete on the fourth floor slab, and then to install concrete on the third floor. It is not clear why this sequence was selected. The collapse occurred when the concrete placement for the fourth floor was being performed. Approximately half of each of third, fourth and fifth floor collapsed (Figure 2 through Figure 4). Immediately following the collapse, the remaining portions of the building were shored to prevent further collapse.

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Figure 2. Overall view of the collapse.

Figure 3. Overall view of the collapse.

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Figure 4. Extent of the collapse (outlined) on floors 3 through 5.

Deficiencies in Design

Initial review of the construction documents, including structural and architectural drawings filed with the department of buildings revealed numerous problems with the design of the exterior light gage metal walls:

Inconsistency in gage and spacing of the exterior wall studs: While the notes on the architectural drawings specified the exterior wall studs as being 16-gage and spaced at 16 inches, the architectural wall sections specified them as being 18-gage and spaced at 16 inches. Moreover, the structural drawings called them differently. The wall stud schedule called for 14-gage studs on the second and third floors and 18-gage studs on the upper floors, and specified a spacing of 12 inches. However, the structural wall sections showed the exterior wall studs as 18-gage and spaced at 16 inches. The as-built studs, on all floors, were 16-gage and spaced at 16 inches.

Non-alignment of wall studs and floor joists: The structural drawings required that the studs align with, and therefore land on, the floor joists. However, the structural drawings specified a spacing of 12 inches for the wall studs and 16 inches for the floor joists, which would make them unaligned. (Figure 5).

Inconsistency in concrete type: The structural drawings specified the concrete type as both normal weight and light weight.

Inconsistency in thickness of concrete slab and depth of metal deck: The architectural drawings called for 2.5 inch thick concrete over 2 inch metal deck. However, the detailed section called for 2 inch concrete over 2.5 inch metal deck. Moreover, the structural drawings specified 3.25 inch thick concrete over 1.5 inch metal deck. The structural

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drawings called for 18-gage deck. The as-built slab was 2.5 inches over a 0.5 inch deck. The as-built metal deck was only 24-gage.

Improper design loads: The structural drawings indicated the wind and earthquake loads were calculated using an incorrect building code.

Lateral system undefined: The structural drawings did not specify the lateral system. However, a note incorrectly indicated it is masonry shear walls.

Incorrect gravity loads: The dead load schedule given in the structural drawings was unrelated to the project.

Unrelated details: The structural drawings included several light gage metal construction details which were unrelated to the project.

Missing detail: The attachment of the exterior stud wall to the second floor framing was not provided.

Deficiencies in Construction

The observations of the authors revealed the following key deficiencies in the construction of the exterior light gage stud walls:

Stud size and spacing: As explained in the previous section, there was inconsistency in the design drawings. The as-built studs did not meet the more stringent requirements on the wall stud schedule.

Slab thickness and metal deck: As mentioned previously, the design drawings were internally inconsistent. The 0.5 inch, 24-gage metal deck that was installed, did not comply with the more stringent requirements. The concrete over the deck was only 2 inches thick, which was not adequate for fire rating.

Wall studs were not aligned with the floor joists: Although the as-built stud spacing of 16 inches matched the joist spacing, the contractor did not align them. Instead, the load-bearing studs landed on a track member between the joists (Figure 5 and Figure 6).

There was no blocking at the joist ends (Figure 5).

No stiffeners had been installed in the headers.

Lack of stiffeners under the posts. Figure 6 illustrates an example.

Studs were attached to only one side of the track (Figure 7).

Studs did not bear at the top, i.e., there was a gap as much as 0.25 inch between the studs and the top tracks.

There was no lateral bracing between the studs (Figure 5).

Inadequate headers: Although the structural drawings required 12-gage joists for the headers, 16-gage joists were used to build the headers.

Joists which were not attached to the joist hangers were present at several locations.

Parts of the metal deck were fastened to the joists with pins instead of screws.

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Figure 5. Wall studs did not align with the floor joists. No bearing stiffeners were installed. No blocking was installed between the joist ends. Missing lateral bracing between the studs. The photograph was taken on the side of the building that did not collapse.

Figure 6. Lack of bearing stiffeners under the jamb post and under the joists. The photograph was taken on the side of the building that did not collapse.

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Figure 7. At many locations, only one flange of the studs was attached to the track member. Twisting of studs was obvious. The photograph was taken on the side of the building that did not collapse.

Other Deficiencies

Shop drawings for the light gage stud walls were required by the structural drawings. However, they were never prepared or submitted to the structural engineer.

Special inspections for the light gage stud walls were required by the structural drawings and the building code. However, the inspections were never performed. The engineer of record had also been retained to provide the special inspections. It is not clear whether the owner notified the inspector that construction had commenced, or whether there was another reason why the inspections were not performed.

Cause(s) of the Collapse

There were numerous significant errors, both in the design and the construction. Also, although many significant construction deficiencies were observed on the side of the building that did not collapse, the extent of these deficiencies on the portion that did collapse is not entirely clear. Because of this, it was not possible to determine with certainty which was the primary cause of the collapse, and which were contributing factors. The following are possible causes and contributing factors:

Lack of lateral system, allowing a sideway collapse.

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Studs with a thinner gage than what was specified, larger spacing than specified, and lack of bracing, which led to buckling of the studs.

Poor detailing at the joist-to-header and header-to-stud connection, which allowed to wall to fail in out-of-plane hinge mechanism.

Fourth floor being installed prior to third floor, which, together with the deficiencies in the joist and stud connections, eliminated the diaphragm action at the third floor level.

Lack of stiffeners under the posts and studs, which overloaded the track members.

Incomplete attachment of studs to track members.

Lack of stiffeners in the box headers, which reduced the buckling strength of the headers.

Lack of blocking at the joist ends, which allowed the joists to rotate.

Construction vibration during concrete installation, which might have triggered the collapse.

Use of normal weight concrete instead of light weight concrete, which might have triggered the collapse.

Lessons Learned

Light gage steel framing has some unique vulnerabilities and unique requirements, which may not be readily apparent to engineers and contractors who are not accustomed to working with this system. This paper has highlighted some of those.

This case study illustrates that design deficiencies, inconsistencies in design drawings, improper construction practices, and/or lack of coordination between the engineer and the contractor may result in serious consequences, such as a structural collapse and fatality.

It also illustrates the importance of performing inspections in a timely manner for light gage construction, preferably by the engineer of record. Construction of light gage framing is sometimes performed by workmen who may not be experienced with this type of construction. Many of the deficiencies that we observed were visually obvious and would have been immediately identified by a qualified inspector.

In order to prevent such events, all parties involved in the construction, including the owner, developer, architect, structural engineer and contractors, should fulfill their responsibilities. The owner and the developer should engage qualified parties in the development, coordinate all the parties during the construction phase to eliminate misunderstandings and have the construction inspected according to the building code requirements. The design consultants should prepare drawings which are consistent, clear, complete and free from unnecessary information, in other words ready for the production of shop drawings without a need for interpretation.

The lack of lateral system can be catastrophic. The lateral system of a structure should be clearly shown on the design drawings and properly designed and detailed using appropriate codes and requirements. The lateral system should be maintained during erection of the structure as well.

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The light gage steel members are vulnerable to buckling, twisting, crippling and hinge mechanisms if adequate bracing, blocking, stiffeners and fasteners are not provided. These types of failures prevent the use of full capacity of the structural members and may occur suddenly without any warning. The design team and the construction team should be experienced in the design of the light gage steel systems. Moreover, there are many technical resources on design and construction of light gage steel system, which need to be promulgated more widely in order to minimize the probability of future failures.

This case study also illustrates that the details, especially the connection details of the light gage stud walls is vital and when proper attention is not paid to the design and construction of the connections, undesired consequences such as local or total collapse may occur. With adequate design drawings, generally accepted construction practices, supervision and inspections, the connections in the light gage steel system can be achieved satisfactorily, and the premature failures can be prevented.

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