Skyline+Plaza+at+Bailey's+Crossroads

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**Introduction**
The Skyline Center is a large complex located in Bailey’s Crossroads, Virginia which includes eight apartment buildings, six office buildings, a hotel, and a shopping center. On March 2, 1973, one apartment building (A-4) and the parking garage adjoining it collapsed. The incident occurred at around 2:30 in the afternoon and resulted in the death of 14 construction workers and the injury of 34 others.

It is concluded that the improper removal of forms supporting the 23rd floor resulted in shear forces which could not be handled by the current capacity of the recently poured concrete. Therefore, the trigger mechanism of the collapse was shear failure around a number of columns on the 23rd story. Without the support of these columns, other columns on that story were overstressed which ultimately led to the collapse of the entire 23rd floor slab onto the floor below. The increased loading on the 22nd floor from the weight of the collapsed floors above was too great and led to a progressive collapse all the way to the ground level. (Delatte 2009)

Project Team
Owner: Charles E. Smith Companiestoc Architect: Weihe Black Jeffries & Strassman Structural Engineer: Heinzman, Clifton, and Kendro General Contractor: Charles E. Smith Construction Concrete Subcontractor: Miller & Long Construction (Ross 1984)

Design and Construction
Building A-4 was a reinforced concrete structure with flat plate floor slabs. It was designed as a 26-story apartment complex with a four-story basement and a penthouse level. All floor slabs were 8" thick and the floor-to-floor height was 9'-0". The design strength of the concrete columns and floor slab is listed in the table below: Figure 1: Design Concrete Strength of Columns and Slabs
 * = Design Concrete Strength ||
 * < Floor: ||< Base-7th ||< 7th-17th ||< 17th-Top ||
 * Column || 5000 psi || 4000 psi || 3000 psi ||
 * Slab || 3000 psi || 3000 psi || 3000 psi ||
 * Slab || 3000 psi || 3000 psi || 3000 psi ||

Both the engineer and architect specified to the contractor that each slab being poured must be shored at least two stories below.

Due to the great length of the building, each floor slab was to be poured in four sections. At the time of the collapse, sections 1 and 2 of the 24th floor had been poured. Section 3 was in the process of being poured and section 4 had not yet been poured. Based upon statements from construction workers and photographs taken, it was concluded that the forms for section 1 and 2 on the 22nd story had been removed prior to the collapse. The forms for section 3 on level 22 (two floors below the location of the current pour) were being removed at the time of the collapse.

There were two cranes used to erect the building, one in section 2 and the other in section 4. At the beginning of construction, these cranes were supported on the 4'-0" thick mat foundation. As construction continued, both cranes were lifted in order to complete the upper levels of the building. On the day of the collapse, the base of the crane is section 2 was located on the 20th floor while the base of the crane in section 4 was located on the 14th floor. (Leyendecker 1977)

Analysis of Failure
On March 5, 1973, three days following the collapse, the Center for Building Technology of the National Bureau of Standards was called in to investigate the collapse of Skyline Plaza. The focus of the investigation was to determine the cause of the collapse. A three-dimensional finite element analysis was conducted on the 22nd and 23rd floors to determine the magnitude of forces exterted on the floor slabs and whether the slabs could properly handle those forces. For completeness, an analysis of three seperate cases was performed in order to cover all possible conditions at the time of the collapse.

Case I: All forms on the 22nd story were removed before the collapse. This essentially means that the 23rd floor slab carried its own weight, the weight of the 24th floor slab, and the weight of the forms underneath the 24th floor slab. The strength of the concrete on the 23rd floor slab used for this calculation was 1200 psi.

Case II: Assumed that the concrete on the 23rd floor slab reached its design strength of 3000 psi.

Case III: Only some of the forms on the 22nd story were removed which results in both the 22nd and 23rd floor slabs sharing the load from above. The strength of concrete on the 22nd floor slab used for this calculation was 1340 psi.

Upon completion of the analysis, it was determined that moments in the column strips of the slab were not great enough to cause failure. On the other hand, the analysis did show that for case I and III column #67, 68, 83, and 84 all experienced shear stress greater than the shear capacity of the concrete slab. This indicates that the partial or complete removal of forms was a major contributing factor to the collapse. The analysis of case II shows that the shear stress in the slab did not exceed the design capacity. This result confirms that the strength of the 23rd floor slab was below the design strength of 3000 psi at the time of the collapse. (Leyendecker 1977)

This type of failure is highly undesirable because it usually strikes without much warning. It also very easily can lead to progressive failure which is defined by the Portland Cement Association as "the local failure of a primary structural component which leads to the collapse of adjoining members which in turn leads to additional collapse." (Polak 2005) In the case of Skyline Plaza, the failure of the local columns on the 23rd floor due to punching shear initiated the failure of floor slabs below it and eventually created a total failure much greater than the original failure.

Non-Compliance Issues
Along with performing a thorough analysis of the structure, the Center for Building Technology of the National Bureau of Standards was asked to assist in the determination of any non-compliance of OSHA regulations during the design and construction of Skyline Plaza. Several infractions were discovered but some of these were not labeled as contributing factors to the collapse of the structure.

According to OSHA regulations, forms are required to be in place for a minimum of 10 days with temperatures greater than 50 degrees F for spans longer than 20'-0". Violation: Areas on the 22nd story had spans greater than 20'-0" but forms on that level were removed before 10 days at 50 degrees F.

According to OSHA regulations, concrete specimens are to be tested in order to confirm that the concrete has obtained the required strength to handle the loading placed upon it. Violation: No specimens were tested.

According to OSHA regulations, bracing and shoring must be designed to handle lateral loads. Violation: (2) nominal 3x4 braces at 16'-0" O.C. were not capable of handling the lateral loads.

According to OSHA regulations, shoring is only permitted to be out of plumb by 1/8" per 3'-0". All damaged or weakened shoring must be removed and replaced. Violation: Shoring on the 23rd and 24th floors exceeded this limitation. They were also not removed.

According to OSHA regulations, an inspection is required before, during, and after the placement of concrete. Violation: No inspection was completed.

According to OSHA regulations, the distance between top and bottom supports of the crane shall not not exceed 18'-0". Standard tower sections used shall not exceed four as recommended by the manufacturer. Violation: Both cranes had a distance of 18'-4" between the top and bottom supports. Crane 2 used five tower sections causing it to exceed the height limitation of 81'-0".

The OSHA violations which contributed to the collapse include the early removal of forms, the lack of specimen testing, and the lack of inspection by the designer. The other violations, while unacceptable, were not major contributors to the collapse of Skyline Plaza. (Leyendecker 1977)

Conflicting Accounts
The only conflicting account surrounding the Skyline Plaza collapse is whether the forms on the 22nd floor had been completely removed at the time of the collapse. Three different responses were obtained through interviews with workers on site. Some believed that all the forms were removed. Others thought that the forms were partially removed. A third group thought that no forms were removed at the time of the collapse. This discrepancy was accounted for by the different cases in the analysis. (Leyendecker 1977)

Preventing Failure and Lessons Learned
Based upon the analysis of case II it is concluded that this collapse could have been prevented if the shoring remained until the concrete reached its full design strength. (Leyendecker 1977)

The tragedy of Skyline Plaza taught the building industry some important lessons.

1. Redundancy within structural design is essential to prevent progressive collapse. (Feld 1997) 2. Construction loads must always be considered during design. There are many instances when these loads will control the design. (Ross 1984) 3. Formwork and shoring needs to be detailed by the contractor. 4. Concrete testing must be performed before the removal of shoring. 5. Inspections must verify that the contractor is properly shoring floors above and that poured concrete is meeting its design strength. (Kaminetzky 1991)

The prevention of progressive collapse is still a big concern for structural engineers. There are now design methods for concrete structures that can be employed to limit the possibility of universal progressive failure. In slab design, it is now encouraged to place rebar continuously through the slab-column intersection at the top and bottom of the slab. If the slab fails in punching shear, the bottom bars act as a catenary and prevent the collapse of the slab onto the structure below. Other provisions include casting the concrete slab monolithically with beams and not splicing reinforcement at midspan or end of slab. (Dusenberry 2007)

Responsibility
The Skyline Plaza collapse became a major landmark in the debate concerning responsibility during construction. At the end of the legal proceedings it was the design engineers and architects who were found guilty of negligence. The general contractor and concrete contractor had some liability but it is ultimately the responsibility of the designers to visit the job site and to warn the contractors of any possible pitfalls due to unforseen environmental conditions. So even though the contractor did not comply with the shoring requirements specified in the construction documents, the collapse was still found to be the fault of the designers. (Feld 1997)

Industry Response
Following this failure, the Portland Cement Association (PCA) and the Prestressed Concrete Institute both issued new design guides with provisions included to prevent progressive collapse. In November of 1974, the ACI Journal reinforced the importance of designing for construction loads as well as normal design loads. This accident also made designers aware of the importance of site inspections and that it is still the responsibility of the designer to make sure the building is constructed properly. All of these changes have helped to prevent the frequency of these kinds of failures. (Ross 1984)

Of course, there is always a chance that a massive failure like Skyline Plaza could happen again. Architects, engineers, and contractors must always be vigilant when it comes to the design and construction of buildings. It is never guaranteed that all legal requirements will be followed on a jobsite or in a design office. However, with improved codes and sound judgement these failures can be easily prevented.