Failure: Precast Beam Collapse
Location: Pittsburgh, Pennsylvania
Year: 1990
Type: Development length of rebar was too short
Deaths: 1

 

terminal

[1] Midfield Terminal as it stands today

On August 14, 1990, workers at the Pittsburgh Midfield Terminal were placing a concrete floor plank on top of a precast concrete beam causing the collapse of the beam. The collapse resulted in the death of one man and the injury of another and shut down the work site for only a couple of days [a].

Investigations immediately followed the mishap as many wondered who was to blame: designers or constructors? When interviewed, the design engineer of the beam, Hanna Ghobrial, said that “the beam was not made according to specifications and had reinforcing rods in the wrong places” [b]. Upon further investigation, Deputy Gilkes of Pittsburgh said that “the accident resulted from the failure of fabricators and inspectors to read the shop drawings in the same way the design engineer and consultant did” [c]. Ultimately, the root of this failure was in the construction of the beam and not in the design.

To understand what went wrong in the design, one must first understand how a reinforced concrete beam is designed. Rebar reinforcement runs throughout much of the tension side of the concrete beam because concrete is weak in tension while steel is very strong in tension. In order for the reinforcement steel to ‘work’ (carry the load it was designed for) it must yield and in order to yield, the concrete must place a certain amount of tension force on the steel reinforcement. The farther a beam is embedded into a concrete beam, the more force the concrete beam places on it–this length of embedment is called the development length as seen in Figure [3]. The development length of rebar reinforcement in a concrete beam is “the shortest distance over which a bar can achieve its full capacity” [d].This is where the Pittsburgh construction workers made a mistake. It was discovered that the reinforcement was embedded a mere 7.5″ into the beam–a much shorter distance than what the development equation in Figure [2] calls for in a beam of this size:

 

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[2] Development length equation for bars in tension depends on a number of factors including the bar size, spacing, and more

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[3] Development length, ld, of a reinforcement steel bar into a concrete beam

Due to the short development length, the precast concrete beam ‘pulled out’ of the wall, collapsing and kill one worker. For a better understand of what development length is please refer to Figure [4]. In this image, the rectangle directly above the Elevation View label is the column and the two rectangles running perpendicular to it are top and side views of the beam. While the image shows a beam as a continuous member on either side of the column, in fact, the columns are the members that extend continuously while the beam must split on either side of the column and anchored safely to the column so that it will essentially function as one whole beam. In reinforced concrete it is common for some reinforcement bars to be shorter than others as the moment capacity (the load the bars need to carry) varies along the beam. Two of the reinforcement bars (the red bars) have a shortened development length (the distance from the dotted line, the face of the column, to the end of the bar) because that is where the moment capacity drops and they are no longer needed throughout the rest of the beam [e]. This is similar to what happened in the Pittsburgh collapse, however, in this figure there are three other reinforcement bars that continue to carry the load throughout the beam while in the Pittsburgh terminal there was not. The lack of reinforcement in the beam caused the beam to pull away from the column causing the ultimate collapse and fatality.

 

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[4] Comprehensive picture of development length, ld

  • [e] Wight, James. Reinforced Concrete Mechanics and Design. 6th Ed. 373-380. Print.