Murrah BuildingAPM Memorial

[10,11]

Building: Alfred P. Murrah Federal Building
Location: Oklahoma City, Oklahoma, USA
Year of Construction: 1977 (completed)
Year of Failure: 1995
Architect: Wendell Locke of Locke, Wright and Associates

Overview
Located in downtown Oklahoma City, Oklahoma, USA, the Alfred P. Murrah Federal Building was a nine-story, reinforced concrete frame structure that housed seventeen government agencies and 361 occupants (some of whom were children) [1]. Its construction was completed in 1976 [8]. To capitalize on available natural light, its north facade featured a full-height curtain wall system [1].

The building was the target of an April 1995 domestic terrorist attack for which Timothy McVeigh was convicted and executed in 2001 [2]. At 9:02 am on April 19, McVeigh detonated approximately 4800 pounds of explosive material contained in a truck outside the north facade of the federal building [3]. One hundred and sixty-eight people, including 19 children, were killed in the attack, and hundreds more were injured [4] . Additionally, about 300 buildings in the vicinity were damaged or destroyed [4].

Structural Failure Mechanisms
Most investigations into the failure mechanism of the building agree that the collapse resulted from four columns on the north side of the building being structurally compromised, whether directly from the bomb or from the failure of other structural components. The building’s structural system was found by the FEMA Building Performance Assessment Team (BPAT) to have been in compliance with all the codes and provisions at the time for the design and construction of a reinforced concrete frame structure [7].
The BPAT reported that since the curtain wall offered no resistance to the blast wave, the wave propagated through the building, resulting in differential pressures on the floor slabs [7]. The pressures were greater on the underside of the floor slabs than on their top surfaces, resulting in upward loading [7]. The floor slabs were reinforced only at the bottom and therefore could not resist the upward force [7]. The uplift caused reverse flexural and shear cracking of the floor slabs (see image below) [7].

After the blast wave, gravity loads caused the slabs to experience catenary action (see image below) [7]. As a result, the slabs failed in punching shear at one line of columns [7].

The catenary action also caused the third-floor transfer girder (which was cast in the slab) to rotate and be pulled inward, resulting in the progressive collapse of the building [7]. (A progressive or disproportionate collapse occurs when an event that should cause localized damage instead causes most or all of the structure to collapse, out of proportion to the initial damage [6].) The collapse of these floor slabs is thought to have caused the buckling of two columns (G16, G24) [7].

Some dispute the generally-accepted theory of the bombing, most notably in The Partin Report, which claims that the damage to the building could not have resulted from the truck bomb alone, but would have required separate demolition charges to destroy columns in the building [3].
The building suffered severe structural damage, as is evident in the picture below, and was demolished about a month after the bombing [5].

Murrah Building 2

Robert Hill, PE of the Dallas-based structural engineering firm Brockette, Davis and Drake determined that the main tower of the building was damaged beyond repair [5]. The mode of demolition chosen was implosion. Because implosion depends partly on the structural integrity of the structure being demolished to control its fall, reconstruction operations had to be carried out to strengthen the remains of the Murrah building before demolition could take place [5]. Less than 150 pounds of explosives were used in the implosion of the building, which took seven seconds [5].

Changes to Design of Reinforced Concrete Structures
In 1997, Hinman and Hammond suggested changes to the design of reinforced concrete structures, based on a case study of the effects of the Oklahoma City bombing on the Murrah building [6]. These suggestions focused on preventing progressive collapse. In the case of the Murrah building, Hinman and Hammond assert that only 4% of the building area was destroyed directly by the bomb blast, but 42% of the building was further destroyed due to progressive collapse [6]. They suggest that redundant load paths and “mechanical fuses” should be provided in the structural system, so that elements can fail without causing progressive collapse [6].

Gitanjali Bhattacharjee

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