In Oklahoma City, at approximately 9:00 AM on April 19, 1995, a truck bomb was detonated on the street right in front of the north side of the Murrah Building, causing extensive damage to the building and various degrees of damage to other buildings in the vicinity of the explosion.
The north side of the Murrah Building, which was exposed directly to the blast, sustained severe structural damage. Most of the north half of the rectangular footprint was destroyed. The damage spread the entire 21.3 m (70 ft) width of the building. Three columns that supported the transfer girder in the third floor were immediately destroyed by the blast, triggering progressive collapse of the upper stories. It was estimated that roughly half of the usable space in the building collapsed. The east end wall suffered significant damage. The blast removed some of the granite panels of the infill walls from the third through the sixth floor. Some the granite panels in the seventh floor were fractured.
Other panels randomly failed in flexure due to inward or outward pressure. All granite panels in the west end wall remained in place but some failed in flexure due to outward or inward pressure from the blast. Damage to the south side of the building was mild and limited to failure of glazing and door frames.
The structure was designed as a reinforced concrete ordinary frame in accordance with ACI 318-71. A building performance evaluation team assembled by the Federal Management Emergency Agency (FEMA) determined that the design was adequately performed following the existing codes at the time and that the building was very well detailed. In agreement with the governing codes, the building was not designed to resist blast, earthquakes, or any other type of extreme loading condition. It was estimated that the explosion had a yield equivalent to approximately 1.8 Mg (4,000 lb) of TNT and that the bomb was located approximately 4.9 m (16 ft) from one of the columns in the north side of the building. The blast immediately removed this column, which supported the transfer girder of the third floor.
The investigating team determined that an ordinary moment frame could not allow for the redistribution of load that resulted from the removal of a first floor column. The assessment team found during the investigation that the removal of a column in the first floor would redirect additional loads to the laterally adjacent columns, exceeding their yield moment and shear capacity. The loss of these columns would leave the transfer girder only partly supported.
It was concluded that an ordinary moment frame could not sustain the level of ductility required to redistribute the loads with three columns missing in the first story. The investigating team suggested that if more recently developed detailing, such as those present in special moment frames used in seismic regions had been in place, the collapsed area would have been reduced at least by 50 % and at most by 80 %.
Oklahoma City, OK, April 26, 1995 -- Search and Rescue workers gather at the scene of the Oklahoma City bombing. (Photo Credit: FEMA News Photo)