In September 2015, a 47-passenger school bus operated by the Houston Independent School District (HISD) in Texas was struck by a passenger vehicle while traveling on an overpass in the right center lane of the four-lane eastbound South Loop East Freeway (also known as Interstate Highway 610 [IH-610]). The school bus veered to the right, crossed the far right travel lane and shoulder, and hit the bridge rail at a ~28-degree angle. The bus went over the concrete portion of the bridge rail, broke through the metal guard rail along the top of the concrete parapet, and fell ~21 ft (6 m) onto Telephone Road. The fatal accident left a ~30-ft (9-m) long opening in the metal guard rail.
In an accident report released earlier this year, investigators with the National Transportation Safety Board (NTSB) (Washington, DC) noted that the overpass above Telephone Road was constructed in 1970 and consisted of four spans. The bridge rail was as a Type C4 (modified) railing that consisted of a 18-in (0.46-m) high concrete parapet with metal posts and rail that brought the total design height to 3 ft (0.9 m). A 3-in (76-mm) bonded overlay applied to the bridge deck in 1987 decreased the concrete parapet height to 15 in (0.38 m) and the total rail height to 33 in (0.84 m). Typical horizontal spacing of the metal rail posts is 10 ft (3 m).
The rail posts were anchored to the concrete parapets using base plates with slotted holes and two U-bolts with four hexagonal nuts and steel washers. The posts were seated on elastomeric pads, one pad at some locations and up to three pads at others. The design plans required all metal components of the rail to be galvanized, including the anchor bolts.
The U.S. Federal Highway Administration specifies that bridge rails should meet the crash test criteria contained in NCHRP Report 350, which requires successful tests of a 1,800-lb (816-kg) car impacting a barrier at a 20-degree angle and a 4,400-lb (1,996-kg) pickup truck impacting a barrier at a 25-degree angle at speeds of 62 mph (100 km/h). The total weight of the HISD school bus was ~16,300 lb (7,394 kg). The Type C4 (modified) bridge railing in its designed condition would not be expected to be able to withstand a collision of the HISD school bus.
NTSB investigators reviewed the bridge inspection reports of the overpass and found evidence indicating severe spalling with exposed reinforcing steel at the anchor bolt connections along the south edge of eastbound IH-610 immediately east of the spot where the school bus broke through the bridge rail. Existing repair records from the Texas Department of Transportation (TxDOT) indicated there also had been a previous severe impact to the bridge rail in the same location where the school bus went over. The previous severe impact resulted in significant damage to the concrete parapet and the anchor bolts. There was evidence that the anchor bolts—bent by the impact—were reshaped and reused rather than being replaced.
The previous severe impact also resulted in significant damage at the posts. Repair mortar had been used to patch spalls at the posts affected by the collision. The quality of the repair mortar was inferior in overall quality compared to the original concrete, and had become completely carbonated in some locations, which significantly increased the corrosion potential for the embedded steel. The combination of compromised galvanizing, poor-quality spall repair material, and contaminants ponding around the anchor bolts within the slotted holes resulted in severe corrosion and section loss in the exact location that the HISD school bus hit the guard rail that day. The U-bolts showed substantial corrosion including reduced diameter, particularly in the area near the lower end of the threads.
TxDOT found that rail segments not previously damaged from a major vehicular impact showed no evidence of significant corrosion or reduced capacity from deterioration. Even in areas where previous vehicular impact caused only minor to moderate damage, the galvanizing was still effectively preventing corrosion from occurring in the metal rail components, including the U-bolt anchors. As a result of the crash, all TxDOT districts have been instructed that the proper procedure for repairing damaged guard rail is to install new anchor bolts as opposed to reshaping and reusing damaged bolts.
To view the NTSB accident docket (HWY15FH010), visit ntsb.gov.