What determined who lived or died in the Cypress Street Viaduct collapse?

Being born three years after this earthquake, the following writing represents my best analysis of the data I have.

On October 17, 1989 at 5:04 pm, a 6.9 magnitude earthquake struck the San Francisco Bay Area in Northern California. An approximately 3/4 mile section of the double-decker Cypress Street Viaduct in Oakland collapsed within 4-6 seconds of the start of shaking. At that moment, 162 people were driving or riding in vehicles in the collapsing section. 85 were on the upper deck and 77 were on the lower deck. A total of 42 people died, and 120 survived, for a death rate of 26 percent.

The freeway was divided into spans, with "bents" - joined by "bent caps" that protruded a few feet from the bottom of the upper deck. The span corresponded to the bent immediately to its south; for instance, Bent 80 joined Spans 79 and 80. The spans and bents were numbered south to north. Span 62 marked the southern end of the collapsed section, and Span 112 was the northern end, for a total of 51 spans collapsed. Each span was generally 72-88 feet long.

For reference:

~Span 63 - 18th St.
~Span 70 - 20th St.
~Span 77-82 - Peralta
~Span 84-87 - 24th St.
~Span 89-92 - Campbell
~Span 96 - 26th St.
~Span 102 - 28th St.
~Span 107 - 32nd St.

Deck - The most important determinant of survival was the deck that one was on. On the southbound upper deck, 7 people died and 78 survived (death rate: 8%), whereas on the northbound lower deck, 35 people died and 42 survived (death rate: 45%).

Seat Belts - None of the 46 people who were wearing lap and shoulder (3-point) seat belts on the upper deck died. Of the 19 unrestrained occupants, 2 died (death rate: 10%), while 5 of the 20 occupants wearing lap belts died, all in the same 1985 Dodge van. Since the damage to this van was primarily frontal and both occupants in the front seats, who were wearing 3-point seat belts, survived, it seems likely that the five occupants who died would have survived had the van had 3-point seat belts in all positions. Most of the vehicles on the collapsed freeway had only lap belts in the rear seats, due to the inexcusably late date of December 1989 that the federal government mandated 3-point seat belts in the rear seats.

On the lower deck, your odds of survival were much higher if you were wearing a seat belt. 7 of the fatalities on the lower deck were in vehicles that were so badly crushed that it could not be determined if the occupant was restrained, while 3 people survived but it could not be determined if they were restrained (two were severely injured small children).

Of the 29 occupants known not to be using their seat belts, 17 died (death rate: 59%), whereas of the 38 occupants known to be using seat belts, 11 died (death rate: 29%). In one vehicle, four people survived and one died; the vehicle had a frontal impact so severe that the occupants in the front seat had to be rescued by firefighters, but the lone fatality was in the rear middle seat and was the lone occupant not wearing a seat belt. This means that your odds of dying on the lower deck were halved if you were buckled up. But not eliminated completely...

Type of Impact (on lower deck)- 53 of the 58 vehicles on the lower deck had an impact that fell into one of three categories. 20 of them drove into a frontal impact with a falling bent cap, 20 were crushed by a falling span with no bent cap involvement, and 13 were crushed by the bent cap itself. Three vehicles sustained only minor damage (see below) and two hit the leading edge of the collapsed section in Span 62.

There were no fatalities in the two vehicles with minor damage or the two that ran into the leading edge of the collapsed section. Of the 70 vehicles that were directly crushed, 35 occupants died; exactly half.

Those that drove into impact with a falling bent cap fared best, likely due to the bent cap's ~4 foot height having a tent-pole effect and preventing a degree of roof crush: many of the vehicles that hit a falling bent cap were only crushed down to the headrests. Still, one-third of people in this scenario died.

Being crushed in a span often meant a worse degree of roof crush; many of those vehicles were crushed down to the dashboard or lower, one particularly bad example was a vehicle crushed down to the frame rail.

While hitting the southern edge of a bent cap often proved beneficial to survival, being directly crushed by the bent cap was often a death sentence; these vehicles' occupant compartments were often obliterated. Just one driver out of 13 survived; her car's engine was directly crushed, but the cap itself narrowly missed the occupant compartment. The other two survivors were the Berumen siblings in the rear seat of their mother's car.

In one of the most famous stories of the earthquake, a mother (Petra Berumen) and her friend (Yolanda Orozco) were killed in the front seats of a Ford crushed by a bent cap. The back seats escaped destruction, and the mother's two children (Julio and Cathy Berumen) survived, although one's leg had to be amputated to extricate him. It's possible that the presence of Yolanda saved one of the children's life. Prior to the mid-1990s, when only one parent was present in the vehicle, a child was often able to sit in the front seat.

Frontal impact with falling bent cap: 11 out of 33 occupants killed (death rate: 33%).
Vehicles crushed in span: 11 out of 21 occupants killed (death rate: 52%)
Vehicles crushed by falling bent cap: 13 out of 16 occupants killed (death rate: 81%)

Span (on lower deck) - Obviously the best area to be in was the second half of 95 or the entirety of 96, a 1 1/2 span section that was the only section out of the 51 that did not collapse. Spans 72 to 79 were only collapsed on the east side. Sadly, only three vehicles were in these areas; all of their drivers were uninjured. Span 62's four survivors all hit the southern end of the collapsed section; they weren't directly crushed.

Anywhere else on the collapsed section, and the fate was far worse. Spans 80 to 89 were relatively "lucky", in this 10-span section, 12 survived and 4 were killed, for a death rate of 25%. Spans 102 to 112, the northernmost section of the collapsed area, was another relatively lucky section with 10 survivors and 5 fatalities for a death rate of 33%.

The spans in the 90s were bad, with the exception of 95-96; 90 to 94 had 2 survivors and 4 fatalities for a death rate of 67%. 97 to 99 were even worse, with 2 survivors and 6 fatalities for a death rate of 75%.

The spans from 63 to 67 had the worst death rate of all. Every single driver who was in this section was killed - regardless of whether they were wearing seat belts or not, the type of impact they had, or any other variable. 11 people were killed and just 2 survived - the Berumen siblings.

Blind Luck - A Honda Accord in span 98 was crushed to a degree that there was only one foot of space between the roof and the bottom of the driver seat. The driver lived.

The driver of a Datsun in Span 97 received only minor injuries despite the fact that his car was crushed to dash level. He was found laying across the front seats.

Another, even luckier survivor was in a 1984 Pontiac 2000 (a badge-engineered Chevy Cavalier) that was crushed to approximately two feet in height. Since the height of a 1984 Pontiac 2000 is 53.6 inches, this means that the car was crushed to less than half its original height. The Pontiac was in Span 90; Spans 89-92 was one of the initiating failure sites, and the Pontiac's driver stated that he had no indication that a collapse was going to happen. The driver sustained only minor injuries and was released from the hospital that same evening.

Here are the applicable reports:
https://archive.org/details/lomaprietaearth1990cali_0
https://archive.org/details/lomaprietaearth1990cali_1
https://archive.org/details/lomaprietaearth1990cali_2
https://archive.org/details/lomaprietaearth1990cali_3
https://archive.org/details/lomaprietaearth1990cali_4
https://archive.org/details/lomaprietaearth1990cali_5


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