 It is the afternoon of the 17th October 1989 and thousands line the seats of the candlestick stadium in San Francisco. Millions more have tuned in to watch the 1989 World Series. The streets of the Bay Area are noticeably quieter than the usual afternoon traffic jams as many stay in to watch the game and others stay late for after work viewings. This is because two local teams are preparing to face off against one another. Those on the road, a quicker journey home, must feel like a godsend as any reduction in commuting time is always greatly appreciated. The many in the Oakland area, navigating the Cypress Street viaduct is essential for those wishing to avoid the congested streets below. But just as the relief of less traffic for those travelling must have been felt, disaster would strike as the 1989 Loma Prieta earthquake began to set in. The viaduct would fail under the stress of the 6.9 magnitude earthquake, killing 42 and burning the image of the fallen roadway into the Californian collective memory of the dangers of living near the San Andreas foot line. Today's subject I'm going to rate here 6 on my disaster scale, but here 8 on my historical legacy scale. In the late 1940s the city of Oakland streets were awash with traffic, especially leading to the Bay Bridge along Cypress Street. A plan was conceived to alleviate the congestion issue with a new viaduct that would be built along the same alignment of Cypress Street. The project would also have a side effect of displacing perceived undesirable slums in West Oakland. The plans were settled upon in 1949, but construction wouldn't start for another few years as contracts were set out to tender. The first phase of the project was completed in 1955 at a cost of $1.7 million. It consisted of the central off-ramp of the future viaduct and was completed by Fredrickson and Watson Company. The second phase would involve the construction of the viaduct proper and was designed under the Association of Highway State Officials specifications, but it did not properly take into account seismic activity and the risks in natural loads on the structure. This is because at the time of its design the concept of lateral loads was less understood than today, and due to this, the viaduct followed the Convention of Design at the time. A new contractor was employed for the main part of the viaduct, with work commencing in February 1956 under contractors Grove, Wilson, Shepard and Krug. The viaduct employed two levels, each with four lanes. The upper deck was used by southbound traffic and the lower deck was used by the northbound traffic. Each roadway has a width of 52 feet and is made up of a deck consisting of a seven cell box girder and is four feet six inches deep. The roadways are supported via column structures called vents. Each one is made of concrete with a metal frame. The concrete for the columns was poured in sections and was supported on one-feet diameter pile foundations, which were then themselves poured concrete inside a metal tube, set between 15 and 50 feet into the ground. Each column had between nine and 35 piles and were set into the fieldland on top of Bay Clay. During the construction several different bent styles were employed as the viaduct snaked its way above Cypress The main types of benches used for the construction were named B1, 2 and 3 respectively. B1 employed two portals mounted on top of one another, with shear keys connecting the upper and lower sections. The keys are used to protect the structure from lateral loads. B2 benches use three such keys to the upper deck where it connects to the upper column and another between the upper and lower column on the viaduct's west side. This means that the east side column was continuous from top to bottom and thus it was the only side that offered resistance to lateral forces. B3 only had shear keys at the top of the upper part of the column and the bottom roadway also had a central support. The remainder of the vents were specialist designs for particular sections, for example exit and entrance ramps and therefore only existed in small numbers. The structure had expansion joints roughly every three columns. These allowed the viaduct to flex and move in relation to the loads placed over it during day-to-day operation. The road was officially open to traffic on June 11th 1957. It was at the time California's first double-decker freeway and represented the controversial urban renewal projects throughout the US during the 1950s to 1970s. All would seem good with the new roadway diverting traffic from the inhabited streets of Oakland below. However in 1971 the San Fernando earthquake would make Caltrans reassess all the roadways and bridges under its control and this would lead to an upgrade project for the Cypress Street installation. Part of this would involve improving the expansion joints to limit the amount the structure can move. This came in the form of seven galvanized steel cables tied to anchor points installed in three points across each section. This leads us up to the 17th October 1989 and the evening of the third game of the World Series. At 1704 the Loma Prieta earthquake struck. The main epicentre was in the Santa Cruz Mountains. The shock lasted approximately 15 seconds, not long you might think, but different areas felt this in different ways. In rocky areas the power of the earthquake was reduced by the unconsolidated soil. But in the clay of Oakland the vibrations were more intense and lasted longer. As the ground shook the viaduct vibrated weakening the structure around the shear keys. The sub part of the lower part of the bent column weakened by lateral forces began to crack and eventually crumble. This forced the upper part of the bent outwards. With nothing holding the upper roadway in place it came crashing down pushing the upper column out further still. As this happened the cars heading north on the Cyprus viaduct in the collapsed areas were crushed under the upper roadway. After a matter of seconds all was over. 48 bent columns had failed with vast sections of the 1.5 mile viaduct collapsed. With no aftershock many residents rushed to assist in finding survivors. Some were able to escape relatively easily but others needed emergency amputation to be cut out of the failed structure. Like the Skyway collapse at the Hyatt Regency the emergency crews bravely worked to rescue commuters from their cars and the wrecked structure. The effort became immensely harder in the evening darkness. Many buildings in the area were damaged by the earthquake and even the San Francisco Oakland Bridge suffered a partial collapse of its upper deck. In total two thirds of the total death toll of the earthquake was from the Cyprus roadway with 42 losing their lives. If it wasn't for the baseball game then the casualty rate would have been much higher. But interestingly the investigation into the disaster also showed that it was pure luck that more of the viaduct hadn't collapsed. Needless to say the root cause of the collapse was the earthquake but how did the viaduct fail so catastrophically especially in an area known for seismic activity? Well post event investigators undertook an examination of the structure including both failed and intact columns. Right from the start investigators found a startling similarity. During the examination many expansion joints were seen cracked around the anchor point hinting that they had experienced extreme force pulling on the seven steel cables. B1 columns were found to have had the highest failure rate followed by the B2 variant although hardly surprising as both were the most commonly employed design. But investigators found that Bentz 96 and 97 had survived the quake. The reference the main part of the collapse was between Bentz 63 and 112. Interestingly these two were of the B3 design which employed shear keys right at the top of the columns. Even more interestingly between 96 and 97 was no modified expansion joint. In addition to this each end of the collapse section was a Bentz design different to B1 and B2. The section of the viaduct that had collapsed also had piles which were the deepest set into the clay. This led investigators to think that the root cause was higher levels of vibrations transmitted into the piles that were then affecting the shear keys on the B1 and B2 Bentz. In the investigation report it concluded that the cause of the collapse was in the stubs of the lower supporting columns which after being subjected to the lateral forces of the earthquake failed and thus the top half of the columns became unsecured. It was also found that reinforcement of this area was inadequate compared to modern day standards and if the earthquake had lasted any longer then the remaining Bentz could have also failed. But this brings up the question of why not incorporate more effective seismic activity protection. Clearly it was understood that the viaduct was vulnerable to extreme lateral forces as expansion joints were modified. Trans-Cow had thought that a double decker bridges and viaducts were stronger due to the extra support provided by the larger columns and as such had proposed upgrade works for its single story structures first. But even if the double decker structures were prioritized no effective measures could be employed on an inherently poor design. Ultimately the issue came down to money as there was no proper way to reinforce the existing structure effectively meaning that replacement was the only way to fix the viaducts. Needless to say it was not a realistic option when the current one cost 10 million dollars in the 1950s. The remains of the viaduct were demolished and a new alignment was employed with a ground level design with a more conventional single story viaduct. This is a plain difficult production. All videos on the channel are creative commons attribution share alike licensed. Plain difficult videos are produced by me John in a currently wet and windy southeast in corner of London UK. Help the channel grow by liking commenting and subscribing. Check out my Twitter for all sorts of photos and odds and sods as well as some future videos. I've also got Patreon and YouTube membership as well so check that out if you fancy supporting this channel financially. And all that's left to say is thank you for watching.