Laurie Dengler | Ignoring submarine fatigue can have consequences – Times-Standard

I’ve been watching the Titan disaster happen. It is likely that the mini-submarine had already exploded, crushed by the enormous pressures of the ocean depths by the time it was first reported missing. We may never know the full story of what happened, but one thing jumps out: cyclic fatigue.

Cyclical fatigue means the damage that occurs when stressing and relaxing a substance repeatedly. No matter the material, the more you press or pull on it, the weaker it becomes. Owned and operated by OceanGate, a private adventure tourism company, Titan has made 13 trips in the past two years to the Titanic site. Each flight produced incredible stresses on the carbon fiber and titanium hull. Each return to the surface reverses that pressure, stressing the structure in the opposite direction.

Fatigue is a known property of materials under stress. Microscopic cracks are formed in response to loading. On repeated loading and unloading cycles, cracks grow. At some point, the strength of the material can be compromised, and crack growth accelerates. The result is often a rapid failure at stress levels well below the strength of the sound material.

I’m not a material science expert, but I am familiar with stress loading, fatigue, and failure from working with earthquake engineers. Structures that are subjected to strong earthquakes are frequently subjected to tension, compression, and twisting.

The seismic effect is complex. The shape and size of the building may resonate and amplify forces to levels even higher than those on the ground. Duration of vibration can lead to fatigue as structural elements are moved repeatedly in one direction and then another. Aftershocks exacerbate the problem. A structure that has withstood the main earthquake may fail in a smaller structure because some structural elements have been compromised.

Other forces also act on structures such as temperature changes, winds, and the movement of people, cars, or other objects. Structural engineers must consider both expected and unexpected forces. The Golden Gate Bridge’s 1987 run, celebrating its 50th anniversary, is a case in point.

Traffic was stopped and about 300,000 people were allowed onto the bridge before officials noticed an apparent flattening. Suspension bridges such as the Golden Gate owe their strength in part to the upward arch that counteracts gravity on the structure. The central arch retracted by seven feet with the unexpected added weight. There were still half a million people who wanted to walk on the bridge when pedestrian access stopped.

In building structures, designers must take into account all potential forces and sometimes one is not aware of all of them until after the fact. All structures must withstand the constant downward pull of gravity, or they won’t stand up in the first place. Dynamic forces such as seismic waves are rare and easy to ignore until the earthquake occurs, and it is too late.

California is a leader in earthquake-resistant design, building codes, and implementation. This is not surprising since we have experienced more devastating earthquakes than any other part of the United States. The 1933 Long Beach earthquake led to specific design requirements for earthquakes. With only a magnitude of 6.4 (the same as the Ferndale earthquake last December), the quake killed 120 and caused nearly a billion dollars in damages in today’s dollars. It was 120 badly damaged brick school buildings that led to the legislation. Fortunately, the earthquake occurred right after school and most of the children left, but the scale of the devastation cannot be ignored.

The Long Beach earthquake starkly indicated the fragility of URMs (Unreinforced Building Structures) and began California’s quest to request reinforcement or removal. Initial efforts focused on schools and hospitals but in 1986, the Unconsolidated Building Act was enacted requiring each county to take inventory and develop plans to reduce the threat of URM. Estimates of its effectiveness vary. A Commission on Seismic Safety report in 1995 reported that three-quarters of California’s communities were in compliance with the law but that progress has been slow and financial barriers high.

In Humboldt County, more than two dozen URMs have been identified. With the demolition of the Lloyd Building this year, nearly all of the URMs have been modified or removed. But URMs are not the only structures at risk during earthquakes. The structures most vulnerable to our M6.4 last December were single-family homes and mobile homes that were not adequately secured to businesses.

Any structure built before 1960 is likely to have foundation problems unless it is reinforced. Our first house in Humboldt County was perched atop piers and piers, a popular and inexpensive building solution for removing the structure from wet earth. Unfortunately, each column/pier will respond differently during vibration causing the structure to tilt or sag. The good news is that this damage will rarely kill you; The wooden structure above is still basically intact. The bad news is that your home will not be safe for occupancy and it is costly to repair.

When our son purchased a home in 1893, his first step in rehabbing the structure was to replace the column base and pier with a continuous perimeter foundation (Not My Fault, Sept. 12, 2019). But the cost of such a retrofit is out of reach for many people. If you own your own home, you may qualify for California’s Brace and Bolt Program (Not My Fault, Jan 21, 2023), which only recently became available in Humboldt County.

Last Sunday was not the first indication of problems with the OceanGate submarine. Titan showed evidence of cyclic fatigue problems in 2020, which prompted repair and boosting. Operating in international waters with few regulations, there were no certification or testing mandates. In contrast, the deep research vessel Alvin, which was first launched in 1964, has made more than 5,000 dives, some deeper than the Titanic. Operated by the US Navy in conjunction with the Oceanographic Institutes, it must meet strict requirements before and after each dive.

It is possible to explore the depths safely by following good science and engineering protocols. The same can be said for living in structures in earthquake country. It costs money, time and energy. Waiting until after the disaster strikes is too late.

To learn more about the Brace and Bolt program in California go to https://www.earthquakeauthority.com/Prepare-Your-House-Earthquake-Risk/Brace-and-Bolt-Grants

Laurie Dengler is Professor Emeritus of Geology at Cal Poly Humboldt, and an expert on tsunami and earthquake risk. Questions or comments about this column, or want a free copy of Living on Shaky Ground readiness magazine? Leave a message at 707-826-6019 or send an email to [email protected].

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