There is an underlying tone contained within this blog that demonstrates a lack of faith in the design, engineering, and construction abilities of Winslow Lewis. Even researchers who feel Lewis was “the right man at the right time”, such as Richard W. Updike, make these assertions with a certain lack of conviction. No matter what opinion one may have of Winslow Lewis, he did accomplish what was thought to be impossible… He was able construct a stable masonry lighthouse structure on the soft soil of the Mississippi River Delta. What makes this achievement more impressive is the fact that he went before Congress and guaranteed that he could do so despite Latrobe’s apparent failure to achieve the same. As a further credit to Lewis in these endeavors, he was able to repeat his success; but not without some failures in between.

The 1823 Frank’s Island Lighthouse stood for 179 years before it collapsed. It had sunken about 3 to 4 feet at the time it was surveyed by Samuel Wilson, Jr.; but it served its purpose without failure or disappointment until it was discontinued in 1856. Between 1831 and 1840, Lewis constructed several other lighthouses on the Mississippi River Delta. These challenges were met with very limited success. Towers at the South and Southwest Passes were undermined by water currents and collapsed. The only other success Lewis had with constructing a masonry lighthouse on the Mississippi Delta is the 1840 Southwest Pass Lighthouse. Even though the tower of this lighthouse was 10 feet shorter than that of the Frank’s Island Lighthouse, the second Southwest Pass Lighthouse began to list shortly after construction. Despite this flaw and a rather disappointing service record, the structure is still standing after 169 years.

Due to the mixed successes and failures of Lewis’ masonry towers on the Mississippi Delta, one could reasonably question whether his success with the Frank’s Island Lighthouse was a fluke. If Winslow Lewis truly knew how to erect a masonry tower on alluvium soil, why could he not faithfully duplicate his earlier success? The Frank’s Island Lighthouse tower was 75 feet tall – at least 10 feet taller than any of the other masonry towers he built along the Mississippi. Therefore, it was the heaviest of the lot. Even though weight was his major concern and criticism with Latrobe’s Lighthouse, Lewis achieved his greatest success in the area with the largest and heaviest structure he built. What kind of foundation did Lewis choose to erect the 1823 Lighthouse? Did he possibly borrow from Latrobe's foundation design, but chose to get it right the second time around? One may never be able to answer these questions without excavating the site now six or so feet under water. The only reference I could find relating to a foundation specification for one of these masonry lighthouses is taken from David Cipra's "Lighthouses & Lightships of the Northern Gulf of Mexico" regarding the original Southwest Pass Lighthouse...

"In 1842, a Congressman charged that the first tower was shoddily built 'on a foundation of old flatboat planks at a cost of $10,011.74' The construction contract had called for a foundation of pilings driven 40 feet, or as far as a 1,400-pound weight falling 26 feet could pound them."

I can only assume that Congress would have specified a foundation based on that of the proven 1823 tower’s design. It would also seem that Lewis’ limited success with these structures may have been hindered by his propensity to take shortcuts as he did with Latrobe’s Lighthouse. Regardless of his haphazard efforts, Lewis did get it right the first time, and this is where Lewis, himself, has inadvertently validated Latrobe’s lighthouse design…

What are you supposed to do if you are walking on the surface of a frozen pond and the ice begins to crack under your feet? You are supposed to lie down and spread your weight across as much of the ice’s surface as you can. By lying down on the ice, do you weigh less? No, you weigh the same; but by lying down, your weight is no longer concentrated within the area of your feet. Instead, it is spread across the entire area of your body. By spreading your weight, you are exerting less pressure across the overall surface of the ice. This is the same thought process that Benjamin Latrobe used in designing his lighthouse. On page 246 of the article, “Benjamin Latrobe’s Designs for a Lighthouse at the Mouth of the Mississippi River”, Dr. Michael W. Fazio offers an extensive analysis of the efforts Latrobe made to lighten the structure and to spread its weight over as large an area as possible. According to the article (Footnote 68), “The area of a 108-ft. diameter circle is 9156 sq. ft. A reasonable pile-mat bearing capacity for the blue clay soil would be about 1500 pounds per square foot; therefore 9156 sq. ft. times 1500 lbs. per sq. ft. equals 13,734,360 pounds, the allowable load that the pile-mat should have supported… If an average weight for the brick and stone masonry is assumed to be 150 pounds per cubic foot, then the total weight of the lighthouse was 36,000 cu. ft. times 150 lbs. per cu. ft. equals 5,400,000 pounds – well below the allowable load. Ruddock, in his report, said that the tower had a masonry volume of 24,667 cu. ft. and weighed 3,154,625 pounds.” Using Dr. Fazio’s analysis, one can divide the weight of Latrobe’s Lighthouse (3,154,625 pounds) into the load bearing capacity of the soil its weight was spread across (13,734,360 pounds) and conclude that the structure came in at only 23% of the soil’s load bearing capacity. Based on this analysis alone, it should be evident that weight was not cause of the structure’s failure.

Now, if one applies the same analysis to Lewis’ lighthouse, a different result comes to light. A truncated cone with a lower diameter of 28 feet, an upper diameter of 22 feet, and a height of 75 feet, has a volume of 37,000 cubic feet. Wilson's survey drawing indicates that Lewis' tower had two 18-inch thick brick walls, one inside the other with a 1-foot space in between. My conservative approximation would suggest that the tower was 25% solid, yielding a masonry volume of 9,250 cubic feet. The masonry weight of the tower would be approximately 1,387,500 pounds. Although the base of the lighthouse was 28 feet in diameter, let's assume that it sat on a foundation platform that measured 30 feet in diameter – less than 1/3rd. the diameter of Latrobe’s structure. This would yield a base surface area of 707 square feet, resulting load bearing capacity of 1,060,500 pounds. If my calculations are even remotely correct, then Lewis' tower weighed 327,000 pounds or 31% over the load capacity of the strata it sat upon! Even though Lewis’s lighthouse weighed about 1/3rd. less than Latrobe’s lighthouse, its weight was spread over a much smaller surface area. Perhaps this is why the structure had sunken 3 to 4 feet over the course of 111 years. However, if weight was the key flaw in Latrobe’s design, then Lewis’ lighthouse should have met a similar fate in as short a time. Instead, his tower, which exceeded the load bearing capacity of the soil it sat upon by 31%, continued to stand for 179 years. Winslow Lewis, through the success of his 1823 lighthouse, proved that it was not the weight of Latrobe's lighthouse which caused its untimely failure!

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## 1 comment:

Nice article. Thank you for this info

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