![]() ![]() I really cannot find anything in any of my statics or strengths textbooks, nor can I find any real information to prove this. Adding strength usually means adding weight.Ī truss has an optimal load distribution when it has an aspect ratio near 2:1 I believe. So that means the middle members will have to be made stronger the taller the bridge is. The amount a piece of wood can support in compression before buckling decreases with length. However, bridge builders have another problem when pieces become longer. Why is this a drawback? By increasing the height, the middle members have to become longer. By increasing the height, the load decreased on the top and bottom chords but remained the same on the middle “truss members”. A decrease in load means you can make it smaller. What does this mean to you?Īs you saw in the example bridges, by increasing the height of the bridge you decrease the load on the top (and bottom) chord. All I did is increase the height of the bridge by one inch. You can see in this second bridge that the middle section of the top chord is only holding 38% of the total load. The only difference is that this bridge is 4 inches tall, one inch taller than before. Now I will show you another bridge, with the same design. ![]() For instance, the very middle of the top chord of the bridge is supporting 50% of the total load. This means the numbers you see act as percentages. I have added two load points with a total load of 100. The bridge is 8 inches long and 3 inches tall. There is no cut and dry answer, as you should evaluate your bridge specifications and guidelines and conduct experiments to reach the best answer for you. John E.All the time I am asked “How tall should I make my bridge?” This article will attempt to answer this question by illustrating a principle in model bridge building. Massachusetts Turnpike's Westfield River Bridge San Francisco–Oakland Bay Bridge (eastern span) San Francisco–Oakland Bay Bridge (western span) Navajo Bridge (dual spans the 1929 span is 467 ft high)īurro Creek Bridge (dual spans) Mike O'Callaghan–Pat Tillman Memorial Bridge The clearance below required under bridges for the largest ships- container ships, ocean liners and cruise ships-is around 220 feet (67 m) so there are often bridges with approximately that height located in coastal cities with bays or inlets, such as New York City's Verrazano-Narrows Bridge and San Francisco's Golden Gate Bridge. Note that the following types of bridges are not included in this list: demolished high bridges historic high bridges such as those over reservoirs-regardless of current reservoir levels-that were filled after the bridge was complete, unless the dam has since been removed and vertical-lift bridges, even those with raised span heights greater than this list's minimum height. The minimum height for inclusion in this list is 130 ft (40 m), which may be either the deck height or the clearance below depending on available references. For bridges that span tidal water, the clearance below is measured at the average high water level. Official figures for a bridge's height are often provided only for the clearance below, so those figures may be used instead of actual deck height measurements. ![]() A bridge's deck height is greater than its clearance below, which is measured from the bottom of the deck structure, with the difference being equal to the thickness of the deck structure at the point with the greatest clearance below. Height in this list refers to the distance from the bridge deck to the lowest point on the land, or the water surface, directly below. This is a list of the highest bridges in the United States by height over land or water. Royal Gorge Bridge, highest bridge in the United States ![]()
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