Design requirements mandated a minimum vertical clearance of 155 feet over a navigable channel of width 700 feet, measured at Mean High Water [See AutoCAD Drawing 2.] Loading is the standard AASHTO (American Association of State and Transportation Officials) HS20-44 or Alternate Military Loading. According to this designation, the HS20-44 represents a typical 20 ton truck (40 kips), with 4 tons distributed to the front axle, and 16 tons to the rear axle, with axles spaced 14 feet apart front-to-back. 44 represents the year of the specifications. Usually, the governing load is a line of such trucks, one per span per lane of traffic, although other patterns of loading may produce the maximum design forces and need to be considered. One should consult the AASHTO specifications directly to learn the details.

As Charleston is in danger of major hurricanes, the structure was designed for a static equivalent wind force based on extreme wind velocity of 155 miles per hour, which represents a return period of 550 years and a 16% probability of exceedance in the 100 year design life.

Perhaps surprising to some, Charleston is in seismic zone 3, (zone 0 reflects very low risk, zone 4 reflects highest risk) and historically has been the victim of strong but relatively infrequent earthquakes. In fact, the strongest shaking reported in the eastern U.S. occurred here in 1886. Estimated at level X on the Modified Mercalli Intensity scale, it caused roughly five million dollars of damage (1886 dollars), killed 60 people, and was felt as far away as Chicago, Boston, and St. Louis (ref. 1). In recognition of the seismic potential, a special "Geological/Seismic" investigation of the site was carried out by private consultants. Based on this, HNTB tentatively recommended a horizontal design acceleration of 0.18g. This value corresponds to a return period of 1000 years and a 10% probability of exceedance in the design life.

All design was based on the Load and Resistance Factor Design (LRFD) method.

The General Notes (from HNTB Drawing No. S-138) contain more information on the design specifications, design methods, design loads, construction materials. Want to download these notes in a Microsoft Word document?

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