DESIGN CRITERIA
Design Speed
During the preliminary design phase, a design speed of 40 mph for the structure was proposed due to the presence of the concrete sidewalk located adjacent to a travel without the physical separation of a barrier. During the detailed design phase, the design speed was lowered to 30 mph to avoid excessive fill embankments and reduce the height of the required gravity walls beyond the bridge. The potential for overtopping and runoff was the driving factor for this reduction as they could easily lead to slope erosion. The design speed for the roadway approaches was selected to be 30 mph. As per the Florida Greenbook, the posted speed was also set at 30 mph, which is acceptable for Low Speed and Very Low Speed roadway facilities.
Lane Width
A lane width of 11 feet on the bridge was selected, using a Low-Speed classification with an ADT of 400 – 2000, as per AASHTO criteria. A lane width of 10 feet for the roadway approaches was selected based on the Ministry of Public Works standard detail for roads on the Family Islands.
Roadway Design Parameters
The geometric design of the roadway on the bridge is based on AASHTO – A Policy on Geometric Design of Highways and Streets 7th Edition 2018, and on the following criteria:
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Transition Slope Rate for Crowned Roadway (FDOT Index No. 510)
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Rate of Cross Slope 1.5% - 2% (two-lane paved highways)
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Superelevation Attainment Method: AASHTO Reverse Crowned Roadway
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Profile Curves: Min K Table: AASHTO Standard
The design parameters for the roadway approaches based upon the recommended 30 mph Design Speed are as follows:
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Minimum length of sag vertical curve: 90 feet
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Minimum length of crest vertical curve: 90 feet
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Minimum k-value of sag vertical curve: 37 feet
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Minimum k-value of crest vertical curve: 31 feet
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Minimum stopping sight distance of 215 feet for 6% downgrade for the north access road
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Minimum stopping sight distance of 184 feet for 6% upgrade for the north access road
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Minimum stopping sight distance of 218 feet for 7% downgrade for the south access road
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Minimum stopping sight distance of 200 feet for ≤ 2% upgrade for the south access road
Cross Section
Based on the parameters laid out above, the roadway cross section on the Glass Window Bridge is as shown above in Figure 16. The roadway cross section for the North Approach Bridge is as shown below in Figure 37. The roadway cross section for the roadway approaches is as seen below in Figure 38.
Figure 37 - North Approach Bridge Cross Section
Figure 38 - Roadway Approach Cross Section