Chapter 4: Geometric Restraints
Anchor: #i1001638Section 1: Bridge Width
Anchor: #i1001643Overview
Bridge width depends solely on the width of the highway except in unusual cases. Geometrically, bridges are just a small part of the highway. From the driver’s standpoint, bridges need to blend inconspicuously into the perception of the road. This need makes bridge widths, alignment, and clearances subject to the requirements of the highway engineer.
Refer to the Texas Department of Transportation Department (TxDOT) Roadway Design Manual for guidance on highway design.
Anchor: #i1001665Background
Roadway widths covered by Bridge Design Standards have ranged from 16 ft. to 48 ft. and from 7.2 m to 13.2 m during the past 80 years. For several years, bridges had curbs located 2 ft. outside of the traffic lane. Texas began providing graveled shoulders on major highways earlier than most states. With continuing increases of highway speed, it became evident that many vehicular accidents happened at the beginning of bridges where the horizontal clearance became restricted. Texas began a campaign for shoulder width bridges in the 1950s, but the recommendation did not appear in the American Association of State Highway and Transportation Officials (AASHTO) Specifications until 1969 because of the considerable cost of the additional bridge width.
Anchor: #i1001675Current Status
Today, virtually all bridges in Texas are as wide as the approach roadway, including shoulders. Curbs are not used except to protect a pedestrian walkway on a low-speed highway. The nominal face of bridge railing is located at the outside edge of the shoulder. Traffic lanes are usually 12 ft. (3.6 m) wide but may be reduced for low-volume or extremely crowded conditions and may be increased for sight distance around a horizontal curve. Shoulders vary from 2 ft. (0.6 m) to 10 ft. (3.0 m) depending on traffic volume and structure function. This results in a large number of bridge widths. The Bridge Division provides standard bridge details for a few of the most repetitive widths that are less likely to have complicated geometry.
Anchor: #i1001687Design Practice
On divided highways, separate bridges are used for each direction of traffic, unless the median is less than 30 ft. (9.0 m) wide between inside lane edges. In this case, bridge surfaces are usually flush with the pavement across the median. A concrete traffic barrier is usually constructed at the center of the median. The resulting bridge can be quite wide when there are multiple lanes in each direction. To ensure against transverse expansion and contraction problems, a longitudinal open joint is recommended when the bridge width exceeds 120 ft. (36.0 m). For simplicity of design, the joint should be at the median centerline. On long span structures, differential deflection can cause cracking in the concrete traffic barrier, which might justify moving the joint to one side of the barrier.
Except for unusual situations, the overall width of bridge decks is 2 ft. (0.6 m) more than the distance between the nominal faces of outside railing. Most standard bridge railings occupy less than 1 ft. (0.3 m) of deck width. The safety-shape or straight-sided traffic railings are slightly wider in their lower part, but this is not considered sufficient encroachment to affect operation of the shoulder. The nominal face of railing is set at 1 ft. (0.3 m) from the deck edge to allow the use of different railing with the same standard details. This dimension is also recommended for all non-standard bridges. Bridge widths for the current Bridge Design Standards are shown in Figure 4.1.
Figure 4-1. Standard Bridge Widths (Online users can click here to view this illustration in PDF.)