231.8 Bridge Width

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Bridge width will vary based on project scope, context of surrounding road, future plans for the route, and Deck Geometry rating (NBI Item 68). Project scope and bridge economics usually determine the treatment strategy for individual projects. Treatment strategies range from new, replacement, widening, redecking and rehabilitation bridge construction.

Major Roads

The desirable width of bridges on Major roads is equivalent to the width of the traveled way and any existing or proposed shoulders. The minimum width of bridges for Major roads is 24 ft., that is, there could be situations where an existing bridge may be rehabilitated and used in place even though it may not be the desirable width.

Minor Roads

The desirable minimum width of bridges on Minor roads is equivalent to the width of the traveled way and, 1) any existing or proposed shoulders or, 2) 2 ft. shoulders, whichever is greatest.

Major and Minor Roads (Based on NBI Deck Geometry Rating)

The bridge width based on Major/Minor Roads classification could conflict with the minimum bridge roadway width based on the Deck Geometry rating of the National Bridge Inventory (NBI) rating system. FHWA requires all bridges on public roads to be inventoried and inspected in accordance with the National Bridge Inspection Standards (NBIS) within which the NBI operates. The FHWA holds MoDOT responsible for collecting and submitting the bridge inventory data for all bridges within the state.

The engineer should determine the roadway width of the bridge based on site-specific conditions. Once this width is determined, it should be checked against the following table in order to evaluate if new, replacement, widening, redecking or rehabilitation bridge construction would be considered to be deficient based on its deck geometry. If the proposed bridge roadway width is less than that shown in the following tables, the width provided in the table should be used or a design exception could be considered. Only under extraordinary circumstances should a design exception be considered for new or replacement bridge construction since the goal at MoDOT and FHWA is to reduce the number of deficient bridges and also since funding sources for bridge construction could be influenced. For other types of bridge construction, design exceptions should be considered under normal circumstances.

Minimum Bridge Width Based on Deck Geometry Rating1

Roadway Classification Facility Type2 Future or Design ADT3Width4 (Ft.)
Major2-Lane/2-Way < 100 18
101 - 40020
401 - 100022
1001 - 200024
2001 - 500028
>500032(28)5
2-Lane/1-WayAll34(29)5
3-Lane/1-Way 45(40)5
4-Lane/1-Way 56(51)5
5-Lane/1-Way 67(62)5
6-Lane/1-Way 78(73)5
Minor2-Lane/2-Way< 10018
101 - 40020
401 - 100022
1001 - 200024
2001 - 500028
>500032(28)5
2-Lane/1-WayAll30
3-Lane/1- or 2-Way 39
4-Lane/1- or 2-Way 50
5-Lane/1- or 2-Way 61
6-Lane/1- or 2-Way 72
Ramp1-Lane/1-WayAll18
2-Lane/1-Way 28
3-Lane/1-Way 40
1 Table based on NBI Item No. 68 - Deck Geometry of the Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges.
2 Where facility type is not listed, see State Bridge Engineer.
3 Includes ADT for both directions of traffic, where applicable.
4 Minimum width is based on a deck geometry code = 4, which is defined as "meets minimum tolerable limits to be left in place as is". Any width less than this will result in a deficient bridge width (i.e., bridge would be considered functionally obsolete).
5 May use value in parenthesis for bridges longer than 200 ft.
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