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Definitions & Method of Measuring Length of Span

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Municipal Small Bridge Program

What is a BRI?

A "BRI" is a highway bridge structure that meets the Massachusetts General Laws (MGL) definition of a bridge but not the federal definition of a bridge. MGL recognize structures having a span greater than 10 feet as bridges, but federal regulations define a bridge as a structure having a span greater than 20 feet. MassDOT uses the category code of "BRI" in order to identify and track MGL definition bridges in its inventory.

References

MGL Chapter 85 Section 35 (relevant provisions):

No bridge on a public highway having a span in excess of ten feet, … , shall be constructed or reconstructed by any county or town except in accordance with plans and specifications therefore approved by the department. Said department shall approve or alter to meet its approval all such plans submitted to it and shall determine the maximum load which any such bridge may safely carry…

Federal regulations, 23CFR Part 650 Subpart C, National Bridge Inspection Standards (NBIS):

Bridge: A structure including supports erected over a depression or an obstruction, such as water, highway, or railway, and having a track or passageway for carrying traffic or other moving loads, and having an opening measured along the center of the roadway of more than 20 feet between undercopings of abutments or spring lines of arches, or extreme ends of openings for multiple boxes

How is a structure determined to be a BRI?

MGL Ch 85 Sec 35 only requires that a bridge have a span in excess of 10 feet. The engineering definition of the term "span" refers to the distance between adjacent centerlines of bearings. However, MGL Ch 85 Sec 35 is primarily concerned with authorizing MassDOT to approve bridge plans prepared by municipalities and to determine a bridge’s safe load carrying capacity. As a result, it provides no guidance on where to measure the span, nor does it provide guidance on how to measure the span for structures that do not have defined centerlines of bearings, such as culverts, arch bridges or in the case of pipes that perform the function of a bridge. To address these questions, MassDOT defaults to the NBIS definition requirements.

Therefore, a BRI is determined as follows:

  1. It must be on and carry a public highway.
  2. All span measurements are taken along the centerline of the roadway.
  3. For bridges that have defined centerlines of bearings, measure the total distance from the centerline of bearings on one abutment to the centerline of bearings on the other abutment. (See Figure 1)
  4. For bridges that do not have defined centerlines of bearings, such as arches, culverts and those bridges where the ends of the beams are encased so that there is no centerline of bearings, measure the clear opening between the breastwalls of abutments, the spring lines of arches, or extreme ends of openings for multiple boxes. (See Figures 2a, 2b, 2c)
  5. For bridges that consist of large size pipes, measure the maximum diameter of the pipe, or, in the case of multiple pipes, the maximum total distance across all pipes provided that the clear distance between pipes is less than half the diameter of the smaller contiguous pipe. (See Figure 3)
Figure 1 description follows

NOTE: Since the MGL Span and the NBIS Span measures are different, the MGL Span measure may exceed 20 feet while the NBIS Span measure may be less than 20 feet. In this instance, the bridge would still be considered a BRI because it does not meet federal definition of a bridge.

Figure 1: Bridges with Defined Centerlines of Bearings
Figure 1 description follows

Figure 2A: Arch Type Bridges

Figure 2B description follows

NOTE: This also includes three sided frame type culverts.

Figure 2b: Culvert Type Bridges
Figure 2C description follows

Figure 2c: Bridges without Defined Centerlines of Bearings

Figure 3 description follows

NOTE: In order for the MGL Span measure to be taken as shown above, W1 must be less than 1/2 D1 and 1/2 D2 and W2 must be less than 1/2 D2 and 1/2 D3. If, for instance, W2 were greater than 1/2 D3, then the span measure would only include D1, W1, and D2. This method of measure also applies to multiple opening Clapper type structures, where a stone slab sits on thick piers.

Figure 3: Multiple Pipe Culvert Structures
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