Section 2: Features Based on Bridge Location

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Highway Grade Separation

Vertical Clearance. Provide 16 ft. 6 in. minimum vertical clearance over the roadway, including usable shoulders on all new highway grade separation structures, and highway-railroad underpasses.

The minimum vertical clearance of 16 ft. 6 inches is required with no exceptions for structures over main lanes of interstate or controlled access highways within cities where this minimum vertical clearance is provided on an interstate loop around the city. Vertical clearances should be provided in accordance with the Roadway Design Manual on all other systems of highways with separations involving interchange facilities, other highways, public roads, or city streets. Where these minimum clearances are impractical or excessively expensive to provide, pursue a design exception. Refer to the Roadway Design Manual and the Project Development Process Manual for information about the vertical clearance design exception process. In such cases, vertical clearance will be held as near as practicable to 16 ft. 6 in. Vertical clearance may never be less than 14 ft. 6 in.

The above-specified clearances apply over the entire width of roadway including usable shoulders and include an allowance of 6 in. for future pavement overlays. Where a sag vertical curve exists on the roadway underneath the structure, vertical clearance should be calculated above the midpoint of a 50-ft. chord.

Horizontal Clearance. Information on horizontal clearance can be found in the Roadway Design Manual.

Airway-Highway Clearances. Where grade separation structures or multilevel interchange structures extend above ground level in the vicinity of airports, obtain clearance in accordance with the Roadway Design Manual.

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Structures Over Streams

Information in the following section provides general reference on common design features of structures over streams. Refer to the Hydraulic Design Manual whenever planning and developing a structure over a stream.

For new locations crossing a stream, conduct a minimum investigation to establish an approximate design high-water elevation. Establishing a high-water elevation early in the design process will aid in identifying any complication concerning the stream crossing while the location of the route is flexible enough to be shifted.

For existing locations, determine the hydraulic adequacy of the existing structure. If the runoff from a storm of documented design intensity has not actually been carried through the structure, the declaration of past adequacy is meaningless. If reliable flood control devices such as National Resources Conservation Service (NRCS) dams have been constructed upstream, an existing structure may be entirely adequate but may also be too large.

The hydraulic adequacy of an existing structure can be verified in only two ways:

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  • documentation that the structure actually has accommodated a flood of at least the approximate design frequency
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  • hydrologic and hydraulic investigation similar to that necessary to design new drainage structures

If investigation is necessary, information from old plans may be used provided that the information is verified and, if necessary, updated. Pay particularly close attention to runoff factors. If no documentable design flood has occurred and old design data are not available, hydrologic and hydraulic information must be furnished with the plans, specifications, and estimates (PS&E) or with preliminary layout submissions, keeping the extent of the investigation in line with the importance of the structure.

When an existing structure is determined to be inadequate, one of two actions may be taken. Either adjust the size of the facility as appropriate, or give the structure a new capacity rating with a corresponding decrease in the hydraulic standards that were previously established. Either action must be documented in the plans. Consult the Design Division’s Hydraulic Section when reducing the hydraulic opening.

The flood frequency used to determine the size of the waterway openings and the desired roadway profile is very important to the design. Base the minimum frequency on economics and risk except on interstate highways, which require a minimum 50-year flood frequency. Do not automatically select the frequency based upon highway classification because other factors can create a need for a higher type hydraulic facility. These factors include land use (both upstream and downstream of the highway), safety to traveling public, debris, environmental concerns, and others. Estimate land use for 20 years into the future.

In addition, apply the 100-year flood event (base flood) on certain proposed highway/stream crossing facilities to determine whether a proposed crossing will cause a flood to damage the highway or any other property beyond damage which would have occurred without the proposed facility. Consider that the flood may be conveyed both over the roadway and through the openings when evaluating whether significant damage occurs to the highway or other property.

Analyze the flood for all highway/stream crossings with one or more of the following:

Bridge-Class Culverts. Follow the procedure for the hydraulic design of bridge-class culverts in accordance with the Hydraulic Design Manual. Additional information on bridge-class culverts such as length, cover, safety treatment, and headwalls can also be found in the manual.

When considering replacing a span bridge with a culvert, ensure that drift accumulation is not a problem.

Railing and safety-end treatment requirements for bridge-class culverts can be found in Chapter 2, Section 7 of the Roadway Design Manual and Chapter 2 of the Bridge Railing Manual.

Existing Bridge-Class Culverts. Follow the procedure for analyzing the hydraulic capacity of existing bridge-class culverts in accordance with the Hydraulic Design Manual.

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Railroad Overpasses

Highway structures over railroads are referred to as railroad overpasses. Most railroad companies require 23 ft. 4 in. of vertical clearance from edge of railroad right-of-way to edge of railroad right-of-way, but vertical clearance requirements should be confirmed with the railroad company during early design stages. For widening of existing structures, it is usually satisfactory to provide no more clearance than is provided by the existing structure. Per Texas Administrative Code, Title 43, Part 1, Chapter 7, Subchapter D, Rule 7.36, the minimum vertical clearance is 22 ft. 0 in. from the top of the track to the lowest part of the bridge, and the minimum horizontal clearance is 8 ft. 6 in. from centerline of tracks to face of pier or other obstruction. These legal minimums include temporary construction clearances as well. However, the desired minimum horizontal clearance is 12 ft. Some railroad companies require greater horizontal clearance. Deviations to the clearance requirements in the Texas Administrative Code may be obtained through a clearance deviation to be approved by the Texas Transportation Commission.

Current American Railway Engineers and Maintenance of Way Association (AREMA) specifications require pier protection or crash walls where the clearance between centerline of tracks and face of pier is less than 25 ft. Place all piers so as not to interfere with drainage. If requested by the railroad company, horizontal clearance will be provided to allow the railroad use of off-track maintenance equipment. All clearances required because of future plans of the railroad company must be substantiated by documented plans and appropriated funding by the railroad company to do the work within the next five years.

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Railroad Underpasses

Highways under railroad structures are referred to as railroad underpasses. Plan and design railroad underpasses in close cooperation with the Railroad Company or companies involved. Most railroad companies do not employ sufficient engineering staff to prepare the detailed plans for such structures. The Bridge Division will prepare the detailed plans for railroad underpass structures when requested by the District and agreed to by the railroad company.

An underpass imposes an added maintenance burden and restricts expansion of the railroad line. An underpass also may result in restricted horizontal or vertical clearance and present a drainage problem for the roadway underneath. Explore all options before resorting to expensive pump stations. For certain conditions an underpass is the only workable solution for highway-railroad separation. In any case, where an underpass is proposed, the District should prepare comparative estimates of an underpass versus overpass and furnish these to the Bridge Division project manager with reasons for proposing the underpass

Clearances. Comply with the required clearances for highways underneath railroad structures as outlined in the Highway Grade Separations section of this manual, except as follows:

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  • For the usual conditions, use the minimum horizontal clearances from the edge of the traffic lane to the face of pier permitted in the Roadway Design Manual. Provide greater clearance where the overall cost of the structure will not be materially increased.
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  • Because the railroad live load can appreciably increase the cost of longer spans use shoulder widths with introduction of guardrail on the approaches to and through the structure. Place the face of the pier or abutment 2 ft. to 6 ft. outside the face of the guardrail.
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  • Consider pedestrian and bicyclist needs at the underpass. When present, pedestrians and bicyclists should be protected from vehicular traffic by a barrier.

Structure Types. Selection of a suitable structure type involves consideration of all facets of an underpass project, but some determination of type must be made early in the preliminary stage of project development. See the design guidelines for the affected railroad company for recommended structure types. The following provides general guidance for this determination:

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  • Decks of underpasses may be either concrete or metal deck plate. While metal deck plate is most expensive, it usually affords the minimum distance from top of rail to lowest point on superstructure and may be necessary where tight clearance conditions exist.
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  • Through-plate girders with floor beam and knee brace system offer the shallowest depth of section below rail. Required grade differential from railroad profile to highway profile is not appreciably affected by an increase in span length. Certain railroads object to this type of structure because of its vulnerability to damage by shifting freight loads or derailments. In cases where vertical clearances are critical, the through-plate girder, although expensive, is sometimes the only logical solution. Do not use this type of structure for more than two tracks.
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  • Deck-type structures may employ simple steel I-beams or plate girders, prestressed concrete I-beams, or prestressed concrete box beams. Continuous steel I-beams or post tension concrete beams may be used in unusual circumstances. Such structures increase in depth as the span length increases. Railroad companies have differing requirements for structures. Arriving at an acceptable structure type is a matter of design and negotiation.

The Bridge Division project manager will assist in specific details for each individual project.

Handling Railroad Traffic. The method of handling railroad traffic during construction usually affects the type of structure that is to be built. Close coordination is necessary with the railroad company through the Rail Safety Section of the Traffic Operation Division (TRF-RSS), assisted by the Bridge Division project manager. Consider the following ways of addressing this issue:

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  • A railroad detour or shoofly track may be constructed. This facility should be as near as practicable to the underpass construction site and as short as acceptable to the railroad company to minimize costs. Where an existing underpass structure is present, the shoofly will require a temporary structure over highway lanes unless highway traffic can be temporarily rerouted.
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  • Avoiding a shoofly track eliminates unnecessary bridge costs. The railroad company and TxDOT, working together, can drop a preconstructed bridge into place with little or no interruption to train traffic. Both parties would share the construction work involved in the project. TxDOT prefers this method, but few railroad companies endorse it. Therefore, the concept should be addressed very early with the railroad company to ensure the project is a candidate for this type of construction.
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  • Always investigate the possibility of constructing the new underpass near the present track and later relocating the track over the new structure. Explore this possibility because it offers the minimum construction cost for the underpass structure. However, a permanent realignment of the railroad may be excessively costly or unacceptable to the railroad company.
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  • Stage construction may be possible where part of the new structure is built to carry rail traffic while the remainder of the bridge is completed.
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  • Consider raising the track if drainage of the underpass section is critical or an increase in vertical clearance is required.
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  • Investigate items such as joint operations between two railroad companies, abandonment of a line, and similar changes in railroad facilities in the planning stage of a railroad underpass.
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Pedestrian Bridges

ADA and TAS Considerations. Design all pedestrian facilities so they are accessible to all persons and in accordance with the American with Disabilities Act (ADA) and the Texas Accessibility Standards (TAS). Always refer to the Americans with Disabilities Act Accessibility Guidelines (ADAAG) and the TAS for complete design requirements.

Structure Width. Construct decks on pedestrian crossings 8 ft. wide, or wider, where pedestrian volumes indicate.

Clearances for Pedestrian Structures. Pedestrian crossover structures are subject to severe damage or collapse when hit by a high load or a loaded truck out of control. Since the probability of loss of life is great under such conditions, these structures have more vertical and horizontal clearance than required for vehicular overpasses. Provide pedestrian crossovers with 17 ft. 6 in. vertical clearance over the travel lanes and shoulders. Consider higher vertical clearances for freight routes.

Railing and Fencing. Provide a 1-ft. high parapet on either side of a pedestrian crossover with a 5 ft. to 6 ft. woven wire fabric type fence mounted on top. In the interest of safety for children using such structures and also to protect the highway traffic beneath, cover portions of the walkway over the highway lanes and shoulders to entirely enclose the walkway. Where such overpasses are near schools or will be used by a substantial number of children, extend the covering to near the grade point at each end of the structure, if feasible.

Ramp Approaches. Ramp approaches may be tangent extensions of the main structure or may be right angles to the structure forming an L, U, or Z shape. Spiral ramps may also be used. Do not use stairs due to limited accessibility.

Illumination. Pedestrian crossovers normally are lighted by street or highway lighting standards placed in the vicinity.

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Overhead Sign Supports

Use TxDOT bridge standards where applicable. The Bridge Division is available to assist in the design of sign bridges not covered by the standards.

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Federally Funded Off-System Bridges

Design off-system bridges that are replaced with federal funds in accordance with the design criteria in Chapter 3 of the Roadway Design Manual for the appropriate roadway classification.

Design off-system bridges that are rehabilitated with federal funds in accordance with the design criteria in Chapter 4 of the Roadway Design Manual for the appropriate roadway classification. However, if the current average daily traffic (ADT) is 400 or less on an off-system bridge to be rehabilitated or replaced, and the facility is not likely to be added to the designated state highway system, then use the design criteria presented in Chapter 6 of the Roadway Design Manual.

For hydraulic design criteria, refer to the Hydraulic Design Manual. Also see Chapter 3, Section 2 of this manual.

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Historic Bridges

Historic bridges frequently cannot be cost-effectively upgraded to meet the usual design standard for roadway width, load carrying capacity, or traffic railing without significantly altering the aspects that make the bridge historically significant. Governments regulatory entities realize the importance of historic structures and have created guidelines that ease some constraints.

The design criteria for on-system historically significant bridges must comply with the design criteria presented in Chapter 4 of the Roadway Design Manual. However, federal law allows flexibility in design criteria on a case-by-case basis when approved as a design exception.

TxDOT and FHWA have developed design criteria for off-system historically significant bridges in order to eliminate the need for some design exceptions. These design criteria can be found in Chapter 2 of the Historic Bridge Manual. Historic off-system bridges that cannot be upgraded to meet or exceed these minimum criteria may be considered for preservation projects on a case-by-case basis when approved as a design exception.

Some important planning considerations concerning historic bridge projects, including coordination with outside divisions and agencies as well as the project letting schedule, are discussed in the section titled Advanced Planning -- Considerations Based on Bridge Location in Chapter 4, Section 2. Moreover, TxDOT has developed specific procedures for the coordination of projects concerning historic bridges. These procedures can be found in the Historic Bridge Manual.

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Bridges Not Funded by TxDOT

Bridges not funded by TxDOT but crossing TxDOT right-of-way must meet TxDOT design criteria. In these cases, the Bridge Division negotiates agreements between the State and the owner. The bridge project manager, in conjunction with the district, will coordinate a satisfactory agreement setting forth the financial responsibility and commitments, including maintenance and liability, of each party involved. Submit the PS&E to the Design Division in accordance with the usual PS&E and construction letting processes.

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