Bridge Project Development Manual: Features Based on Bridge Location

Section 2: Features Based on Bridge Location

Highway Grade Separation

Vertical Clearance. All new highway grade separation structures, including railroad underpasses, should provide 16 ft. 6-in. minimum vertical clearance over the roadway, including usable shoulders.

No exceptions will be made to this policy for structures over main lanes of interstate or controlled access highways except within cities where the 16 ft. 6 in. vertical clearance is provided on an interstate loop around the particular city. On all other systems of highways with separations involving interchange facilities, other highways, public roads, or city streets vertical clearance should be provided in accordance with the Roadway Design Manual. Where these minimum clearances are impractical or excessively expensive to provide, design exceptions may be pursued. Such exception requires approval from Bridge Division and Design Division (DES) with the Federal Highway Administration (FHWA) and/or the Military Traffic Management Command Transportation Engineering Agency (MTMCTEA) of the Department of Defense. In such cases, vertical clearance will be held as near as practicable to 16 ft. 6 in. In no case may vertical clearance 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 mid-point of a 50-ft. chord.

Horizontal Clearance. Information on horizontal clearance can be found in Table 2-11 of the Roadway Design Manual.

Airway-Highway Clearances. Where grade separation structures or multilevel interchange structures extend above ground level in the vicinity of airports, clearance should be obtained 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. When planning a structure over stream, however, always refer to the Hydraulic Design Manual for additional and more detailed information.

For new locations crossing a stream, conduct a minimum investigation to establish an approximate design highwater elevation. This high-water elevation should aid in the determination of any complication concerning the 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.

Basically, there are only two ways to verify the hydraulic adequacy of an existing structure:

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

If investigation is necessary, information from old plans can be helpful but information should be verified and, if necessary, updated. Runoff factors especially should be given close attention. 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 discovered to be inadequate or oversized, one of two actions may be taken. Either the size of the facility is adjusted as appropriate, or the structure size remains unchanged and the structure is given a new capacity rating with a corresponding increase or decrease in the hydraulic standards that were previously established. Either action must be documented in the plans.

The flood frequency used to determine the size of the waterway openings and the desired roadway profile are very important to the design. The minimum frequency should be based on economics and risk except on interstate highways, which require a minimum 50-year flood frequency. The selection of frequency based upon highway classification should not be automatic 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. Land use should be estimated for 20 years into the future.

In addition, the 100-year flood event (base flood) must be applied 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 over and above damage which would have occurred without the proposed facility. One important fact is that the flood may be conveyed both over the roadway and through the openings. The test is whether significant damage occurs to the highway or other property.

The flood must be analyzed for all highway/stream crossings with one or more of the following:

bridge classification type structures,
some feature within the influence of the 100 year flood plain to which significant damage could occur, or
federally established 100-year flood plain boundaries.

Bridge Class Culverts. The procedure for the hydraulic design of bridge class culverts must be 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.

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

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

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Railroad Overpasses

Clearances. Highway structures over railroads are referred to as railroad overpasses. Vertical clearance for new structures over railroad tracks must be 23 ft. minimum. In cases where electric powered trains are involved, a vertical clearance of 24 ft. 3 in. or 26 ft. may be required. For widening of existing structures, it is usually satisfactory to provide no more clearance than is provided by the existing structure. By law the minimum vertical clearance is 22 ft. 0 in. from the centerline of the track, 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. 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. It is also desirable that all piers be placed 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

Railroad underpasses involve structures carrying rail traffic over highways and must be planned and designed 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 structure is usually undesirable to railroad companies because such a structure imposes an added maintenance burden and restricts expansion of the railroad line. From the highway standpoint, an underpass usually results in restricted horizontal and/or vertical clearance and presents a drainage problem for the roadway underneath. All options should be explored before resorting to expensive pump stations. For single-track separations an underpass may be more economical than an overpass, but for multiple track separation an overpass will usually be more economical. For certain conditions an underpass is the only workable solution for highway-rail separation. In any case, where an underpass is proposed, the district should prepare comparative estimates of an underpass versus overpass and furnish to the Bridge Division project manager with its reasons for proposing the underpass.

Clearances. Clearances for the highway underneath a railroad structure should usually follow those outlined for Highway Grade Separations (Chapter 3, Section 2) except as follows:

For the usual conditions, the minimum horizontal clearances from the edge of the traffic lane to the face of pier permitted in Table 2-11 of the Roadway Design Manual are recommended. Greater clearance should be provided where the overall cost of the structure will not be materially increased.
Because the railroad live load can appreciably increase the cost of longer spans, guidelines recommend adequate shoulder widths with introduction of guardrail on the approaches to and through the structure. The face of the pier or abutment should be placed 2 ft. to 6 ft. outside the face of the guardrail.

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. The following provides general guidance in such determination:

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.
Through-plate girders with floor beam and knee brace system offer the shallowest depth of section below rail. Required grade differential from rail 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. This type of structure is not recommended for more than two tracks.
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 and arriving at an acceptable structure type is a matter of design as well as negotiation.

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

Handling Rail Traffic. The method of handling rail traffic during construction usually affects the type of structure that is to be built, and this item should be coordinated closely with the railroad company through the Railroad Operations Section of the Traffic Operations Division, assisted by the Bridge Division project manager. The principal ways of handling this problem are as follows:

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.
Avoiding a shoofly track eliminates unnecessary bridge costs. The railroad company forces and TxDOT, working together, can drop a preconstructed bridge into place with little or no interruption of train traffic. Both parties would share the construction work involved in the project. TxDOT prefers this method and some railroad companies endorse it.
It is sometimes possible to construct the new underpass near the present track and to later relocate the track over the new structure. This possibility should always be explored 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.
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.
Consideration to raise the track should be given if drainage of the underpass section is critical or an increase in vertical clearance is required.
Items such as joint operations between two railroad companies, abandonment of a line, and similar changes in rail facilities should be investigated in the planning stage of a railroad underpass.

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Pedestrian Bridges

The following section provides guidance on pedestrian bridges to the bridge planner or designer. Additional information can be found in Chapter 7 of the Roadway Design Manual.

ADA and TAS Considerations. Pedestrian facilities must be accessible to all persons and designed 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.

General information on ADA and TAS can be found in the information on Advanced Planning -- General Considerations (see General Features, Section 1).

Structure Width. The decks on pedestrian crossings are usually 8 ft. wide but may be 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. The probability of loss of life is great under such conditions. These structures have more vertical and horizontal clearance than required for vehicular overpasses. Pedestrian crossovers must have 17 ft. 6 in. vertical clearance over the travel lanes and shoulders.

Railing and Fencing. A 1-ft. high parapet should be provided 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, portions of the walkway over the highway lanes and shoulders should be covered to entirely enclose the walkway. Where such overpasses are near schools or will be used by a substantial number of children, the enclosed covering should extend 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 or stairs may also be used, but stairs are not recommended due to limited accessibility.

Illumination. Pedestrian crossovers normally are not lighted except by street or highway lighting standards placed in the vicinity.

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Overhead Sign Supports

Use TxDOT design 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

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

Off-system bridges that are rehabilitated with federal funds must be designed 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 the design criteria presented in Chapter 6 of the Roadway Design Manual may be used.

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

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Historic Bridges

Normally, historic bridges 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. The federal and state governments 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 Advanced Planning -- Considerations Based on Bridge Location (see Chapter 4, Section 2) section of this manual. 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. Agreements between the State and the owner must be negotiated. 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.

The PS&E should be submitted to the Design Division in accordance with the usual PS&E and construction letting processes.�