Chapter 3: Preliminary Design Features

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Section 1: General Features

Bridge Standard Drawings

Bridge standard drawings are available for many structure types, skews, and common bridge widths. These standard drawings contain systems and details that can be used in bridge plans without modification.

Many standard drawings are available in the Texas Department of Transportation (TxDOT) main web site at http://www.dot.state.tx.us/insdtdot/orgchart/cmd/cserve/standard/bridge-e.htm. (Use the browser’s “Edit-->Find” menu to locate individual drawings.) The web site also contains instructions about the use of these graphics files.

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Bridge Widths

For all new and replacement projects (4R) including freeway rehabilitation, all bridges will carry the full usable shoulder width of the approach roadway across the structure. Bridge widths must conform to the requirements in Chapter 3 of the Roadway Design Manual in which the design criteria for 4R projects are represented for various roadway functional classifications and traffic volumes. Bridge widths for structures in complex interchanges containing flares, gores, etc. should be constructed to full width of the approach roadway, as well.

For non-freeway rehabilitation projects (3R) where the bridge structures are to be modified, bridge widths should meet the approach roadway width as a minimum. Otherwise, bridge widths must conform to the requirements in Chapter 4 of the Roadway Design Manual in which the design criteria for 3R projects are represented for various roadway functional classifications and traffic volumes.

Minimum bridge width requirements for special facilities, such as off-system bridge replacement and rehabilitation projects, Texas Parks and Wildlife Department projects, and bicycle facilities can be found in Chapter 6 of the Roadway Design Manual. Minimum bridge width requirements for off-system historically significant bridge projects, can be found in the Historic Bridge Manual.

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Bridge and Span Lengths

Every project is unique. Thus, in planning stages, the length of the bridge is an approximation based on available preliminary information. As the project progresses the actual design length becomes more apparent. The length of a bridge generally depends on the existing topographical conditions at the site, the width of the obstruction being crossed (other roads, waterway, railroad tracks, etc.), roadway alignment, highway design criteria (sight distance, maximum grades, etc.), and economics. The future plans of the area also have an impact on the structure length. If possible, the “begin bridge” point and “end bridge” point should be located at whole station numbers and on a tangent alignment. This can be accomplished by moving the point of curvature (PC) or the point of tangency (PT) off the bridge, if allowable.

Once the bridge length is determined, the number of spans, bent locations, and span lengths can be determined. Existing site conditions, economy, and aesthetics are usually controlling factors. If location of the bents is arbitrary and site conditions allow the bents to be placed anywhere along the length of the bridge, the interior span lengths should be equal. The bent locations should, if possible, be placed at whole station numbers.

The span length requirements limit available options for superstructure type. Economy and aesthetics often govern at this point. Recommended span lengths and approximate depths for various superstructure types can be found on the TxDOT website. Bridge costs for various superstructure types also are posted on the TxDOT website.

This entire process, like many tasks in engineering, is iterative steps that take place during development of preliminary bridge layouts. The district, the Bridge Division project manager, the Bridge Division Field Operations Section, the Bridge Division Design Section consultants, and others discuss options until they develop a plan for an economically feasible, aesthetically pleasing structure that serves its design purpose.

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Vertical Curvature

Vertical curvature of structures should generally conform to curvatures permitted on sections of roadway for the same conditions of traffic and terrain. Basic design criteria for vertical alignment can be found in Chapter 2 of the Roadway Design Manual.

Some important factors to be aware of when determining the vertical alignment of a structure include the following:

On controlled-access highways where crossover roads intersect frontage roads near the main lanes, the vertical curvature of the crossover structure should be set to allow adequate sight distance for crossover and frontage road traffic. It may be necessary to locate intersections farther away from bridges to provide adequate sight distance on steep crossover grades.
In areas where icing is prevalent, structures should have somewhat flatter grades than comparable sections of roadway because they are more susceptible to icing and likely to present a traffic hazard in the early stages of an ice storm.
On long flat grades a small crest vertical curve is recommended throughout the bridge length to prevent an illusion of sag and to improve deck drainage.

Additional design recommendations concerning alignments can be found on the TxDOT website.

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Horizontal Alignment

A bridge structure should be on tangent alignment if such can be accomplished without sacrificing the overall geometric design of the highway. Tangent alignment affords easier plan preparation and easier bridge construction, thereby resulting in lower structure cost. In urban areas where high right-of-way costs prevail and in some rural areas, it is not always feasible to build structures on a tangent alignment. Where curved structures are built, their geometry should fit the curve geometry for the roadway sections. Tightly curved alignments can significantly restrict the type of superstructure. Basic design criteria for horizontal alignment can be found in Chapter 2 of the Roadway Design Manual.

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Skew

Structures may be built on a skew if necessary to match alignment of roadways, railroad tracks, or stream flow. If a skew is required, the following guidelines should be considered:

Skews should normally be limited to the minimum angle practicable. Standards for several beam types and roadway widths are available in skews of 30 degrees and 15 degrees. Skews in excess of 30 degrees will usually require special design considerations.
For railroad overpasses, place bents parallel to railroad track alignment, if possible.
For railroad underpasses, each railroad company may have its own limitations on acceptable skew angle.
Skewed structures that have horizontal curvature require special geometric and structural design, and additional time will be required for plan preparation.
Slab breakbacks and special slab reinforcing details may be required. Refer to the Bridge Detailing Manual.

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Superelevation, Transitions, and Cross Slopes

Several factors can affect the allowable rate of superelevation (e) including climate, terrain, type of traffic, design speed, and alignment, among others. Thus, no single maximum superelevation rate (emax) is applicable to all situations and ranges of emax are necessary. The maximum superelevation rate on structures should usually be held to 8% regardless of the degree of curvature due to the tendency of vehicles to slide toward the inside of the curve when icing conditions exist. Otherwise, amounts of superelevation and transition rates on structures should be the same as those specified for the sections of roadway. Basic design criteria on superelevation can be found in Chapter 2 of the Roadway Design Manual.

Spiral transitions should not be used on bridges. The same effect as a spiral curve can be achieved by compounding smaller degree curves into the principle curve. On preliminary bridge layouts the rate of transition from full superelevation to normal crown should be specified in enough detail to enable the designer to define the roadway surface.

The cross slopes on bridges may be set to match the approach roadway. Usual crown is 1.5% or 2%. In some cases 1% is used on bridge standards. In this case the approach roadway slope is transitioned to fit the bridge.

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Bridge Medians

Median widths are measured between the inside edges of opposing travel lanes. Where narrow medians (4 ft. to 16 ft.) are used, it is desirable to carry the median uninterrupted across the bridge structure.

When the median width is 30 ft. or less and a median barrier is used on the approaches, a single structure with a closed flush median and a median barrier extended uninterrupted throughout the structure length should be used. When the median width exceeds 30 ft., dual structures should be constructed with an open median and suitable guardrail connected to the bridge railing.

Additional design considerations for determining median width can be found in Chapter 2 of the Roadway Design Manual.

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Sidewalks and Curbs on Bridges

Pedestrian and driver safety must be taken into consideration when sidewalks are provided on bridges. Pedestrian facilities must be accessible to all persons and designed in accordance with the Americans with Disabilities Act (ADA) and the Texas Accessibility Standards (TAS). Information on ADA and TAS, including general concerns and basic design criteria, can be found in Advanced Planning -- General Considerations (see Chapter 4, Section 1) of this manual and Chapter 2 of the Roadway Design Manual.

Sidewalks on Bridges. When pedestrians need to be accommodated, suitable sidewalks must be provided. The minimum clear sidewalk width should normally be 5 ft. In no case should a sidewalk not protected by a traffic railing be less than 3 ft. 6 in. wide. Refer to Chapter 2 of the Roadway Design Manual for other appropriate design criteria.

The need for sidewalks usually occurs in an urban area where a depressed highway crosses under a city street or on frontage road bridges. A suitable barrier rail or combination railing should be provided, if required. The use of barrier rail to separate vehicle from pedestrian traffic is governed by the following criteria:

Appropriate barrier rail is required when the design speed equals or exceeds 50 mph.
If the design speed equals or exceeds 40 mph but less than 50 mph, appropriate barrier rail may be considered where bridge site specific conditions would allow without interference to pedestrian movements, intersecting roadways, or other features.

Additional features of sidewalk and rail include:

American Association of State Highway and Transportation Officials (AASHTO) height requirements for pedestrian railing do not apply to the traffic barrier rail. However, engineering judgment should be exercised.
Ends of barrier rail must be properly protected.
A light pedestrian rail or chain link fence may be used on the outside of the sidewalk when a barrier rail is provided on the inside of the sidewalk.

Curbs on Bridges. Curbs are not normally used on bridges except in conjunction with sidewalks. They are prohibited directly in front of guard fence, barrier rail, or traffic rail.

If curbs are used, they are governed by the following criteria:

Curb height must meet or exceed that of the approach roadway.
Curb height must not be less than 5-3/4 in. or greater than 8 in.

Refer to Chapter 2 of the Roadway Design Manual for curb types and considerations.

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Bike Paths

If a bike path is provided on a bridge, the design is governed by the current AASHTO Guide for the Development of Bicycle Facilities.

Additionally, TxDOT has designated the following minimum design criteria:

For new shared lanes on a signed, designated bicycle route, the minimum lane width must be 14 ft.
Widths less than 14 ft. require approval by the Roadway Design Exception Committee.

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Illumination

Lighting of bridge structures should be coordinated with lighting of approach roadways, and location of suitable brackets for mounting light standards should be spaced in cooperation with the roadway design engineer. Suitable conduit and brackets can be provided in the bridge plans to serve those light standards which are to be mounted on the structure.

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Railings

The selection of bridge railing is based on its adequacy to accommodate the design vehicle under design impact conditions. The rail must meet the requirements of the National Cooperative Highway Research Program (NCHRP) 350.

TxDOT has developed many railing types and created standard drawings for use in different situations. These standard drawings are readily available for use on highway bridge projects and must be included in the bridge details. The Bridge Railing Manual discusses the usage of TxDOT railing types.

Under certain conditions, barriers or combination rails must separate sidewalks from vehicular traffic. Refer to Sidewalks and Curbs on Bridges in this section for guidance.

Refer to Chapter 2 of the Roadway Design Manual concerning usage of railings on bridge class culverts.

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Beginning and End of Bridges

For stream crossing structures, slopes of embankments at bridge ends should normally be 2:1 in a direction normal to the abutment cap, and the side slopes normal to the roadway should be no steeper than 3:1. Riprap should be used under the bridge and wrapped around the embankment, terminating when the slope becomes 3:1 or flatter. Steeper slopes may be used for special conditions but should be avoided where possible to allow for easier placement of riprap and greater slope stability. Customary practice calls for 5 in., Class B concrete riprap. For structures in reservoirs the riprap at bridge ends should be the same as that used to protect the roadway embankment, which is usually rock riprap with granular bedding or soil cement riprap.

The slopes indicated above are satisfactory for highway separation structures except that a 3:1 slope may be used in a direction normal to the abutment cap. The flatter slope is encouraged where greater sight distance under a structure is needed. When the 3:1 slope is used, only a shadow type riprap is to be used extending no more than 2 ft. beyond the horizontal projection of the structure. For skewed structures the acute side may be constructed normal to the abutment cap to prevent erosion at edge of riprap. Curbs may also be used at the edges of riprap to prevent erosion. The riprap for highway interchange structures is normally 4 in. Class B concrete riprap.

U-type abutments are not normally used because they have proven more expensive and are difficult to widen; however, in cases of restricted right of way, the U-type abutment may be indicated. Bridge approach slabs should be used at bridge ends for vehicular structures on all major highways. The approach slab covers an area behind the abutment backwall where good compaction of base and sub-base is difficult to obtain. The approach slab will reduce maintenance due to settlement adjacent to abutment backwalls. Such settlement causes increased impact and roughness at bridge ends, subsequent horizontal movement, and cracking of abutment. The approach slab standards are available from the Design Division.

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Design Exceptions, Waivers, and Variances

A design that complies with the department’s design manuals is easy to defend because the standards are generally based on documented engineering research or practice. Any design that is an exception to usual standards requires a documented, logical, evaluation process explaining why the standards are not met. The design exception, design waiver, and design variance procedures establish this documentation.

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Design Exceptions

Design exceptions are required whenever certain controlling criteria for the different types of construction projects (i.e., 4R, 3R, 2R, or Special Facilities) are not met. When less than the recommended guidelines for these controlling criteria are anticipated in the design phase, a formal design exception request should be made to the appropriate design exception committee. Three different committees have been formed to review the different criteria. These are the Roadway Design Exception Committee (RDEC), the Bridge Design Exception Committee (BDEC), and a joint subcommittee, the Roadway/Bridge Design Exception Committee (R/BDEC), composed of selected members from both the RDEC and BDEC.

The composition of the RDEC is as listed in Chapter 3 of the Design Division’s Project Development Policy Manual.

The Chair of the BDEC is the Director of the Bridge Division and the membership is composed of:

Director, Project Development Section, Bridge Division
Director, Design Section, Bridge Division
Director, Field Operations Section, Bridge Division
Project Manager, Project Development Section, Bridge Division- serves a two-year term
Member from a district bridge office recommended by the Director of the Bridge Division and appointed by the District Engineer - serves a two-year term

The Chair of the R/BDEC is the Director of the Design Division and the membership is composed of:

Director, Field Coordination Section, Design Division, serves a two-year term
Director, Roadway Design Section, Design Division, serves a two-year term
Director, Design Section, Bridge Division
Director, Field Operations Section, Bridge Division
Two district members from the RDEC - serve six month terms

These committees will serve as the final arbiter on all design exception requests on projects with state oversight. Design exceptions on projects with federal oversight will be submitted to the Federal Highway Administration (FHWA) for approval. The Bridge Division project manager is the district point of contact when requesting design exceptions that are the responsibility of BDEC or R/BDEC.

The following construction project types will have controlling criteria that dictate a design exception. The responsible design exception committee is listed after each criterion.

New Location and Reconstruction Projects (4R). The list below gives the controlling criteria that will require a design exception:

Design speed (RDEC)
Lane width (RDEC)
Shoulder width (RDEC)
Bridge width (R/BDEC)
Structural capacity (BDEC)
Horizontal alignment (RDEC)
Vertical alignment (RDEC)
Grades (RDEC)
Stopping sight distance (RDEC)
Cross slope (RDEC)
Superelevation (RDEC)
Vertical clearance (R/BDEC)

Resurfacing, Restoration or Rehabilitation Projects (3R). The list below gives the controlling criteria that will require a design exception:

Deficient bridge rails (for high volume roadways) (BDEC)
Design speed (for high volume roadways) (RDEC)
Horizontal alignment (for high volume roadways) (RDEC)
Vertical alignment (for high volume roadways) (RDEC)
Lane width (RDEC)
Shoulder width (RDEC)
Bridge width (R/BDEC)
Structural capacity (BDEC)

Resurfacing or Restoration Projects (2R). Design exceptions are required for 2R projects any time the existing geometric features (RDEC) or bridge features (BDEC) for the proposed project will be reduced.

Low Volume Off-System Bridges. For off-system bridge replacement and rehabilitation projects with current average daily traffic (ADT) of 400 or less, the following design elements must meet or improve conditions that are typical on the remainder of the roadway or a design exception will be necessary:

Design speed (RDEC)
Lane width (RDEC)
Shoulder width (RDEC)
Structural capacity (BDEC)
Horizontal alignment (RDEC)
Vertical alignment (RDEC)
Grades (RDEC)
Cross slope (RDEC)
Superelevation (RDEC)
Minimum structure width, face to face of rail: 24 ft. (R/BDEC)

Off-System Historically Significant Bridge Projects. The list below gives the controlling criteria that will require a design exception if the minimum design criteria listed in the Historic Bridge Manual cannot be met:

Roadway width (R/BDEC)
Load carrying capacity (operating rating) (BDEC)

Bicycle Facilities. Design exceptions are necessary when the minimum requirements given in the AASHTO Guide for the Development of Bicycle Facilities for bicycle lanes cannot be met. (RDEC)

Hydraulic Design Criteria. The Hydraulic Design Manual outlines recommended and required design procedures, criteria, and documentation. If the required design procedures, criteria, etc., are not met, a design exception will be required. A departure from recommended procedures or criteria will not require a design exception. However, the Design Division’s Hydraulics Branch should be consulted to ensure that alternate procedures are appropriate. The BDEC will consider requests for exceptions to required hydraulic design criteria.

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Design Waivers

When criteria are not met in a non-controlling category, a design exception will not be required. In these cases design waivers at the district level will handle the variations from the recommended design criteria. Design waivers will be granted as the district authorizes. The complete documentation should be retained permanently in the district project files, or it may be forwarded to the Design Division for retention. For a listing of the types of non-controlling criteria that will require a design waiver, see Design Exceptions in the Roadway Design Manual.

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Design Variances

A design variance is required whenever the design guidelines specified in the Americans with Disabilities Act Accessibility Guidelines (ADAAG) and the Texas Accessibility Standards are not met. Design variances should be sent to the Design Division for forwarding to the Texas Department of Licensing and Regulation for approval.

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Corrosion Protection Systems

Protecting reinforcing steel is critical to the design life of a concrete structure. The following methods of inhibiting corrosion, either in combination or alone, are currently used by TxDOT:

Additional concrete cover
Low permeable concrete (with slag or pozzolans such as fly ash)
Epoxy coated rebar
Linseed oil treatment
Corrosion inhibitors
Cathodic protection
Concrete class specification

Climatic conditions usually determine which structures to protect. Guidelines for the appropriate use of corrosion protection systems within the state can be found on the TxDOT website.

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Loads on Bridge Decks

Design Loads. All new bridges must be designed for a minimum of HL93 loading. This load must be used in the design of a bridge as specified in the latest edition of the AASHTO Load and Resistance Factor Design Bridge Specifications.

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Excavation Protection Requirements

In accordance with state law, whenever a project involves trench excavations deeper than 5 ft., a bid item must be included in the contract to compensate the contractor for determining or providing the specified safety precaution system. If the excavation is for linear installations such as pipe and conduit, then Standard Specification Item 402, “Trench Excavation Protection,” should be used.

However, if there are special shoring requirements, Standard Specification Item 403, “Temporary Special Shoring,” should be used. The limits of the temporary special shoring should be clearly shown on the plans. The special shoring should be designed in accordance with Item 403.

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Bridge Joints

Bridge deck joints have proven to be both a construction and a maintenance problem and, as such, should be used only as required by design. Where they are necessary, the type and size of the joint will be determined by the type of superstructure, the length of structure that is contributing to the expansion that must be handled at the joint location, and the need to seal the joint against water leakage.

Bridge deck continuity, which minimizes the number of expansion joints, is recommended. All expansion joints in deicing zones should be sealed or drained. Stream crossing structures may use open joints in most cases, although environmental concerns may necessitate sealed joints for some structures. Joints for all grade separation structures should be sealed.

For new construction requiring sealed joints, the recommended joint types for various superstructures are as follows:

Pan Form Girder Units: SEJ-A, sealed elastomeric concrete, or sealed armor joints
Prestressed Box Beam Units: SEJ-A, sealed elastomeric concrete, or sealed armor joints
Prestressed Concrete Beam Units: SEJ-A, or SEJ-P for structures with heavy truck traffic
Steel Girder Units: SEJ-P for required movement of 5 in. or less, or SEJ-A (Mod) Finger joints with drainage troughs for larger movements

For new construction not requiring sealed joints, the recommended joint types for various superstructures are as follows:

Slab Spans and Units: Type “A” Joint with preformed expansion joint material and poured top. Use 1 in. thickness for continuous units; 1 1/2 in. for simple spans.
Pan Form Girder Units: Open steel plate armor joints
Prestressed Box Beam Units: Open steel plate armor joints
Prestressed Concrete Beam Units: Open steel plate armor joints
Steel Girder Units: Open steel plate armor joints or finger joints

For the retrofit or repair of existing expansion joints that are leaking or otherwise malfunctioning, the following methods have been successfully employed:

Asphaltic Plug. A slab of rubberized asphaltic concrete is placed over a 1/8 in. plate attached to the open joint. Total movement of 1 1/2 in. has been accommodated.
Elastomeric Concrete or Polymer Nosing. A specially designed and constructed material is used to rebuild spalled joints. It can be used with various sealing systems to waterproof the joint.
Class 7 Silicone. A rapid curing sealing material is placed on a foam backer rod in the sand blasted armor joints. This should be limited to 3 in. opening and 1/2 in. total movement.

Expansion joints should utilize the latest standard drawings for armor joints and sealed expansion joints (SEJ). Contact the Bridge Division Construction/Maintenance Branch for details of polymer nosing and other retrofit type joint systems.

In addition to transverse bridge deck joints, occasionally the design parameters are such that a longitudinal joint will be required in the bridge deck. This can be the result of extreme bridge width, jumps in elevation across the width of the deck, or construction phasing requirements. In all cases these longitudinal joints should be placed next to a bridge rail or concrete traffic barrier (CTB), where possible, and should not be placed in traffic lanes due the potential hazard to motorcyclists. Longitudinal joints may be required to be sealed against leakage in a manner similar to transverse joints.

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Stage Construction-—Existing Structure Removal

The removal of the existing structure is in accordance with Standard Specifications Item 496, “Removing Old Structures.”

The partial removal of an existing structure begins with cutting and removing the slab. The location of the cut is called the breakback. The approximate location of the breakback is determined through coordination with the traffic and highway engineer and is based on lane width requirements of both the new structure and the partial structure to remain in place. The bridge designer should determine the exact breakback point and base it on the structural capacity of the existing structure.

The breakback is generally located over a beam and must be supported by a stable substructure. After the slab is cut and removed, the beams are removed and the substructure, or a portion thereof, is demolished. If necessary, footings are removed and drilled shafts and piles are cut and removed to a distance a minimum of 2 ft., or as specified in the plans, below the proposed ground.

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Stage Construction—New Substructure

The following are guidelines for the design of the new substructure.

Foundations. Minimum drilling and pile driving clearances must be observed. They are as follows.

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	Minimum Vertical Clearance	Minimum Horizontal Clearance
Drilling	12 ft.	2 ft.
Pile Driving	20 ft.	2 ft.

If possible, avoid locations of existing foundations. For widenings, foundations should be of similar type as those remaining in use.

Abutments. At the stage construction joint, it is difficult to leave reinforcing steel projecting from the abutments for splicing because of the conflicts with temporary shoring that must retain the fill. Instead, locate foundations (drilled shafts or piling) close to the stage construction joint and dowel the two sides of the cap together, or provide a sealed expansion joint.

Interior Bents. If possible, use independent bents. If a single structure is required, the reinforcing steel can be spliced together using a lap, a mechanical coupler, or butt weld. If splicing is used, adequate horizontal and vertical clearances must be provided to account for the projecting reinforcement. The exposed reinforcement must be protected. If available clearances are limited, use mechanical couplers or butt welds. Due to the complexity of couplers and welds, accurate details and proper structural detail notes are essential.

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Stage Construction—New Superstructure

The following are suggestions for the design of the new superstructure.

The location of the stage construction joint in the slab and the available clear distance for splicing the mat reinforcing are critical factors in the slab design.

The stage construction joint can be placed over a supporting beam or in a bay between beams. Placing the stage construction joint over a supporting beam is the preferred method. When placing the joint over a supporting beam, the joint must be located 2 in. beyond the centerline of the beam. When placing the joint between beams, locate the joint at the quarter point of the beam spacing. The bay containing the joint must not utilize prestressed concrete panels.

The available construction clear distance may limit the available length required for adequate lap length. If the clear distance is inadequate, mechanical couplers can be utilized. However, there are concerns about the performance of a construction joint using couplers in both mats, particularly in areas susceptible to salt contamination. In such instances, consider raising the grade a few inches to allow lapping the top mat bars to clear above the existing deck. If couplers are used, be sure the appropriate specifications are supplied.

The design of the beam nearest to the stage construction joint should be given additional attention. At first an exterior beam, this beam becomes an interior beam once the adjacent stage is constructed. The beam spacing between stages may also be different. Given these considerations, the designer must account for the worst case live load distribution factor in the design of the beam.

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Temporary Railing

For guidelines on selection and placement of temporary railing, refer to the Bridge Railing Manual.