Chapter 3: Preliminary Design Features

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Section 1: General Features

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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 on the Texas Department of Transportation (TxDOT) main website (use the browser’s “Edit-->Find” menu to locate individual drawings.) The website 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, carry the full usable shoulder width of the approach roadway across the structure. Conform bridge widths to the requirements in Chapter 3 of the Roadway Design Manual, which presents the design criteria for 4R projects for various roadway functional classifications and traffic volumes. Construct bridge widths for structures in complex interchanges containing flares, gores, etc. to full width of the approach roadway, as well.

For non-freeway rehabilitation projects (3R) where the bridge structures are to be modified, set bridge widths at least to the approach roadway width. Otherwise, conform bridge widths to the requirements in Chapter 4 of the Roadway Design Manual, which presents the design criteria for 3R projects 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

In planning stages, the length of the bridge is an approximation based on available preliminary information which becomes more refined as the project progresses. The length of the bridge depends on such factors as existing topographical conditions at the site, the width of the obstruction being crossed (other roads, waterway, railroad tracks, etc.), the roadway alignment, highway design criteria (sight distance, maximum grades, etc.), economics, and plans for future development. When determining preliminary bridge lengths, set the “begin bridge” point and “end bridge” point at whole station numbers and on a tangent alignment, if possible. This geometry can be accommodated by moving the point of curvature (PC) or the point of tangency (PT) off the bridge, if allowable.

The number of spans, length of spans, and bent locations can be determined once the preliminary bridge length is set. Where bridge geometry and site conditions allow, place bents such that interior span lengths are equal. If possible, locate the bents at whole station numbers. If the bridge is crossing a stream, spanning the channel is recommended to decrease the probability of future scour issues.

Span length requirements limit the available options for superstructure. Select the most economic superstructure type that meets span length requirements and satisfies aesthetic needs at the site. Recommended span lengths, approximate depths, and associated bridge costs for various superstructure types can be found on the TxDOT Bridge Division (BRG) website. The process of setting bridge geometry consists of iterative steps that take place during development of preliminary bridge layouts. During this process, the district and divisions coordinate to develop a plan for an economically feasible, aesthetically pleasing structure that serves its design purpose.

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Vertical Curvature

Conform bridge vertical curvature 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,

Be aware of the following important factors when determining the vertical alignment of a structure:

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  • On controlled-access highways where crossover roads intersect frontage roads near the main lanes, set the vertical curvature of the crossover structure to allow adequate sight distance for crossover and frontage road traffic. Locate intersections farther away from bridges to provide adequate sight distance on steep crossover grades, if necessary.
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  • In areas where icing is prevalent, design structures with a flatter grade 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.
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  • On long, flat grades, use a small crest vertical curve throughout the bridge length to prevent the illusion of sag and to improve deck drainage.
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Horizontal Alignment

Place a bridge structure on tangent alignment if this can be accomplished without sacrificing the overall geometric design of the highway. Tangent alignment results in lower structure costs by simplifying plan preparation and bridge construction. Consider overall project economics when setting horizontal alignment for the structure. While building structures on a tangent alignment is generally more economical, this may not be feasible in areas with high right-of-way costs. Build curved structures where their geometry fits 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

Build structures on a skew if necessary to match the alignment of roadways, railroad tracks, or stream flow. If a skew is required, consider the following:

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  • Normally, skews should 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 usually will require special design considerations.
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  • For railroad overpasses, place bents parallel to railroad track alignment, if possible.
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  • For railroad underpasses, each railroad company may have its own limitations on acceptable skew angle.
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  • Skewed structures that have horizontal curvature require special geometric and structural design, and additional time will be required for plan preparation.
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  • Slab breakbacks and special slab reinforcing details may be required. Refer to the Bridge Detailing Guide for more information.
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Superelevation, Transitions, and Cross Slopes

Minimize the superelevation rate (emax) whenever possible. Ranges of emax are necessary. Hold the maximum superelevation rate on structures 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. Where roadway superelevation rates are less than 8%, match the superelevation and transition rates on structures to those specified for the sections of roadway. Basic design criteria on superelevation can be found in Chapter 2 of the Roadway Design Manual.

Do not use spiral transitions 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, transition the approach roadway slope to fit the bridge.

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Bridge Medians

Use a 6-foot median width for pedestrian refuge in accordance with Public Rights-of-Way Accessibility Guidelines (PROWAG) in urbanized settings on new construction projects and where practical on reconstruction projects.

Median widths are measured between the inside edges of opposing travel lanes. Where narrow medians (4 ft. to 16 ft.) are used, 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, use a single structure with a closed flush median and a median barrier extended uninterrupted throughout the structure length. When the median width exceeds 30 ft., construct dual structures 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

Consider pedestrian and driver safety when sidewalks are provided on bridges. Make pedestrian facilities accessible to all persons and design them 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 in Chapter 4, Section 1 of this manual and Chapter 2 of the Roadway Design Manual.

The need for sidewalks usually occurs in urban areas, on frontage road bridges, or where a depressed highway crosses under a city street. In urban areas, consider placing sidewalks on both sides of any new construction or reconstruction bridge project. Provide a suitable barrier rail or combination railing, if required. The use of barrier rail to separate vehicular from pedestrian traffic is governed by the following criteria:

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  • Appropriate barrier rail is required when the design speed >= 50 mph.
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  • For design speeds >= 45 mph, but < 50 mph, consider appropriate barrier rail where bridge site specific conditions allow without interference to pedestrian movements, intersecting roadways, or other features.

Consider the following additional information when selecting barrier rails at sidewalks:

Curbs on Bridges. Do not use curbs on bridges except in conjunction with sidewalks. Do not use curbs directly in front of guard fence, barrier rail, or traffic rail.

If curbs are used:

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 AASHTO’s current Guide for the Development of Bicycle Facilities.

Additionally, TxDOT has designated the following minimum design criteria:

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  • The minimum lane width is 14 ft. for new shared lanes on a signed, designated bicycle route.
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  • The 14-ft. usable lane width for shared use in a wide curb lane is measured from the edge stripe to the lane stripe or from the longitudinal joint of the gutter pan to the lane stripe. The gutter pan should not be included in the usable width. Do not include the curb offset as part of the usable lane width for a shared use in a wide curb lane.
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  • Widths less than 14 ft. require a design exception.
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  • Provide a 5-ft. shoulder (4-ft. shoulder and 1-ft. barrier offset) on the structure and along the adjacent barrier for all projects involving bridge replacements or bridge deck replacements/rehabilitations of on-system roadways or off-system roadways with greater than 400 ADT.
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Illumination

Coordinate lighting of bridge structures with lighting of approach roadways. Cooperate with the roadway design engineer to determine the locations of light mounting brackets. Include details for conduit and brackets for all lighting mounted to the structure. Standards for bridge lighting details can be found on BRG’s standards web page.

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Railings

Select bridge railing that is adequate to accommodate the design vehicle under design impact conditions. The rail must meet the requirements of the AASHTO Manual for Assessing Safety Hardware (MASH), or the National Cooperative Highway Research Program (NCHRP) 350. All newly implemented rails should meet MASH requirements.

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 should be included in the bridge details. The Bridge Railing Manual provides further guidance on 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 and Chapter 2 of the Bridge Railing Manual for information on the use of railings on bridge-class culverts.

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Beginnings and Ends of Bridges

Stream Crossings. For stream crossing structures, make the slopes of embankments at bridge ends a maximum of 2:1 in a direction normal to the abutment cap. Side slopes should be normal to the roadway and no steeper than 3:1. Use stone riprap (preferred) or concrete riprap 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 revetment and greater slope stability. The Bridge Division recommends using flexible revetment (stone protection, interlocking articulated concrete blocks, gabion mattresses, etc), where possible. If flexible revetment is not possible, then use 5-in., Class B concrete riprap (RR8) for stream crossings. For structures in reservoirs, make the revetment at bridge ends the same as that used to protect the roadway embankment at stream crossings, which is usually stone protection or soil cement riprap.

Overpasses. The slopes indicated above are also satisfactory for highway overpass structures, except that a 3:1 slope may be used in a direction normal to the abutment cap. Use a flatter slope where greater sight distance under a structure is needed. When the 3:1 slope is used, it is common to use only a shadow type revetment, extending no more than 2 ft. beyond the horizontal projection of the structure. However, various factors (geometry, soil conditions, etc.) could cause the revetment to extend a greater distance beyond the horizontal projection of the structure. If using the shadow type revetment, for skewed structures, construct the acute side normal to the abutment cap to prevent erosion at the edge of riprap. Curbs may also be used at the edges of riprap to prevent erosion. Use 4-inch Class B concrete riprap (RR9) for highway overpass structures.

Approach Slabs. Use bridge approach slabs 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. Using approach slabs 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 the abutment. The approach slab standards are available on BRG’s standards web page.

For roadways that cross streams that could potentially meander, consider designing the abutment(s) as interior bents. This would allow the backwall to be removed and the bridge extended if needed. This does not add any additional cost to the structure.

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Design Exceptions, Waivers, and Variances

A design that complies with the Department’s design manuals is 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

As of May 5, 2016, the Federal Highway Administration’s (FHWA’s) guidance memorandum, available in docket FHWA-2015-0020, has revised the policy on the controlling criteria for design and documentation of design exceptions. In 1985, FHWA designated the original 13 controlling criteria for geometric design. This new revision reduced the criteria to ten, which focuses the application on NHS, high-speed roadways (i.e. design speed >= 50 mph). Only two criteria apply on NHS, low-speed roadways (i.e., design speed < 50 mph).

Design exceptions are required whenever the controlling criteria mentioned above are not met. A formal design exception should be processed when the designer anticipates that the recommended guidelines for these controlling criteria cannot be met. The determination of whether a design exception exists rests with either the district engineer or the Bridge Design Exception Committee (BDEC), depending on the controlling criteria in question.

The only criteria requiring the BDEC’s design exception approval are structural capacity and bridge rails. All other criteria require the district engineer’s approval. Refer to the Roadway Design Manual and the Project Development Processes Manual for more information on these criteria.

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

The district engineer or BDEC will serve as the final arbiter on all design exception requests on projects with state oversight. Design exceptions on projects with federal oversight or on the interstate system will be submitted to the FHWA for approval. The Bridge Division project manager is the district point of contact when requesting design exceptions that are the responsibility of the BDEC.

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Design Waivers

When criteria are not met in a non-controlling category, a design exception is not required. In these cases design waivers at the district level will handle the variations from the recommended design criteria. Design waivers will be granted at the District’s authority. Permanently retain the complete documentation in the district project files and furnish a copy to the Design Division. 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

Request a design variance whenever the design guidelines specified in the Americans with Disabilities Act Accessibility Guidelines (ADAAG) and the Texas Accessibility Standards are not met. Send design variances to the Design Division to be forwarded 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:

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

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Loads on Bridge Decks

Design all new bridges for a minimum of HL93 loading according to the most current edition of the AASHTO Load and Resistance Factor Design Bridge Specifications.

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Excavation Protection Requirements

In accordance with state and federal laws, whenever a project involves excavations for linear installations such as pipe and conduit, equal to or deeper than 5 ft., include Item 402, "Trench Excavation Protection," in the contract to compensate the contractor for determining or providing the specified safety precaution system.

If the existing embankment is removed more than five feet below the existing ground, then Item 403, "Temporary Special Shoring," is required as a pay item. Clearly show the limits of the temporary special shoring on the plans.

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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, determine the type and size of the joint by the type of superstructure, the length of structure that is contributing to the expansion to be handled at the joint location, and the need to seal the joint against water leakage.

Use bridge deck continuity, which minimizes the number of expansion joints, when possible. Seal or drain all expansion joints in deicing zones. Use open joints in most stream crossing structures; however, environmental concerns may necessitate sealed joints for some structures. Seal joints for all grade separation structures.

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

Design parameters occasionally require a longitudinal joint in the bridge deck to accommodate extreme bridge width, jumps in elevation across the width of the deck, or construction phasing requirements. In all cases, place these longitudinal joints next to a bridge rail or concrete traffic barrier (CTB). Do not place them in traffic lanes due to the potential hazard to motorcyclists. If needed, seal longitudinal joints against leakage in a manner similar to transverse joints.

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Stage Construction-—Existing Structure Removal

Specify removal of the existing structure is in accordance with Standard Specifications Item 496, “Removing 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 location of the breakback point based 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 minimum distance a minimum of 2 ft., or as specified in the plans, below the proposed ground.

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Stage Construction—New Substructure

Below are a few general rules of thumb for staged substructure design. Please refer to the Geotechnical Manual for more information about substructure design for stage construction.

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Work Type

Minimum Horizontal Clearance

Drilling

2 ft.

Pile Driving

2 ft.



Foundations. If possible, avoid locations of existing foundations. For widenings, see the Geotechnical Manual, Chapter 5, Section 1, for guidance.

Abutments. At abutments, temporary special shoring should be located at a sufficient distance to allow the reinforcing steel to be projected from the abutments for splicing. If this is not possible, 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 using a lap, a mechanical coupler, or butt weld, depending on space constraints. If splicing is used, provide adequate horizontal and vertical clearances to account for the projecting reinforcement. Protect the exposed reinforcement. 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 critical factors in slab design are 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. Please refer to the TxDOT BRG web page for more information on superstructure design for phased construction.

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Temporary Railing

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

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