Bridge Design Manual-Texas Department of transportation

Bridge Design Manual-Texas Department of transportation Load and Resistance Factor Design (LRFD) is a design methodology that makes use of load and resistance factors based on the known variability of applied loads and material properties. In 1994, the American Association of State Highway and Transportation Officials (AASHTO) published the first AASHTO Load and Resistance Factor Bridge Design Specifications. The Federal Highway Administration (FHWA) mandated the use of LRFD for all bridges for which the Texas Department of Transportation (TxDOT) initiated preliminary engineering after October 2007.

Bridge Design Manual

December 2001

© by Texas Department of Transportation

(512) 416-2055 all rights reserved

Manual Notice 2001-1

To: Districts, Divisions and Offices

Deputy Executive Director

Effective Date: December 1, 2001

Purpose

This manual provides policies and guidelines set forth by TxDOT regarding the design ofbridges It has been developed to help bridge designers working on TxDOT projects

Instructions

This is a new manual containing new and significantly reorganized material It supersedes

the 1990 Bridge Design Guide and the 1990 Bridge Design Examples, both first editions.

Contents

The manual contains ten chapters – Organizational Overview, TxDOT and Bridge Design,Design Specifications, Geometric Restraints, Preliminary Considerations, General DesignControls, Superstructure Design, Substructure Design, Special Designs, and FoundationDesign The manual also has four appendices

Contact

For more information regarding any chapter or section in this manual, please contact theDesign Section of the Bridge Division

Chapter 1 Organizational Overview

Contents:

Section 1 — This Manual 1-2Section 2 — Evolution of the TxDOT Bridge Division 1-4

Chapter 1 — Organizational Overview Section 1 — This Manual

Section 1 This ManualOverview

This manual was developed to provide bridge designers working on Texas Department ofTransportation (TxDOT) projects with the policies and guidelines set forth by TxDOTregarding the design of bridges

Its purpose is to improve the bridge design and detailing process by promoting uniformityamong bridge designers working on TxDOT projects

This manual is subject to revision as conditions, experience, or research data warrant

Changes will be issued by Manual Notice Changes may be in the form of new sheets to beadded, revised sheets to replace superceded ones, or sheets to be deleted

The manual is not intended to be a complete substitute for engineering experience,

knowledge, or judgment Special situations may arise that appear to call for variation fromthe policy requirements herein Such variation will be subject to approval of the

administration of the TxDOT Bridge Division

Direct any questions or comments on the content of the manual to the Director of the DesignSection of the Bridge Division, Texas Department of Transportation

Bridge Design Manual Format

The manual begins with this overview of the manual and a description of the evolution ofthe TxDOT Bridge Division The chapters that follow include information on TxDOTDivisions/Sections, design specifications, geometric restraints, preliminary considerations,general design controls, superstructure design, substructure design, special designs, andfoundation design The following paragraphs briefly discuss these chapters:

Chapter 2 presents a description of the TxDOT Divisions/Sections primarily involved inbridge design, planning, construction, and maintenance and provides descriptions of theresponsibilities of the TxDOT Bridge Division’s Bridge Design Section

Chapter 3 lists and briefly describes the governing design specifications, or “Rule Books,”involved in bridge design The chapter includes information on mandatory specifications,guide specifications, and industry recommendations

Chapter 4 discusses the common roadway geometric restraints inherent in bridge design.Bridge widths, span lengths, clearances, and alignment are discussed A section on theconstraints involved during stage construction is also included

Chapter 5 presents some common aspects a designer/planner must consider during thepreliminary planning and design process These aspects include materials, structure type,

Chapter 1 — Organizational Overview Section 1 — This Manual

economics, and aesthetics A discussion on bridge railing and use of corrosion protection isalso included

Chapter 6 discusses in greater detail some of the more common design specifications

involved during the design of a bridge, giving the designer additional information on theapplication and usage of common design specifications and criteria

Chapter 7 presents design criteria and design guidance for the most commonly used

superstructure types, including cast-in-place, precast, and steel superstructures Backgroundinformation on the development of each superstructure type is also included

Chapter 8 presents design criteria and design guidance for the most commonly used

substructure items, including caps, columns, and foundations Background information onthe development of some of these items is also included

Chapter 9 presents design criteria and design guidance for designs that inherently involveunique aspects, culverts and drainage, bridge appurtenances, sign bridges, and some

common bridge items Background information on the development of some of these

designs is also included

Chapter 10 discusses in greater detail the relationship between structural design and

geotechnical design Some guidance on bridge foundation designs and retaining wall

designs is included, as well as background information on the development of some of theseitems

Chapter 1 — Organizational Overview

Section 2 — Evolution of the TxDOT Bridge

Division

Section 2 Evolution of the TxDOT Bridge DivisionOrigin of the TxDOT Bridge Division

The Texas Highway Department was established in 1917 and is responsible to the Governor

of Texas to design, construct, and maintain an adequate system of highways in the state In

1918, a Bridge Office was created with the primary responsibility of preparing standarddesigns and drawings in an attempt to bring some uniformity to the bridges being

constructed by the counties The Bridge Division appeared in 1928, retaining bridge design

as a big part of its mission

The Bridge Division continued to maintain standards and design non-standard bridges Intime, advance planning, railroad negotiations, and plan review capabilities were developed.Construction management was provided for some of the more complicated structures

1940s and 1950s

Activities were curtailed during the war years, but in the late 1940s and 1950s increaseddemand for improved infrastructure produced a large volume of expressways, for whichspecial design offices were established in the affected cities Some of the groups adoptedtheir own design and detailing standards

When welding began to replace rivets for field splices in steel beams and girders in the early1950s, the Bridge Division sent qualified welders to the larger projects to help with qualityassurance and quality control

In the middle 1950s the Bridge Division, with the cooperation of precast manufacturers,developed a group of standard pretensioned concrete beams, which quickly proved to be themost economical way to construct medium-span length bridges

When the Interstate Highway System was inaugurated in the middle 1950s, the design

workload increased dramatically and has remained generally good to date Between theexpressway offices, district design groups, and the Bridge Division, plan preparation washandled for several years with a minimum of help from consulting engineers

Chapter 1 — Organizational Overview

Section 2 — Evolution of the TxDOT Bridge

Division

Bridge Division continues to prepare its share of structure plans while attending to a

growing number of non-engineering responsibilities

Chapter 2 TxDOT and Bridge Design

Contents:

Section 1 — Coordinating with TxDOT Divisions and Sections 2-2Section 2 — Primary Responsibilities of the Bridge Design Section 2-3Section 3 — Coordination Responsibilities of the Bridge Design Section 2-7Section 4 — Contractive Responsibilities of the Bridge Design Section 2-10

Chapter 2 — TxDOT and Bridge Design

Section 1 — Coordinating with TxDOT Divisions

and Sections

Section 1 Coordinating with TxDOT Divisions and SectionsOverview

Before a bridge is designed, critical preliminary functions described in the

Bridge Project Development Manual must be completed The planning and design of abridge project involves several divisions and sections within the Texas Department of

Transportation (TxDOT) Some of the contributing entities are:

♦ Bridge Division

♦ Design Division, Field Coordination Section

♦ Transportation Planning and Programming Division

♦ Traffic Operations Division, Railroad Section

♦ Environmental Affairs Division

♦ Construction Division, Materials Section

♦ Maintenance Division, Maintenance Operations Section

Within the Bridge Division, the Bridge Design Section is responsible for functions thatinclude engineering and non-engineering aspects of bridge design These responsibilitiesare discussed in this chapter

Chapter 2 — TxDOT and Bridge Design

Section 2 — Primary Responsibilities of the Bridge

Design Section

Section 2 Primary Responsibilities of the Bridge Design SectionOverview

The primary responsibilities of the Bridge Design Section are structural design and thepreparation of working drawings or plans These primary responsibilities involve a

procedure that begins with a concept to construct a highway facility and concludes with thesubmission of finalized plans, specifications, and estimates (PS&E) The procedure includesmany steps and it is important to know these steps to fully grasp the responsibilities of theBridge Design Section Generally, the procedure for preparation of plans by the BridgeDesign Section is as follows

Consultation Consultation between the Bridge Design Section, bridge project development

manager, and the district design engineer, district bridge engineer, and/or area engineershould precede determination of structure type and scheduling of the letting

Note: This is a good time to make a preliminary decision about who will prepare the bridge

plans

Preliminary Bridge Layouts The area engineer or the project’s designated consulting

engineer prepare preliminary bridge layouts These layouts are usually complete with

geometric controls, type, size, length of spans, hydraulic data, required clearances, soil testboring data, classification of highway, and projected traffic At this time, type of foundationshould be proposed and conveyance of water through stream crossings and scour analysisshould be addressed and coordinated with the Hydraulics Section

Note: Area engineers and consulting engineers are encouraged to contact the Geotechnical

Branch for advice if there is any question regarding the proper foundation

The layouts are sent to the bridge project development manager who will forward them tothe Design Division, the Federal Highway Administration (FHWA) on federal oversightprojects, or other agencies that may exercise review authority

Bridge Plan Preparation When approval has been secured from all the appropriate

agencies, timing for the plan work is re-negotiated with the district and the job of bridgeplan preparation is given to the bridge design engineer or to the consultant

The following steps apply to the Bridge Design Section, but the routine for consultants will

be similar

Note: If consultants are unsure about the current design or detailing standards for an item,

they are encouraged to contact the director of the Bridge Design Section

♦ The director of the Bridge Design Section assigns the work to a Design Group

according to its particular expertise in that type of design, and primarily on its ability tocomplete the plans in the required length of time

Chapter 2 — TxDOT and Bridge Design

Section 2 — Primary Responsibilities of the Bridge

Design Section

♦ The Design Group leader schedules engineering and detailing work for the job

according to the target completion date and the Design Group’s other commitments

♦ The Geotechnical Branch is contacted early if there is any doubt about the foundationtype When foundation loads have been determined, the Geotechnical Branch will beasked to establish founding elevations

♦ When the plans are complete, the bridge project manager sends prints to the district

♦ Reviewed prints are returned to the bridge project manager and the Design Group

makes any revisions required by the district review

♦ Originals, including all reproducible standard drawings are sent to the district

♦ Project plans, specifications, and estimates are sent to the Design Division Any

revision required by the pre-letting review are made by the Design Group

♦ After letting, optional designs for prestressed concrete beams are submitted by thefabricator and checked early Other shop drawings follow

Note: Consulting engineers should be aware that shop drawings require a significant

amount of checking time They should budget their labor accordingly

♦ Occasionally, bridge members that were fabricated beyond specification tolerances arereviewed by the Bridge Design Section for structural adequacy when properly repaired.Also, occasionally, construction problems arise that require review for structural

Stream Crossings Stream crossings carry highway traffic over creeks, rivers, bayous,

channels, and bays Hydraulic considerations are usually involved Clearances for marinetraffic may be required

Grade Separation Structures Grade separation structures occur where one roadway must

cross over another Clearance for the overpassed traffic is critical Highway over highwayseparations are called overpasses if the project highway passes over; otherwise they arecalled underpasses

Railroad Underpasses Railroad underpasses are where a highway passes under an

intersecting railroad Design is under strict control by the railroad companies

Miscellaneous Structures Miscellaneous structures include sign bridges, illumination

Chapter 2 — TxDOT and Bridge Design

Section 2 — Primary Responsibilities of the Bridge

Design Section

Culverts and End Treatment Culverts carry storm water under highways The larger of

these culverts are reinforced concrete boxes Most culverts are constructed from standarddrawings Under some conditions Safety-End Treatment is required to protect errant

vehicles These conditions are outlined in the TxDOT Roadway Design Manual

Retaining Walls Some types of retaining walls require structural design Mechanically

stabilized earth (MSE) wall designs require mostly geotechnical considerations Detailedplans for MSE walls are prepared by the successful wall supplier

Geotechnical Design A special group within the Bridge Technical Services Section

performs geotechnical design

Preparation and Approval of Working Drawings

Preparation and approval of working drawings involve assuring that the various

requirements of the design are shown on plan-size (22 x 34 in.) or half-size (11 x 17 in.)drawings as completely and accurately as necessary to allow the structure to be built

according to the design Half-size sheets are preferred by the department The use of size sheets is being phased out

full-Bridge Plans Drawings for construction projects must contain accurate quantities of the

various items of work so that the contractor can be adequately reimbursed according to theunit bid prices

Bridge Standards Bridge standards are maintained by the Bridge Standards Branch of the

Technical Services Section Standard drawings contain often-used systems and details thatcan be used in bridge plans without modification Standards are indexed on the main

TxDOT website under Business/TxDOT CAD Standard Plan Files (See

http://www.dot.state.tx.us/insdtdot/orgchart/cmd/cserve/standard/disclaim.htm.) The websitecontains instructions about the use of the graphics files

Preliminary Bridge Layouts Preliminary bridge layouts are reviewed and approved by the

Bridge Design Section Layouts are initiated by the district, sometimes with assistance by aconsulting engineer See the Bridge Project Development Manual for the submittal process

Note: For major structures, coordinate with the Bridge Design Section through the project

development manager as early as possible

PS&E Plans, specifications, and estimates contain structural details prepared by the

district, consulting engineers, or the Bridge Design Section In any case a review is

necessary to determine that the structure is safe and reasonably economical

Shop Drawings The Field Operations Section of the Bridge Division coordinates and

checks shop drawings The Bridge Design Section assists the Field Operations Sectionwhen necessary and will check shop drawings for structures that they have designed

Consultants must check their own shop plans

Chapter 2 — TxDOT and Bridge Design

Section 2 — Primary Responsibilities of the Bridge

♦ Segmental prestressed concrete

♦ Retaining wall systems requiring shop drawings by specification

♦ Sound barrier walls

♦ Bearing pads and other structural bearings

♦ Various bridge joints (armor joints, sealed expansion joints, finger joints, etc.)

♦ Bridge protective assemblies

♦ Overhead sign bridges

♦ Concrete piling

♦ Prefabricated pedestrian bridges

Chapter 2 — TxDOT and Bridge Design

Section 3 — Coordination Responsibilities of the

Bridge Design Section

Section 3 Coordination Responsibilities of the Bridge Design Section

of the effects of these vehicles on the bridges along the proposed route The Bridge

Construction Section will approve or disapprove accordingly

Structures Research Management Committee (RMC 5)

TxDOT carries on a very extensive research program, primarily through the Highway

Planning and Research Program In the area of Structures and Hydraulics, the Bridge

Design Section and some districts provide technical support for the various projects Overmany years, this program has generated a significant library of reports on various aspects ofstructural and hydraulic design Many of the findings have been incorporated in the designspecifications or procedures

Preliminary Consultation Regarding Structure Type

Preliminary consultation regarding structure type is a very important duty, especially whenbridge plans are to be prepared by the districts or the consulting engineers Once the PS&Eare submitted it is usually too late to change systems or major details

Note: For major bridges, especially, interaction with the Bridge Design Section is desirable

during the preparation of preliminary layouts

Consultation During Plan Preparation by Others

Consultation during plan preparation by others, especially a consulting engineer, can

alleviate misunderstandings and avert delays

Note: No matter how busy the workload, the Bridge Design Section will try to make

someone available for consultation in a timely manner

Chapter 2 — TxDOT and Bridge Design

Section 3 — Coordination Responsibilities of the

Bridge Design Section

Scheduling Plan Work

Scheduling plan work is usually a negotiation process with the district The Bridge DesignSection will consider workload and available personnel, allowing the details to be completed

to meet a realistic target date Chapter 5 of the TxDOT Bridge Project Development Manual

contains suggested lead times for submitting bridge layouts

Oversight

The determination of federal or state oversight for highway projects is discussed in

Chapter 3 of the TxDOT Bridge Project Development Manual When the state has oversight

responsibilities, the Bridge Design Section will exercise oversight of all aspects of bridgestructural design

Interaction with Outside Agencies

Outside agencies are often involved in various aspects of bridge planning and design

Chapter 3, Section 3 of the TxDOT Bridge Project Development Manual lists and describes

many of these agencies Additionally, the Bridge Design Section has close working

relationships with the following agencies:

Federal Highway Administration One of the most important interactions since the advent

of the Interstate Highway System has been with the Federal Highway Administration Theyhave maintained strong bridge sections in Washington (Headquarters), Atlanta (SouthernResource Center), and Austin (Texas Division)

National Committees The Bridge Design Section represents TxDOT on several national

committees and organizations that furnish information and develop procedures for structuraldesign

♦ The American Association of State Highway and Transportation Officials (AASHTO)The director of the Bridge Division represents Texas on the prestigious American

Association of State Highway and Transportation Officials Highway Subcommittee onBridges and Structures This organization is responsible for writing and revising thestructural design specifications to be followed by all 50 states They publish

construction specifications also, but these are modified heavily by our own TxDOT

Standard Specifications for Construction of Highways, Streets, and Bridges The

director of the Bridge Division and/or their representatives meet annually with the fullbridge committee The group is divided into several subcommittees, each with a

specific structural system to monitor for possible improvements to the specification.Specification revisions usually originate in these subcommittees Texas has been able

to influence the specifications by the results of local research

♦ The American Railway Engineering and Maintenance-of-Way Association (AREMA)AREMA is the organization that controls everything associated with railway

engineering and maintenance and publishes a specification that must be followed when

Chapter 2 — TxDOT and Bridge Design

Section 3 — Coordination Responsibilities of the

Bridge Design Section

♦ Transportation Research Board (TRB)

The Transportation Research Board is a federal agency that manages transportationresearch projects contracted by universities and other research organizations in the U.S.and Canada

♦ The American Concrete Institute (ACI)

The American Concrete Institute publishes a specification for reinforced concrete that iswidely used for building construction This is a very important institute, with

contributors from many universities nationwide AASHTO sometimes draws on theexperience of ACI in revising its specification

♦ Prestressed Concrete Institute (PCI)

The Prestressed Concrete Institute publishes a manual and keeps up-to-date on

developments in prestressed concrete

♦ Post-Tensioning Institute (PTI)

The Post-Tensioning Institute publishes a manual and keeps up-to-date on

developments in post-tensioned concrete

♦ The American Institute of Steel Construction (AISC)

The American Institute of Steel Construction publishes a specification for structuralsteel design that is widely used in building construction AISC is active in trying tokeep AASHTO current in steel design and publishes a manual that contains much usefulinformation regarding availability and capability of steel components

♦ The American Iron and Steel Institute (AISI)

The American Iron and Steel Institute is a nonprofit service organization for the

fabricated steel industry in the United States and is dedicated to presenting the mostadvanced information available to the technical professions

♦ The American Segmental Bridge Institute (ASBI)

The American Segmental Bridge Institute is a nonprofit organization that provides aforum where owners, designers, constructors, and suppliers can meet to further refinecurrent design, construction, and construction management procedures, and evolve newtechniques that will advance the quality and use of segmental concrete bridges

♦ The American Society of Testing and Materials (ASTM)

The American Society of Testing and Materials develops and publishes specificationsfor all types of materials used in highway construction

Chapter 2 — TxDOT and Bridge Design

Section 4 — Contractive Responsibilities of the

Bridge Design Section

Section 4 Contractive Responsibilities of the Bridge Design Section

Overview

The Bridge Design Section manages a pool of consultants that expands its capabilities tomeet the demands of large letting volumes The contracting process is subject to rulesgoverning all TxDOT engineering contracts

Chapter 3 Design Specifications

Contents:

Section 1 — Mandatory Specifications 3-2Section 2 — Guide Specifications 3-4Section 3 — Industry Recommendations 3-6

Chapter 3 — Design Specifications Section 1 — Mandatory Specifications

Section 1 Mandatory SpecificationsOverview

There are many specifications available that have a bearing on the design of bridges andother highway structures This section identifies those specifications that the Texas

Department of Transportation (TxDOT) considers mandatory for use

AASHTO Standard Specifications for Highway Bridges

The Standard Specifications for Highway Bridges adopted by the American Association of

State Highway and Transportation Officials (AASHTO) is the most important control overbridge design It is usually published in full every four years In the intervening years,Interim Specifications are distributed, which contain the revisions approved on a ballotfollowing the last meeting of the AASHTO Highway Subcommittee on Bridges and

Structures The Federal Highway Administration (FHWA) may, at any time, review designsand details for compliance with these specifications for projects using federal money

Note: Although these specifications are considered mandatory, a few deviations are made

based on long-time local practice or research

Copies of these specifications, as well as other AASHTO publications, may be purchasedfrom AASHTO by calling 1-800-231-3475, or at their website at

treatment of wind effects on structures

ANSI/AASHTO/AWS Bridge Welding Code (D1.5)

The American National Standards Institute (ANSI)/AASHTO/American Welding Society(AWS) Bridge Welding Code combines the recommendations of the three agencies withregard to welding details, methods, and quality tests Bridge design is no better than thedetails that are generated, and structural steel is particularly detail oriented because of

fatigue considerations

Chapter 3 — Design Specifications Section 1 — Mandatory Specifications

AREMA Specifications

The American Railway Engineering and Maintenance-of-Way Association (AREMA)

Specifications cover many aspects of railway engineering, including the design of bridgesand culverts that carry railway traffic

Individual Railroad Company Requirements

Individual railway companies may have their own supplemental requirements, which should

be investigated for each project

Chapter 3 — Design Specifications Section 2 — Guide Specifications

Section 2 Guide SpecificationsOverview

Specifications that are not binding to bridge design, but may be useful or even vital to thedesign process are referred to as guide specifications This section identifies and describesguide specifications recommended by TxDOT

AASHTO Guide Specifications

AASHTO publishes a number of specifications and manuals, other than those discussed inSection 1, Mandatory Specifications, relating to bridge design that may be useful to thedesigner

Standard Specifications for Highway Bridges — Load and Resistance Factor Design.

This document is a complete rewrite of the bridge specification, with load and resistancefactors based on probability analyses In coming years, bridges in Texas will be designedusing this specification This places it in the guidance category

Guide Specifications for Design and Construction of Segmental Concrete Bridges This

document contains guidelines for the design and construction of segmental concrete bridges.The guidelines are the recommendations of a team of nationally recognized experts,

composed of consulting engineers, contractors, academicians, researchers, state highwayagencies, and federal agency representatives from throughout the United States as well asrepresentatives from Canada, France, Switzerland, and Germany

The guidelines are comprehensive in nature and embody several new concepts that aresignificant departures from previous design and construction provisions They are

formulated and based on both observed performance of bridges of this type and on recentresearch conducted in the United States and abroad

This document was originally prepared by the Post-Tensioning Institute under NationalCooperative Highway Research Program (NCHRP) Project 20-7/32 with the title, “Designand Construction Specifications for Segmental Concrete Bridges,” in February 1988 It wassubsequently studied and approved as a guide specification by the AASHTO HighwaySubcommittee on Bridges and Structures in 1989

Additional AASHTO Publications There are several other guide specifications, standard

specifications, and manuals published by AASHTO Some of those include:

♦ Guide Specifications for Alternate Load Factor Design Procedures for Steel BeamBridges Using Braced Compact Sections

♦ Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members

♦ Guide Specifications for Horizontally Curved Highway Bridges

Chapter 3 — Design Specifications Section 2 — Guide Specifications

♦ Guide Specifications for Fatigue Design of Steel Bridges

♦ Guide Specifications for Fatigue Evaluation of Existing Steel Bridges

♦ Guide Specifications for Strength Evaluation of Existing Steel and Concrete Bridges

♦ Guide Specification and Commentary for Vessel Collision Design of Highway Bridges

♦ Guide Specifications for Strength Design of Truss Bridges (Load Factor Design)

♦ Guide Specifications for Distribution of Loads for Highway Bridges

♦ Guide Specifications for Structural Design of Sound Barriers

♦ Guide Specifications for Aluminum Highway Bridges

♦ Guide Design Specifications for Bridge Temporary Works

♦ Guide Specifications for Design of Pedestrian Bridges

♦ Standard Specifications for Movable Highway Bridges

♦ Manual for Maintenance Inspection of Bridges

♦ Manual for Condition Evaluation of Bridges

Chapter 3 — Design Specifications Section 3 — Industry Recommendations

Section 3 Industry RecommendationsOverview

Industry recommendations are valuable to the structural designer The specifications

produced usually have the backing of leaders in the industry and also experts from

universities and other research agencies nationwide

Concrete

Concrete design is the subject of these publications:

♦ American Concrete Institute Specifications(ACI)

♦ Prestressed Concrete Institute Manual (PCI)

♦ Post-Tensioning Institute Manual (PTI)

Steel

Structural steel design is the subject of these publications:

♦ American Institute of Steel Construction Specification and Manual (AISC)

♦ American Iron and Steel Institute Manual (AISI)

Materials

Specifications for a wide range of materials are contained in several volumes of AmericanSociety of Testing and Materials publications

Chapter 4 Geometric Restraints

Contents:

Section 1 — Bridge Width 4-2Section 2 — Bridge Span Length 4-4Section 3 — Horizontal and Vertical Clearances 4-5Section 4 — Alignment 4-8Section 5 — Stage Construction 4-13

Chapter 4 — Geometric Restraints Section 1 — Bridge Width

Section 1 Bridge WidthOverview

Bridge width depends solely on the width of the highway except in unusual cases

Geometrically, bridges are just a small part of the highway From the driver’s standpoint,bridges need to blend inconspicuously into the perception of the road This need makesbridge widths, alignment, and clearances subject to the requirements of the highway

engineer

Refer to the Texas Department of Transportation Department (TxDOT)

Roadway Design Manual for guidance on highway design

Background

Roadway widths covered by Bridge Design Standards have ranged from 16 ft to 48 ft andfrom 7.2 m to 13.2 m during the past 80 years For several years, bridges had curbs located

2 ft outside of the traffic lane Texas began providing graveled shoulders on major

highways earlier than most states With continuing increases of highway speed, it becameevident that many vehicular accidents happened at the beginning of bridges where the

horizontal clearance became restricted Texas began a campaign for shoulder width bridges

in the 1950s, but the recommendation did not appear in the American Association of StateHighway and Transportation Officials (AASHTO) Specifications until 1969 because of theconsiderable cost of the additional bridge width

Current Status

Today, virtually all bridges in Texas are as wide as the approach roadway, including

shoulders Curbs are not used except to protect a pedestrian walkway on a low-speed

highway The nominal face of bridge railing is located at the outside edge of the shoulder.Traffic lanes are usually 12 ft (3.6 m) wide but may be reduced for low-volume or

extremely crowded conditions and may be increased for sight distance around a horizontalcurve Shoulders vary from 2 ft (0.6 m) to 10 ft (3.0 m) depending on traffic volume andstructure function This results in a large number of bridge widths The Bridge Divisionprovides standard bridge details for a few of the most repetitive widths that are less likely tohave complicated geometry

Chapter 4 — Geometric Restraints Section 1 — Bridge Width

exceeds 120 ft (36.0 m) For simplicity of design, the joint should be at the median

centerline On long span structures, differential deflection can cause cracking in the

concrete traffic barrier, which might justify moving the joint to one side of the barrier.Except for unusual situations, the overall width of bridge decks is 2 ft (0.6 m) more than thedistance between the nominal faces of outside railing Most standard bridge railings occupyless than 1 ft (0.3 m) of deck width The safety-shape or straight-sided traffic railings areslightly wider in their lower part, but this is not considered sufficient encroachment to affectoperation of the shoulder The nominal face of railing is set at 1 ft (0.3 m) from the deckedge to allow the use of different railing with the same standard details This dimension isalso recommended for all non-standard bridges Bridge widths for the current Bridge

Design Standards are shown in Figure 4.1

Figure 4-1 Standard Bridge Widths (Online users can click here to view this illustration in PDF.)

Chapter 4 — Geometric Restraints Section 2 — Bridge Span Length

Section 2 Bridge Span LengthOverview

Bridge length depends on terrain, hydraulics, prescribed clearances, or aesthetics, and iscontrolled by economics and the capability of available structural systems

Grade Separations

Prescribed clearances and header slopes govern highway grade separation span lengths.Sometimes aesthetic considerations may dictate longer-than-necessary spans to give theseparation a more open look underneath The larger prestressed concrete beams have

become so economical that they are used where shorter spans would suffice

Stream Crossings

Stream crossing span lengths usually have a main span that straddles the stream For smallstreams, where hydraulic considerations are minimal, the bridge is often divided into equalspans for ease of construction However, no bents or piers should be placed in erodablestreambeds River crossings almost always have a main span across the center of flood flow;the purpose being to discourage an accumulation of drift on the piers There are no warrantsfor determining the length of this span; it is a matter of engineering judgment

Chapter 4 — Geometric Restraints Section 3 — Horizontal and Vertical Clearances

Section 3 Horizontal and Vertical ClearancesOverview

Clearances are established by AASHTO, American Railway Engineering and of-Way Association (AREMA), FHWA, U.S Army Corps of Engineers, U.S Coast Guardand, to some extent, local authorities Minimum horizontal and vertical clearances forhighway bridges are established in the TxDOT Roadway Design Manual, repeated in theTxDOT Bridge Project Development Manual, and tabulated here for ready reference

Maintenance-Clearances to railroads are specified by AREMA Intracoastal canal clearances are

determined by the U.S Army Corps of Engineers International shipping lane clearancesmust be negotiated with the U.S Coast Guard

Highway Grade Separations

Clearance measurements for highway grade separations are depicted in Figure 4-2 For acomplete listing of horizontal and vertical clearances for specific highway functional

classifications, see the Roadway Design Manual

Highway Grade Separation Clearances Vertical 14'-6" (4.42 m) Absolute minimum

16'-6" (5.03 m) To be provided over all roadways if possible

and mandatory for new construction over interstate highways

Horizontal 1'-6" (0.46 m) Absolute minimum from face of curb or

barrier 10'-0" (3.05 m) From edge of travel lane on low-speed, low-

volume roadways 16'-0" (4.88 m) From edge of travel lane on medium-volume

roadways and freeway ramps 30'-0" (9.14 m) From edge of travel lane on high-volume

roadways and all freeway main lanes

Note: Special conditions that would severely increase structure cost may justify negotiation of these

clearances with the TxDOT Bridge Division.

Stream Crossings

Stream Crossing Clearances Vertical 2'-0" (0.61 m) Desired, above design high water

1'-0" (0.30 m) Absolute minimum, above design high water

Horizontal As determined by topography and hydraulics

Chapter 4 — Geometric Restraints Section 3 — Horizontal and Vertical Clearances

May be required for electric powered trains

Horizontal 9'-0" (2.59 m) Absolute minimum (12' crash walls required)

12'-0" (3.66 m) Desirable minimum (6' crash walls required) 25'-0" (7.62 m) Minimum to eliminate crash walls

Railroad Underpasses

Railroad Underpass Clearances Vertical Same as for Highway Grade Separation Structures

Horizontal Same as for Highway Grade Separation Structures

Note: Although the horizontal clearance criteria for a railroad underpass are the same as for a highway grade

separation structure, consideration should be given to using appropriate barrier railing through the railroad underpass, allowing for a reduced horizontal clearance Given the high cost of this structure type, this

practice will greatly reduce bridge cost by reducing the required span length.

Pedestrian Bridges and Non-redundant Bridge Supports

Pedestrian Bridge and Non-redundant Bridge Support Clearances Vertical 17'-6" (5.33 m) Minimum

Horizontal Desirably greater than highway grade separation structures

Chapter 4 — Geometric Restraints Section 3 — Horizontal and Vertical Clearances

Intracoastal Canal Bridge Clearances Vertical 73'-0" (22.25 m) Above mean high water

Horizontal 125'-0" (38.10 m) From center of channel

Bridges over International Shipping Lanes

Clearances are subject to negotiation with the U.S Coast Guard

Figure 4-2 Clearance Measurements (see following explanatory notes) (Online users can click here to view this illustration in PDF.)

Explanatory Notes for Figure 4-2

1 Horizontal clearance from edge of curbed roadway to obstruction Refer to the TxDOTRoadway Design Manual for specific criteria of roadway functional classification

2 Horizontal clearance from outside edge of exterior lane (uncurbed) to obstruction

Refer to the TxDOT Roadway Design Manual for specific criteria of roadway

functional classification

3 Horizontal clearance from the centerline of tracks to obstruction Use 25'-0" or greater

to avoid the need for crash walls Minimum horizontal clearance of 9'-0" with crashwalls that are 12'-0" above track elevation Minimum horizontal clearance of 12'-0"with crash walls that are 6'-0" above track elevation

Chapter 4 — Geometric Restraints Section 4 — Alignment

Section 4 AlignmentOverview

The subject of alignment covers horizontal and vertical curvature of the profile and/or

station line and the cross-slope of the deck surface

Background

In the early days, highway engineers were satisfied with bridges that were straight, square,and relatively flat It gradually became evident that bridges could handle other types ofalignment, although with considerably more complexity in the details Presently, curves,skews, variable widths, and crown rollouts are normal About the only alignment that is notcompatible with bridges is the spiral curve Spirals are still used occasionally for highwayalignment, but they are usually approximated by three centered circles for use in bridgeframing

Current Practice

Highway alignment follows the guidelines given in the TxDOT Roadway Design Manual.Bridge alignment conforms to these alignments and is usually a “given” on the preliminarylayouts

Design Recommendations

Horizontal Curvature Horizontal curvature up to 5 degrees on wide bridges and 10 degrees

on narrow connection structures can be expected Curves up to 20 degrees have

occasionally been used on “button hook” ramps and turnarounds Horizontal curvature ofbeams, with the exception of pan form girders, can be handled gracefully in cast-in-placestructures, but these have not been economical in Texas for many years The preferredsupport system is precast prestressed beams Since the beams must be straight, overhangwidth to the curved deck edge may limit the span length Figure 4-3 shows this relationship

If the curvature/span length combination exceeds the capability of the deck slab, the spanmust be decreased or other measures must be considered, such as the use of curved steelgirders

Vertical Curvature Extreme vertical grade can cause construction problems but seldom

influences structure type Grades over 5 percent call for extra care during concrete

placement The concrete tends to flow downhill during finishing operations Thicker slabspans are more sensitive than deck slabs Elastometric bearing details for prestressed

concrete beams require special consideration for grades over 5 percent, Extreme verticalcurvature can seriously affect forming methods for deck slabs on prestressed beams,

especially if precast concrete deck panels are used Crest curves cause extra deck depth in

Chapter 4 — Geometric Restraints Section 4 — Alignment

beam Even so, it is necessary to set the haunch depth carefully to avoid construction

problems

Based on AASHTO Slab Grade 60 Reinforcing

*Adjust to 1/4 pt of Flanges for Steel Beams

Figure 4-3 Guidelines for Horizontal Curvature Using Prestressed Beams (Online users can click here to view this illustration in PDF.)

Gradeline Current practice is to set haunch depths that will keep the top of beam at or

below the bottom of the slab Extra reinforcing is required if the haunch depth exceeds3.0 in (75 mm) If precast concrete deck panels are used, the problem is more critical.Special grading details may be required to accommodate tall haunches Bearing seat

elevations may require lowering for sag curves Variable camber in prestressed beamsaggravates the problem It may not be possible to cover all these variations in the designstage Contractors have become accustomed to adjusting the gradeline after taking

elevations on the tops of the erected panels

Cross-slope or crown for bridges is 1 percent minimum, 2 percent desirable If the structure

is more than two lanes wide, the outer lanes are usually sloped 2.5 percent to facilitatedrainage Cross-slopes can transition into superelevations as much as 8 percent on curvedstructures Superelevation above 5 percent can cause problems with concrete placement thesame as steep grades If such deck slopes cannot be avoided, the construction engineersshould be alerted to the possible need for special concrete placement requirements

Chapter 4 — Geometric Restraints Section 4 — Alignment

Superelevation also affects clearances between deck slab and beam, especially when precastconcrete panels are used Superelevation creates an apparent sag vertical curve along theprestressed beam, which is a chord to the curvature

When vertical curvature and superelevation exist, and the effect of beam camber is added,drastic measures may be required, especially with sag curves and panel deck construction.All of this can be accommodated, but extreme caution should be exercised and detailedgeometric computations made

Roadway Design System Deck dimensions, beam framing, bearing seat elevations, web

cutting, and bent locations must be accurately calculated to fit the prescribed alignment.This calculation is the responsibility of the bridge designers The Roadway Design System(RDS) is a geometric computer program, originally developed in Texas and formerly usednationwide The program has several bridge oriented capabilities for slabs, beams, andgirders and is used exclusively by the Bridge Design Section for prestressed concrete beamspans on curves The more important bridge routines in RDS are the following:

♦ SLAB Computes and tabulates edge dimensions and areas for deck slabs of all

configurations Slab edges can be plotted

♦ SLEL Will produce a tabulation of distances, surface elevations, bottom of slab

elevations, and bottom of slab plus dead load deflection along the boundaries of theslab

♦ FOPT Computes and tabulates framing dimensions for beam spans or continuous

girders according to one of several programmed options Framing diagrams can beplotted

♦ BMGD Will produce a tabulation of surface elevations, bottom of slab elevations, and

bottom of slab elevations plus dead load deflection along the centerline of each beam

♦ VCLR Computes vertical distance from a roadway surface to chorded beam lines It is

used to calculate vertical clearances and to check beam haunch within a span

Contour plotting is also available in the program Refer to the Roadway Design System

Manual for further details.

There is a company that maintains the RDS program for national use However, the

Information Systems Division of TxDOT has performed most of the maintenance of theprogram’s bridge commands in the past few years The program has also been made

compatible with metric dimensions Consultants should be careful to use the most recentversion of the program

Other Software IGRDS, a computer software roadway design system, is an

AASHTOWARE product available from AASHTO

A new geometry program, called GEOPAK, is being used in most highway applications It

is very useful in performing the usual highway plan functions, though the company has notyet been able to provide good bridge routines If there are significant bridges in a project,

Chapter 4 — Geometric Restraints Section 4 — Alignment

One problem with RDS is that the roadway surface must be defined by radial cross-slopesfrom only one profile grade line In varying roadway widths, where ramps are converging

or diverging, it may be necessary to adjust cross-slopes at close intervals along the mainprofile grade line to provide a smooth transition to the ramp grade and avoid edge profileproblems Contours can be used to advantage in this situation

Superelevation Transition Superelevation transition across a varying width roadway can

cause unsightly lines on the outside railing This usually occurs where ramps enter or leavethe main structure Relative grades between the two also have an influence Highwayengineers are better able to work out this problem, but it appears often to be overlooked orloosely handled It is recommended that bridge engineers consider this situation carefullybefore setting cross-slopes for framing computations Contour plots and a plot option ofSLEL can be useful in these considerations

Under certain conditions, a combination of superelevation transition and vertical curvaturewith a constant roadway width can cause sags or humps on the outside of the bridge Bothare unsightly, and sags can pond water on the roadway surface This problem is usuallycorrected by highway engineers, but it would be advisable for the bridge designers to verifythe outside lines by contours or pavement edge profile plots

Superelevation transitions can have an adverse effect on beam haunch This effect can beminimized by starting and ending a transition at a bent Figure 4-4 shows the built-in andrecommended optional methods for handling superelevation transition in RDS

Chapter 4 — Geometric Restraints Section 4 — Alignment

Figure 4-4 Superelevation Transition According to Roadway Design System (Online users can click here to view this illustration in PDF.)

Chapter 4 — Geometric Restraints Section 5 — Stage Construction

Section 5 Stage ConstructionOverview

Stage construction is required when traffic must be diverted onto a portion of an existingbridge while part of the new structure is built, then moved over for reconstruction of the firstpart This section is provided to give the bridge planner/designer some guidelines that

generally apply for all staged construction Topics include existing structure removal, newsubstructure, new superstructure, and temporary railing

Existing Structure Removal

Texas Standard Specifications, Item 496 “Removing Old Structures,” outlines requirementsfor the removal of existing structure

The partial removal of the existing structure begins with the cutting and removal of 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 onlane width requirements of both the new structure and the partial structure to remain inplace The exact breakback point should be determined by the bridge designer and is 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 anddrilled 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

New Substructure

The following are guidelines for the design of the new substructure

Foundations Consideration must be given to the room required for drilled shaft and piling

installation Both drilled shafts and piling require a 1 ft minimum horizontal clearance fromedge of foundation to the obstruction Ideally, there should be no vertical obstruction aboveeither type of foundation Special drilled shaft rigs are now available that can work with aslittle as 6 ft of headroom This equipment is quite expensive, and placement of reinforcingsteel and concrete is very difficult Contact the TxDOT Bridge Division - GeotechnicalBranch for information on the practicality and cost of these types of shafts

The only way to install piling in limited headroom is to drive and splice short sections ofsteel piling This process is seldom practical or cost effective, and should be avoided

If possible, avoid the location of the existing foundations that remain For widenings,

foundations should be of similar type as those remaining in use

Chapter 4 — Geometric Restraints Section 5 — Stage Construction

Abutments At the stage construction joint, it is difficult to leave reinforcing steel projecting

from the abutments for splicing because of the conflicts with the temporary shoring thatmust retain the fill Instead, locate foundations (drilled shafts or piling) close to the stageconstruction joint and dowel the two sides of the cap together, or provide a sealed expansionjoint

Interior Bents If possible, use independent bents If a single structure is required, the

reinforcing steel can be spliced together using a lap or mechanically coupled together Ifsplicing is used, adequate horizontal and vertical clearances must be provided to account forthe projecting reinforcement The exposed reinforcement must be protected If availableclearances are limited, use mechanical couplers or butt welds Due to the complexity ofcouplers and welds, accurate details and proper structural detail notes are essential

New Superstructure

The following are guidelines for the design of the new superstructure

The location of the stage construction joint in the slab and the available clear distance forsplicing the mat reinforcing are critical factors in the slab design The stage constructionjoint can be placed over a supporting beam or in a bay between beams However, placingthe stage construction joint over a supporting beam is the preferred method When placingthe joint over a supporting prestressed beam, the joint must be located 2 in beyond thecenterline of the beam to grab the R-bars with the first pour Prestressed concrete panels aretypically not allowed in the second placement in the bay adjacent to the construction joint.When placing the joint between beams, locate the joint at the quarter point of the beamspacing

Joints should be located so that space for minimum reinforcing steel laps and 1 in of coverbeyond the ends of the bars is provided The available construction clear distance may limitthe available length required for an 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 salt areas If couplers areused, be sure the appropriate specifications are supplied Consideration of raising the grade

a few inches to allow the top mat to be lapped should be given Shorter laps might be

justified based on the AASHTO provision (As required /As provided) in areas where theslab has excess capacity

Chapter 5 Preliminary Considerations

Contents:

Section 1 — Materials 5-2Section 2 — Structure Type 5-9Section 3 — Economics 5-26Section 4 — Aesthetics 5-27Section 5 — Corrosion Problems 5-28Section 6 — Railing 5-29

Chapter 5 — Preliminary Considerations Section 1 — Materials

Section 1 MaterialsOverview

Availability of materials is generally not a factor in determining the most suitable type ofstructure for a given location Concrete and steel are the basic ingredients of most structures,and they are available to every county in the state While bridges are primarily concrete andsteel, aluminum is used very sparingly in railing and pipe; plastics are used for small

diameter pipe; asphalt is used for overlays; neoprene is used for bearings; and butyl rubber isused for railroad underpass waterproofing

Except for reinforcing steel, only brief descriptions are given here

Concrete

Concrete is described by class, which identifies its strength, cement content, water/cementratio, and coarse aggregate type according to the item “Portland Cement Concrete” of the

Texas Department of Transportation (TxDOT) Standard Specifications for Construction of

Highways, Streets, and Bridges Concrete may be made from many different sources of

cement, fine and coarse aggregate, and water, but all materials must meet the requirements

of the specification

Various additives are allowed or required for certain conditions of use The use of fly ash toaugment or replace some of the cement is gaining acceptance Silica fume has also beenused It has been demonstrated that high strength concrete, around 13,000 psi compressivestrength, can be produced from Texas aggregates and successfully placed in the forms forcertain bridge members These ingredients are also being used to develop a “high

performance concrete” (HPC) with emphasis on density to provide better resistance to

chloride attack

Reinforced concrete is the term applied to concrete containing reinforcing bars designed toresist any tension that may occur in the member Virtually all bridges contain some

reinforced concrete The concrete is usually mixed nearby and trucked to the job site

Prestressed concrete is the term applied to strength concrete containing very strength steel that has been stretched and anchored to the concrete with sufficient force tosignificantly reduce tension from occurring in the member When the concrete is placedbefore the steel is stretched, the member is said to be “post-tensioned.” When the steel isstretched before the concrete is placed, the member is said to be “pretensioned.” Post-tensioned structures are used sparingly, but pretensioned precast concrete beams are themainstay of Texas bridge construction Type IV beams up to 135 ft long are possible tomanufacture and transport Greater lengths are possible with the use of high performanceconcrete Type VI (MOD) beams can be used for spans up to 175 ft in special

high-circumstances Use of long beams, however, depends on the accessibility of the bridge site

Chapter 5 — Preliminary Considerations Section 1 — Materials

Structural Steel

Structural steel is available in many shapes and sizes Much of the structural steel is

manufactured elsewhere, but fabrication is usually performed in or near the state Furtherdiscussion of structural steel can be found in Chapter 7 of this manual

Prestressing Steel

Prestressed steel is a very high-strength material, which is discussed further in of this

manual

Reinforcing Steel

Background There have been many changes in the strength and configuration of

reinforcing bars in the history of TxDOT Smooth bars cold twisted to improve bond wereused early, but soon outlawed by the specifications All bars were square for awhile and,even into the late 1940s, #9, #10, and #11 bars were square Oil well sucker rods were usedoccasionally during World War II because of a scarcity of regular reinforcing bars For non-specification work it was possible to find anything from barbed wire to old car parts

reinforcing the concrete

Variations through the years in the specification requirements for reinforcing bars are shown

in tables for years 1918-1953 and 1953-1988

Ductile structural grade steel was used until the early 1950s Rail steel was added, only to

be removed in the late 1970s and added back in the 1980s Deformations were a big

concern of the 1940s but the questions were put to rest by ASTM 305-47T The early 1960ssaw the availability of #14 and #18 bars established The 1973 American Association ofState Highway and Transportation Officials (AASHTO) specification ushered in high-strength reinforcing steel and put a limit on stress range to avoid fatigue problems

Weldable reinforcing steel was covered by ASTM A706 Grade 75 bars were considered forconcrete but abandoned because of the absence of a yield plateau in the stress/strain

diagram Grade 75, size #18S bars were used for anchor bolts by one light pole

manufacturer Epoxy coating of reinforcing bars was introduced in the late 1970s

Design of reinforcing steel requires analysis of the complex interaction with concrete slabs,beams, columns, and footings For service load design, an allowable stress is specified thataccounts for a reasonable factor of safety Load factor design, which has become the

standard method, allows the reinforcing steel to reach yield under the action of loads

factored up to provide safety With load factor design, service load stresses must usually becalculated to insure that crack width and fatigue stress limits are not exceeded