101 - Introduction

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101.1 Purpose

The Delaware Department of Transportation (DelDOT) has developed this Bridge Design Manual (this Manual) to provide guidance and assistance in the standard practice of design related to bridges and all structures on or over a public roadway in the State of Delaware. The Manual documents DelDOT policies and prescribes procedures for design. It is intended to be a technical manual, providing engineers and technicians guidance in:  

  1. Structure design practices specific to the State of Delaware;
  2. Delaware preferences and interpretation of American Association of State Highway and Transportation Officials (AASHTO) specifications necessary to provide consistent structure designs; and
  3. The minimum criteria and information necessary to produce documents for the fair procurement of construction services.

101.2 Limitations of the Design Manual

Although this Manual attempts to unify and clarify bridge and structure design policy performed by or for DelDOT, it does not preclude justifiable variances; variances are subject to the approval of the Bridge Design Engineer, provided the variances are based on sound engineering principles. Good design practice will always require a combination of basic engineering principles, experience, and judgment to produce the best possible structure, within reasonable economic limitations, to suit an individual site. The policies in this Manual have been established primarily for application to typical highway structures using conventional construction methods with additional applications, such as Accelerated Bridge Construction (ABC). These policies are subject to re-examination and may not be applicable to long-span, complex-curved, or high-clearance structures, such as major river crossings or multi-level interchange structures.

101.3 Modifications to the Design Manual

Updates and Revisions to the Manual will be released as needed based on changes to practice or referenced manuals and publications. Revised text and previous versions can be reviewed in the current manual by using the View History tab for each page in the manual. See Revision History for instructions. All revisions will be compiled in the Changelog.

Based on the urgency of an update or revision, the Department may issue a Design Guidance Memorandum, which provides technical guidance on a specific issue during an interim period. Direction included in these memos will then be incorporated into the next update of the Manual.

101.4 Policy

The AASHTO LRFD Bridge Design Specifications is the basis for highway bridges designed for DelDOT. Users of this Manual should be completely familiar with the AASHTO LRFD. Refer to Section 101.5.2 Design Specifications for current adopted design specifications.

101.5 Applicable Design Specifications and Standards

101.5.1 Design Specification Reference Nomenclature

All references to AASHTO LRFD sections, articles, equations, figures or tables carry the prefix A.

References to AASHTO commentary carry the prefix AC.

References to the sections within this Manual carry no prefix.

References to commentary to sections within this Manual carry the prefix C.

101.5.2 Design Specifications

The following specifications, unless otherwise modified or amended in this Manual, shall govern the design of highway structures:

  1. AASHTO LRFD Bridge Design Specifications, 2017, 8th Edition
  2. AASHTO/American Welding Society (AWS) D1.5M/D1.5:2015 – Bridge Welding Code, 7th Edition including interims through 2019
  3. AASHTO Manual for Bridge Evaluation, 3rd Edition, 2018
  4. AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, 1st Edition, 2015, including interims through 2020

101.5.3 AASHTO Interim Specifications and New Editions

As AASHTO interim specifications and new editions are published, DelDOT will review the interims and incorporate them into this Manual as appropriate.

101.5.4 Deviations from Specifications

Any deviations from the specifications and standards listed above, or the Department’s design criteria described hereafter, require the Bridge Design Engineer’s approval. The approved design criteria shall be shown on the bridge plans. Refer to Section 102.5.4 – Design Exceptions and Design Variances for additional discussion on obtaining a design variance.

101.5.5 Order of Precedence

The design criteria given in this Manual supersedes any criteria given in the referenced design specifications in Section 101.5.2 Design Specifications. In case of conflict or where clear precedence cannot be established, the Bridge Design Engineer shall establish governing specifications.

For this Manual and AASHTO LRFD, the final interpretation shall be made by the Bridge Design Engineer.

101.5.6 Additional Reference Manuals and Documents

The following references contain material that is relevant to bridge project development and design. These documents contain certain provisions that pertain to a particular type of bridge or part of the bridge project process. Bridge designers should consider these documents where applicable.

DelDOT references, along with additional materials pertinent to project development, can be found on the DRC portion of DelDOT’s website and are referred to as follows in this Manual:

  1. DelDOT Project Development Manual (PDM)
  2. DelDOT Road Design Manual
  3. DelDOT QC/QA Manual
  4. CADD Standards Manual (Wiki Format)
  5. DelDOT Standard Specifications for Road and Bridge Construction (Standard Specifications)
  6. DelDOT Standard Construction Details
  7. DelDOT Pedestrian Accessibility Standards for Facilities in the Public Right of Way

The adopted version of the reference is the edition used throughout this Manual unless specifically noted above. Updated editions will be incorporated in future updates of the Manual.

101.6 Terms

Design exception – a request to deviate from the Department’s governing criteria and AASHTO’s new construction criteria for the 13 Controlling Design Elements as may be warranted by special or unique project conditions.

The 13 Controlling Design Elements are:

  1. Design Speed
  2. Through lane and auxiliary lane widths
  3. Shoulder widths
  4. Stopping sight distance on vertical and horizontal curves
  5. Horizontal alignment (radius of curves)
  6. Vertical Alignment
  7. Minimum and maximum grades
  8. Cross slopes
  9. Superelevation rate
  10. Horizontal clearance
  11. Vertical clearance
  12. Bridge width
  13. Structural capacity

Additional information related to design exceptions and the justification of design exceptions is found in the Road Design Manual, Chapter 3.2, Departure from Design Criteria.

Design Resource Center (DRC) – the DRC is a page on DelDOT’s website that contains a variety of data related to the development of transportation projects in the State. The DRC can be located at: [1].

Design Variance – a request to deviate from the Department’s governing standards excluding the 13 Controlling Design Elements, which may be warranted by special or unique project conditions.

101.6.1 Bridge Types

The following bridge-related terms are used throughout the Manual to provide reference to the anticipated level of design oversight and/or submission standards associated with various structure types and complexities.

In Delaware, a bridge is defined as a structure, including supports, erected over a depression or an obstruction, such as water, a road, or a railroad, for carrying traffic or other moving loads that has an opening exceeding 20 square feet. Bridges with a clear span greater than 20 feet are included on the National Bridge Inventory (NBI).
Major bridges
Major bridges are defined as bridges with an estimated construction cost of $40 million or more. This criterion also applies to individual units of separated or dual bridges.
Complex bridges
Complex bridges are curved girder bridges, moveable bridges, stayed girder bridges, segmental bridges, and any structure having a clear unsupported length in excess of 350 feet, or bridges classified as complex by the Bridge Design Engineer on the basis of type, size, and location (TS&L) or conceptual review. Complex bridges also include those with difficult or unusual foundation problems, new or complex designs involving unusual structures or operational features, or bridges for which the design standards or criteria may not be applicable. Use of new products and experimental or demonstration projects are also considered as unusual structures.

101.6.2 Roadway Types Functional Classification

Delaware has adopted a system of classifying and grouping highways, roads, and streets as to their purpose and the character of service they provide in accordance with the Federal Highway Administration’s (FHWA’s) Traffic Monitoring Guide (2013). To determine certain bridge design elements, knowing and understanding the functional classification of the roadway facility supported is essential. The standard functional classifications recognized by DelDOT are indicated below. Additional information related to functional classification can be found in the PDM and the Road Design Manual. DelDOT maintains a map identifying the functional classification of all Delaware roads. This map can be found on the DRC – Highway Design.

  1. Rural System
    1. Principal Arterial – Interstate
    2. Principal Arterial – Other
    3. Minor Arterial
    4. Major Collector
    5. Minor Collector
    6. Local
  2. Urban System
    1. Interstate
    2. Freeways and Expressways
    3. Principal Arterial
    4. Minor Arterial
    5. Major Collector
    6. Local National Highway System

A prominent feature of the statewide planning process is maintaining the integrity of the National Highway System (NHS). Intermodal Surface Transportation Efficiency Act Section 1006 created the NHS as required by the National Highway System Designation Act of 1995. This directive was further defined and expanded by the Moving Ahead for Progress in the 21st Century Act (or MAP-21) legislation of July 6, 2012.  

The purpose of the NHS is to provide an interconnected system of principal arterials that serve major population centers, internal border crossings, ports, airports, public facilities, and other intermodal transportation facilities and major travel destinations; meet national defense requirements; and serve interstate and interregional travel. To determine certain bridge geometry and submission requirements, knowing whether the structure is located on an NHS-designated roadway. Additional information related to the NHS can be found in the PDM and the Road Design Manual. A map of all NHS roadways in the State of Delaware can be obtained on DelDOT Gateway Maps.

101.6.3 Project Types

New Construction and Reconstruction Projects
Projects in this category include the construction of new bridges and/or complete bridge replacement.
Intermediate Projects
Intermediate project types consist of bridge rehabilitation projects and/or bridge superstructure replacement projects.
Preventative Maintenance
Preventative maintenance projects include rehabilitation or restoration of specific elements of a bridge when such activities are a cost-effective means of extending bridge service life. The majority of the work for these projects is usually maintained between the existing curb lines or outer edges of the shoulders. Preventive maintenance activities include, but are not limited to, bridge painting, deck rehabilitation, joint replacement or repair, bearing replacement, installation of pile jackets, placement of scour countermeasures, and seismic retrofit.

101.7 FHWA Stewardship and Oversight Agreement

The intent and purpose of the Stewardship and Oversight (S&O) Agreement is to document the roles and responsibilities of the FHWA’s Delaware Division Office and DelDOT with respect to project approvals and related responsibilities, and to document the methods of oversight that will be used to efficiently and effectively deliver the Federal Aid Highway Program.

DelDOT may assume FHWA’s Title 23 responsibilities for design; plans, specifications, and estimate (PS&E); contract awards; and inspections, with respect to Federal-aid projects on the NHS if both DelDOT and FHWA determine that assumption of responsibilities is appropriate.

FHWA may, in its discretion and on a case-by-case basis, retain any specific approval or related activity for any project located on the NHS. Those projects for which FHWA retains certain project-specific actions or related responsibilities will be identified as Projects of Division Involvement (PoDIs). Project approvals and related activities retained by FHWA will be identified in individual project oversight plans. FHWA, in coordination with DelDOT, will use a risk-based approach to determine which NHS projects are considered PoDI and which project areas warrant FHWA approval or oversight. An updated PoDI list will be maintained in a manner that is easily accessible and readily available to both FHWA and DelDOT project staff. Criteria for identifying PoDI projects are further outlined in Section IX of the S&O Agreement.

DelDOT may assume FHWA’s Title 23 responsibilities for design, PS&Es, contract awards, and inspections, with respect to Federal-aid projects off the NHS (non-NHS) unless DelDOT determines that assumption of responsibilities is not appropriate (Title 23 the United States Code [U.S.C.] 106(c)(2)). Project approvals and related activities for which DelDOT has assumed responsibilities are outlined in Attachment A of the S&O Agreement.

DelDOT assumption of responsibilities under 23 U.S.C. 106(c) covers six areas:  design; PS&E; contract awards; and inspections, which are defined more specifically in Section VI of the S&O Agreement.

Any approval or related responsibility not listed in Attachment A cannot be assumed by the State without prior concurrence by FHWA. A list of the most frequently occurring approvals and related responsibilities that may not be assumed by DelDOT are listed in Section VII of the S&O Agreement.

For projects that have FHWA oversight, Section XI outlines the criteria that FHWA must follow. For DelDOT administered projects, DelDOT is responsible for demonstrating to FHWA how it is carrying out its responsibilities in accordance with the S&O Agreement. DelDOT oversight and reporting requirements are outlined in Section XII of the S&O Agreement.

All Federal-aid projects on the NHS should be reviewed with the Bridge Design Engineer at initiation to determine the level of FHWA involvement.

101.8 Computer Software

The Bridge Design Engineer should approve a specific computer software before use. The Department has the discretion to either accept or reject the use of any commercially available or consultant-developed software proposed for use on any project. In all cases, the designer is responsible for the accuracy of all computer software programs utilized on a project.

101.9 Feedback

Users of this Manual should direct any questions, comments, or recommendation for modifications to the content of the Manual directly to the Bridge Design Engineer, DelDOT.

101.10 References

AASHTO, 2011. Manual for Bridge Evaluation, 2nd Edition with 2012, 2013, 2014, and 2015 Interim Revisions.

AASHTO, 2015. AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, 1st Edition.

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

DelDOT, n.d. Standard Construction Details.

DelDOT, 2020. Standard Specifications for Road and Bridge Construction, August.

DelDOT, 2022. Quality Assurance/Quality Control Plan.

DelDOT, n.d. CADD Standards Manual, (Wiki Format).

DelDOT, 2015. Project Development Manual, July.

DelDOT, 2022. Road Design Manual, September.

DelDOT, 2018. Pedestrian Accessibility Standards for Facilities in the Public Right of Way. February.

FHWA, 2013. Traffic Monitoring Guide, Office of Highway Policy Information, September.