101 - Introduction

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 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 101.4.2 -  Design Specifications for current adopted design specifications.

101.4 Applicable Design Specifications and Standards

101.4.1 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.

101.4.2 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, 7^th Edition including interims through 2019
3. AASHTO Manual for Bridge Evaluation, 3^rd Edition, 2018
4. AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, 1st Edition, 2015, including interims through 2020

101.4.3 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.

101.4.4 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.

101.4.5 Order of Precedence

The design criteria given in this Manual supersedes any criteria given in the referenced design specifications in Section 101.4.2 – 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.

101.4.6 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. The revised text will be marked with a single vertical line in the right hand margin of the page. All revisions will be compiled in a spreadsheet located on the Design Resource Center (DRC).

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.

101.4.7 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 Quality Assurance/Quality Control Plan
4. DelDOT 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 updated of the Manual.

101.5 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 3.1.3, Departure From Standards.

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:  http://www.deldot.gov/Business/drc/index.shtml.

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.