Chapter 4: Advanced PlanningAnchor: #CIHBAJEG
Section 1: General ConsiderationsAnchor: #i1015227
Follow the Bridge Division standard drawings in the planning of all structures when applicable.
A bridge consists of a superstructure and a substructure. The superstructure includes the bridge deck and beams. The substructure includes the cap and foundations of the abutments and the cap, columns, and foundations of the interior bents. Review the guidelines for selecting an appropriate superstructure and a discussion of substructures on the TxDOT website .
Superstructure. The superstructure is critical in the performance and cost effectiveness of a bridge. Many types of superstructure are used by TxDOT. Choosing an appropriate superstructure depends on factors such as:
- Span length
- Vertical clearance
- Hydraulics (freeboard)
- Speed of construction
Span length requirements and vertical clearance are generally the controlling criteria when choosing the superstructure. Span lengths are determined based on bridge location, geography, and structural limitations. Vertical clearance is based on bridge location and federal and state requirements. General design criteria concerning span lengths, clearances, and other design features are discussed in Chapter 3, Preliminary Design Features, of this manual.
Speed of construction, economics, and aesthetics also influence the choice of superstructure. Construction times and costs vary for superstructures. The location of the bridge often influences use of aesthetics. (For more information, see the Landscape and Aesthetics Design Manual). Generally, the more aesthetically pleasing the bridge, the more it costs.
Substructure. The structural elements used in the superstructure often influence the design of the substructure. Substructure caps can be either steel or concrete. Generally the bent cap material is concrete. Concrete bent caps are rectangular or inverted tees. Steel is used for integral caps and box caps of straddle bents that span a large distance.
Also, the substructure generally consists of either single or multiple reinforced concrete columns. Available construction space, right-of-way limitations, bridge width, stage construction, and aesthetics are often factors in this decision. Consider use of three column bents where appropriate, such as for bridges over a body of water where there is a potential for scour or significant drift, and for bridges over roadways where there is a potential for vehicular impact with the columns.
The bent configuration and subsurface conditions determine an appropriate foundation type. Abutment foundations can be prestressed concrete piles, steel piles, or drilled shafts. Foundations for multiple-column bents generally consist of concrete drilled shafts at each column. Single-column bent foundations consist of rectangular footings supported by drilled shafts or piles. Make the choice of foundation type as flexible as possible in preliminary planning to allow an economic design in the detailed plan preparation stage.
Chapter 4 of the LRFD Bridge Design Manual discusses the use and design of substructures used by TxDOT.
Design Loads and Design Specifications. 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 Design Bridge Specifications. The Federal Highway Administration (FHWA) has mandated the use of LRFD for all bridges on which preliminary engineering is initiated after October 2007.
Use HL-93 design live load as described in the AASHTO LRFD Bridge Specifications unless design for a special vehicle is specified or warranted. Design widenings for existing structures using HL-93. Rate existing structures using AASHTO Standard Specifications and HS20 loading. Show load rating and design loads on the bridge plan, for example HS-21.5 (Existing) and HL-93 (New).
For routes where heavy truck traffic is expected, such as on North American Free Trade Agreement (NAFTA) routes, use a design load of HL-93. LRFD and HL-93 provide a more rational design and a better model of live loads expected along NAFTA routes than previously used loadings such as HS-25.
Currently, not all structure types can be designed by LRFD. If considering using LRFD, contact the Bridge Division for the latest status on LRFD implementation. The Bridge Division has developed many standard drawings for HL-93.Anchor: #i1017012
Modification of Existing Structures
Modifications such as widening, strengthening, or raising a structure are often required to meet increasing traffic demands at existing bridges. Modifications of bridge projects funded by any category of funding may occur only after an analysis of the following:
- An appraisal of the structural adequacy and condition of the existing structure
- An economic study of replacement versus modification
- A study of the method and handling of traffic during construction
Appraising the Existing Structure. Use the following procedure to determine the structural adequacy and condition of the existing structure:
- Inspect the as-built plans to determine the load capacity.
- Perform a load rating if plans indicate a design less
than H20. A load rating is not required if plans indicate a design
of H20 or greater unless the bridge was designed using Supplement
No. 1 to 1944 AASHO Design Specifications for Texas Bridges (THD
No. 1). For bridge-class culverts, a load rating is required only
if the culvert carries direct traffic (2 ft. of fill or less). At
a minimum, the bridge must be able to carry or be improved to carry
an HS20 operating loading and must have condition ratings as follows:
- Item 58 (Deck). Rating greater than or equal to 4.
- Item 59 (Superstructure). Rating greater than or equal to 5.
- Item 60 (Substructure). Rating greater than or equal to 5.
- Item 62 (Bridge-class Culvert). Rating greater than or
equal to 5.
These criteria are minimum load rating criteria for an on-system bridge that does not require load posting. Do not widen and/or rehabilitate a bridge if load posting would still be required after work is completed, with the exception of historically significant bridges discussed later in this chapter.
- Submit the load rating report to the Bridge Division project manager. Include the load rating calculations in the report, signed and sealed by a licensed professional engineer. Planning can continue only if the Bridge Division approves the load rating for the bridge.
- Perform a condition survey if the Bridge Division approves the load rating. Contact the Bridge Division project manager if division services are required.
- Submit the condition survey report to the Bridge Division
project manager. The condition survey report must be signed and
sealed by a professional engineer. The Bridge Division Design Section
director will also consider the following factors in determining
whether to rehabilitate or replace a structure:
- HS inventory loading
- Condition of the bridge as determined by the condition survey
- Type of structure
- Intended use (for example, average daily traffic, percent of truck traffic, location, etc.)
Plan development can continue only if the Bridge Division concurs with the condition survey recommendations. These criteria apply to all bridge types, including bridge-class culverts. Replace bridges with a load capacity that cannot be economically strengthened to an HS20 operating loading. Contact the Bridge Division project manager if division services are required.
The Load Rating. The load rating represents the heaviest loads that can safely use the bridge for an indefinite period of time. Use the load rating to determine if the structures can be considered a candidate for widening (rehabilitation pending a satisfactory condition survey). The district may perform the load rating or request the Bridge Division to perform the load rating.
Provide the following information to the Bridge Division project manager when requesting the Bridge Division to perform the load rating:
- Job number of the original bridge project
- Job numbers for any subsequent work performed on the structure
- Indication of existing and proposed overlay thickness
- National Bridge Inventory (NBI) structure number
- Map of bridge location
- Description of proposed work
- Project contact person and phone number
Include the following in the load rating:
- A review of the as-built bridge
details, bridge details from subsequent work at the bridge, and bridge
inspection database records. Assume the load rating to equal the
design loading indicated on the as-built bridge details under the
- As-built bridge details accurately represent the bridge
- Damage or deterioration has not weakened the bridge
- Overlay does not exceed 2 in.
- Bridge was not designed using Supplement No. 1 to 1944 AASHO Design Specifications for Texas Bridges (THD No. 1)
- A signed and sealed report by the engineer of record.
The report will include the following:
- The load rating
- Rating calculations; note method (load factor/working stress) used in the calculations
The Condition Survey. The condition survey identifies structural deficiencies that prohibit a reasonable service life with normal maintenance. Note conditions of the foundation, substructure, and components of the superstructure in this survey. The Bridge Division or qualified and experienced representatives of the district will conduct the condition survey.
Include the following in the condition survey:
- If the condition survey suggests that bridge elements are contaminated with salt, further analyzed the deck to determine the degree of contamination and internal deterioration of the concrete. For decks, take two random cores from within a 2 ft. radius circle from each span of the structure and send to the TxDOT Construction Division (CST)-Materials and Pavements Section. CST-Materials and Pavements Section uses one core for chemical analysis to determine the chloride content and the other to determine the compressive strength of the concrete.
- Test the substructure if it exhibits signs of corrosion damage. Take samples from caps and columns by collecting the cuttings from holes drilled in the concrete. Contact CST-Materials and Pavements Section, Concrete Pavement and Structures branch or Bridge Division’s Construction and Maintenance branch for sample procedures.
- Send all samples to the CST-Materials and Pavements Section. They will issue a report showing concrete strength and chloride content. All available information is then reviewed and a recommendation is made by the Bridge Division whether or not the concrete needs to be replaced. Generally, a chloride content of 2 pounds per cubic yard near the reinforcing steel is considered to be a high level of salt contamination.
- Visually examine the structure for the following:
- Settlement of the foundation
- Spalling, cracking, or deterioration of the concrete and corrosion of the reinforcing steel in the substructure
- Any damage or defects of the beams or girders and shoes
- Unsound concrete, cracking, delaminations, or efflorescence indicating salt contamination and depth and corrosion of the reinforcing steel in the top and bottom of the deck
- Deterioration of the overlay due to defects or damage in the underlying concrete
- Take photographs to include the following:
- Bridge ends
- Bridge elevations
- Bridge approaches
- Problem areas
- Views upstream and downstream, if applicable
- Review and analyze of the extent of the deficiencies and the feasibility of repair. The structure should probably be replaced if a 20-year service life cannot be predicted
- Review the hydraulic adequacy, if applicable
Where the condition survey indicates that restoration of the bridge deck is warranted, a more detailed field appraisal to further define the deficiencies may be recommended by the Bridge Division. This more detailed appraisal may require one or more of the following:
- Delamination detection to determine the extent of internal fractures of the concrete
- Determination of the extent of reinforcing steel corrosion
- Determination of areas with inadequate concrete cover over the reinforcing steel
- Chemical analysis to determine extent of chloride contamination
Important Considerations. Additional considerations include the following:
- Superstructure -- Bridge widenings should be of similar type construction to that of the existing structure. Prestressed beams are satisfactory for use in widening some common structures.
- Substructure -- The proposed foundation should be similar to that of the existing structure. This is particularly necessary where differential vertical movement in the foundation material can damage the widened facility.
- Bridge railing -- Replace or retrofit the railing on both sides to meet AASHTO Manual for Assessing Safety Hardware or National Cooperative Highway Research Program (NCHRP) 350 standards. This is required even if widening is to be done on one side only.
- Minimum design criteria such as vertical and horizontal clearances -- Modifications should normally not encroach on the waterway, highway, or railway clearances beneath the existing structure. Make every effort to maintain or improve the existing clearances; however, a design exception may be requested if the minimum clearances are not met.
- The effects of widening one side only versus widening both sides.
- Establish the proposed roadway centerline -- Consult with the Bridge Division to determine an appropriate centerline. Factors that affect the location of the proposed centerline include: existing roadway alignment, embankment widening, and traffic control.
- Removing existing structures -- When an existing structure is replaced, the district determines if the state will retain salvageable material such as structural steel, railing, or timber. If the district elects not to retain salvageable material, transfer the material to the contractor in accordance with Texas Standard Specifications, Item 497, Salvageable Material.
- Removing paint - Follow the procedures for determining whether the paint should be considered hazardous. If hazardous paint is identified, let a painting contract in a separate contract from the structural work required. Contact the Construction Division in the early stages of the planning process if the presence of hazardous paint is suspected or if there are any questions concerning the necessity of a separate painting contract.
- Abating asbestos - Follow the procedures for the identification, notification, and abatement of asbestos. If asbestos is identified, make proper notification to the Texas Department of State Health Services. Perform asbestos abatement separately from the prime contract wherever possible. If possible, abate asbestos-containing material before construction begins. Contact the Construction/Maintenance Branch of the Field Operations Section in the early stages of the planning process if the presence of asbestos containing material is suspected or if there are any questions concerning the proper procedures to be employed.
Replacing an existing bridge with a new bridge or widening an existing bridge often requires stage construction. Keeping lanes of the existing bridge open while the first phase of the construction of the new bridge takes place maintains minimum traffic needs. Each project may require unique solutions or have individual needs. These unknowns make the planning and design of stage construction a challenging process. The uniqueness of each project requires engineering judgment and experience in developing the bridge plans. Resolve any uncertainties early in the preliminary plan preparation stages.
The guidelines below on early planning, bridge layout and structural details, and design assist planners and designers with some aspects common to most stage construction projects.
Early Planning. Consider the following early factors:
- Determine the need for stage construction early in the planning stages. Due to the complexity of stage construction, other solutions may be preferable.
- Identify the traffic control needs of the project prior to the development of the bridge layouts. Communication between the engineer responsible for traffic control and the design engineer is critical during the preparation of the bridge layouts and construction sequence process. Temporary single lane crossings over a structure are used occasionally. Refer to Traffic Control Plan TCP (2-8)-12 standards.
- Leave exact breakback locations up to the designer, if possible.
The Bridge Layout and Structural Details. Proper plan preparation is essential in both producing a quality product and adhering to the letting schedule.
- The individuality of each project necessitates the need for greater detail in the structural details. Do not leave decisions up to the contractor unless as specified in the structural detail notes. Fully detail complex construction or unique solutions to avoid any confusion.
- Delays often result due to the lack of information in the layouts. The Preliminary Layout Approval Process -- Layout Considerations and Requirements (see Chapter 5, Section 2) section of this manual contains guidelines for preparing bridge layouts for projects that include staged construction.
Design Guidelines. Design guidelines can be found in the Preliminary Design Features -- General Features (see Chapter 3, Section 1) section of this manual.Anchor: #i1020969
Detour/Temporary Crossing Structures
If a project must maintain traffic during bridge replacement and staged construction is not feasible, provide a temporary structure to handle traffic during construction of the new structure.
Temporary bridge structures are not a common element in most bridge projects. Careful coordination between the bridge designer, the traffic control engineer, the environmental coordinator, and the bridge project manager should occur early in the process to properly design these structures.
A common type of temporary structure is the pre-engineered, pre-manufactured, modular structure Pre-manufactured bridges come in a variety of span lengths, widths, and load carrying capabilities. Depending on the size involved, they can usually be erected with a minimum of heavy construction equipment and a minimal amount of labor. Due to their modular nature, they are usually easy to transport and erect. TxDOT owns several of these bridges that are stored at various maintenance warehouses around the state. Contact the Maintenance Division for more information on the availability of these bridges.
Temporary structures are sometimes constructed of the same types of structural elements found in permanent bridges, for example, prestressed box beams as well as steel I-beams. It is almost impossible to determine in advance what type of structure will be most economical in a particular situation. It is, therefore, best not to design a particular type of temporary structure but to allow the contractor to provide the temporary structure needed.
If the project allows the contractor to provide the temporary bridge for a project, the plans must include certain items:
- Provide the required number and width of traffic lanes and the required design loading.
- The alignment of the temporary structure.
- Any special requirements, such as limits on fill or other environmental restrictions.
Economic Comparisons and Alternate Designs
A cost comparison among structure types during the initial planning stages of a project will assist in determining the most economical structure type. A cost per square foot comparison is a simple and quick way to compare structure types. The Bridge Division may assist in furnishing current structure costs.)
Providing alternate bid items is sometimes warranted. This allows the contractor to bid the most economical design. This may require multiple designs of an item to be shown on the plans. Examples include drilled shafts versus piles and double tee beams versus pan form girders. Alternate foundation designs are suggested for structures over $2 million unless experience has shown a particular foundation is warranted.Anchor: #i1021933
State versus Federal Oversight
Each year the Federal Highway Administration (FHWA) and TxDOT select a list of projects for which FHWA will have oversight through plan review and approval. All other projects will be TxDOT’s responsibility. A list of each year’s project selection can be found in the current FHWA oversight agreement.
If FHWA has oversight, submit the preliminary bridge layouts to the FHWA through the Bridge Division project managers. Add one month to the total lead time for projects including major bridges or unusual structures requiring FHWA headquarters approval.Anchor: #i1022305
FHWA is responsible for assuring that the projects it funds do not have significant environmental impacts or, if they do, that appropriate action is taken. The following Environmental Affairs Division assessments, listed in order of investigative detail from least to most, may be requested:
- Categorical Exclusion (CE)
- Environmental Assessment (EA)
- Finding of No Significant Impacts (FONSI)
- Environmental Impact Statement (EIS)
For more information, see the Environmental Manual.
The Environmental Affairs Division (ENV) supports the district environmental coordinator. The environmental coordinator conducts assessments and works closely with the Bridge Division project manager when evaluating environmental concerns. This coordination should occur as early as possible in the project development process. The Environmental Issues, Permits, and Commitments (EIPC) sheet is initiated in the district in the preliminary project development stage to ensure that all environmental issues are addressed. To obtain information concerning access to the Texas Environmental Oversight System (Texas ECOS) and EIPC, contact ENV. Such concerns may include the following:
Proximity to Hazardous Sites.An assessment of all potential right-of-way properties that could be contaminated with hazardous substances, as well as adjacent properties from which contamination could migrate. Do this early in the planning stages when time and options remain to address these critical problems.
Hazardous Paint. Many of the early paint formulations used to paint bridges contained lead and chromium. Blast-cleaning operations may create hazardous waste according to federal and state regulations. For all projects for cleaning and painting bridges, identify the type of paint on the structure prior to plans, specifications, and estimates (PS&E) submittal to the Bridge Division for review. If complete painting records, including any spot-painting dating back to initial construction and painting, are not available to identify the type of paint on the structure, submit a sample (approx. 50 grams) to the CST-Materials and Pavements Section to determine the potential for hazardous waste.
Asbestos. Identify and address asbestos issues early in the project development to minimize impacts to construction and project costs. Many TxDOT projects are regulated under the National Emission Standards for Hazardous Air Pollutants (NESHAP) found in 40 CFR 61 Subpart M. These Federal standards were originally developed to address asbestos found in buildings, but have been expanded to include demolition and renovation work on bridges. The Department of State Health Services is responsible for administering these regulations in Texas. Asbestos Containing Material (ACM) is defined as any material that contains greater than one percent asbestos based on examination by an approved laboratory method. Regulated Asbestos Containing Material (RACM) is ACM that is found in the following quantities: 260 linear feet of pipe, 160 square feet on other components (coatings), or 35 cubic feet where length or area could not previously be measured. Special specifications and special provisions are available for typical asbestos abatement work.
Hydraulic Impacts. In order to comply with 23 CFR Part 650 Subpart A, perform complete hydraulic studies at the time of environmental documents.
Wetlands Impact. Under Section 404 of the Clean Water Act, the U.S. Army Corps of Engineers (COE) regulates the discharges of dredged or fill material into “waters of the U.S., including wetlands.” Prior authorization is required from the COE to deposit dredged or fill material into wetlands or any “waters of the U.S.” Refer to Advanced Planning -- Agreements and Permits in Chapter 4, Section 3 of this manual for additional information concerning COE requirements and Section 404 permits.
Storm Water Runoff. Bridge projects may be subject to storm water abatement requirements. The TxDOT publication Storm Water Management Guidelines for Construction Activities (TxDOT, 1993) details the department’s procedures and recommended best management practices to be included in a Stormwater Pollution Prevention Plan for proposed projects. The Hydraulic Design Manual contains useful information as well. Federal requirements can be found in 23 CFR Part 650 Subpart B.
Mitigation of Environmental Impacts. Bridge projects may require mitigation of environmental impacts by replacement of trees and other vegetation. This mitigation is eligible for federal funding under the same category of work as the original bridge project.
Because mitigation projects are usually let sometime after the bridge contract has been let or completed, it may be necessary to separate the mitigation contract and the bridge contract. Mitigation contracts are typically kept open for a period of about two years after planting to ensure that the plants take root and become established. Link the mitigation contract to the bridge project in order to receive reimbursement from FHWA.
The mitigation portion of the project will have its own FHWA project number and its own control-section-job (CSJ) number. FHWA form Federal Project Authorization and Agreement (FPAA) ties the bridge and mitigation contracts together. This form has a comment field to indicate that both contracts are connected to the same project. When the FPAA form is sent to the FHWA, explain in the cover letter that the mitigation contract is part of the bridge contract.
Although the mitigation project can be let in a different fiscal year than the bridge project, let the mitigation contract for construction as soon as it is reasonably practical in order to maintain continuity with the bridge project.
Historically Significant Bridges, Property, and Archeological Coordination. With the exception of most of the interstate system, federally funded projects involving historic bridges must comply with Section 4(f) of the United States Department of Transportation (U.S.DOT) Act of 1966 and with Section 106 of the National Historic Preservation Act of 1966. Therefore, TxDOT must coordinate with the State Historic Preservation Officer (SHPO) of the Texas Historical Commission and the FHWA to assess the effects of federally funded projects on historic resources. In addition, for state funded (non-federal) projects involving bridges that have been designated State Archeological Landmarks, coordinate efforts with the SHPO to assess the impact of the project on the landmark structure in accordance with the State Antiquities Act. Additional information concerning the requirements of Section 4(f) of the U.S.DOT Act of 1966, Section 106 of the National Historic Preservation Act of 1966 and the State Antiquities Act can be found in the Historic Bridge Manual.Anchor: #i1023938
Pedestrian bridges, bridges with sidewalks, and highway rest and picnic areas are the most common highway facilities that require Americans with Disabilities Act (ADA) and Texas Accessibility Standards (TAS) compliance. Features that must meet specific requirements include the following:
- Maximum curb ramp slope
- Cross slope and grade on sidewalks
- Minimum sidewalk clear width
- Sidewalk passing space
- Objects protruding into the sidewalk
- Location of curb ramps and sloped areas
- Diagonal curbed ramps
- Raised curbed islands
- Drop-offs (or curb heights) greater than 9 in.
Additional information on ADA and TAS requirements can be found in Chapter 2 of the Roadway Design Manual. However, always refer to the current Americans with Disabilities Act Accessibility Guidelines and Texas Accessibility Standards for complete ADA and TAS requirements.