Noise barriers can be designed by working-stress design methods or load factor design. For the working-stress design method, the following load combinations should be considered:

Group I: D + E + SC

Group II: D + W + E + SC

Group III: D + EQD + E

Group IV: D + W + E + I

dead load

lateral earth pressure live load surcharge wind load seismic load ice and snow load

For load combination I, the stresses are limited to 100 percent of the basic allowable stresses. For load combinations II, III, and IV, the stresses are limited to 133 percent.

9.9.2 Design Criteria

The AASHTO Guide Specifications state that, for the design of noise barriers in concrete, timber, or steel, the design should conform to either the AASHTO Bridge Specifications or an industry-recognized design specification. Such sources may be referred to for allowable stress values and other details. For masonry walls, detailed design criteria are presented in the AASHTO Guide Specifications. Other materials can be designed using established engineering principles and appropriate industry specifications.

9.9.1 Noise Barrier Design Loads

Wind Loads. In most cases, the wind load represents the main load. The design pressure depends upon the wind velocity, which should be based upon a 50-year mean recur­rence interval (Fig. 9.6). The wind pressure is applied perpendicular to the wall surface to develop the design wind load. On the basis of AASHTO Guide Specifications for Structural Design of Sound Barriers, the pressure may be calculated from

U. S. Customary units: P = 0.00256(1.3 V)2 Cfc (9.1a)

SI units: P = 0.613(1.3V)2 CdCc (9.1b)

where P = wind pressure, lb/ft2 (N/m2)

V = wind velocity, mi/h (m/s)

Cd = drag coefficient = 1.2 for noise walls

Cc = combined height, exposure, and location coefficient

The factor of 1.3 in Eq. (9.1) provides for wind gusts. Values of Cc and calculated wind pressures are given in Table 9.3 A and B. The following four conditions with increasing levels of wind pressure are included:

1. Noise barriers not located on structures and having exposure B1. This includes urban and suburban areas with numerous closely spaced obstructions having the size of single-family dwellings or larger that prevail in the upwind direction from the noise wall for a distance of at least 1500 ft (457 m).

2. Noise barriers not located on structures and having exposure B2. This includes urban areas with more open terrain that does not meet exposure B1.

3. Noise barriers located on bridge structures, retaining walls, or traffic barriers (exposure C). This is based on open terrain with scattered obstructions.

4. Noise barriers not located on structures and having exposure D. This includes coastal regions.

The interpretation of the surrounding terrain and identification of local conditions that may have increased effect on wind loads are left to the design engineer.

TABLE 9.3A Design Wind Pressures on Noise Walls

Pressure for indicated wind velocity, lb/ft2

Pressure for Indicated wind velocity, N/m2

Seismic Loads. AASHTO requires that, where structures are designed for seismic load, noise walls also be designed for such. They define the seismic load (EQD) as

EQD = A X f X D (9.2)

where A = acceleration coefficient (varies from 0.05 to 0.40 depending on geographical location; see AASHTO Guide Specifications, Fig. 1-2.1.3)

D = dead load

f = dead load coefficient (2.50, on bridges; 0.75, not on bridges; 8.0, connections of prefabricated walls to bridges; 5.0, connections of prefabricated walls to retaining walls)

The product of A and f must not be taken as less than 0.10.

Other Loads. In addition to dead load, other loads that might be encountered include earth load, live load surcharge, and ice and snow load. When encountered, these loads can be developed from information in the AASHTO Standard Specifications for Highway Bridges. Increased allowable stress levels may be used for certain combinations, as discussed below.

Preliminary Engineering. During the preliminary engineering step, the following actions should take place:

• Develop a basic noise abatement plan, and determine barrier height and location.

• Develop alternative methods of abatement such as walls, earth berms, berm-wall combinations, etc.

• Develop alternative locations for abatement facilities.

• Develop alternative material types such as concrete, timber, masonry, or steel.

• Develop a conceptual landscaping plan for each alternative.

• Develop cost estimates for alternatives.

• Develop a general environmental plan.

• Make preliminary arrangements for public informational meetings.

Items to be considered in selecting proposed alternatives include aesthetics, traffic safety, sight distance, drainage, maintenance, existing utilities, lighting, signing, potential soil problems, compatibility with surrounding terrain and land use, and restrictions imposed by available right-of-way. Consider any requirements for snow storage, future construction of sidewalks, trails, etc.

Layouts, cross-sections, and wall profiles should be prepared for each alternative. Aerial photography contour maps should provide sufficient accuracy for determining ground elevations. Supplementary field information may be required in problem areas. Drainage away from both sides of the noise barrier should be provided, with a minimum slope of 0.04. Ditches or culverts may be required where walls or berms alter natural drainage patterns.

Public and Municipal Involvement. Local officials and the affected public should be informed of the scope of the proposed work and the alternative methods being con­sidered to achieve noise abatement. Work through these groups to achieve a consensus. Provide sketches, renderings, plan drawings, and other visual aids to assist in the process. With this input, a public corridor plan should be developed with a consistent theme that considers aesthetics and avoids conflicts with adjacent barriers.

Preparation of Preliminary Plans. Preliminary plans must be prepared for design and safety review. The plans should include a layout with the wall placement and profiles of the ground line and the top of the barrier. Supplemental layouts for sight distance requirements may be required.

Preliminary Approvals. Local approval of the preliminary plan developed is sought at this time. Where applicable, municipal acceptance of maintenance responsibility of back slopes or other areas outside the noise barrier should be obtained. Subsequent approval by the state DOT and FHWA is then sought.

Final Design. Information on soil conditions at the final noise barrier location should be obtained from the soils engineer. The required depth of the investigation should correspond to the depth of post embedment or depth of spread footings. For construction in new embankment areas, care must be taken to avoid excessive differ­ential settlement, because of concern for wall tilting, rotation, or cracking (of rigid systems). If a combination wall and berm is to be constructed, consider specifying an embankment material that will result in an economical wall design. It may be desirable to use a cohesive material of uniform thickness that does not move when saturated with water for the upper portion of the berm.

Wall alignment can be modified slightly when necessary to make adjustments for standard panel sizes or material sizes; to fit with existing features such as trees, signs, lights, or utilities; or to better meet safety or drainage requirements.

Often, wall designs are based on standard agency plans. Special designs may be required where a wall ties into a bridge abutment or retaining wall, where the wall height exceeds the standards, where lights or signs are constructed integrally with the wall, where the wall must also serve as a retaining wall, or where soil properties are outside the range of those anticipated in the design standards.

State and local government agencies sometimes mandate that noise wall corridors be developed. As part of roadway improvement, they anticipate a need by local residents that will help approve the roadway system.