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
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Location/
* Height refers to distance from average level of adjoining ground surface to centroid of loaded area in each height zone. fStructure refers to noise walls on bridge structures, retaining walls, or traffic barriers. Source: Adapted from AASHTO Guide Specifications for Structural Design of Sound Barriers, 1989, and Interim Specifications, 1992 and 2002, Washington, D. C. |
Pressure for Indicated wind velocity, N/m2
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TABLE 9.3B Design Pressures on Noise Barriers Location/
*Height refers to distance from average level of adjoining ground surface to centroid of loaded area in each height zone. fStructure refers to noise walls on bridge structures, retaining walls, or traffic barriers. Source: Adapted from AASHTO Guide Specifications for Structural Design of Sound Barriers, 1989, and Interim Specifications, 1992 and 2002, Washington, D. C. |
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.
