8. STRUCTURAL CONSIDERATIONS

Proper design of noise barrier systems requires the consideration of a variety of structurally related factors. This section is not intended to provide either a standard or a recommended process for the structural design of noise barriers. Rather, its goal is to identify structural issues which should be addressed and considered in the design of barrier systems. Specific application and interpretation of appropriate structural criteria is the responsibility of the respective responsible organization in charge of designing and constructing the barrier system.

8.1 Expansion and Contraction of Barrier Materials
All materials used in the construction of noise barriers expand and contract with temperature and moisture variation. Such expansion and contraction must be appropriately considered in the design of all elements of noise barrier systems. Failure to do so can result in both structural, acoustical, and aesthetic problems. The individual barrier elements themselves must be designed and constructed to preclude unacceptable deformation, cracking etc. Conditions where consideration of such expansion and contraction effects is most essential include:

Post to panel connections - Expansion/ contraction is normally accommodated by allowing sufficient space or gaps between the post web and the panel. Some designs may call for full or partial caulking or shimming of the panel/post flange contact point to assure adequate load transfer and/or to avoid sound leakage. Care must be taken to assure that the caulking and/or shimming material does not restrict panel expansion or contraction (see Figure 203).

Figure 203. Expansion and contraction of materials: post to panel connections photo #8038

Panel to panel connections - Such connections occur both horizontally (such as between stacked panels or tongue and grove panels) and vertically (such as with post-less or vertical tongue and grove barrier systems). Such connections must allow sufficient movement while maintaining tight joints (see Figure 204).
	Figure 204. Expansion and contraction of materials: post to panel connections photo #181

• Expansion joints on cast-in-place and brick/masonry noise barrier systems - Vertical expansion joints are required at sufficient intervals to preclude cracking of the wall system. Designing of such joints in a manner which assures aesthetic and acoustical integrity is often a challenge.

Connections between ground-mounted and structure-mounted barriers - It is sometimes necessary for a ground-mounted barrier to continue onto a structure (bridge or retaining wall). In such instances, special detail barrier sections are required in order to accomplish an adequate connection which is both structurally and acoustically sound while maintaining the desired barrier aesthetics (see Figures 205 to 207).

Figure 205. Expansion and contraction of materials: connections between barriers photo #463

Figure 206. Expansion and contraction of materials: connections between barriers photo #1716	Figure 207. Expansion and contraction of materials: connections between barriers photo #1720

• Structure-mounted barriers - In addition to the expansion considerations discussed above, barriers mounted on structures must also accommodate expansion/contraction at the structure's expansion joint locations (see Figures 208 and 209).

Figure 208. Expansion and contraction of materials: structure barriers photo #414	Figure 209. Expansion and contraction of materials: structure barriers photo #1711

8.2 Noise Barrier Loadings
Design of noise barrier systems must include consideration of a variety of design loads, both individually and in combination with each other. Such loads include:

• Dead Load - The weight of the barrier itself must be considered in all barrier design calculations. Weight considerations are particularly critical in the design of structure-mounted barriers and can require modifications to the structure design itself. Lightweight barrier materials are often utilized in situations where existing or proposed structures are limited in the amount of additional weight which they can accommodate. Ice loads represent a special type of dead load caused by water freezing and building up on exposed barrier surfaces.

• Wind Load - Wind loads vary with geographic location and can be influenced by elevation in relation to existing topography. They affect the overturning moment or rotational force placed upon the barrier, its foundation, and/or the structure to which the barrier is attached. Unlike dead loads, wind loads are essentially the same, regardless of barrier material type.

• Snow Loads - Unlike ice loads, snow loads are not considered to be dead loads placed upon the barrier. Rather, in barrier design, considerations related to snow relate to the generally horizontal forces of both plowed and stored snow which can be placed on the vertical surface of the barrier. Design of the barrier to accommodate such loadings should consider the area available for safe storage of plowed snow as well as the relationship (both horizontally and vertically) of the barrier to the location of snow clearing and snow removal equipment ( plows, front end loaders, melters, and blowers).

• Impact Loads - Impact loads can be classified as loads placed on the barrier due to errant vehicles and airborne debris. Noise barriers are not themselves designed to withstand the full force of a vehicle impact. Rather, either a protective metal guardrail or a Jersey type barrier placed in front of the noise barrier is usually relied upon to keep errant vehicles away from the barrier. Placement of a noise barrier on a structure is usually restricted to the structure's parapet. In such cases, options for barrier mounting to the parapet (either top or face mounting) should be reviewed in light of the potential for the barrier being impacted by all portions of vehicles, including the potential impact resulting from a tall truck tilting toward the barrier after hitting the protective barrier. Airborne debris such as retreads, stones, vehicle parts, etc., can also strike the barrier, regardless of what type of protection is provided against vehicle impact. The impact of such strikes upon the barrier is mainly a function of the durability of the noise barrier's material, especially its surface. While lightweight materials have an obvious positive influence in terms of dead load design factors, they may not be as durable in terms of impact as compared to heavier barrier materials.

8.3 Barrier Height Considerations
Barrier heights can be influenced by one or all of the above load related factors and by other conditions such as the presence of overhead utilities or other restrictions, cost, aesthetics, and foundation requirements.

8.4 Foundation Requirements
Footings and foundations for ground-mounted noise barriers are typically limited to concrete cylinders (caissons), spread footings, and continuous footings. When designing these, the following factors contribute to the selection of the type of footing to be used as well as its depth and size:

• The bearing capacity and compressibility characteristics of the surrounding soil or rock;
• Possible ground movements;
• Anticipated future excavation activity adjacent to the foundations;
• Ground water levels;
• Extent of frost penetration;
• Extent of seasonal volume changes of cohesive soils;
• The proximity and depth of foundations of adjacent structures; and
• Overall ground stability, particularly adjacent to cut or fill slopes.

8.4.1 Concrete Footings in Earth.
Concrete for drilled or augured footings should be cast entirely against undisturbed soil. If other than drilled footings are necessary, the footings should be formed and the excavation should be backfilled with granular materials and properly compacted. The tops of all footings should be shaped to provide for full horizontal seating of panels with the remaining surface area to be sloped away from the post so as to shed water. Stepped footings may be required to suit grade changes (see Figure 210). To avoid premature failure of the concrete in the footings, the concrete should have an opportunity to cure properly before the noise panels are installed.
	Figure 210. Stepped concrete footings in earth photo #2939

8.4.2 Concrete Footings in Rock.
When rock is encountered, a different technique should be considered to ensure a stable foundation. A typical example would be to construct the footing in the same manner as for footings in earth with partial embedment into solid rock. All excavations into rock should be backfilled with either concrete or other suitable material. The excavation above the top of rock may be backfilled with granular material.

Section Summary