Practices for High-Tension Cable Barriers (2016)

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8 In both figures, the cross-hatched area indicates those loca- tions where barrier underride or override, as indicated, is a possibility. Placement for Horizontal Curves Based on numerous crash studies, there is a higher frequency of vehicles leaving the roadway on the outside of the curve versus the inside of the same horizontal curve. Therefore, when installing barrier offset from the ditch bottom, the barrier is to be placed toward the inside rather than the outside of the curve (Texas DOT), as shown in Figure 4 (Cooner et al. 2009). Currently, TxDOT guidance recommends closer post spac- ing through curves, as shown in Table 1. Locating the barrier on the convex side provides clear space for deflection when impacted on that side and maximum recovery area for any vehicle running off the outside of the same curve. Although such placement will reduce nuisance hits and can provide an obstacle-free recovery area in a wide median, it may allow a vehicle in a high-speed encroachment to overturn in an uneven or soft median or possibly deflect the cable into the inside lane of opposite direction traffic. Figure 5 shows that placing the HTCB on the concave side of a curve, while increasing the likelihood of impacts, results in good performance when it is struck. On a sharp curve, loss of several posts can result in the cables falling onto the shoul- der when several posts are hit (as seen here) or, in extreme cases, onto the roadway itself. Anchor Placement and Run Length End anchors are critical in any cable barrier system, because they provide the means to tension the cables. Standard termi- nal anchors for each HTCB system vary from designs with each cable connected to individual anchors to designs where all the cables are attached to a single anchor. Cable anchors are gating, so they will not prevent a vehicle from going into the area behind and beyond the anchor. They should be placed in locations where direct impacts are least likely to occur when- ever practical. In situations where the cables are switched from one median side to another, they should be overlapped to prevent possible crossovers (Cooner et al. 2009). The distance between anchor terminals for cables is referred to as a cable run. The maximum run of cable barrier between This chapter summarizes the findings of the literature search regarding high-tension cable barrier practices, with the study’s primary objective being to summarize transportation agency specifications and special provisions. The reports collected for this study were from individual states, including: • Development of Guidelines for Cable Median Barrier Systems in Texas, TTI • The Advisability of Expanding the Use of Cable Median Barriers in Illinois, IDOT • Study of High-Tension Cable Barriers on Michigan Roadways, MDOT • Cable Median Barrier Program in Washington State, WSDOT • Evaluation of Wisconsin Cable Median Barrier Systems: Phase 2, WIDOT • NCHRP Report 711: Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems. The relevant information regarding HTCB specifications, maintenance, and construction is provided here. It can be noted that there is some amount of overlap in the areas covered; therefore, there is not a clear distinction between reports. DESIGN ISSUES Barrier Placement To achieve the highest level of performance, it is important that barriers be placed on near-level terrain and with adequate clear space behind to allow for dynamic deflection during impacts. Generally, this location is found close to the travel lane where the barrier may experience frequent nuisance hits. Michigan DOT notes minimum offsets from the edge of the travelled way ranging from 8 ft to 12 ft and on slopes ranging from 4H:1V to 10H:1V. On a 6H:1V slope, barriers should not be placed from 1 ft to 8 ft from the ditch centerline (NCHRP Report 711) (Marzougui 2012). Barriers placed along the centerline of median ditches in poor or saturated soil conditions have experienced cable barrier foundation and anchor failures. The most common recommendations are “center of median” and/or “greater than 8 ft from the bot- tom of the ditch,” which is consistent with the Texas report and the AASHTO Roadside Design Guide (2011). NCHRP Report 711 contains specific guidance on the placement of cable barriers on slopes (Marzougui 2012). Figures 2 and 3 show examples of placement guidelines from this report. chapter two LITERATURE REVIEW

9 anchors that would allow for proper tensioning is approx- imately 10,000 ft and the minimum run length is 1,000 ft (Cooner et al. 2009). NCHRP Report 711 recommends a minimum 1,000 ft spacing based on economics since the cost of anchorages is relatively high compared with the cable runs themselves (Marzougui 2012). Emergency Vehicle Access Crossover locations must be provided at regular intervals to allow access for emergency vehicles to respond to an incident or an emergency, necessitating a break in the run of cable. The Texas DOT report noted that the maximum distance between emergency turnarounds varied from 1 to 5 miles (Cooner et al. 2009). Soil Conditions NCHRP Report 350 requires that all systems be installed in standard soil for full-scale crash testing (Ross et al. 1993). Highly plastic and/or highly saturated soils have been prob- lem areas. It is important that transportation agencies deter- mine the soil properties before placement of systems (Cooner et al. 2009). MAINTENANCE AND CONSTRUCTION Anchors Under certain soil conditions, the anchor sizes recommended by the manufactures were inadequate and were pulled from the ground after an impact. Wisconsin noted that when barriers are installed in the median centerline or ditch line where the soil is unstable and saturated anchors and post bases tend to dis- place. Illinois DOT (IDOT) has since revised their specifi- cations to require a specific design calculation and method; this required steel reinforced concrete drilled foundations from 10 to 15 ft deep and two and a half feet or more in diameter at some locations. Posts Replacing posts after an impact was an initial concern, with the degree of difficulty based on how the post bent, sheared, (a) (b) FIGURE 2 Underride criteria for V-shaped medians (Figure 6.1 from NCHRP Report 711) (Marzougui et al. 2012). FIGURE 3 Override criteria for V-shaped medians steeper than a 6H:1V slope (Figure 6.2 from NCHRP Report 711) (Marzougui et al. 2012).

10 or buckled. Socketed posts are generally easier to remove than those placed directly in concrete or driven. Illinois found that the initial construction cost for driving posts into the ground or through a relatively thin, paved mow strip with hydrau- lic equipment was less compared with the socketed posts. However, replacing driven posts generally required more time and machinery, and exposed maintenance personnel to heavy traffic for extended periods of time, whereas socketed posts can be removed and replaced manually without the need for post-driving equipment (CH2MHILL 2009). Frozen or Wedged Posts Posts can be difficult to remove with the socketed systems when they become wedged into the sleeves after an impact or ice and freezing temperatures makes them difficult to remove; this can be an issue for all colder climate states. Illinois tried prying techniques, heat application to the post, and salt solu- tions around the post in order to remove damaged posts. Sock- eted posts are still preferred by most Illinois districts and are called for in the standard specifications; however, individual districts are allowed to choose driven posts, if preferred by their maintenance personnel. Posts that fail as a result of shear rather than bending are often more difficult to remove from sockets (CH2MHILL 2009). Mowing and Weed Control Illinois has placed extra pavement underneath the barrier to reduce mowing and weed control. The Texas DOT report notes the following typical guidance on use for mow strips (Cooner et al. 2009): • Width—4 ft (widths ranging from 2 to 6 ft); • Thickness—3 in. (thickness ranged from 2 to 6 in.); and • Material—concrete (asphalt and aggregate were other materials). Resurfacing and Pavement Rehabilitation Illinois noted that in locations where cables are placed near the shoulder; for example, placed within 4 ft of the shoulder of a relatively steep slope (steeper than 6:1), several challenges were presented (CH2MHILL 2009): • For resurfacing projects—the height of the cables needed to be adjusted accordingly. FIGURE 4 Recommended cable barrier placement on a horizontal curve (Figure 2-26 from Cooner et al. 2009). Radius (feet) Post Spacing 650–2,500 6’-8 ” 2,501–5,500 10’ >5,500 As shown in details Table 2-8 of TxDOT Report. TABLE 1 TXDOT’S RECOMMENDED POST SPACING VS. CURVE RADIUS FIGURE 5 Impacted cable on outside of curve.

11 • Spot pavement repairs, such as patching—cable barrier can interfere with the space needed for the construc- tion equipment and may require removal, resulting in an increase in construction cost for the pavement patch- ing work. • Enforcement vehicle—this restricts the space available for enforcement vehicles to safely park on the inside (median) shoulder. Enforcement and Emergency Response Police and other emergency responders need information and training to avoid cutting the cables after a crash except in extreme emergency situations. This saves the time and expense of replacing entire segments of cables (CH2MHILL 2009). Based on the response of emergency responders in Michigan, their main issues were the increased time needed to reverse direction resulting from the large distances between cross- overs, difficulty removing vehicles in the event of a crash, and the need to close lanes because of the barrier’s close proxim- ity to the edge of the roadway. MDOT requires that the cable manufacturers provide training to agency staff and local emer- gency first responders as part of every cable barrier installation (Savolainen 2014). NCHRP Report 711: Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems (Marzougui et al. 2012) summarized the use of cable barriers and the introduction and use of HTCB. Also included in the report are detailed descrip- tions and information on the systems currently implemented in the United States. Based on state transportation agency surveys, specific concerns with HTCB were identified. Finite element analysis (FEA) was used to develop solutions to many of the expressed concerns, most notably barrier lateral placement in sloping medians. A summary of recommended guidelines was included as Appendix E in the report and were categorized as follows: • Planning and Feasibility (Use) • Cable Barrier Design • Deployment • Cost and Benefits Analysis • Construction • Maintenance and Operations.. Many of the proposed guidelines are general or address- specific design concepts and details that are decided by the state transportation agency before a project goes to bid. Those Items that would normally be included in the specifications accompanying the bid package (PS&E) are listed here: • Acceptance criteria (NCHRP Report 350 or MASH) • Test Level (normally TL-3 or TL-4) • Number of cables • Post spacing • Post footings • Maximum deflection • End anchor and post footing designs (based on soil analyses) • Mow strips • Construction tolerances (cable heights) • Cable tension • Training emergency responders. NCHRP Report 711 is currently under review by the AASHTO Technical Committee for Roadside Safety with the expectation that its pertinent findings and recommendations will be included in the next edition of the AASHTO Roadside Design Guide.