Practices for High-Tension Cable Barriers (2016)

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SUMMARY From the first experimental installation of the high-tension Brifen wire rope safety fence in 2000 in the median of the Lake Hefner Parkway in Oklahoma City, Oklahoma, the use of high-tension cable barriers (HTCB) has significantly increased throughout this country. As more state agencies became alarmed at the number of serious cross-median crashes, exist- ing warrants for the use of median barriers were reexamined and many states realized that previous guidelines were inadequate. Because HTCB was found to be the most cost-effective barrier to install in existing medians, its use proliferated, along with the number of proprietary systems that soon became available. With this increased usage, some design, installation, and performance issues came to light. This synthesis study was conducted to identify and report on the current state of the practice for HTCBs used in medians of access-controlled roadways in the United States. Information on HTCB systems related to state agency specifications, special provisions, design standards, and installation and maintenance concerns was collected. Because all HTCB systems currently eli- gible for use on public roads in the United States are proprietary, information was also obtained from each of the manufacturers of these systems. Most of the information presented in this report was obtained through a survey sent electronic- ally, through the AASHTO Subcommittee on Design, to all state highway agencies. Responses to the questionnaire were received from all 50 states, for a response rate of 100%. Additional information was obtained through a literature review. The survey revealed that HTCB was allowed or in use in all but eight states and the District of Columbia. Although the majority of users allow a contractor to select the system that is ultimately installed, some limit the systems to those on their approved products lists. Because all of the currently eligible HTCB systems are proprietary, the states cannot specify only one particular design without a formal request for a public interest finding from FHWA. Typically, each state does call for a NCHRP Report 350 or AASHTO Manual for Assessing Safety Hard- ware (MASH) Test Level (TL) 3 or 4 design and most require a 4-cable system. Some states specify a design deflection and, in many cases, a maximum post spacing. Warrants for median barriers are based on those in the AASHTO Roadside Design Guide, separate state warrants, crash histories, or some combination of these. Several states reported initial problems with terminal anchor movement owing to poor soil conditions, high static loads, and impact loading. As a result, most state agencies now require the manufacturer to design the anchors based on on-site soil conditions. When the line posts are set in socketed concrete foundations, these foundations are generally designed for each project as well. Such foundations may vary from 12 to 15 in. in diameter and from 2 to 4 ft in depth. Because of the extremely high tension in the cables, posts that are not in a direct line with the end anchors in a run may have a tendency to deflect when along a horizontal curve or pull out of the ground when the posts traverse a sag vertical curve. In such cases, adjustments can be made to the design by decreasing the post spacing throughout horizontal curves and by increasing the resistance of the posts to uplift in sag vertical curves. PRACTICES FOR HIGH-TENSION CABLE BARRIERS

2 The location of the HTCB anchors is also an important consideration. Cable runs must be terminated to provide emergency vehicle access to the opposite traffic lanes and whenever median bridge piers or highway overpasses are shielded with semi-rigid or rigid barriers. In the first instance, offsetting the downstream run of cable from the upstream run will normally provide adequate overlap to prevent a cross-median crash. When the existing barrier is in place, the HTCB can be terminated in advance of the stiffer barrier, or extend behind or in front of the existing barrier, depending on site conditions such as existing offsets and side slopes. A fourth option is to connect the cables directly to an existing w-beam installation such as one used to shield a bridge end. One state noted issues on its first HTCB installation as a result of a general unfamiliarity with the system, both on the part of the contractor as well as state agency personnel. Because most state agencies now require training by the manufacturer on each project and because HTCB is no longer a new technology in most states, a lack of experience no longer appears to be a significant concern. Anchor block movement was also noted as an initial concern by some agencies; however, most now require a custom design based on existing soil conditions. Unlike other barrier systems, HTCB designs require some degree of on-going maintenance. In addition to repairing crash damage, the tension in each cable must be periodically monitored and adjusted as needed. Each contractor must tension the cables after installation according to manufacturer specifications and recheck it after several weeks. Once the project is accepted, state or contract maintenance personnel are expected to recheck the tension on a periodic basis and record the results on a tension log. Twelve states reported that maintenance is done exclu- sively by their own employees and seven that all such work was done by contract. For the remainder of the states, maintenance work was divided between state and contract personnel. More than half of the survey respondents reported that they have developed specifications or special provisions for HTCB. These documents are not product-specific, but generally apply to any eligible cable system that is selected by a contractor. Several state agencies rely entirely on the manufacturer specifications and installation and maintenance manuals, hav- ing none of their own. Example state documents were selected from the material provided by the states. These ranged in length from one or two pages to more than a dozen. The most comprehensive special provisions addressed the following items: • A description of the work to be performed. • Materials specifications, often including the test level and minimum number of cables, as well as a copy of the FHWA eligibility letter(s). • A manufacturers’ representative on site to oversee the installation and address any issues that may arise. • Manufacturer training provided to construction and state personnel. Some states require that every crew member be certified that he or she has been trained on all aspects of the system’s installation. Many require that maintenance and emergency responders also receive appropriate training. • Plans and shop drawings must be site-specific, particularly regarding end anchor and post-foundation designs. Some agencies require these designs to be signed and sealed by an engineer licensed in their state. • Limited geotechnical information may be provided by a state; however, it remains the contractor’s responsibility to coordinate with the system manufacturer to determine if additional soil borings and related data are needed to design the concrete foundations. • General HTCB system design specifies the criteria for foundation designs such as design loads, factors of safety, and, in some cases, the design method to be used to check for foundation movement, uplift, and overturning. • Concrete foundation construction is generally not specified separately in an HTCB spe- cial provision; however, at least one state includes details usually found in a concrete specification.

3 • HTCB construction and installation essentially requires the installation to be in confor- mance with the plans, any special provisions that exist, and shop drawings prepared by or for the manufacturer of the system being installed. • At least one state requires reference markers to be placed on the anchors with control reference markers nearby to monitor possible anchor movement over time. Movement is not to exceed 1 in. over the first 12 months following installation. • Measurement and payment is similar in the majority of states. Typically, the HTCB itself is paid for per linear foot installed, exclusive of the end anchors that are paid per each. One state pays per linear foot, including the end anchors, and one state pays for the steel portions of the end anchors per each, but the foundation concrete is paid per cubic yard. Much has been learned about HTCB design and performance since 2000. Foundation pullouts can be minimized by designing these features for the specific soil conditions in which they are placed. The performance of cable barriers on slopes has been investigated and placement guidelines have been established to minimize the likelihood of vehicle overrides and underrides. NCHRP Report 711: Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems, provides detailed information on all aspects of HTCB design and placement. Additional information may be acquired by a comprehensive in-service perfor- mance evaluation to determine if there remain significant concerns or issues that might be addressed through additional research.