Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems (2012)

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103 Although strong-post cable barriers were in use in some states as early as the 1950s, New York pioneered the development and testing of the weak-post design in the 1960s. This design, included in the 1977 AASHTO Guide for Selecting, Locating, and Designing Traffic Barriers as the G1 Cable Guardrail, may have been the first design guidance for a cable barrier system. This system was originally used primarily as a roadside barrier in a few states that were concerned with snow collecting in front of more solid barrier systems such as the W-beam guardrail. In the 1990s, several states became aware of an increase in cross-median head-on crashes along sections of freeways and began to install cable barriers to minimize the likelihood of these severe crashes. Today, there are several thousand miles of cable median barriers in the United States – both the initial low-tension generic designs and the more recent proprietary high-tension designs. The following sections summarize the principal findings of this research effort and translate them into guidelines that can be used by highway agencies to design cable barrier projects or formalize their own policies and procedures. 6.1 Warrants Barrier warrants address the need for, and cost-effectiveness of, any type of traffic barrier system for specific highway situations. This study made no attempt to assess, develop, or revise any existing warrants for either median barriers or roadside barriers. Current warrants, either as recommended in the AASHTO Roadside Design Guide or individual state guidelines, were considered appropriate and adequate for determining if a barrier is needed. Once the decision to install a barrier has been made, the next step is to select the most appropriate barrier type to use. In this regard, cable barrier is often a cost-effective choice for installation in wider medians because it is relatively lower in cost than other designs, can be installed on non-level terrain (within limitations), and can often be maintained relatively easily. 6.2 Structural Details and Crash Performance Characteristics Over the years, various cable barrier designs have been developed for use in medians and on roadsides. Early designs, known as generic, low-tension cable barriers, were adopted by state DOTs such as New York, Washington, North Carolina, Missouri, and others. These designs were found to be an effective means of preventing most median crossover events where deployed. The designs had limitations that prompted efforts to develop the several proprietary designs that emerged in the early 2000s. These new designs, known as high-tension systems, have higher cable tension, stronger cable-to-post connections, and, consequently, lower barrier deflection than generic systems. These new designs were also found to be a cost-effective means for reducing C h a p t e r 6 Guidelines for Cable Barriers

104 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems run-off-the-road fatalities and severe injuries. High-tension cable barrier systems are available from five manufacturers: Brifen, Gibraltar, Nucor Steel Marion, Gregory Industries (Safence systems), and Trinity (CASS systems). The available cable barrier systems vary in number of cables, cable heights, post size and geometry, cable-to-post connections, post spacing, and end-anchoring systems. Descriptions of these systems are presented in Chapter 4 and Appendix B of the contractors’ final report. A summary of the full-scale crash tests conducted on cable barrier systems is presented in Appen- dix D of the contractors’ final report. 6.3 Cable Barrier Guidelines The cable guardrail guidelines developed under this project addressed seven areas of concern: barrier lateral placement on sloped surfaces, influences of post spacing and end-anchor spacings on barrier deflection, post and terminal anchoring, interconnection with other barrier systems, construction and maintenance tolerances, placement along horizontal curves, and installation and maintenance costs. The principal recommendations or guidance that evolved for each of these areas are discussed below. A summary of the developed guidelines as well as others from the literature are listed in Appendix E of the contractors’ final report, which is published herein. Lateral Barrier Placement Vehicle dynamics analyses were performed to develop guidelines for effective cable barrier placement on sloped medians. These guidelines were developed for systems where the top cable is at 838 mm (33 in.) or higher and the bottom cable is at 533 mm (21 in.) or lower. This covers most currently available cable barrier systems. For systems that do not meet these requirements, vehicle dynamics analysis for the specific median profiles and specific barrier systems, follow- ing a similar approach to the one described in Section 5.1, can be used to find the effective placement locations. Nomographs for many basic median configurations and barrier designs are provided in Appendix C of the contractors’ final report and can be used to determine effec- tive locations where there are consistent conditions. Additionally, similar analysis can be per- formed to determine effective cable barrier placement if this cannot be achieved based on the following guidelines. The analysis showed that changes in slopes are the main factors contributing to median barrier penetrations and vehicle intrusion into opposing traffic lanes. A negative change in slope (down slope), typically located at the edge of the median or roadside, can lead to vehicle over- rides for high-speed and high-angle impacts. A positive change in slope (upward slope) typically located near or at the center of the median, can lead to vehicle underrides. The greater the change of slope, the more critical effect it has on barrier performance. The guidelines listed below provide recommendations for reducing vehicle underrides and overrides. Because these guidelines were based on a relatively limited number of vehicle types, impact speeds, and impact angles, and do not factor driver inputs such as braking and steering, it is important to note that adherence to these guidelines will not prevent all penetrations of the barrier. General Placement Guidelines • Cable barrier systems should not be placed on slopes steeper than 4H:1V (unless the system has been designed for and successfully crash-tested under these conditions). • Cable barrier systems can be used on 4H:1V or shallower sloped medians or roadsides (6H:1V or shallower sloped medians or roadsides are preferable), provided the placement guidelines listed below are followed.

Guidelines for Cable Barriers 105 • The gentler the slope, the lower the effects on the trace of the vehicle interface area and the greater the potential for an effective vehicle-to-barrier interface. • Wider medians offer more effective lateral placement options. • Median cross-section shape influences the effective lateral placement areas. Symmetric V-Shaped and Rounded-Bottom Medians • To avoid underrides of small and mid-size vehicles, the barrier should not be placed in the region between 0.3 m (1 ft) and 2.4 m (8 ft) from either side of the center of the V-shaped section (Figure 6.1a). For medians with slopes steeper than 6H:1V, the region between -0.3 m (-1 ft) and 0.3 m (1 ft) from the center of the median should also be avoided (Figure 6.1b). The cross- hatched areas in the figures indicate the lateral positions where the cable barrier may not be effective. • To avoid override of larger vehicles (SUVs and pickup trucks), the barrier should not be placed in a region between 1.2 m (4 ft) and 6.0 m (20 ft) from the edge of the median (breakpoint) when the median slope is steeper than 6H:1V (Figure 6.2). Symmetric Flat-Bottom Medians • To avoid underrides of small and mid-size vehicles, the barrier should not be placed in the region between 0.3 m (1 ft) and 2.4 m (8 ft) from either side of the flat-bottom breakpoints (Figure 6.3a). If the median slope is steeper than 6H:1V or if its flat-bottom section is less than 2.4 m (8 ft) in width, the region between -0.3 m (-1 ft) and 0.3 m (1 ft) from the flat- bottom breakpoint should also be avoided (Figure 6.3b). -1 ft -8 ft 1 ft 8 ft (a) Medians shallower than 6H:1V slope -8 ft 8 ft (b) Medians steeper than 6H:1V slope Figure 6.1. Underride criteria for V-shaped medians. 4 ft 0 ft 20 ft 0 ft 4 ft 20 ft Figure 6.2. Override criteria for V-shaped medians steeper than 6H:1V slope.

106 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • To avoid override of larger vehicles (SUVs and pickup trucks), the barrier should not be placed in a region between 1.2 m (4 ft) and 6.0 m (20 ft) from the edge of the median (breakpoint) when the median slope is steeper that 6H:1V (Figure 6.4). Non-Symmetrical Medians • It is preferable to place the cable barrier on the side of the median with the shallower slope. • The guidelines for V-shaped and flat-bottom medians should be followed to avoid vehicle underrides or overrides. Shoulders and Superelevations • Shoulders with slopes of 6 percent or flatter do not lead to any negative effects on vehicle-to- barrier engagement and consequently do not affect barrier performance. • For roads with superelevation steeper than 3 percent, the barrier should be placed not farther than 0.6 m (2 ft) from the edge of the median for medians steeper than 6H:1V slope and not farther than 1.5 m (5 ft) for medians shallower than 6H:1V. The barrier could be placed on the opposite side of the median (away from the superelevation). Cable Barrier Deflection Computer simulations using validated cable barrier models were conducted to study the effects of end-anchor spacing and post spacing on the dynamic deflection. The simulation -1 ft 1 ft -8 ft 0 ft -8 ft -8 ft 0 ft -8 ft (a) Medians shallower than 6H:1V slope and flat bottom wider than 2.4 m (8 ft) (b) Medians steeper than 6H:1V slope or flat bottom width less than 2.4 m (8 ft) Figure 6.3. Underride criteria for flat-bottom medians. 4 ft 0 ft 20 ft Figure 6.4. Override criteria for flat-bottom medians steeper than 6H:1V slope.

Guidelines for Cable Barriers 107 results, as expected, indicated that in impacts with cable barriers, the maximum dynamic deflec- tion is affected significantly by the end-anchor spacing and post spacing. Greater end-anchor spacing leads to increased barrier deflections. The rate of increase in deflection is greater for end-anchor spacing of less than 500 m (1,640 ft). Similarly, the results showed that post spacing affects the barrier deflection with longer post spacing leading to increased deflection. The results indicate that these effects vary for different types of cable barrier systems. Based on these findings the following guidelines are recommended. • Design deflection should not be based solely on deflection data from full-scale crash tests. • Highway designers should investigate the differences in deflections associated with a specific design (e.g., end-anchor spacing, post spacing, post type, cable-to-post connection) of a cable barrier system. It is important to know that the “design deflection” distances noted by each manufacturer are based on the deflection that resulted from a 100 km/h (62 mph), 25° test with a pickup truck impacting a cable barrier with specific post and end-anchor spacings. In the field, deflections can be greater depending on the specific impact conditions that occur and the installation setup. A hypothetical plot, showing the effect of end-anchor and post spacings on barrier deflection, is shown in Figure 6.5. Similar plots should be generated for the different cable barrier systems. It should be the individual manufacturer’s responsibility to ensure that such plots are available for each of its systems. End Anchors and Post Embedment End anchors perform a critical function for cable barriers by keeping the cables in tension to minimize deflection upon impact by a vehicle. Some problems have been noted in the field when these anchors have moved in the soil, either because of initial cable tension, increased cable tension Figure 6.5. Hypothetical plot of barrier deflection vs. end-anchor and post spacings.

108 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems caused by lower temperatures, or as a result of crashes into the barrier. Movement of the anchor in the soil decreases the tension in the cables which, when extreme, may result in unsatisfactory barrier performance. Therefore, anchors should be designed based upon an analysis of the soil where the cable barrier will be placed. Based on the soil data and climate information, static and dynamic geotechnical analyses or testing should be performed to determine the appropriate size for the end anchor. Likewise, many designs use posts set in concrete foundations to facilitate removal and replacement of damaged posts. These foundations also must be sized properly, based on existing soil and climate conditions, so they are not damaged or pulled out of the ground in a crash. Below are guidelines to ensure adequate size for the end-anchor and post footings. • Soil analysis must be performed to determine the appropriate size for the end-anchor and post footings. • The anchors should withstand a dynamic and static load (due to temperature variation) of 140 kN (31 kips) if all cables are connected to a single anchor or 90 kN (20 kips) if the cables are individually anchored. The anchor should not be damaged, and anchor movement should be less than 50 mm (2 in), preferably less than 25 mm (1 in.) under this load. • Fitting hardware used to connect the cable ends to the end terminal and/or end anchors should be properly installed to avoid cable pull-out from the connector during impact. • When a concrete footing is used for the posts, the footing should be designed to withstand a load higher than the plastic capacity of the post (i.e., the post would bend before the post footing breaks or moves significantly). Interconnection with Other Systems Because of the deflection distances inherent in a cable barrier, there will often be locations where cable cannot be used to shield a fixed object hazard effectively, such as where bridge piers are set in a narrow median. In such cases, a median cable barrier must be terminated and a semi-rigid barrier introduced. FHWA has issued acceptance letters for several cable-to-W-beam transition designs as meeting NCHRP Report 350 test conditions at Test Level 3. The designs were accepted based on full-scale crash testing conducted under NCHRP Report 350 with a South Dakota design in which a generic 3-cable barrier was carried over and under a W-beam guardrail (which was anchored separately) and anchored independently behind the metal beam rail. Subsequent to this testing, several of the manufacturers of proprietary cable systems proposed similar transitions, with the basic dif- ference being the cables were attached directly to the W-beam rail element, thus eliminating the need for a separate anchor for the cables. At this time, only transitions where the cable barrier system is in line with the other system have acceptance letters and all accepted transitions connect to semi-rigid (strong-post W-beam) barriers. Also, all transition acceptance letters are based on one full-scale crash test (South Dakota design). The new transitions have been accepted without additional full-scale crash testing. Based on the findings described in Section 5.4, the following guidelines are recommended: • Cable barrier should not be transitioned into rigid (concrete) or flexible (weak-post W-beam) barriers unless further analyses and/or testing are conducted and adequate performance is achieved. • Transitions achieved by overlapping a section of the cable barrier in front or behind semi- rigid systems should not be used until further analyses and/or testing are conducted to ensure adequate performance is achieved. • The semi-rigid barrier to which the cables are connected must be long enough and adequately anchored at its downstream end to resist the tension in the cables. The semi-rigid barrier should withstand a 140 kN (31 kips) load with less than 25 mm (1 in.) movement of its end

Guidelines for Cable Barriers 109 post or end anchor. Under this study, simulations were conducted to identify the minimum length needed for strong-post W-beam systems, and it was found to be 23 m (75 ft). • The connection between each cable and the rail of the semi-rigid barrier should be designed such that it can withstand 90 kN (20 kips). • The semi-rigid barrier should be appropriately flared back with adequate offset between it and the cable barrier. A minimum of 1.2 m (4 ft) flare is recommended. • The heights of the cable barrier system should be compatible with the semi-rigid barrier over the length of the transition. • The cable barrier needs to be transitioned from its lateral position to that of the semi-rigid system. Cable Barrier Tolerances The tolerances listed in Table 6.1 are recommended to minimize the effects of installation variations on cable barrier performance (see Section 5.6). Placement along Horizontal Curves There are two concerns that must be considered when cable barrier is located along a curved section of roadway. The first is that the high-tensioned cables can exert significant lateral pressure on the support posts and may cause them to bend over time, regardless of whether the barrier is on the inside of a highway curve to the left (convex) or on the outside (concave). The second concern is crash performance. When a convex installation is impacted, the tension in the barrier immediately decreases as the cables are separated from the posts and become slack, resulting in deflections in excess of the barrier’s design deflection. Limited research has been conducted to study the effects of horizontal curvature on the performance of cable barrier systems. To investigate this effect, finite elements simulations were performed. Barrier models with varied curvature radii, 150, 300, and 400 m (492, 984, and 1,312 ft), and varied post spacing, 1.6, 3.2, and 6.4 m (5, 10, and 20 ft), were created. The models are based on one of the high-tension systems used in analysis described in Section 5.2 (a 4-cable CASS system). It is expected that similar results with other high-tension systems would be observed. The simulations were conducted for impacts with a 2000P vehicle traveling at 100 km/h (62 mph) and 25° angle. The barrier length used for all simulations was 200 m (656 ft). The deflections of the barrier from these simulations were extracted, and the results are plotted in Figure 5.48. The simulations clearly indicate that convex curvature leads to increased deflection. The increase in deflection was as high as 70 percent. Thus, it is important to consider this effect when selecting barrier location. The simulation results suggest the following guidelines: • Shorter post spacings should be used to account for the increase in deflection when cable barriers are placed on horizontal curves with a radius less than 400 m (1,300 ft). Cable Barrier Parameter Tolerance Top Cable Height -0,+25 mm (-0, +1 in.) Bottom Cable Height -25,+0 mm (-1, +0 in.) Middle Cable(s) ±25 mm (±1 in.) Barrier Lateral Position ±150 mm (±6 in.) Average Post Spacing ±150 mm (±6 in.) Consecutive Post Spacing ±600 mm (±2 ft) Cable Tension ±2 kN (±0.45 kips) Table 6.1. Suggested cable barrier tolerances.

110 Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems • Shorter post spacings should also be used to reduce the bending of the posts over time due to lateral forces applied by the cables on the posts. • The lateral placement guidelines developed under Section 5.1 should be followed to ensure adequate vehicle-to-barrier interface. • The barrier should be placed as close as possible to the convex side to allow more space behind the barrier for the added deflection. This will also reduce the number of cable barrier hits since it is placed farther away from the more frequent impacts with vehicles coming from the concave side [1]. • For narrower medians, the use of two cable barriers (one on each side of the median) or a more rigid barrier should be considered to reduce the likelihood of vehicles impacting the convex side and intruding into on-coming traffic. Placement on Vertical Curves Placing cable barriers on vertical curves also creates potential concerns for barrier performance. In such installations, the tensioned cables exert upward vertical forces on the posts, which are significantly lower in elevation than the end anchors, particularly at the bottom of the vertical curve. Depending on the specific design details of the barrier, i.e., post embedment and cable- to-post connections, these upward forces could pull the posts out of the ground, pull the posts out of the sockets, or pull the cables off the posts, resulting in less cable tension and cables at the incorrect height in relation to the ground. Designers should be aware of this possibility. Placing the end anchors as close as possible to the minimum and maximum points of the vertical curvature reduces the upward vertical forces. Similarly, placing the end anchors at locations where there is a sudden change in the vertical slope would reduce the vertical variation effects. Another possible solution is to use closer post spacing through sharp vertical curves to reduce the vertical component of force on individual posts. Suggested guidelines for cable barrier when placed on vertical curves are listed below. • End anchors of the cable barrier should be placed as close as possible to the bottom and top points of the vertical curvature. • End anchors should be placed at points where there is a sudden change in vertical slope. • Post spacings should be reduced in sharp vertical curves. • Regrading short sections of the median that have sharp vertical curvatures should be considered. • Regular checks should be performed at the bottom and top of vertical curves to ensure that the cables have not moved up or down from their specified heights. Installation and Maintenance Costs An analysis of installation and maintenance costs for cable barriers was conducted to ensure effective use of limited safety resources (Section 5.7). Based on these findings, the following guidelines have been developed: • Life-cycle cost analyses should be used to evaluate cable barrier systems rather than selecting systems on the basis of lowest bid for installation. All FHWA-accepted systems do not per- form the same. • End-anchor and post foundation specifications should require designs based on in situ soil conditions and expected climate conditions that are reflected in the costs. • If a maximum design deflection has been set for a project, then the combined effects of post spacing and anchor-to-anchor spacing for each specific type of high-tension cable barrier should be considered to ensure that the required maximum deflection will not be exceeded.

Guidelines for Cable Barriers 111 • Because of the small marginal cost of adding an extra cable and the likelihood of improved safety performance of the barrier with an extra cable, benefit/cost analyses should be used to determine when a fourth or fifth cable is justified. • Periodic inspection schedules based on expected frequency of impacts should be established for each cable barrier installation. • Maximum response times for repairing damaged barriers should be established based on the class of highway and the extent of damage. • Cable heights on barriers at the bottom and top of vertical curves should be checked periodically to make sure they remain at design heights. • Post spacings should be reduced for horizontal curves with a radius 400 m (1,300 ft) or less. • A benefit/cost analysis should be conducted to determine when a mow strip is justified. • Cable barriers should be inspected after severe weather events, including flooding and extremely cold temperatures, to check for erosion, end anchor movement, and cable heights on vertical and sharp horizontal curves. • Driven posts should not be used except when the expected rate of impacts is too low to justify socketed posts. • Manufacturers should provide training on barrier installation, maintenance, and repairs for highway agencies and emergency response crews.