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The Guidelines for Road Design 684 Appendix 12.A - Barrier Placement 12.A.1 LATERAL OFFSET Where a roadside barrier is needed to shield an isolated condition, adherence to the uniform clearance criteria is not critical It is more important in such cases that the barrier be located as far from the traveled way as conditions permit The distance a barrier will deflect upon impact is a critical factor in its selection as well as in its placement, especially if the obstruction being shielded is a rigid object Figure 12-A-1 illustrates the two basic situations where deflection distance must be considered The barrier-to-obstruction distance for rigid objects shall not be less than the dynamic deflection of the barrier for impact by a full-sized automobile at impact conditions of approximately 250 and 100 km/h (Some reduction in deflection distance can be justified if the operating speed is less than 100 km/h.) In some cases, the available space between the barrier and the object can not be adequate In such cases, the barrier shall be stiffened in advance of and alongside the fixed object The effects on deflection of reduced post spacing are shown in Table 122-4 with the individual barrier descriptions 12.A.2 SLOPES As a car leaves the traveled way and crosses the shoulder and the embankment, the bumper path deviates from the normal bumper height as shown The primary area of concern is the zone of higher than normal bumper height A barrier placed in this zone can be expected to be hit at a higher than normal bumper position, and unless it has been designed for such impacts, its performance can be inadequate Table 12-A-1: Example Bumper Trajectory Data (see text) Five parameters have been selected to describe the embankment data With reference to Figure 12-A-2 these are Hs, HM, H2, LM and L Values of Hs, and H2 are important because most roadside barriers are placed between the edge of the shoulder and 0.6 m off the shoulder Table 12-A-1 contains trajectory data for rounded embankments for 100-km/h encroachments at angles of 250 and 15o These numbers were obtained primarily from computer simulations and are included mainly to illustrate the problem rather than to provide design guidelines The type of barrier also comes into play Strong post w-beam and thrie-beam installations were tested on 1:6 slopes and found to be only marginally 685 The Guidelines for Road Design satisfactory, due to the tendency of the rail element to bend backward and “ramp” the vehicle Based on these results, it is recommended that existing installations of these barrier systems can be retained (within the placement guidelines of Figure 12-A-3), but new installations on 1:6 slopes are not generally recommended 686 The Guidelines for Road Design Figure 12-A-1: Recommended Barrier Placement for Optimum Performance Figure 12-A-2: Design Parameters for Vehicle Encroachments on Embankments Figure 12-A-3: Recommended Barrier Location on 1:6 Slope 687 The Guidelines for Road Design The Guidelines for Road Design 688 12.A.3 SELECTION FACTORS It is critical that a vehicle contact most types of barriers with its center of gravity at or near its normal position This reduces the tendency for a vehicle to wedge under or go over the barrier Thus, the slopes between a barrier installation and the roadway shall be 1:10 or flatter or the barrier shall be far enough from the road that a vehicle is on the ground with its suspension system neither compressed nor extended at the time of contact Figure 12-A-3 approximates the acceptable location of a traffic barrier for approach slopes as steep as 1:6 A second reason for installing a barrier as far as practical from the roadway is to keep the barrier from causing drivers to slow down, change lanes, or shift positions within their own traffic lanes It is also worth noting that median barriers can be set closer to the edge of the driving lane without affecting vehicle placement When the barrier is to the left, the driver can clearly see how close the barrier is, whereas for a right shoulder installation, depth of perception becomes more of a problem for many drivers and they tend to position their vehicles further from the barrier than is necessary The tangent length of barrier immediately upstream from the area of concern, L1, is a variable length selected by the designer If a semi-rigid railing is connected to a rigid barrier, the tangent length shall be at least as long as the transition section to reduce the possibility of pocketing at the transition and to increase the likelihood of smooth redirection if the guardrail is struck immediately adjacent to the rigid barrier The final variable to be selected by the designer to calculate the required length of guardrail at a specific location is the flare rate Recommended maximum flare rates for semi-rigid and rigid barriers are shown in Table 12-2-6 Note that the recommended flare rate for barriers within the shy line is approximately twice that for barriers located outside the shy distance Once the appropriate variables have been selected, the required length of need, X, in advance of the area of concern, for straight or nearly straight sections of roadway, can be calculated with the following equation: X L A (b / a )(L )  L b / a  (L A ) / (L F ) Note that for a parallel installation, i.e., no flare rate, the first equation reduces to: X LA  L2 (L A ) /(L R ) The lateral offset, Y, from the edge of the traveled way to the beginning of the length of need can be calculated using the following equation: Y L A  (L A ) (X) (L R ) 689 The Guidelines for Road Design Since metal beam guardrail is manufactured in nominal 3.8 or 7.6 meter lengths, the amount of rail installed shall be a multiple of these lengths The above formulas will serve to locate the beginning of the length of need for an approach barrier To this length, a crashworthy end treatment must be added if this end treatment is located within the clear zone or in a location where it is likely to be struck As an alternative to computing a length of need, design charts have been developed to enable a length of barrier to be selected directly, based on standard conditions Examples of such charts are shown in Figures 12-A-4 and 12-A-5 for flared and parallel installations respectively 690 The Guidelines for Road Design Figure 12-A-6 illustrates the layout variables of an approach barrier for opposing traffic The length of need and the end of the barrier are determined in the same manner as previously described, but all lateral dimensions are measured from the edge of the traveled way of the opposing traffic, e.g., from the centerline for a two lane roadway If there is a two-way divided roadway, the edge of the traveled way for the opposing traffic would be the edge of the driving lane on the median side There are three ranges of clear zone width, Lc, that deserve special attention for an approach barrier for opposing traffic (Refer to Figure 12-A-6.) If the barrier is beyond the appropriate clear zone, no additional barrier and no crashworthy end treatment is required If the barrier is within the appropriate clear zone but the area of concern is beyond it, no additional barrier is required but a crashworthy end treatment shall be used If the area of concern extends well beyond the appropriate clear zone (e.g., a river), the designer can choose to shield only that portion which lies within the clear zone by setting LA equal to Lc Figure 12-A-4: Example Design Chart for a Flared Roadside Barrier Installation The lateral placement of the approach rail shall satisfy the criteria for embankment slopes If the existing slope is steeper than 1:10, it is suggested that the slope be flattened as illustrated in Figure 12-A-7 In lieu of flattening the slope, the designer can decrease the flare rate of the barrier so the embankment criteria is not violated The Guidelines for Road Design 691 Figure 12-A-5: Example Design Chart for a Parallel Roadside Barrier NO FLARE RATE Figure 12-A-6: Approach Barrier Layout for Opposing Traffic Figure 12-A-7: Suggested Roadside Slopes for Approach Barriers The Guidelines for Road Design 692 Figure 12-A-8: Example of Barrier Design for Bridge Approach Flanred Installation Farallel Installation Given: ADT = 6200 Speed= 110 kmlh Embankment slopes: 1:5 (right); 1:10 (median) Select: Clear Zone, LC, = 11.5-14 m (for 1:5 slope from Figure 11-A-4 in Appendix A Chapter 11) Clear Zone, Lc, = 9-10.5 m (for 1:10 median slope from Figure 11-A-4) Runout length, LR, = 145 m (Table 12-2-7) Transition, L1, = 7.6 m Barrier offset, L2 = 3.6 m (right); 2.4 m (median) Flare rate= 15:1 (Table 12-2-6) 693 The Guidelines for Road Design Discussion: For the right shoulder installation, the designer can scale 145 m back from the bridge rail end and 11.5 m laterally from the same point The hypotenuse of this triangle approximates a vehicle’s runout path To shield the bridge end and the river to the edge of the clear zone, the barrier installation must intersect this line Based on the variables selected, a barrier length of 57.1 m is required If this were an existing bridge and the approach embankment slopes were 1:2, the barrier would most commonly be installed parallel to the shoulder to minimize earthwork and approximately 84 m would be needed to shield the same area Calculations for the flared installation are as follows: 11.5  (1/15)(7.6) - 3.6 Length of need  57.6 m (1/15)  (11.5/145) (use 57.1) Note that on the median side, the designer can shield the entire opening even though this distance slightly exceeds the recommended clear zone for the 1:10 slope This emphasizes that the clear zone distance is not a precise number and that engineering judgement must be used in its application