Design of Roadside Barrier Systems Placed on MSE Retaining Walls (2010)

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32.1 Design of MSE Wall MSE walls are made of alternating layers of soil (fill) and rein- forcement (Figure 2.1) (4). The fill must satisfy specifications (e.g., plasticity index limits, percentage passing #200 limits) and are generally sandy or rocky fills. The reinforcement is tied to panels erected vertically at the front of the wall. The reinforce- ment can be made of steel strips, bar mats, or geosynthetics. Each layer between reinforcement is compacted to the required compaction level (Figure 2.2). The idea of an MSE wall is to create a reinforced earth mass that is equivalent to a gravity wall. As such, the basic design con- sists of two parts: external stability design and internal stability design. 2.1.1 External Stability The external stability ensures that the wall is safe against slid- ing, overturning, bearing capacity failure, and slope stability failure (see Figure 2.3): • Sliding design consists of ensuring that the active force devel- oping behind the wall does not represent an unreasonable risk of overcoming the friction resistance at the base of the wall. • Overturning design consists of ensuring that the moment created by the active force around the bottom of the front of the wall does not represent an unreasonable risk of overcoming the resisting moment due to the weight of the wall mass. • Bearing capacity design consists of ensuring that the pres- sure due to the wall mass does not represent an unreason- able risk of overcoming the ultimate bearing capacity of the soil. • Slope stability design consists of ensuring that the overall wall configuration does not represent an unreasonable risk of failing by general deep seated rotation. 2.1.2 Internal Stability The internal stability ensures that the wall mass is a coher- ent solid block with tensile resistance. This design addresses the issues of the load on the reinforcement, the required length of the reinforcement, and the stress in the reinforcement (see Figure 2.4). • The load on the reinforcement is obtained by using a semi- empirical equation developed from experience. This equa- tion expresses that the reinforcement must safely resist the pressure on the panel that would develop in the soil if the reinforcement were not there. • The length of the reinforcement is equal to the sum of the length required to safely resist in friction the load calculated in the previous step plus the length in the failing zone behind the wall. This length is usually calculated by a prescriptive approach, L = 0.7H (height of the wall). • The stress in the reinforcement is the load divided by the reinforcement area after discounting the corrosion thick- ness and other factors if appropriate. This stress is checked to ensure that it is safely below the yield stress of the material used. In AASHTO LRFD (2), to satisfy the internal stability, the static factored resistance (φP) to pullout of the reinforcement should be at least equal to the static factored load (γT) due to the earth pressure. The static resistance (P) per unit of reinforcement width is calculated using the following equation (LRFD Equation 11.10.6.3.2-1): where F* = Pullout friction factor as shown in Figure 2.5 α = Scale effect correction factor (LRFD Table 11.10. 6.3.2-1) P F CLv e= * ( )ασ 2 1- C H A P T E R 2 State of the Practice

4Source: Elias et al. (4) Figure 2.1. Principal elements of MSE wall. Source: Elias et al. (4) Figure 2.2. Construction of MSE wall. σv = γ × h, where γ = Soil unit weight, h = Height of the strip from the roadside C = Overall reinforcement surface area geometry factor based on the gross perimeter of the reinforcement and equal to 2 for strip-, grid-, and sheet-type rein- forcements Le = Length of reinforcement in resisting zone To obtain the static load (T) expected per unit of wall width due to the soil, the following equation in AASHTO LRFD is used (LRFD Equation 11.10.6.2.1-2) where σh = Horizontal stress due to the soil, σh = Kr × σv, where Kr = lateral earth pressure coefficient Sv = Vertical spacing of the reinforcement Example applications of the AASHTO LRFD MSE wall design procedures are presented in Appendix A, which is available from the NCHRP Report 663 summary web page T Sh v= σ ( )2 2- on the TRB website (www.trb.org) by searching for “NCHRP Report 663”. 2.2 Design of Barrier This section includes background regarding roadside barrier crash testing criteria, a history of the design loads, and design practice of roadside barriers. 2.2.1 Background of Barrier Crash Testing Guidelines Guidelines for testing roadside appurtenances originated in 1962 with a one-page document—“Proposed Full-Scale Test- ing Procedures for Guardrails” (5). This document included four specifications on test article installation, one test vehicle, six test conditions, and three evaluation criteria. NCHRP Report 153 (6), published in 1974, provided the first complete test matrix. Parameters to be measured were specified along with methods and limiting values, and limited guidance on

Source: Elias et al. (4) Figure 2.3. External stability considerations. Source: AASHTO (2) Figure 2.4. Internal stability considerations. 5

reporting formats was included. These procedures gained wide acceptance following their publication, but it was recognized at that time that periodic updating would be needed. In 1978, Transportation Research Circular 191 (7) was published to provide limited interim changes to NCHRP Report 153. An extensive revision and update was made in 1981 with the publication of NCHRP Report 230 (8). This doc- ument specified different service levels for evaluating longitu- dinal barriers whose test matrices included vehicles ranging from small passenger cars to intercity buses. NCHRP Report 350 (3), which was published in 1993, pro- vides current guidance on testing and evaluating roadside safety features. This 132-page document represented a com- prehensive update to crash test and evaluation procedures. It incorporated significant changes and additions to procedures for safety performance evaluation, and updates reflecting the changing character of the highway network and the vehicles using it. NCHRP Report 350 selected a 2,000 kg (4,409 lb) pickup truck as the design test vehicle to reflect the fact that over one half of new passenger vehicles sales in the United States were in the light truck category. This change was made recognizing the differences in wheel bases, bumper heights, body stiffness and structure, front overhang, and other vehicular design fac- tors associated with light trucks. NCHRP Report 350 further defines other supplemental test vehicles including an 8,000 kg (17,637 lb) single-unit cargo truck and 36,000 kg (79,366 lb) tractor-trailer vehicles to provide the basis for optional testing to meet higher performance levels. Six test levels are defined for longitudinal barriers (e.g., bridge rails, median barriers, guardrails) that place an increas- ing level of demand on the structural capacity of a barrier sys- tem. The basic test level is Test Level 3 (TL-3). The structural adequacy test for this test level consists of a 2,000 kg (4,409 lb) pickup truck impacting a barrier at 100 km/h (62 mph) and 25 degrees. At a minimum, all barriers on high-speed road- ways on the National Highway System (NHS) are required to meet TL-3 requirements. Many state transportation depart- ments require that their bridge railings meet TL-4, which requires accommodation of an 8,000 kg (17,637 lb) single-unit truck hitting a barrier at 80 km/h (50 mph) and 15 degrees. Higher containment barriers are sometimes used when condi- tions such as a high percentage of truck traffic warrant. Such barriers are necessarily taller, stronger, and more expensive to construct. Since publication of NCHRP Report 350, changes have occurred in vehicle fleet characteristics and testing technology. NCHRP Project 22-14(2) (9), “Improved Procedures for the Safety-Performance Evaluation of Roadside Features,” was initiated to take the next step in the continued advancement and evolution of roadside safety testing and evaluation. The results of this research effort culminated in the new document 6 Source: AASHTO LRFD Figure 11.10.6.3.2-1 (2) Figure 2.5. Default values for the pullout friction factor, F*.

Manual for Assessing Safety Hardware (MASH) (10) that was published by AASHTO and supersedes NCHRP Report 350. Changes in the new guidelines include new design test vehi- cles, revised test matrices, and revised impact conditions. The weight and body style of the pickup truck changed from a 2,000 kg (4,409 lb), 0.75-ton, standard cab pickup to a 2,270 kg (5,000 lb), 0.5-ton, four-door pickup. For TL-4, the weight of the single-unit truck increased from 8,000 kg (17,637 lb) to 10,000 kg (22,000 lb) and the speed increased from 80.47 km/h (50 mph) to 90.12 km/h (56 mph). Although still a draft, many user agencies have already begun applying the MASH criteria in their crash test programs. 2.2.2 Background of Barrier Design Loads Historically, the design of bridge rails has followed guidance contained in the AASHTO Standard Specifications. Prior to 1965, the AASHTO Standard Specifications required very sim- ply that “Substantial railings along each side of the bridge shall be provided for the protection of traffic.” It was specified that the top members of bridge railings be designed to simultane- ously resist a lateral horizontal force of 2.19 kN/m (150 lb/ft) and a vertical force of 1.46 kN/m (100 lb/ft) applied at the top of the railing. The design load on lower rail members varied inversely with curb height, ranging from 7.3 kN/m (500 lb/ft) for no curb to 4.4 kN/m (300 lb/ft) for curb heights of 0.23 m (9 in.) or greater. It was further specified that the railing have a minimum height of 0.69 m (27 in.) and a maximum height of 1.07 m (42 in.) above the roadway surface. These loads are only a fraction of what is used today. Based on a poor accident history, accentuated by increased exposure due to dramatically increasing travel volumes, the engineering community came to realize that these criteria were inadequate. There was a recognized need (and, in the words of some, an “urgent necessity”) for a railing specification that established loading requirements more in line with the weights and increased speeds of vehicles of that day. In 1962, the U.S. Department of Commerce, Bureau of Public Roads (BPR), now the Federal Highway Administration (FHWA), developed proposed revisions to the specifications for bridge railings. It was proposed that bridge railings and parapets be designed for a transverse load of 133.4 kN (30 kips) using plastic design procedures. This load was distributed among the horizontal railing members. A figure with 10 differ- ent railing types/configurations was provided to assist with dis- tribution of the load. The difficulty of defining a static load that would be equivalent in effect to a vehicle impact on a railing was recognized. As part of the rationale for selecting the load of 133.4 kN (30 kips), reference was made to designs that met the proposed specification and which experience indicated would be adequate to resist the usual anticipated forces of impact. Based on information received from a retired Texas Depart- ment of Transportation (TxDOT) bridge engineer involved in review of this proposal, many AASHTO members were un- familiar with plastic design procedures and there was “great objection” to using it. Ultimately, after considerable discus- sion, comment, and revision, the AASHTO Committee on Bridges and Structures approved a revision to the railing spec- ification in 1964. The revised railing specifications were subsequently pub- lished in 1965 in the ninth edition of the AASHTO Standard Specifications for Highway Bridges (11). It required that rails and parapets be designed for a transverse load of 44.5 kN (10 kips) divided among the various rail members using an elastic analy- sis. The force was applied as a concentrated load at mid-span of a rail panel with the height and distribution of the load based on rail type and geometry as provided in an accompanying fig- ure. Posts were designed for the transverse loading applied to each rail element plus a longitudinal load of half the transverse load. The transverse force on concrete parapet walls was dis- tributed over a longitudinal length of 1.52 m (5 ft). Guidance on the effective length of slab resisting post loadings was pro- vided for rail designs with and without a parapet. The height of the railing was required to be no less than 0.69 m (27 in.). It was noted that railing configurations successfully crash tested were exempt from the design provisions. The rationale for changing the 133.4 kN (30 kips) force proposed by the BPR to the 44.5 kN (10 kips) force ultimately adopted by AASHTO is not fully known. However, it can be shown that a 44.5 kN (10 kips) load with the rail resistance defined by elastic analysis is roughly equivalent to a 133.4 kN (30 kips) load with the rail resistance defined by plastic analy- sis following the BPR procedure. Such an equivalency may have been established to permit more familiar design proce- dures to be followed. The provisions in the 17th edition of the AASHTO Standard Specifications for Highway Bridges (1), published in 2002, are essentially the same as the revised spec- ification adopted in 1965. These requirements are intended to produce bridge rails that will function adequately for passenger cars for a reasonable range of impact conditions. The reserve load capacity of the rail, beyond its elastic strength, offers some degree of protec- tion for more severe impact conditions or for heavier vehicles. Several catastrophic crashes involving large vehicles increased awareness of design requirements for bridge rails and the need to extend protection beyond passenger cars. In the first of two such studies, an instrumented concrete wall (shown in Figure 2.6) was designed to, for the first time, measure the magnitude and location of vehicle impact forces (12). The wall consisted of four 3.05 m (10 ft) long panels lat- erally supported by four load cells. Each of the 1.07 m (42 in.) tall × 0.61 m (24 in.) thick panels was also instrumented with an accelerometer to account for inertia effects. Surfaces in 7

contact with the supporting foundation and adjacent panels were Teflon coated to minimize friction. In this first study, eight full-scale crash tests were conducted using various sizes of passenger cars and buses. In the second such study (13), a new wall with a height of 2.29 m (90 in.) was constructed using similar design details, and crash tests with a variety of trucks (up to and including a 36,300 kg (80,000 lb) tractor with tank- type trailer) were conducted. Speeds in these tests ranged from 80.5 km/h (50 mph) to 69.6 km/h (60 mph) and the impact angles ranged from 15 degrees to 25 degrees. The data from the instrumented wall tests were analyzed to determine the resultant magnitudes, locations, and distribu- tions of the contact forces. Maximum forces were obtained by averaging the data over 0.05-second (sec) intervals to reduce the effect of force “spikes” in the data that were believed to have lit- tle consequence to the required structural integrity of the bridge railings due to their short duration. Two forces were determined for each test—one associated with the initial impact of the front corner of the vehicle, and one associated with the second impact or “backslap” as the rear of the vehi- cle rotates (yaws) into the rail as it is redirected. An example is shown in Figure 2.7. The pressure of these resultant forces was assumed to be distributed as half a sine wave in both the horizontal and vertical directions (see Figure 2.8). The length of the con- tact area was measured from high-speed film. An example of the longitudinal distribution obtained in this manner is shown in Figure 2.9. Because the force measurements were obtained from a nearly rigid barrier, they are considered to represent the upper bound of forces that would be expected on an actual bridge railing. Any deformation of the bridge rail during impact will tend to reduce the magnitude of the impact forces below those obtained on the “nearly rigid” instrumented concrete wall. 8 Source: Noel et al. (12) Figure 2.6. Instrumented wall. Source: Noel et al. (12) Figure 2.7. Magnitude and location of average resultant force (4,740 lb vehicle, 60 mph, 24 degrees).

9Source: Noel et al. (12) Figure 2.8. Distribution of contact pressure. Source: Noel et al. (12) Figure 2.9. Longitudinal distribution for initial and final impacts (4,740 lb vehicle, 60 mph, 24 degrees).

Data from the instrumented wall studies were used to derive barrier design loads for various impact conditions included in the AASHTO Guide Specifications for Bridge Railings (14) and subsequently, the AASHTO LRFD Bridge Design Specifications: Section 13, Railings (2). 2.2.3 Barrier Design Practice As previously mentioned, the AASHTO Standard Specifica- tions for Highway Bridges (1) specifies an elastic, allowable stress analysis methodology for designing bridge rails using a static load of 4,536 kg (10,000 lb) distributed among the various rail elements. These requirements have existed since their adoption in the ninth edition of the AASHTO Standard Specifications for Highway Bridges (11) in 1965. It can be observed that measured dynamic impact forces obtained from full-scale vehicle crash tests into an instrumented concrete wall are significantly higher than static loads used in the design of bridge rails for passenger cars. Yet, this observa- tion does not necessarily mean that railings designed for a static load of 4,536 kg (10,000 lb) following the AASHTO Standard Specifications for Highway Bridges are inadequate, because a railing system will generally have an ultimate strength well above that indicated by allowable stress design procedures. However, the amount of reserve capacity will vary depending on materials and design details and is not predicted when allow- able stress design methods are used. Ultimate strength design procedures provide a more accurate indication of the actual strength of a rail. In 1984, Buth et al. (15) recommended that bridge rails be designed based on ultimate strength procedures using yield strength of the material with a factor of safety equal to 1.0. The capacity determined in this manner is compared to the dynamic impact loads determined from data measured in the instru- mented wall testing programs. Such a design procedure is intended to produce yielding, but not ultimate failure/fracture, when a design impact collision occurs. This premise should hold true provided the materials and structural elements have sufficient ductility and ultimate strength substantially greater than yield strength. Such analyses are based on bending moments induced in the structure and the formation of plastic hinges at points of high bending moment. Thus, the failure mechanism of the rail must be known or assumed. The failure mechanism and the number of posts involved in the mechanism are dependent on how the load applied by the vehicle is distributed to the system. Inves- tigation of several different failure mechanisms for a given rail system is typically required to determine the controlling mech- anism (i.e., the mechanism that develops at the lowest load). One-span, two-span, and three-span failure mechanisms are idealized in Figure 2.10. The validity of an ultimate strength failure mechanism requires the structure to be able to deform enough to actually develop the failure mechanism. Ultimate strength design procedures were widely used by roadside safety researchers in the 1980s to develop bridge rails capable of containing buses and trucks. In most cases, the impact performance of the rails was verified through full-scale crash testing. In 1989, these procedures were incorporated into the AASHTO Guide Specifications for Bridge Railings. This specifi- cation prescribed three performance levels for bridge rails and warrants for their use. The test matrices associated with these performance levels included tests with trucks which, up to this time, had not been given consideration in testing documents such as NCHRP Report 230. Impact conditions associated with Performance Level 1 (PL-1) included a 2,500 kg (5,400 lb) pickup truck hitting at a speed of 72.4 km/h (45 mph) and an angle of 20 degrees. For PL-2, the speed of the pickup truck test was increased to 10 Source: Buth et al. (15) Figure 2.10. Idealized span-based failure mechanisms.

96.5 km/h (60 mph) and a test with an 8,165 kg (18,000 lb) single-unit truck impacting the barrier at a speed of 80.5 km/h (50 mph) and an angle of 15 degrees was added to the test matrix. The highest performance level, PL-3, incorporates a test with a 22,680 kg (50,000 lb) van-type tractor trailer impact- ing the barrier at a speed of 80.5 km/h (50 mph) and an angle of 15 degrees. The design impact loads prescribed for each per- formance level were determined based on data measured in the previously described instrumented wall crash tests (12, 13). In 1993, NCHRP Report 350 was published. This report contains six test levels for longitudinal barriers. Test Levels 1 through 3 relate to passenger vehicles and vary by impact speed. Test Levels 4 through 6 retain consideration of passen- ger cars, but also incorporate consideration of trucks. The impact conditions of TL-4 in NCHRP Report 350 are similar to those associated with PL-2 in the 1989 AASHTO Guide Specifications for Bridge Railings. TL-4 is the test level used by most states to qualify the impact performance of their bridge rails—a fact that may be a holdover from prior use of the 1989 AASHTO Guide Specifications for Bridge Railings. Ultimate strength design procedures were subsequently adopted in the first edition of the AASHTO LRFD Bridge Design Specifications published in 1996 (16). Rather than perpetu- ate two sets of impact performance criteria, the test levels of NCHRP Report 350 were adopted over the performance levels of the 1989 AASHTO Guide Specifications. Section 13, Railings, of the AASHTO LRFD Bridge Design Specifications applies to the design of railings for bridges. Yield line theory considers the plastic strength of all the railing sys- tem components with consideration given to barrier ge- ometry, material strengths, applied loading, and strength of the supporting bridge structure. Steel rail systems, concrete rail systems, or a combination rail composed of a steel rail on a concrete parapet can be evaluated using these design proce- dures. Based on the yield line theory, the limiting ultimate capacity of the railing system is calculated. This ultimate capac- ity is then compared to design forces derived from vehicular loads measured in actual crash testing. Typically, capacities of the railing system are calculated at both mid-span of the railing system and at a joint or end of the rail system. The controlling yield line failure mechanism for a vertical concrete parapet loaded at mid-span is shown in Fig- ure 2.11. The failure mechanism for loading at a joint or end is theoretically similar but involves only a single “hinge” as shown in the illustration presented in Figure 2.12. For safety- shaped barriers, such as the New Jersey (N.J.) and F-shape bar- riers, the hinges or failure planes are often isolated in the upper, narrower portion of the barrier as shown in Figure 2.13. (17) Post-and-beam types of bridge parapets are fabricated from concrete, structural steel, or aluminum components, or a com- bination of these materials. Failure mechanisms in post-and- beam parapets can occur in several different modes. As the name implies, the impact loads must be transferred to the deck through discrete posts rather than through a continuous rail section. This can result in higher concentrations of load that can result in severe localized damage to the deck or slab if not properly designed. The calculated ultimate strength or capacity of the rail is then compared to applicable design forces to assess its structural adequacy. The prescribed impact loads for different test lev- els are presented in Table A13.2-1, Section 13 of the AASHTO LRFD Bridge Design Specifications. The loads are considered to be short duration, one-time loads. The barrier is sized such that it will have an ultimate strength, based on a yield line analysis, that is equal to or greater than the specified load with no “factor of safety.” 2.3 Design of the Barrier on Top of the MSE Wall AASHTO allowable stress design (ASD) (1) and LRFD (2) use the same basic procedure to design a barrier on top of an MSE wall even though the impact specification was increased from 44.5 kN (10 kips) to 240 kN (54 kips) for the design of the traffic barrier. This section summarizes the current AASHTO LRFD design procedure for barriers mounted on 11 Source: AASHTO (2) Figure 2.11. Idealized mid-span failure mechanism.

the edge of MSE walls, compares the AASHTO ASD and LRFD procedures, and describes previous test results. 2.3.1 Design of MSE Wall for Barrier Impact In AASHTO LRFD Bridge Design Specifications Section 11.10.6.2.1, the following equation is presented to calculate horizontal stress due to the soil weight and the impact load: where σh = horizontal stress due to the soil weight = kr × σv, kr is the horizontal earth pressure coefficient given by 1.7 ka, σv is vertical stress due to the soil weight Δσh,max = horizontal stress due to the impact load (Ph1) on the barrier = 2Ph1/l1, l1 is the depth of influence of the impact load down the wall face as shown in Figure 2.14. σ σ σH h h= + Δ ,max ( )2 3- 12 Source: AASHTO (2) and Alberson et al. (17 ) Figure 2.12. Failure mechanism at barrier joint or end. Source: Alberson et al. (17) Figure 2.13. Typical failure pattern for safety-shaped barriers.

2.3.2 Comparison between ASD and LRFD AASHTO is in the process of changing from ASD to LRFD. The 2002 AASHTO ASD makes use of a 44.5 kN (10 kips) load for the design of the traffic barrier and for the impact load that is distributed into the MSE wall below (in the form of added load for the reinforcement). The 2004 AASHTO LRFD speci- fies a 240 kN (54 kips) design load (corresponding to TL-3 and TL-4) for the traffic barrier and a 44.5 kN (10 kips) load for the design of the MSE wall. Therefore, there has been a significant increase in the design load for the barrier. The 240 kN (54 kips) load level comes from measurements made on an instrumented barrier during impact and, there- fore, is a dynamic load. The increase from 44.5 kN (10 kips) to 240 kN (54 kips) for the structural design of the barrier does not increase the size of the barrier significantly because the 44.5 kN (10 kips) load is used with an elastic design analysis while the 240 kN (54 kips) load is used with an ultimate strength analysis. However, for the moment slab design, the change from 44.5 kN (10 kips) static to 240 kN (54 kips) dynamic requires a proportional increase in the width of the moment slab if the 240 kN (54 kips) is used as a static load in the stability analy- sis of the barrier system. Indeed one would calculate a 1.37 m (4.5 ft) wide moment slab with AASHTO ASD and a 1.37 m (4.5 ft) × 54/10 = 7.4 m (24.3 ft) wide moment slab with AASHTO LRFD. This difference arises because that 54 kips is taken as a static load when in fact it is a dynamic load. From experience, a 7.4 m (24.3 ft) wide moment slab would be unrea- sonably conservative. The objective is to find out how to take into consideration the 240 kN (54 kips) for overturning and sliding of the barrier. The design of the barrier against overturning consists of applying the load to the barrier at the prescribed height and then using moment equilibrium to find out how wide the moment slab has to be while satisfying a factor of safety against overturning equal to 2. This factor of safety of 2 is consistent with the requirement for overturning of an MSE wall but is not explicitly written in the AASHTO ASD for overturning of barriers. The design of the barrier against sliding consists of applying the load to the barrier and then using horizontal equilibrium to find out how wide the moment slab has to be while satisfy- ing a factor of safety of 1.5. This factor of safety of 1.5 is consis- tent with the requirement for sliding of an MSE wall but is not explicitly written in the AASHTO ASD for sliding of barriers. In LRFD, the recommendations are not as detailed. The load factor γ is taken as 1.0 for the load combination of Service I and the resistance factor for sliding as 0.8 for cast-in-place concrete on soil. There are no recommendations for the resistance fac- tor against overturning. 2.3.3 Previous Crash Test of Barrier on Edge of MSE Wall In 1982, Terre Armée Internationale (TAI), which is closely related to the Reinforced Earth Company (RECO) in the United States, performed a crash test of a barrier on top of an MSE wall (18). The test vehicle was a 12,020 kg (26,500 lb) bus that impacted the barrier at a speed of 71.2 km/h (44.2 mph) and an angle of 20 degrees. The impact severity was esti- mated to be 30% larger than the AASHTO PL-2 (19) loading condition. The barrier was an N.J. shape barrier approximately 0.81 m (32 in.) high as shown in Figure 2.15. The barrier reinforce- ment was minimal, consisting of two longitudinal No. 4 bars. The precast barrier units were 1.52 m (5 ft) long and tied to the moment slab through rebars. The moment slab was cast in place with a joint every 9.15 m (30 ft). The width of the moment slab was 1.25 m (4.1 ft), and its thickness was 254 mm (10 in.). The 254 mm (10 in.) of cover over the moment slab consisted of compacted soil and a layer of bituminous mix. 13 Source: AASHTO LRFD Figure 3.11.6.3-2 a (2) Figure 2.14. Distribution of stress from concentrated horizontal loads.

The MSE wall was 3.05 m (10 ft) high with two rows of 1.52 m (5 ft) tall panels. The reinforcement strips were 5 m (16.4 ft) long and the layers of strips were located at depths of 380 mm (15 in.) and 1.14 m (45 in.) below the bottom of the moment slab (best guess) and were 0.76 m (2.5 ft) apart in the horizontal direction (best guess). A horizontal gap of 19 mm (0.75 in.) was purposely left between the coping and the traffic face of the wall panels to avoid lateral contact with the wall panel during impact. Figure 2.16 shows the cracking on the front and back side of barrier after the crash test. The test was considered successful. The bus was redirected and stayed upright. The barrier was damaged but the wall and the moment slab were not damaged. The upper part of the bar- rier was broken over a length of 2.2 m (7.2 ft) and a height of 508 mm (20 in.). The top panel of the wall moved 5 mm (0.19 in.) dynamically during the event and had 1.5 mm 14 Source: RECO (18) Figure 2.15. Precast barrier and coping with cast-in- place slab. Source: RECO (18) Figure 2.16. Barrier damage after RECO crash test.

(0.06 in.) of residual movement after the impact. The bot- tom panel did not move. No wall damage occurred. The max- imum deceleration on the front and rear axles of the bus was 8g (moving average) and 14g, respectively. The maximum dynamic force recorded on the most loaded strip was 28.91 kN (6.5 kips). The minimum reinforcement density for MSE walls gives a resistance of 42.3 kN/m (2.9 kips/ft) of wall at the top layer of strips. Pulling the strips out of the wall would require move- ment of the moment slab unit. For a joint spacing of the moment slab equal to 6.1 m (20 ft), the maximum load that the strips can resist at impact is 6.1 m (20 ft) × 42.3 kN/m (2.9 kips/ft) = 258 kN (58 kips; static). The 1982 TAI test leads to a load of 28.91 kN (6.5 kips) × 6.1 m (20 ft) / 0.76 m (2.5 ft) = 231.3 kN (52 kips; dynamic) if all strips within the 6.1 m (20 ft) section of barrier and moment slab were stressed at the maximum observed value. The value 258 kN (58 kips; static resistance) is much higher than the 44.48 kN (10 kips; static) value required by AASHTO. Therefore, RECO con- cluded that the minimum reinforcement density is adequate to resist the impact load. 2.4 Survey of State Transportation Agencies A comprehensive survey of the nation’s state transportation agencies was conducted to obtain information regarding the design, construction, and performance of barriers mounted on top of MSE walls. Major categories of the survey included MSE walls, barriers, barrier connection to wall/pavement, design, and performance. A total of 18 states responded to the survey: Alaska Department of Transportation (DOT) and Public Facil- ities, Arizona DOT, Arkansas State Highway and Transporta- tion Department, Connecticut DOT, Georgia DOT, Hawaii DOT, Illinois DOT, Kansas DOT, Maryland State Highway Administration, Minnesota DOT, Mississippi DOT, Nevada DOT, New York State DOT Structures, South Carolina DOT, TxDOT, Utah DOT, Washington State DOT, and Wisconsin DOT. The blank survey instrument is shown in Appendix B, which is available from the NCHRP Report 663 summary web page on the TRB website (www.trb.org) by searching for “NCHRP Report 663”. The data reduction of these responses is provided in two manners: (1) a weighted average based on the percentage of usage indicated by each state, which provides an indication of national usage of different alternatives within a given cate- gory (herein referred to as Weighted Percentage of Usage from Responding States), and (2) the number of states indicating usage in a certain category (herein referred to as Number of States Responding Positive Usage). For example, in the MSE wall section of the survey, the respondents were asked not only if they use a certain type of wall reinforcement in their state, but also what percentage of each type of reinforcement is used. The percentage of usage (e.g., 45% steel strips, 45% bar mats, 10% geosynthetics) is used to compute a weighted average for all respondents and is presumably indicative of average usage across the country. Additionally, the number (and correspond- ing percentage) of states indicating use of a given type of reinforcement is reported. Note that in the above example, the respondent indicated use of all three types of wall reinforce- ment and, therefore, positive usage would be indicated for each. When appropriate, the data has been presented in the form of pie charts for easier visualization of the responses. The survey question associated with each chart is provided for reference purposes. Certain data are presented in tables and/or in a written format. 2.4.1 MSE Walls The survey section on MSE walls includes questions regard- ing the percentages of the type of reinforcement, the type of facing panels, and type of facing-panel connections used in MSE walls in the responding state. Figures 2.17 through 2.19 present the results for this survey section in the Weighted Per- centage of Usage from Responding States format. Figure 2.17 indicates that approximately 57% of the MSE walls constructed within the responding states utilize steel strips as the means of reinforcement. Usage of steel strips is followed by usage of steel bar mats (24%) and geosynthetic grids (18%). As shown in Figure 2.18, 80.7% of MSE wall construction incorporates concrete panels, while 19% are composed of 15 Concrete Panel 80.7% Modular Block 19% Other 0.3% Figure 2.18. Type of facing panels in MSE walls (Question 2). Steel Strips 57% Bar Mats/Wire Mesh 24% Geosynthetic Grids 18% Other 1% Figure 2.17. Type of reinforce- ment in MSE walls (Question 1).

modular blocks. Entries made by responding states in the “other” category for facing panel types noted use of wire-face walls, cast-in-place concrete walls, Gabion/exposed rock, and two-stage walls. In regard to the type of panel-to-panel connec- tions utilized in MSE walls, Figure 2.19 indicates the majority (55%) use dowels, followed by tongue-and-groove (16%) and ship-lap (12%) connections. In the “other” category for facing panel connection type, states noted use of cast-in-place clips, friction or mesa, block lip, modular blocks, and RECO-lap. It should be noted that Georgia indicated 100% usage for both dowels and ship lap, and dowels were used in the analyses pre- sented herein. 2.4.2 Barriers The survey section for barriers included questions regard- ing the percentages of barrier categories used on MSE walls, types of guardrail and bridge rail used, and whether the bar- rier is precast or cast in-place. The survey also asked for the minimum segment length permissible for the precast barrier option. Figures 2.20 through 2.24 present the survey results of eighteen responses for the question on category of barriers, six responses for the question on type of guardrail, and eighteen responses for the question on type of bridge rail. Unless other- wise noted, the results are reported in the Weighted Percentage of Usage from Responding States format. As shown in Figure 2.20 (which is presented in the Number of States Responding Positive Usage format), thirteen of the eighteen states responding to the survey (72%) use only bridge 16 Guardrail Only, 0, 0% Bridge Rail Only, 13, 72% Both, 5, 28% Figure 2.20. Percentage of states using different barrier categories (Question 4). Guardrail (Post Mounted) 10% Bridge Rail (Slab/Pavement Attached) 90% Figure 2.21. Category of barriers (Question 4). Strong Post W- Beam (%) 56% Weak Post W- Beam 16% Thrie Beam 10% Box Beam 18% Other 0%Cable 0% Figure 2.22. Type of guardrail (Question 5). Cast-In-Place Coping and Barrier 76% Precast Coping with Cast-In- Place Barrier 8% Precast Coping and Barrier Unit 16% Other 0% Figure 2.24. Precast barrier vs. cast-in-place barrier (Question 7). Concrete Safety Shape 91% Concrete Parapet w. Steel Rail 2% Steel 1% Concrete Beam and Post 0% Vertical Concrete Wall 6% Other 0% Figure 2.23. Type of bridge rail (Question 6). Tongue and Groove 16% Ship Lap 12% Other 17% Dowels 55% Figure 2.19. Type of facing-panel connection (Question 3).

rails atop MSE walls, while five states (28%) indicated use of both guardrail and bridge rail. There were no states that used only guardrail on MSE walls. When weighted averages of use are computed (see Figure 2.21), the results indicate that 90% of the MSE walls constructed with barriers on top utilize some type of bridge rail connected to a moment slab or pavement, while only 10% of such installations use guardrail mounted on soil-embedded posts. Figure 2.22 shows the type of guardrail used by the six states indicating use of guardrail on MSE walls in Question 5 of the survey. Strong post W-beam is used 56% of the time, followed by box beam (18%), weak post W-beam (16%), and thrie beam (10%). The median offset from the edge of the MSE wall reported for post-mounted guardrail was 0.91 m (3 ft). As mentioned earlier, all states responding to the survey indicated use of some percentage of bridge rail atop MSE walls. As indicated by the weighted averages shown in Figure 2.23, the vast majority (91%) of such installations incorporate some form of concrete safety shape barrier (e.g., N.J., F-shape). This type is followed by vertical concrete parapets (6%) and con- crete parapets combined with a steel railing (2%). Figure 2.24 provides information regarding precast versus cast-in-place construction practices followed by the respond- ing states. Seventy-six percent of barrier construction on MSE walls uses cast-in-place coping and barrier. Precast coping and barrier segments are used 16% of the time, while use of a precast coping with cast-in-place barrier is limited to 8%. The median minimum segment length for the six states indicating use of precast barrier segments was 4.57 m (15 ft). 2.4.3 Barrier Connection to Wall/Pavement The survey section dealing with the barrier connection to wall/pavement included questions regarding the percentage of the different types of pavement used on top of MSE walls, the offset of post-mounted guardrails from the edge of the wall, and asphalt concrete pavement (ACP) and reinforced concrete pavement (RCP) applications. As shown in Figure 2.25 (which is presented in the Number of States Responding Positive Usage format), 11 of 17 states responding to this question (64%) use both RCP and ACP on MSE walls. Four states (24%) indicated use of only RCP, while another two states (12%) use only ACP on MSE walls. When weighted averages of use are computed (see Figure 2.26), the results indicate a nearly 50-50 split between asphalt and reinforced concrete pavement applica- tions in regard to MSE wall construction. For slab-attached bridge rails, the barrier connection to wall/pavement section of the survey is divided into asphalt con- crete pavement and reinforced concrete pavement applica- tions. Because of the nature of these questions, the results are reported using the Number of States Responding Positive Usage format. The survey responses related to the use of ACP on MSE walls with barriers are presented in Figures 2.27 through 2.30. Supporting information for some of these questions and fig- ures is presented in Table 2.1. With reference to Table 2.1 (Question 11), the median thickness of the moment slab for MSE wall applications with ACP is 343 mm (13.5 in.). The median width of the moment slab used by the responding states is 6.5 ft (Table 2.1, Ques- tion 12). Figure 2.27 (which is based on survey Question 13) indicates that 50% of the responding states use continuous 17 ACP Only, 2, 12% Both, 11, 64% RCP Only, 4, 24% Figure 2.25. Use of different pavement types on MSE walls (Question 9). RCP 49.6% ACP 50.4% Figure 2.26. Pavement type (Question 9). Flush, 4, 33% Offset, 8, 67% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.28. Barrier flush or offset from face of wall (ACP, Question 13). Continuous, 6, 50%Jointed, 6, 50% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.27. Continuous or jointed barrier slab/footing (ACP, Question 13).

barrier slabs and 50% use jointed barrier slabs. Those states indicating use of jointed slabs were asked a follow-up ques- tion regarding joint spacing. The median response, shown in Table 2.1 (Question 14), was 6.1 m (20 ft). The mean, standard deviation, median, and number of responses are reported for all such questions in which length or distance was requested. Note that, if the state responded in metric units, the value was converted to U.S. customary units, and when ranges were reported, an average value was used when com- puting the descriptive statistics mentioned above. As shown in Figure 2.28, 67% of responding states report they offset their barriers from the face of the MSE wall and 33% install the barrier flush with the MSE wall. As shown in Table 2.1 (Question 16), the median barrier offset for those states that practice offsetting the barrier from the face of the MSE wall is only 140 mm (5.5 in.). Figure 2.29 indicates that 92% of responding states recess the top wall panel into the bottom of the coping. This practice is followed to provide support for precast coping and barrier sections prior to their connection to cast-in-place slabs and as an aesthetic treatment to cover the “steps” in the panels along the top edge of the wall. The median distance that the top wall panel is recessed into the coping is 216 mm (8.5 in.) (see Table 2.1, Question 18). Additionally, 85% of states responded that lateral and vertical movement of the barrier system is iso- lated from the wall panels (see Figure 2.30). Although the percentages are slightly different, the responses obtained for MSE wall applications with RCP show the same trends as the MSE wall applications in which ACP is used. The survey responses related to the use of RCP on MSE walls with barriers are presented in Figures 2.31 through 2.35. Additional information for RCP applications is presented in Table 2.2. 18 Continuous, 8, 57% Jointed, 6, 43% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.31. Continuous or jointed barrier/slab footing (RCP, Question 22). Coped/Recessed, 11, 92% Not Coped/ Recessed, 1, 8% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.29. Wall panel coped/ recessed (ACP, Question 17). Percentages derived from number of states using the category shown divided by the total number of states responding Disconnected/Isolated, 11, 85% Connected, 2, 15% Figure 2.30. Lateral and vertical barrier movement connected or disconnected/isolated from wall panel (ACP, Question 19). Survey Question No. Description Mean StandardDeviation Median No. of Responses 11 Thickness of barrier/slab footing (in) 15.0 4.2 13.5 12 12 Width of slab/footing (ft) 6.6 1.8 6.5 11 14 Joint spacing (ft) 32.9 28.0 20.0 6 16 Barrier offset from face of wall (in.) 7.4 9.3 5.5 8 18 Wall panel recess distance into bottom of coping (in.) 8.4 2.4 8.5 9 Table 2.1. Survey responses related to MSE walls with ACP.

19 Flush, 6, 40% Offset, 9, 60% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.32. Flush or offset barrier from face of wall (RCP, Question 24). Coped/recessed, 12, 80% Not Coped/ Recessed, 3, 20% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.33. Wall panel coped/ recessed (RCP, Question 26). Connected, 2, 17% Disconnected/ Isolated, 10, 83% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.34. Lateral and vertical barrier movement connected or disconnected/isolated from wall panel (RCP, Question 28). Integrally Poured 45%Doweled 55% Percentages derived from number of states using the category shown divided by the total number of states responding Figure 2.35. Integrally poured or doweled into pavement (RCP, Question 29). Survey Question No. Description Mean StandardDeviation Median Number of Responses 20 Thickness of barrier/slab footing (in.) 13.9 4.6 12.0 15 21 Width of slab/footing (ft) 6.7 1.2 6.6 12 23 Joint spacing (ft) 18.8 3.8 20.0 5 25 Barrier offset from face of wall (in.) 4.9 3.6 5.5 8 27 Wall panel recess distance into bottom of coping (in.) 6.9 3.8 7.0 11 Table 2.2. Survey responses related to MSE walls with RCP. Table 2.2 shows that the median barrier slab thickness (Question 20) and width (Question 21) are 305 mm (12 in.) and 2.01 m (6.6 ft), respectively. Figure 2.31 shows that 57% of the MSE walls constructed with RCP incorporate continu- ous barrier slabs and 43% use jointed barrier slabs. The median joint spacing for those states indicating use of jointed slabs was 6.1 m (20 ft) (see Table 2.2, Question 23). As shown in Figure 2.32, 60% of responding states report they offset their barriers from the face of the MSE wall, while the remaining 40% install the barrier flush with the MSE wall. The median barrier offset for those states that offset their bar- riers from the face of the MSE wall is 140 mm (5.5 in.) (see Table 2.2, Question 25). The practice of recessing the top wall panels into the bot- tom of the wall coping is followed by 80% of the responding states (see Figure 2.33). The median distance that the top wall panel is recessed into the coping is 178 mm (7 in.) (see Table 2.2, Question 27). As shown in Figure 2.34, 83% of

states responded that lateral and vertical movement of the bar- rier system is isolated from the wall panels, while the remain- ing 17% indicated that the wall panels and barrier system are connected to one another. A question specific to RCP applications (Question 29) is whether the barrier slab is integrally poured with the concrete pavement or doweled to it. Figure 2.35 shows that 55% of responding states use dowels to connect the barrier slab to the pavement, and 45% follow the practice of integrally casting the slab and pavement. 2.4.4 Design For the design section of the survey, only a few of the responses can be presented in graphical format. The responses to questions referring to NCHRP Report 350 test level (Ques- tion 31), adherence to Section 13, Railings, of AASHTO LRFD Bridge Design Specifications (2) for bridge railing design (Ques- tion 32) and whether design procedures include calculation of bending moment in the pavement slab due to impact load on barrier (Question 38) are presented below in the Number of States Responding Positive Usage format. Question 30 regarding the magnitude of the barrier impact load transferred to the top of the MSE wall was not included in this summary because of the high variation in the numerical value of the responses. The varying responses may have been due to confusion regarding the intent of the question. As shown in Figure 2.36, 76% of responding states use TL-4 barriers in conjunction with MSE wall applications. TL-3 and TL-5 barriers are both used by 12% of responding states. Fig- ure 2.37 indicates that 58% of responding states use Section 13, Railings, of the AASHTO LRFD Bridge Design Specifications for bridge rail design, and 42% do not. Compliance with the AASHTO LRFD Bridge Design Specifications is not required if a railing is successfully crash tested. The median impact load and impact location reported by the states specifying they do not follow AASHTO LRFD for bridge rail design are 44.48 kN (10 kips) and 0.84 m (2.75 ft), respectively (see Table 2.3). Only 41% of responding states reported that they calculate the bend- ing moment in the barrier slab due to vehicular impact load (see Figure 2.38). 20 Survey Question No. Description Mean StandardDeviation Median No. of Responses 33 Magnitude of barrier design (kips) 8.4 3.6 10.0 5 34 Height of the applied design load (ft) 2.8 0.2 2.75 5 Table 2.3. Barrier design load and location. TL-3, 2, 12% TL-4, 13, 76% TL-5, 2, 12% Figure 2.36. Use of NHCRP Report 350 test levels (Question 31). Yes, 11, 58% No, 8, 42% Figure 2.37. Use of AASHTO LRFD Bridge Design Specifications for rail design (Question 32). Yes, 7, 41% No, 10, 59% Figure 2.38. Calculation of bending moment in pavement slab due to barrier impact load (Question 38).

21 Yes, 3, 17% No, 15, 83% Figure 2.40. Other perfor- mance issues associated with MSE walls or barriers atop MSE walls (Question 40). Yes, 1, 6% No, 17, 94% Figure 2.39. Failures of MSE walls or barriers atop MSE walls due to vehicular impact (Question 39). 2.4.5 Performance The last section of the survey, performance, included ques- tions inquiring about the in-service performance and failure history of MSE walls and barriers on top of MSE walls. The sur- vey responses for these questions are presented in Figures 2.39 and 2.40 in the Number of States Responding Positive Usage for- mat based on a total of 18 responses. The only participating agency reporting failure of an MSE wall or barrier atop an MSE wall during vehicular impact was Georgia. Georgia DOT reported that “a semi- tractor trailer knocked off a section of barrier that was lacking strap anchorages.” Minnesota, New York, and Washington reported various performance issues and/or design questions associated with MSE walls or barriers atop MSE walls. Minnesota DOT reported it had seen some con- nection details between the barrier and the slab that are not adequate.