Development of Guidelines for Anchor Design for High-Tension Cable Guardrails

Transcription

1 Duplication for publication or sale is strictly prohibited without prior written permission of the Transportation Research Board Paper No Development of Guidelines for Anchor Design for High-Tension Cable Guardrails by Ling Zhu, M.S.M.E. Midwest Roadside Safety Facility University of Nebraska-Lincoln 527 Nebraska Hall Lincoln, Nebraska Phone: (402) Fax: (402) John R Rohde, PhD, PE Associate Professor Midwest Roadside Safety Facility University of Nebraska-Lincoln 527 Nebraska Hall Lincoln, Nebraska Phone: (402) Fax: (402) [email protected] (Corresponding Author) resubmitted to Transportation Research Board 88 th Annual Meeting January 10-14, 2010 Washington, D.C. November 15, 2009 Length of Paper: 2,327 (text) (10 figures) = 5,827 words

2 ABSTRACT High tension cable guardrail is becoming increasing popular in median and roadside applications due to the promise of reduced deflections upon impact and reduced maintenance. As the performance of these systems is observed in service, there is a growing concern over the end anchorage foundation performance of current systems. Foundations for high tension systems must not only be capable of restraining the impact load of a vehicle but must also restrain the initial pretension on the cable system as well as temperature induced loads. While it may be acceptable for many roadside safety devices to require foundation repair after impact, foundation failure due to environmentally induced loads would be a serious maintenance problem. As initial tension and temperature induced loads can be greater than those loads applied during impact these loading must be considered in foundation design. Foundation deflection can reduce cable tension, increasing deflection of the system during impact and letting the cables sag after impact. The soil conditions in which these foundations are placed vary significantly. This paper considers the potential impact, tension, and temperature loads and develops a set of suggested foundation designs to accommodate a range of in situ soil conditions. These designs will vary significantly in different areas around the nation due to variations in both weather and in situ soil conditions. Deflection during full-scale crash tests may not accurately represent the foundation deflection that will be experienced in the field. Keywords: Roadside Safety, High-Tension Cable Barrier, Anchorage, Crash Testing, BARRIER VII simulation, LPILE simulation.

3 INTRODUCTION High tension cable guardrail is becoming increasing popular in median and roadside applications due to the promise of reduced deflections upon impact and reduced maintenance. As the performance of these systems is observed in service, there is a growing concern over the end anchorage foundation performance of current systems. Foundations for high tension systems must not only be capable of restraining the impact load of a vehicle but must also restrain the initial pretension on the cable system as well as temperature induced loads. While it is acceptable for a many roadside safety devices to require foundation repair after impact, this is not acceptable for high tension cable systems as the temperature induced loads can be greater than those loads applied during impact. This paper considers a range of foundation soils that may be encountered on the roadside to develop a set of recommendations for sizing foundations for tensioned cable systems. For simplicity, this paper will utilize a single cylindrical foundation with a four cable system, other configurations can be considered by factoring these loads. Initially, the design load acting on the end anchors is determined from a combination of the static cable pre-tensions, the vehicle impact load, and the thermal load due to the temperature fluctuation. Based on these loads, a series of LPILE simulations were performed to establish foundation deflection in different soils. These simulations were correlated with full scale testing to assure efficacy. Barrier VII simulations are then performed to evaluate the effect of foundation deflection on pretension and ultimately the high-tension cable s redirecting capability. Based on this analysis, suggested guidelines for proper anchor designs of high-tension cable guardrails are presented. These guidelines are based on the premise of foundations that will have limited deflection under thermal, pretension and impact loads. This performance will reduce the necessity of cable tension maintenance and the possibility of needing to repair the foundation. These designs are very conservative and should not be utilized to indicate the inadequacy of existing foundation designs. COMBINED LOAD ACTING ON THE END ANCHOR For the high-tension cable barrier system, the ultimate lateral force acting on each end anchor is the combination of the vehicle impact load, static cable tensions, and the thermal load. Dynamic Load During the development of a 4-cable high-tension cable barrier system for the Midwest States Pooled Fund at the MwRSF, cable loads were directly measured utilizing in-line load cells 1. A full-scale tests under MASH criteria utilizing a Dodge Ram 1500 (2270P) yielded the highest cable loads as shown in Figure 1, with the observed peak load on the anchor of 47 kips. Considering the initial pre-tension in each cable was around 4200 lbs (total = 16,800 lb), the actual dynamic load from the vehicle impact was around 31 kips.

4 Thermal Load Figure 1. Dynamic High-Tension Cable Load The contraction of the cable due to the temperature drop can generate significant load on the end anchorage, which may cause permanent deflection in the foundations, reducing cable tension and potentially affecting the high-tension cable s performance. The impact of these thermal loads will be especially significant during seasons where soil stiffness is reduced by high moisture content. If it is assumed that the foundation is infinitely stiff, the thermal load change in the cable can be calculated usingerror! Reference source not found.; this load is independent of the cable s original length. Assuming the cable s design tension is 4200 lb at 110 F, the tensile load in the cable versus temperature is shown in Figure 2, the cable tension can reach up to 7000 lb when the temperature drops to -20 F. Thus, for the 4-cable barrier, the load on the anchor caused by the temperature could be as high as 28 kips. F=A*E*α* T Eq. 1 F: Cable Load A: Cable Cross Section Area E: Cable Material Young s Modulus

5 α: Thermal Expansion Coefficient T: Temperature Change Figure 2. Thermal Load in Fixed Length Cable PREDICTION AND IMPLICATIONS OF ANCHORAGE DEFLECTION The performance of the high-tension cable guardrail depends on the soil/anchor interaction. If the anchor design is not adequate to resist thermal load, the anchor might move, reducing cable tension and affecting the high-tension cable s performance. As discussed above, it is not unreasonable in colder climates to expect that thermal loads are essentially equivalent to impact loads under the current MASH criteria. Anchor Movement under Thermal Load Without restraint, the change in cable length can be calculated using Eq. 2. Since the impact of foundation movement will be affected by cable length four different installation lengths were analyzed: 1 mile, ½ mile, ¼ mile, and 1/8 mile. L=α*L* T L: Cable Original Length α: Thermal Expansion Coefficient T: Temperature Change: Eq. 2

6 L: Cable Length Change The tension loss due to the anchor movement can be calculated using Eq. 3. As demonstrated in Figure 3, for a cable system with the installation of mile, an anchor movement of 4.8 in. or more can release the entire 4200-lb pre-tension in the cable. Thus, the high tension cable barrier system will lose its advantages if the anchor is not sufficiently designed. T=E* L *A/L T- Cable Tension E- Cable Young s Modulus L- Cable Length Change A-Cable Cross Section Area L- Initial Cable Length Eq. 3 Redirection Performance Figure 3. Anchor Movement vs. Cable Tension Change To evaluate the impact of loss in cable tension associated with anchor deflection a series of BARRIER VII simulations were performed to assess working width changes in the system 4. A baseline high-tension cable barrier model was built using BARRIER VII to replicate the fullscale test 4CMB-1; the validation results are shown in Figure 4. The model was run with various cable pre-tensions and the cable s maximum lateral dynamic deflections for each of the pre-

7 tension levels were recorded; the results are plotted in Figure 5. The impact of the increased lateral deflection on system design depends on the requirements for a given installation. Figure 4. Validation of BARRIER-VII Model Figure 5. Maximum Lateral Deflection-MwRSF HT Model

8 Post Impact Performance A significant advantage of the high-tension cable systems is that redirective capability is not always lost after an impact. With sufficient pretension, after impact the cable will still have sufficient tension to hang in the air. Cleary, if sufficient anchor movement occurs reducing or negating pretension, this advantage will be lost as shown in Figure 6. Figure 6. Low-Tension Cables after Impact ANCHOR DESIGN Lateral deflection of a relatively short pile foundation is affected by both of its diameter and embedment length. Both increases in diameter and depth increase lateral load capacity and decrease deflection. Optimization of foundation strength/stiffness versus cost must also consider construction equipment typically available on site and the expertise of the contractor. The

9 performance foundation is significantly influenced by in situ soils. Most existing systems were evaluated under the requirements in NCHRP 350 Report, meaning that the deflection of the foundations due to cable pretension and impact load are significantly less than may be anticipated with typical soils found along the roadside. To rationally design an anchor capable of reasonable deflections under temperature induced loads it is necessary to evaluate the anchor s performance in various soil types. To consider the range of foundation diameters, embedment lengths and soil conditions a parameter study was conducted to evaluate each of their relative influences. The study was performed using LPILE, a widely accepted analysis program for evaluating laterally loaded piles 2, 3. LPILE has been validated for various foundations evaluated at the MwRSF over the past several years and the confidence in modeling standard strong soil under NCHRP Report 350 is high. This software is generally utilized by State Design Engineers for evaluation of piles in various structures so most States should have confidence in assessing soil conditions for application to evaluating cable foundations discussed herein. LPILE Analysis of Shaft Anchor This evaluation was conducted under the worst-case load, assuming the foundation was subjected to the impact of a 2270P pickup truck while the pretension was very high (-20 F). The lateral load resulting from this scenario is about 40 kips. Since the temperature and impact loads are approximately equal, this evaluation represents a factor of safety of about 2 in respect to either load independently. Three different soil conditions were investigated in the LPILE simulation: well graded crushed stone as defined in NCHRP Report 350, stiff clay, and sand. As drilled concrete piles have become the foundation of choice with many of the Midwest Pooled Fund States this configuration was selected for evaluation. Three diameters (12, 18 and 24 in.) were evaluated at a variety of embedment depths. As shown in Figures 7, 8 and 9 for crushed stone, clay and sand, respectively, there is a pile embedment depth for which the deflection becomes asymptotic. The failure of foundation was considered at the point when the pile was pulled out, or plowed through the soil. For a cylindrical concrete pile in the three soils analyzed herein, recommendations are based on acceptable deflection of the anchor as defined by the author (4 in.) and anticipated loads. Based on a particular States weather, soil conditions, and post impact maintenance goals the results and subsequent recommendations will be dramatically different. Therefore the results in Table 1 are only presented as a methodology to approach design and not specific design embedment for a given location.

10 Figure 7. Concrete Shaft Head Deflection vs. Embedment Depth in NCHRP 350 Soil Figure 8. Concrete Shaft Head Deflection vs. Embedment Depth in Stiff Clay.

11 Figure 9. Concrete Shaft Head Deflection vs. Embedment Depth in Sand Table 1. Recommended Embedment for Cylindrical Concrete Foundation 350 Soil 12-in. Diameter Shaft 18-in. Diameter Shaft 24-in. Diameter Shaft Minimum Embedment Minimum Embedment Minimum Embedment Depth (ft) Depth (ft) Depth (ft) Stiff Clay Sand CONCLUSION Because high-tension cable foundations are subjected to static loads that may induce deflection that can negatively impact the performance of the system, it is necessary to take steps beyond evaluation of full-scale crash testing to evaluate the efficacy of the anchor design. As shown herein, consideration of weather, soil and system details all play a critical role in the design of an appropriate foundation. Based on a design load selected from initial cable tension, potential temperature induced and impact loads, an LPILE analysis using appropriate soil conditions

12 yielded an effective evaluation of required foundation diameter and embedment. This approach is certainly applicable for a range of different foundation configurations and cable geometries. In many cases the cost of incrementally increasing embedment depth to account for changing soil conditions may not be a cost effective, as the engineering cost of evaluating specific site conditions will often exceed the cost of the foundation depth. Many States have selected a particular foundation design based on principals discussed herein for a worst case scenario and applied this design universally across all sites. These design decisions will of course be based on the breadth of soil conditions that are anticipated.

13 REFERENCES 1. Thiele, J.C., Bielenberg, R.W., Faller, R.K., Sicking, D.L., Rohde, J.R., Reid, J.D., Polivka, K.A., and Holloway, J.C., Design and Evaluation of High-tension Cable Barrier Hardware, Final Report to the Midwest State s Regional Pooled Regional Pooled Fund Program, Transportation Research Report No. TRP , Project No.: SPR-3(017), Project Code: RPFP-01-05, RPFP-04-01, RPFP-06, and RPFP Years 11, 14, 16, and 18, Midwest Roadside Safety Facility, University of Nebraska-Lincoln, February 25, ENSOFT, Inc, LPILE Plus 5.0, User s Manual, Austin, Texas 3. ENSOFT, Inc, LPILE Plus 5.0, Technical Manual, Austin, Texas 4. Powell, G.H., BARRIER VII: A Computer Program For Evaluation of Automobile Barrier System, Prepared for the Federal Highway Administration, Report No. FHWA RD-73-51, April Lymon C. Reese, and William F. Van Impe, Single Piles and Pile Groups under Lateral Loading, Brookfield, 2001