Project 1 – Completed

Finite Element Evaluation of Two Retrofit Options to Enhance the Performance of Cable Median Barriers

Sponsor: North Carolina Department of Transportation
Duration: July 1, 2007 – June 30, 2009
Research team: Howie Fang (PI), David C. Weggel (Co-PI), Jing Bi, Michael E. Martin, Amol Gaikwad

Introduction:

Commonly used barrier systems include concrete barriers, W-beam guardrails, thrie-beam guardrails, and cable barriers. Cable barriers are cost-effective, flexible systems that are ideal for retrofit designs in existing, relatively wide medians to prevent cross-median crashes. North Carolina currently has approximately 550 miles of low-tension cable median barriers (CMBs) that have dramatically reduced cross-median crashes and fatalities since their installation in 1994.

Although the current CMBs have very good safety performance (less than five percent penetration rate), there is the potential to make the current CMBs safer by retrofitting the current systems. Due to the high cost and destructive nature of crash testing, it is not possible to use physical experiments to explore good retrofit designs.

In this project, finite element (FE) models are used to perform crash simulations for evaluating two retrofit options for the current North Carolina CMBs: 1) determine the optimum locations of the middle and bottom cables in the current three-cable system; and 2) add a fourth cable to the current three-cable system and determine the optimum location of the fourth cable. The results of this project can be used to develop economical and effective retrofit solutions for North Carolina’s existing CMBs to enhance their safety performance.

Retrofit Options:

Option 1: Lower bottom                          Current Design                                   Option 2: Add
two cables.                                                                                                           fourth cable.

Major Research Findings

  • Cable heights affect the cable-vehicle interactions and engagements. Lower cables typically engage better with smaller vehicles (i.e., vehicles with lower profiles). The top cable is ineffective for small vehicles while it is effective for large vehicles.
  • Upon impacting the CMB placed four feet from the bottom of the ditch, the vehicle’s front-end is significantly lower in back-side impacts than in front-side impacts due to compression of the suspension upon crossing the ditch.
  • The resistance of a cable from bearing against posts plays an important role in the cable’s ability to redirect a vehicle. Cables on the sides of the posts facing the impacting vehicle have larger resistance than cables on the opposite sides of the posts.
  • Cable-vehicle engagement depends upon the impact locations between two adjacent posts. For example, the cable-vehicle engagement can be totally different when a vehicle impacts the CMB midway between two posts from that when it impacts a post first.
  • Cable-vehicle engagement is also affected by the impact speed and impact angle.

Simulation Visualizations

Neon

Isometric view of Neon impact. Cables are highlighted in orange and meshes on front portion of car are shown for detail.

Visualizations

Use .gif for slower connections.

 

Front-Side Impact

Back-Side Impact

  .GIF
.AVI
.GIF
.AVI
20° 55 MPH 55 MPH 55 MPH 55 MPH
65 MPH 65 MPH 65 MPH 65 MPH
70 MPH 70 MPH 70 MPH 70 MPH
75 MPH 75 MPH 75 MPH 75 MPH
30° 55 MPH 55 MPH 55 MPH 55 MPH
65 MPH 65 MPH 65 MPH 65 MPH
70 MPH 70 MPH 70 MPH 70 MPH
75 MPH 75 MPH 75 MPH 75 MPH
40° 55 MPH 55 MPH 55 MPH 55 MPH
65 MPH 65 MPH 65 MPH 65 MPH
70 MPH 70 MPH 70 MPH 70 MPH
75 MPH 75 MPH 75 MPH 75 MPH