National Academies Press: OpenBook

Permanent Signs Mounted on Median Barriers (2014)

Chapter: CHAPTER FOUR Testing and Research

« Previous: CHAPTER THREE Existing Sign and Barrier Combinations
Page 24
Suggested Citation:"CHAPTER FOUR Testing and Research." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
Page 24
Page 25
Suggested Citation:"CHAPTER FOUR Testing and Research." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
Page 25
Page 26
Suggested Citation:"CHAPTER FOUR Testing and Research." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
Page 26

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

22 CHAPTER FOUR TESTING AND RESEARCH This chapter provides a summary of past and current research related to the safety performance of barrier-mounted sign supports. Although there have not been a large number of studies on this topic, several projects include results of full- scale crash tests and computational analyses that were con- ducted to quantify the size and shape of ZOIs or to evaluate the effect on safety performance of objects mounted within the ZOI. Some of the earlier studies did not specifically address sign supports; however, they identified the conse- quences of placing fixed objects on top of or directly behind rigid barriers, thereby establishing the concept of the zone of intrusion. Subsequent research has evaluated the effects of a greater variety of barrier-mounted appurtenances, includ- ing sign supports, and has provided a better understanding of which practices have greater potential for compromising safety performance. EARLY RESEARCH One early study by the New York State DOT focused on cir- cular concrete bridge piers located directly behind half-sec- tion NJ safety-shape barriers (Phillips 1984). Although the term ZOI had had not yet been established, full-scale crash tests showed that piers located behind the barrier caused roll- over of a 4,500-lb (2,041-kg) sedan impacting at 25 degrees and approximately 60 mph (97 kph). The piers were clearly located in the ZOI for that particular combination of bar- rier design and impact conditions. The researchers went on to design a modification to the barrier in which box-beam guardrail extends along the face of the barrier in front of the piers. This modification significantly reduced vehicle climbing during impact and kept the vehicle from hitting the piers with enough force to destabilize its trajectory. The result was much smoother redirection of the vehicle, with significantly less pitch and roll. Survey responses indicated that the box-beam treatment is still used by a few agencies, and it is shown as a design option for reducing ZOI exposure in the AASHTO Roadside Design Guide (AASHTO 2011). The term ZOI was first used by researchers from the Mid- west Roadside Safety Facility (MwRSF), who conducted a study to develop guidelines for placement and design of vari- ous attachments on or near traffic barriers (Keller 2003). The project included an extensive review of previous crash tests of different types of barriers to evaluate the extent of vehicle intrusion observed above and behind the barriers for Test Level 2, 3, and 4 impact conditions. The ZOI configurations focused on the areas into which substantial structural com- ponents of the impacting vehicle extended during an impact event. Zones that were intruded by relatively weak external components of the vehicle (such as mirrors) were not con- sidered to be critical to defining the ZOI, because they yield under low loads and would not cause significant deceleration forces if they struck an object mounted on or near the bar- rier. Structural components of the vehicle that snag on bar- rier attachments are likely to cause high deceleration forces and occupant compartment deformation, which are pri- mary evaluation criteria for assessing safety performance. Results of the crash test analyses were used to establish ZOI diagrams for various barrier types and impact conditions. These diagrams were subsequently included in the RDG to illustrate the concept of ZOI and provide preliminary guide- lines for addressing objects located in the ZOI (AASHTO 2011). The report also identifies a range of attachments that might be located within a traffic barrier ZOI, including over- head and small sign supports (Keller et al. 2003). MORE RECENT RESEARCH MwRSF conducted a series of full-scale crash tests to eval- uate the practice of placing luminaire poles on top of and behind 32-in. (813-mm) high single-slope concrete barri- ers to evaluate their effects on safety for both occupants of the impacting vehicle and adjacent traffic (Wiebelhaus et al. 2008). Although the project focused on luminaire poles rather than sign supports, the results are directly applicable to evaluating safety issues related to ZOI impacts. Two of the three tests were conducted on a top-of-barrier-mounted luminaire pole. NCHRP Report 350 Test No. 4-11 consisted of a 4,430-lb (2,009-kg) pickup truck hitting the barrier at a speed of 61.7 mph (99.3 kph) and an angle of 23.4 degrees. NCHRP Report 350 Test No. 4-12 consisted of a 17,605-lb (7,985-kg) single-unit truck hitting the barrier at a speed of 50.4 mph (81.0 kph) and an angle of 15.6 degrees. For Test No. 4-11, the 2000P vehicle was adequately contained and redirected by the barrier, and all applicable performance evaluation criteria were met. However, the front hood of the vehicle made significant contact with the pole, which likely increased damage to the vehicle compared with the same impact conditions without the pole mounted on the barrier.

23 For Test No. 4-12, the 8000S vehicle was adequately con- tained and redirected by the barrier, and all applicable per- formance evaluation criteria were met. However, the pole was fractured and dislodged from its base plate and came to rest behind and parallel to the barrier. This could have caused a hazard to adjacent traffic if the barrier had been used for a narrow median application, with traffic present on both sides and narrow inside shoulders. A subsequent ZOI study for the Florida DOT included an analysis using LS-DYNA simulation that was performed to investigate the ZOI of an NCHRP Report 350 2000P pickup truck impacting a 40-in. (1,016-mm) high F-shape rigid bar- rier (Reid and Sicking 2010). For TL-3 impact conditions [62 mph (100 kph), 25 degrees], the simulation predicted the lateral dimension of the ZOI to be 5 in. (127 mm). A majority of the intrusion was from the front hood of the 2000P vehi- cle, because it extended just over the 40 in. (1,016 mm) bar- rier height. The researchers suggested that some intrusion can be expected at almost all impact speeds; however, they concluded that the ZOI for this barrier design was inconse- quential compared with that of a 32-in. (813-mm) high rigid barrier, in which a significant amount of vehicle structure will overhang the barrier. The California DOT conducted a full-scale crash test on a permanent small sign support mounted on a 36-in. (914-mm) high, single-slope concrete median barrier (Caldwell 2011). The sign support was a 4-in. (102-mm) diameter steel post welded to a 3/8-in. (10-mm) thick saddle-style base plate with horizontal through-bolts securing the support assem- bly to the barrier. Impact conditions in the test were in com- pliance with NCHRP Report 350, Test 3-11 [2000P pickup truck, 62 mph (100 kph), 25 degrees]. The barrier success- fully redirected the vehicle. However, a significant portion of the vehicle structure impacted the sign support, including the front hood, which was pushed into the windshield toward the driver. The hood penetrated the windshield, causing sig- nificant occupant compartment deformation. In addition, the horizontal bolts securing the sign support caused a jagged slice down the side of the vehicle as a result of its sustained contact with the barrier. The sign support was bent toward downstream of the impact. Because of the substantial dam- age to the vehicle, the occupant risk for this test was deemed unacceptable. It was also noted that the excessive amount of debris could have become a hazard to oncoming traffic. On the basis of these test results, the saddle-mounted sign support is not considered crashworthy in accordance with NCHRP Report 350 evaluation criteria. The researchers rec- ommended improving performance by modifying the bar- rier in one or more of the following ways: increase the height of the barrier, increase the width of the barrier, or attach the sign panel directly to the barrier with no support post. A project to design and conduct full-scale crash testing of a sign support assembly mounted on portable concrete traf- fic barrier was completed by the Texas A&M Transportation Institute (Williams and Menges 2011). Although this type of concrete barrier is unanchored, some of the results may be valuable for evaluation of rigid barrier as well, especially as available crash test data for any type of barrier-mounted sign supports are limited. In addition, the test was conducted in accordance with The Manual for Assessing Safety Hard- ware; the availability of MASH test results is also compared with those for NCHRP Report 350 (Ross et al. 1993; AAS- HTO 2009a). The barrier used for the project is a freestand- ing, 33-in. (838-mm) high safety-shape, composed of 30-ft (9.1-m) sections. The sign support connection consisted of a 6 x 2 x 1/4 in. (152 x 51 x 6.3 mm) x 72 in. (1,829 mm) long structural steel tube mounted horizontally on top of the barrier. A 3-in. (76-mm) diameter x 4.5-in. (114-mm) high collar was welded to the steel tube, and a 2.5-in. (64-mm) diameter signpost was inserted into the collar and secured with a ½-in. (13-mm) diameter bolt. The sign support design was intended to limit movement of the adjacent barrier sec- tions and thus improve performance. (See the project report for specific details of the test article.) MASH Test 3-11 was selected as the critical test, owing to likely interaction between the pickup truck and the sign support. Results of the test met all the applicable MASH evaluation criteria. The maximum dynamic deflection of the barrier was 3.9 ft (1.2 m), which would not have occurred with a rigid bar- rier installation. Lateral movement of the barrier may have allowed less snagging on the sign support than would have occurred with a rigid barrier of the same height. Subsequently, the Texas A&M Transportation Institute completed a comprehensive research project sponsored by the Texas DOT that covers several aspects of signs mounted on concrete median barriers (Abu-Odeh et al. 2013). The project includes an assessment of current practices used by the Texas DOT, various types of analyses, and full-scale crash testing to identify crashworthy sign support designs and develop guidelines for their placement on permanent rigid barriers. In an effort to improve performance of a typ- ical small sign support mounted on top of and within the ZOI of the barrier, six experimental design concepts were presented and evaluated as potential solutions. Evaluations included analytical analyses and computer simulations to assess both impact and environmental wind-loading levels on typical sign supports. The four highest ranked concepts were selected for further analyses, which included addi- tional simulation and full-scale crash testing. The simula- tion indicated that all four concepts would pass MASH Test 3-11 test conditions, and they did. All the designs are rated for supporting sign panels up to 4 ft x 6 ft (1.2 m x 1.8 m) with a mounting height of 7 ft (2.1 m) measured from the pavement surface to the bottom of the sign panel. One of the concepts emerged as the preferred design; it consists of a 2.5-in. (64-mm) diameter Schedule 80 pipe support attached to a sliding base and chute that is mounted on top of the rigid barrier. The support assembly will move longitudinally

24 with the base, sliding within the chute when load is applied by an impacting vehicle. Displacement of the post reduces snagging of the vehicle and thereby improves performance compared to that of a rigidly attached support post. Although these are valuable findings, the literature search showed that these studies represent most of the limited research that has been conducted on ZOIs for rigid barriers and on the consequences of mounting rigid sign supports within the ZOI. However, much of the research has been conducted within the past few years, indicating that this subject is being recognized as a roadside safety issue wor- thy of further investigation. Furthermore, the most recent study described earlier hardware designs that are available to potentially alleviate safety concerns for new or retrofit- ted installations. More research and development may be needed to cover a wider range of applications, but it appears that research on barrier-mounted sign supports is increasing.

Next: CHAPTER FIVE Conclusions and Research Needs »
Permanent Signs Mounted on Median Barriers Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 465: Permanent Signs Mounted on Median Barriers reports on the current state of practice for mounting permanent highway signs on top of rigid median barriers throughout the United States.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!