National Academies Press: OpenBook

Aesthetic Concrete Barrier Design (2006)

Chapter: Chapter 7 - Final Design Guidelines

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Page 66
Suggested Citation:"Chapter 7 - Final Design Guidelines." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Page 66
Page 67
Suggested Citation:"Chapter 7 - Final Design Guidelines." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
×
Page 67
Page 68
Suggested Citation:"Chapter 7 - Final Design Guidelines." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
×
Page 68
Page 69
Suggested Citation:"Chapter 7 - Final Design Guidelines." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
×
Page 69

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66 CHAPTER 7 FINAL DESIGN GUIDELINES GUIDELINES FOR AESTHETIC SURFACE TREATMENTS OF SAFETY SHAPE CONCRETE BARRIERS As described in the previous chapter, the internal energy of the floorboard of the pickup truck was used as a surrogate measure of OCD. Due to limited test data, the internal energy threshold associated with the maximum allowable OCD was not well defined. Consequently, the preliminary guidelines contained a large region of asperity configurations for which impact performance was unknown. The crash test data were evaluated and used to make adjustments to the preliminary guidelines. Each asperity con- figuration that was crash tested has an associated level of truck floorboard internal energy that was derived from the simulation study. A summary of these data is presented in Table 8. The verification crash test with the 820-kg passen- ger car (Test 6) is excluded from the table because the small car is not critical in terms of the performance assessment of the asperities. Test 2 evaluated the same asperity configura- tion used in Test 6. The tentative energy failure threshold upon which the pre- liminary guidelines were based was 10.7 kJ. Test 2 and Test 7 confirmed that this level of floorboard internal energy was indeed unacceptable. The highest level of energy associated with a successful test can conservatively be used as a pass/ fail threshold. Based on this rationale, a floorboard internal energy of 8.5 kJ was selected as the pass/fail threshold. With reference to Table 8, the asperity configurations used in Test 3, Test 4, and Test 5, which were all successful tests, had internal floorboard energies ranging from 8.4 kJ to 8.9 kJ. Given that the highest OCD among these successful tests was 120 mm, using 8.5 kJ as the internal energy threshold pro- vides good confidence in the validity of the “acceptable” or crashworthy region of the guidelines. The failure curve associated with each asperity angle (i.e., 30, 45, and 90 degrees) was shifted to correspond to the revised energy threshold of 8.5 kJ. The final design guidelines for aes- thetic surface treatment of safety shape concrete barriers based on the revised threshold are presented in Figure 82. For each asperity angle, the guidelines show regions of “acceptable” asperity configurations and regions of asperity configurations that are “not recommended” due to a high probability of failure during a design impact event resulting from excessive OCD. It can be observed that for a given asperity width, the acceptable asperity depth varies with the asperity angle. For example, at an asperity width of 500 mm, the acceptable asperity depths are 6 mm, 35 mm, and 99 mm for 90 degree, 45 degree, and 30 degree asperity angles, respectively. The guidelines do not include asperity spacing as an addi- tional design parameter. In the opinion of the researchers, the degree of variation in the asperity configurations that are acceptable for the two asperity spacings investigated did not justify adding another level of complexity to the guidelines. Even though wider asperity spacing results in less spalling of the concrete between asperities, the net change was a reduction in overall snagging severity. Therefore, the final guidelines, which were based on an asperity width of 25 mm, are slightly conservative for wider asperity spacings. COMPARISON WITH GUIDELINES FOR SINGLE- SLOPE AND VERTICAL-FACE BARRIERS AND STONE MASONRY GUARDWALLS Guidelines developed for the safety shape barriers under this research were compared, to the extent possible, with the previously developed guidelines for single-slope and vertical- face barriers and stone masonry guardwalls. In the case of guidelines for safety shape barriers, the use of finite element simulation studies in conjunction with crash testing enabled the researchers to define relationships over a range of asperity parameters. Previously existing guidelines for single-slope and vertical-face barriers and stone masonry guardwalls were developed using crash testing alone and therefore were not in the form of relationships defined over a range of asperity pa- rameters. Moreover, a significant portion of the information contained in these guidelines cannot be displayed graphically. Figure 83 shows an overlay of the guidelines developed for the safety shape barriers and some of the information from the guidelines for single-slope and vertical-face barriers that could be displayed graphically. This figure shows lines for 45- and 90-degree asperities that were suggested for single-slope and vertical-face barriers. For the 90-degree asperities on single-slope and vertical- face barriers, the maximum depth and width allowed were 13 mm and 25 mm, respectively. At the same time, the guidelines allow gaps, slots, grooves, or joints of any depth with a maximum width of 20 mm. This amounts to the ver-

tical line shown in Figure 84 for 90-degree asperities (see Figure 85 for English units). A similar vertical line has been suggested for the safety shape barriers, but with a slightly larger maximum asperity width (30 mm as opposed to 20 mm). In addition to an “acceptable” region for asperity widths of less than 30 mm, guidelines developed for the safety shape barriers show an “acceptable” region at higher asperity widths, which was identified through simulation and later verified by crash testing. For the 45-degree asperities on safety shape barriers, the asperity depth versus width relationship allows for smaller depths when asperity widths are small. The acceptable maxi- mum asperity depth increases with the increase in width for these guidelines. However, in the case of guidelines for single- 67 slope and vertical-face barriers, a single maximum asperity depth value of 25 mm was set, irrespective of the width of the asperities. The comparison thus shows that for smaller widths, guidelines for safety shape barriers allow for shallower asper- ities, whereas for larger widths, deeper asperity widths are acceptable when compared with the guidelines for the single- slope and vertical-face barriers. The two guidelines are reasonably similar to each other. The differences highlighted above stem from the differences in the development approach. In the case of safety shape bar- riers, finite element simulations allowed for developing rela- tionships as a function of asperity parameters. Moreover, a greater number of crash tests were conducted for the 45-degree asperities so as to allow verification and readjust- ment of these relationships. The single-slope and vertical- face barrier guidelines, however, were developed primarily through crash testing, and finite element simulations were not performed. This restricted the guidelines to single maximum asperity depth values for different asperity angles. Initially, the comparison was done so as to generate a single graph of relationships between asperity depths and widths for all types of barriers. However, such a generalized graph can become very confusing for the designer. In addi- tion, a significant portion of the information contained in the guidelines for single-slope and vertical-face barriers and stone masonry guardwalls can only be displayed textually. TABLE 8 Floorboard internal energy associated with crash-tested asperity configurations Test No.* * Test 6 with 820C excluded 1 2 Internal Energy (kJ) Test Outcome 3 4 5 7 6.9 8.9 8.9 8.4 11.8 10.3 Pass Pass Pass Pass Fail Fail Figure 82. Final design guidelines for aesthetic surface treatment of safety shape concrete barriers.

68 Figure 83. Comparison of guidelines for single-slope and vertical-face barriers and stone masonry guardwalls. Figure 84. Final design guidelines for aesthetic surface treatment of safety shape concrete barrier (metric).

69 Figure 85. Final design guidelines for aesthetic surface treatment of safety shape concrete barrier (English). For the convenience of an aesthetic barrier designer, all three guidelines have been consolidated into a single, standalone section, which appears in the appendix. The guidelines for safety shape barriers have been presented in graphic form, whereas the guidelines for single-slope and vertical-face and stone masonry guardwalls have been presented in textual form. This appendix also includes examples of the use of the guidelines developed for safety shape barriers.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 554: Aesthetic Concrete Barrier Design provides guidance for the aesthetic treatment of concrete safety shape barriers.

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