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Structural Analysis of Approach Transitions

Question
State WI
Description Text

I have a major project team that is challenging my requirement that they provide structural analysis for their transitions.  They indicate the following:

 

  1. "AASHTO LRFD defines analytical procedures for structural design of barrier (aka bridge parapet) connection to bridge decks. The intent is to ensure that the connection to the deck, and the deck itself, offers greater resistance than the barrier (i.e., make sure the bridge deck is not the weak link). As far as we know, AASHTO does not establish analytical procedures for barrier design for purposes of load classification (e.g. TL-3) and physical crash testing is required. There is however some history of FHWA accepting analytical procedures (structural calculations) used to demonstrate that a customized bridge parapet will perform at least as well as a similar crash-tested version."

 

  1. If it is desired and/or required to adopt an analytical procedure for designing barrier transitions, what will the basis of those procedures be? From a structural engineering perspective, behavior of reinforced concrete barrier under static loads is predictable enough. Behavior of the foundation (structure interaction with subgrade below and pavement adjacent) is more difficult to predict and normally involves assumptions which are quite conservative. Structure response to dynamic loading (vehicular crash) is very complex and difficult to predict even when materials and construction are well controlled. Because of this complex behavior and variability in conditions, as well as unknowns associated with the crash vehicle itself, a purely analytical method to assess barrier performance may necessarily be very conservative. The adjacent pavement and subgrade would offer substantial resistance to overturning, but this is proven with confidence empirically (crash test) and not so easy to demonstrate analytically (as mentioned above). Could barriers be treated similar to gravity retaining walls, using the TL-3 equivalent static loading forces from the AASHTO LRFD.

 

  1. What precedents exist for either analytical methods or empirical methods for designing barrier and barrier transitions? I think the team would benefit from a historical perspective, and also perhaps a wider geographic (national) perspective, as well as local precedent.

 

 

In your opinion there is little difference between designing a roadside barrier and a bridge parapet(i.e. the impact forces and how to deal with them are about the same).  The fact that one is in the soil, verse connected to a deck, may allow for different methods to handle overturning moments (e.g. a roadway barrier could be wedged between lifts of asphalt or tied into a footing). 

Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date August 13, 2010


Response
Response

Please see my comments in red!

 

I have a major project team that is challenging my requirement that they provide structural analysis for their transitions.  They indicate the following:

 

  1. "AASHTO LRFD defines analytical procedures for structural design of barrier (aka bridge parapet) connection to bridge decks. The intent is to ensure that the connection to the deck, and the deck itself, offers greater resistance than the barrier (i.e., make sure the bridge deck is not the weak link). As far as we know, AASHTO does not establish analytical procedures for barrier design for purposes of load classification (e.g. TL-3) and physical crash testing is required. There is however some history of FHWA accepting analytical procedures (structural calculations) used to demonstrate that a customized bridge parapet will perform at least as well as a similar crash-tested version."

**The AASHTO LRFD Bridge Design Specifications provides guidance for designing bridge railings for use on bridge decks as well as those attached to bridge approach slabs. This guidance is intended to help engineers properly configure bridge railings as well as their attachment to reinforced concrete decks. Both solid and open concrete parapets can be configured as well as metallic beam and post systems. Combination concrete and metal systems are also addressed. Limited discussion is provided for timber railings. Yield-line analysis procedures have been provided for addressing the design of reinforced concrete parapets and railings. Inelastic design procedures are available for most metal systems. These rail design procedures were developed and/or documented in a 1978 study report by TTI researchers and have been consistently used for a large share of railing systems. Upon design, it has been common practice for the design to be verified through the use of full-scale crash testing. Actually, full-scale crash testing has also been used for demonstrating the system's structural adequacy and safety even when the prior noted design procedures were not used. If crash testing has been shown to corroborate a design based on the noted procedures, then these procedures have also been used to modify other parapets as long that they provided equivalent or greater strength and did not pose increased risk for vehicle snag, rollover, or override.

For reinforced concrete parapets, the noted design procedures have also been to ensure that sufficient strength is provided at critical locations within the barrier, such as at barrier ends and at expansion joints. At such locations, the number of yield lines that can be developed  is much reduced, thus potentially lower the redirective strength of the parapet. Therefore, it is imperative that these equations be utilized to modify a barrier's capacity to ensure that an impacting vehicle can be safely contained and redirected along the entire barrier length. Basically, the entire barrier must act as though it is continuous even though weakened sections may exist therein. End buttresses that are used to anchored approach guardrail section must also provide adequate structural strength so as to not allow for vehicles to penetrate directly behind the bridge railing if the entire length plus AGT must shield the hazard.

  1. If it is desired and/or required to adopt an analytical procedure for designing barrier transitions, what will the basis of those procedures be? From a structural engineering perspective, behavior of reinforced concrete barrier under static loads is predictable enough. Behavior of the foundation (structure interaction with subgrade below and pavement adjacent) is more difficult to predict and normally involves assumptions which are quite conservative. Structure response to dynamic loading (vehicular crash) is very complex and difficult to predict even when materials and construction are well controlled. Because of this complex behavior and variability in conditions, as well as unknowns associated with the crash vehicle itself, a purely analytical method to assess barrier performance may necessarily be very conservative. The adjacent pavement and subgrade would offer substantial resistance to overturning, but this is proven with confidence empirically (crash test) and not so easy to demonstrate analytically (as mentioned above). Could barriers be treated similar to gravity retaining walls, using the TL-3 equivalent static loading forces from the AASHTO LRFD.

**As noted above, the yield-line and inelastic design procedures are appropriate for designing the barrier systems that are anchored to both the bridge decks and approach slabs. These procedures have also been used for designing similar parapets to soil grade beams. In most cases, full-scale crash testing has demonstrated that the procedures are effective. However, when we use such procedures, we use a load factor of 1 using our MwRSF loads and not necessarily the loads noted in AASHTO. In addition, we would use the appropriate reduction factor for determining the various capacities, such as bending of reinforced concrete. These equations may not always work in every case due the various types of anchorage or support. In such cases, approximations are sometimes made for certain parameters based on experience and historical crash testing results under review. In some special cases, the published dynamic design loads have also resulted in overdesigned moment slabs for concrete parapets placed on MSE walls when used in static overturn analysis and design.

  1. What precedents exist for either analytical methods or empirical methods for designing barrier and barrier transitions? I think the team would benefit from a historical perspective, and also perhaps a wider geographic (national) perspective, as well as local precedent.

**Both analytical methods, computer simulation, and full-scale crash testing have to be used by themselves, or in combination, when developing and verifying the safety performance of guardrails, transitions, and bridge railings/median barriers. In most cases, crash testing was used but not in all. After researchers, designers, and engineers have become familiar with these methods, the more experienced personnel know when to apply one or more than one method to ensure that a system is properly configured.

 

In your opinion there is little difference between designing a roadside barrier and a bridge parapet(i.e. the impact forces and how to deal with them are about the same).  The fact that one is in the soil, verse connected to a deck, may allow for different methods to handle overturning moments (e.g. a roadway barrier could be wedged between lifts of asphalt or tied into a footing). 

 

The procedures are generally the same. The foundation systems could vary between roadside and bridge applications.

 

 

Date August 13, 2010


Response
Response

I have to summarize your response to me about yield-line analysis. Am I on the mark with this comment? I want to say:


An errant vehicle imparts the same amount of force into roadside barrier or bridge parapet. Yield-line analysis has been used to develop both roadside and bridge parapets. Crash testing has proven yield-line analysis can provide a structural adequate roadside barrier or parapet. Some of these crash tests may have had failing crash test results because of the roadside barrier or parapet was not functionally adequate.


This design methodology provides that the barrier itself has:


  • Sufficient reinforcement so that the force of vehicle impact can be withstood by the barrier or transition (i.e. the barrier does not shatter and allow the vehicle to pass through the barrier)



  • Sufficient reinforcement and footing to prevent the barrier from shifting during (e.g. provide a snag point or pocket) or pivoting during an impact (e.g. if the barrier tips over during an impact or provides a ramp to launch a vehicle in to the air has it done its' job?).


How forces get absorb by a deck, footing or soil may be different. However, a structural design engineer should have the necessary skill set to develop a design.


Yield-line analysis is only required at special transitions and unique situations (e.g. sign bridge integrated into barrier...). A "normal section" of single slope barrier with end anchorages does not need to be analyzed. However, it does need sufficient longitudinal steel to prevent shrinkage cracking.



Is this correct? I'm having difficulties defending this topic because I'm not a structural engineer. An I know that structural engineers will be present at my meeting. So I want to run this past someone who knows more about barrier design than I do.

Date August 16, 2010


Response
Response

See my comments below in red!


An errant vehicle imparts the same amount of force into roadside barrier or bridge parapet. (This would be true if both barriers were rigid. If one barrier is allowed to displace, then the impact load would likely be reduced.) Yield-line analysis has been used to develop both roadside and bridge parapets. (If configured with reinforced concrete.) Crash testing has proven yield-line analysis can provide a structural adequate roadside barrier or parapet. (Yes.) Some of these crash tests may have had failing crash test results because of the roadside barrier or parapet was not functionally adequate.


This design methodology provides that the barrier itself has:



  • Sufficient reinforcement so that the force of vehicle impact can be withstood by the barrier or transition (i.e. the barrier does not shatter and allow the vehicle to pass through the barrier)


  • Sufficient reinforcement and footing to prevent the barrier from shifting during (e.g. provide a snag point or pocket) or pivoting during an impact (e.g. if the barrier tips over during an impact or provides a ramp to launch a vehicle in to the air has it done its' job?). (Do not allow vehicle override or rollover for passenger vehicles.)


How forces get absorb by a deck, footing or soil may be different. However, a structural design engineer should have the necessary skill set to develop a design.



Yield-line analysis is only required at special transitions and unique situations (e.g. sign bridge integrated into barrier...). A "normal section" of single slope barrier with end anchorages does not need to be analyzed. However, it does need sufficient longitudinal steel to prevent shrinkage cracking. (Yield-line analysis is used at all locations, including interior regions, ends, gaps, special shape transitions, etc. However, experience may help determine if one really needs to perform the analysis at each location. The use of different types of footings may require that the certain terms in the yield-line analysis equations be neglected or minimized. Prior crash testing results may be used to support those changes.)



Date August 16, 2010


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