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We have a project in the KC area that involves reconstruction of two high-speed (65 mph) major highways; however, the existing bridge columns are to remain in place. These columns will not accommodate the AASHTO LRFD requirement for impact load (1800 kN or about 400 kips). We plan to protect these median columns with a 51" or 54" barrier. The attached detail provides details for an application that was previously done for a TL-3 design; we used 32" barrier for that application.
On this project we plan utilize a barrier that will meet TL-5 criteria. We propose to use a tall wall and construction diaphragms between the barrier to isolate the columns if a TL-5 level impact is experienced. The thought is that if the columns are not isolated then the impact load will be transmitted through the barrier and aggregate backfill to the column. Is the last statement true, if not is there an approximation on the amount of load that gets transmitted to the column.
Can we meet TL-5 by tying the barrier into the concrete shoulder and having a granular backfill with diaphragms near the column area?
Can we go vertical for the entire height? If not any recommendations on the shape to address head slap?
|Date||September 17, 2008|
|Attachment||CSB Type II.PDF|
|Response||You note below that you desire to use either 51" or 54" tall barriers to shield the bridge piers. Currently, there are several 42" tall, reinforced concrete barriers that meet the TL-5 impact conditions of NCHRP Report No. 350. Years ago, MwRSF also developed details for tall, TL-5 single-face, safety shape barriers for the WsDOT. The use of 42" high, concrete barriers placed in front the bridge piers would prevent a head-on collision into the piers when a truck leaves the roadway at 15 degrees or less. Using an adequate length of need upstream of the piers would reduce the tendency for the truck to get behind the barrier. A gradual flaring of the barrier toward the median (i.e., away from the roadway) would reduce the required barrier length. The available, 42" reinforced concrete median barriers can be placed on a aggregate base with asphalt placed on both sides or doweled into a RC slab or footing. For the 50+" single-face, TL-5 barriers, anchorage options have consisted of RC slabs or footings.
For these barrier and anchorage options, the RC barriers resist the heavy truck lateral impact loads. With 42" tall barriers, a portion of the tractor-trailer vehicle extends over the top of the barriers. For this truck and trailer-box lean over the barrier top, this vehicle portion could potentially impact the exposed portion of the pier above the top of the parapet. However, the truck and trailer box would not be expected to provide a significant impact event against the stout bridge pier columns. To further protect against this truck-pier impact event, RC diaphragms could be installed between the piers (i.e., parallel to the roadway) in order to stiffen and strengthen the piers to resist truck, trailer lean and subsequent pier impact.
Do you plan to use TL-5 barriers placed forward from the piers? If yes, can you use any of the existing double-face or single-face designs? Do you also want to mitigate pier impact with those vehicle components that lean over 42" tall parapets? Or, do you want to use 50+" parapets to reduce the lean over lower height parapets.
If you want to use a vertical shape parapet, you certainly could do so. If you want to mitigate any tendencies for head ejection and head slap against taller parapets for passenger vehicle impacts, modifications to the new TL-5 barrier could be made. However, the basic top geometry (setback) should be followed and as published in the research report.
With regard to placing traverse diaphragms or compacted fill between the parapets, I do not think either is necessary as long as the appropriate barrier is selected for use.
Please provide any clarifications, comments, and/or questions regarding the information provided above. Once we receive that information, we will continue brainstorming solutions for your situation.
|Date||September 18, 2008|
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