Presentation on theme: "Barrier End Treatments and Crash Cushions"— Presentation transcript:
1Barrier End Treatments and Crash Cushions RDG Chapter 8
2Terminal Classifications A gating deviceA non-gating deviceNOTE: All of the guardrail terminals that you are about to see in this lesson are considered gating, except for the buried-in-the-back slope design.A gating device is one designed to break when impacted between the nose and the beginning of the length of need (the point where a vehicle will be redirected), thus allowing the vehicle into the area behind the installationA non-gating device is just the opposite: a vehicle does not usually go behind the device but is stopped or redirected toward the roadway
3A Brief History of Non-Proprietary Terminals Blunt EndTurned-Down End or Texas TwistBCT (Breakaway Cable Terminal)MELT (Modified Eccentric Loader Terminal)Buried-in-BackslopeNon-proprietary terminals are not patented, and usually not expensive.These were the some of the first terminals developed.Let’s start with a little history and look at a few pictures ...
4Blunt EndBlunt EndsThe first terminals we put out -- were considered blunt ends. Because if a vehicle hit the end.....
5Hot knife through butter ...the end would slice through the vehicle just like the proverbial hot knife through butter.This may be a little hard to see but the guardrail is actually in the car you can see it through the windshield… just like in the introductory slide for this lesson.
6Texas Twist Turned-Down End So, the next thing the industry tried to do was eliminate the “spearing” potential by burying the end in the ground this was known as the turned-down end or Texas Twist.Burying the end in the ground certainly eliminated spearing … but it caused vaulting instead. If you hit the turned down terminal at the end, it wouldn’t slow you down much, might let you slide along the top of the rail until you reached the bridge rail or some fixed object hazard, or it might even send your car into a violent rollover.For these very good reasons, turned down ends are no longer allowed on the approach end of GR on high-speed, high-volume roads.
7BCTBCTNext came an attempt to design a terminal that would neither spear nor vault an impacting vehicle - the Breakaway Cable Terminal, or BCT.It has 2 weakened end posts and requires a 4’ parabolic-type flare which is critical to the proper performance of the terminal.The rail is designed to bend away when hit head-on and alows the car to go behind and beyond the end without a rollover.
8Guardrail So, what were they thinking? Here we have a utility pole, mail box, curb, guardrail. The GR does have some value as a delineator -- if you don’t move the utility pole.This situation wouldn’t be any worse than without the GR -- it just looks dumb -- especially to someone who knows how breakaway devices are intended to work !
9MELTMELTSince the BCT was not deemed crashworthy under Report 230, the Modified Eccentric Loader Terminal or MELT, shown here, was developed.This design has more eccentricity than the BCT, uses six weakened posts instead of two and utilizes a ground strut to provide additional anchorage.Developed under NCHRP Report 230, the MELT did not meet NCHRP 350 evaluation criteria when tested under these new guidelines. However, existing units do not require upgrading because the expected difference in performance from a similar-type Report 350 terminal is negligible.Ground strut allows two posts to work together.
10Buried Guard Rail Buried-In-Backslope ...a guardrail. This is the best way to terminate the rail. If there isn’t an end to hit, then you don’t have to worry about the breakaway characteristics.If you’re burying a w-beam, you need to keep the rail height relatively constant in relation to the shoulder and you will also need to use a rub rail across a deep ditch to prevent vehicle snagging on the exposed posts. Several buried terminal designs meet NCHRP 350 evaluation criteria at TL-3.
11Report 350 Terminals Video This video is also unnarrated --- this covers one of the 350 tests for several different end terminals -- and crash cushions, which we haven’t talked about yet.1. ET The bumper wrapped around the nose. It didn’t spew out the rail as cleanly as it usually does, but it did slow the vehicle.2. SRT 3503. CAT -- This uses a slotted w-beam rail which telescopes the system when hit end on.4. REACT It basically can be be pulled back into shape.5. QuadGuard -- This crash cushion replaces the GREAT system.6. ADIEM II -- It’s basically made of a crushable concrete and terminals concrete barrier.7. BEST -- One of the tangential terminals.8. MELT - This is the only test that passed.9. Anchored in the Backslope -- The rail needs to be kept at a constant height above the road. If there’s a ditch, a rub rail can be added to keep vehicles away from the posts.
12Some Proprietary W-beam Terminals BRAKEMASTERCAT (Crash Cushion Attenuating Terminal)SRT 350 (Slotted Rail Terminal 350)RegentHere’s a list of just some of the current proprietary terminals. Proprietary simply means that there is a patent on the terminal and it is usually made and sold by only one company. It’s more common these days to find proprietary terminals in use rather than non-proprietary ones. All of these devices have passed NCHRP 350 at TL-3. Let’s look at the crash performance of a couple of them and then discuss these and other in more detail.
13CAT Crash Cushion Attenuating Terminal (CAT) This is the CAT - or the Crash Cushion Attenuating Terminal. The rail is made by Syro/Trinity, and it has passed Report 350 at TL-3.The rail includes fabricated slots so the system, when hit end on, folds up like a telescope. An angle impact will either allow the vehicle to pass through the system or it will redirect depending on where the terminal is hit. The CAT’s redirection capabilities begin at the 4th post. It is best used to terminate a double-faced w-beam median barrier as shown here.The CAT cannot be directly attached to a rigid barrier. A transition is needed to go from the flexible system to the rigid barrier.
14Slotted Rail Terminal 350This terminal is quite similar to the non-proprietary MELT design, but uses a somewhat different post layout and a w-beam section weakened by the addition of horizontal slots cut into the rail. This terminal has a 4’ offset at the nose, a ground strut between the first two posts, and a parabolic shaped flare. This is a gating terminal and redirection begins at post 3. Variations on the original design allow installation with only a 3’ offset. The manufacturer, Trinity Industries also has designed a straight-flare system, using steel hinged breakaway posts. It is not an energy-absorbing device.
15Slotted RailsThe best way to tell any of the slotted rail systems from the others are the slots in the rail. You know -- you have to wonder how these people come up with such fancy names.There are 3 slots at different points in the rail.
16Energy-Absorbing Proprietary Terminals ET PLUS (Extruder Terminal PLUS)SKT 350 (Sequential Kinking Terminal 350)FLEAT (Flared Energy Absorbing Terminal)A significant improvement in terminal development came with the advent of various energy-absorbing extruder heads. Whereas most earlier terminal designs allowed the vehicle to go through and behind the guardrail installation when struck nearly head-on, these terminals absorb much of the energy of the crash under the same impact conditions. As a result, they can stop many vehicles safely in about 50 feet, compared to well over the 100 feet needed for terminals that merely break away cleanly when hit. HOWEVER, FOR ANGLE HITS AT OR NEAR THE END, THERE IS NO SIGNIFICANT DIFFERENCE IN THE PERFORMANCE OF ENERGY-ABSORBING TERMINALS AND BREAKAWAY DESIGNS.
17ET- 2000This is the Extruder Terminal 2000 or ET 2000 Plus. This is a Trinity product.The best way to distinguish this terminal from other flared terminals is the flat end and that it’s parallel to the road. This has met 350 to TL-3. This is a terminal available with a 50’ length and 37.5’ lenth-- it has a 2 soil tube, 4 tube, and an 8 tube option. You’ll notice that this terminal does have a ground strut.Make sure the ground strut is actually on the ground. This needs to satisfy the same 4” stub height requirement we discussed back in the sign supports section.This has a 1-2’ flare over the length of the system.
18How the ET-2000 worksThe ET-2000 basically works so that when hit head on, the rail gets flattened and curled through the extruder head away from traffic. The flattening absorbs the energy from the crash.You can see where the extruder head ended up -- in the middle of the rail.
19SKT 350 Sequential Kinking Terminal (SKT) Sequential Kinking Terminal or SKT 350Has center horizontal brace and two vertical braces.
20SKT-350 After HitThis is what the SKT-350 looks like after a successful hit. Note the kinks in the rail element. This was probably a near 60 mph hit as it appears that most of the unit was “used up”. This happened on Hwy. 47 near Gaston. Next slide shows the vehicle.
21SUV vs. SKTFor an assumed 60 mph impact, this could be characterized as “slight damage”
22FLEAT Flared Energy Absorbing Terminal (FLEAT) The FLEAT or the Flared Energy Absorbing Terminal has a straight flare and it can be installed with any end offset between 2.5’ to 4’ at the nose. This terminal is 37.5’ in length.Fleat has a tube bracket on top
23Installation Problems with Gating Terminals Some of the problems that you see not only with gating terminals, but with any installation is that you don’t have enough area behind the terminal for the vehicle to go except down and on it’s roof -- not a good situation.In crash tests, the pickup truck went over 100’ behind the terminal in head-on crashes into gating terminals and in most 15 degree hits on all terminals. There needs to be additional grading behind as well as in front of the terminal.Need to make sure that there is a 5’ pad or platform for the terminal to sit on – but have to be careful about discontinuity in terrain.
24Good installation? Is this a good installation? Hardly! Although the guard rail here was primarily used to shield the end of the bridge railing, the designer needs to recognize other problems behind the rail. The gating end is not long enough. The concrete channel is clearly a hazard and that’s exactly where a motorist would end up - even though the breakaway terminal works just as it’s supposed to….
25Good installation 2?How about this installation? Any good? Again, not really.When these terminals are hit, they gate and push out of the way.One works fine. I’m not quite sure what two would do, but these two installations would definitely interfere with each other. Maybe the vehicle could be skewered from two directions.The “best” solution would depend on what was being shielded here, but usually a gore area is a great place for a crash cushion.