October 13, 2009
FOCUS | STEEL
Alberta Highway Services installing new type of barrier
RED DEER, Alta.
The Alberta government is hoping to increase safety with the installation of North Amercia’s longest post-and-cable median barrier system along 119 kilometres of the Queen Elizabeth II (QEII) Highway.
The stretch of road between Airdrie and Red Deer should reduce injuries on both sides of the roadway, whenever a vehicle hits the median.
The $8.2 million project, which includes a 3.5-km stretch through the City of Leduc, is currently being undertaken by Alberta Highway Services using a four-strand system manufactured by Gibraltar Cable Barrier Systems of Texas.
The median system is made up of high tension flexible steel cables that are threaded through collapsible steel posts.
When a vehicle makes contact with the system, the vehicle gets caught up in the cables, which act like the strands of a spider web, preventing the vehicle from crossing the median into oncoming traffic.
Additionally, the post-and-cable system reduces the likelihood of the hard impact typical of conventional concrete barriers and metal guardrails.
Ron Faulkenberry, general manager of Gibraltar Cable Barrier Systems said the backbone of the system is a <0x00BE> inch diameter wire rope.
“It’s three bundles of seven strands each,” Faulkenberry said.
“Those three bundles are woven together like a regular rope.”
However, unlike regular rope, the galvanized steel cables have a 39,000 pound minimum breaking strength and are supported by the fittings, which are rated at 36,800 pounds.
“I don’t know of any instances with our system, or even some of the other systems, where the cable has actually broken,” Faulkenberry said, noting he has seen fittings break.
“We believe that’s how it should be,” he said.
“It should be like a circuit breaker. If you overstress the cable, it becomes like over-hardened steel and becomes brittle.”
But while the cable is not designed to break, the posts which they are attached to certainly are.
Each of the posts, spaced about 20 feet apart, is a roll-formed C-section post with an open seam down one side to create a socket for a hairpin clip that allows the contractor to attach the four cable strands in about 30 seconds.
These posts are set into a galvanized sleeve which is mechanically vibrated 42 inches into the ground.
Within the sleeve is a post stop, set 15 inches below ground.
Faulkenberry said that the open post seams are offset in an alternating pattern two inches to the right and left of the cable line.
“The cable actually runs straight through the middle of the system, and you have posts on alternating sides,” he said, noting that the system’s design allows for the post to release from the cable.
“By alternating the posts, it actually becomes a bi-directional barrier,” he said.
“It can be hit from either side.”
The Gibraltar general manager said that in good soil, the vibrating machine is able to install a sleeve in about 45 seconds.
However, sleeve installation isn’t the only fast part of the installation process.
Faulkenberry said the contractor is installing and tensioning about 15,000 to 20,000 linear feet of barrier per day, considerably more than the 12,000 linear feet that is the norm.
Faulkenberry said that although the cable is supplied in 2,000-foot spools, installation requires connecting the cable every 1,000 feet with a turnbuckle.
This allows the contractor to provide the right amount of tension, but also allows easier access to the opposite side of the highway for emergency vehicles.
The QEII and Leduc installation is Alberta’s second foray into post-and-cable median technology.
The province previously spent $1.4 million installing Gibraltar’s three-strand system with cast-in-place concrete post supports in the spring of 2007, along 10 kilometres of Calgary’s Deerfoot trail.
It is a stretch of road that had seen several deaths in recent years, due to vehicles crossing the median.
Alberta Transportation reports that no cars have crossed the Deerfoot since the installation of the new system.
Since the original installation, changes in American Association of State Highway and Transportation Officials (AASHTO) regulations provided the province with the opportunity to save about $5 million on the project by going to the less costly driven post system.
“As long as the barrier system met their crash-testing requirements, then they would qualify under our standards,” said Mike Damberger, construction manager for Central Region, Alberta Transportation.
Damberger said that with the reduced cost of the system, it is likely that more post-and-cable barriers will make it into Alberta’s construction program in the future.
“This is such a large project, we’re going to be very interested to see how often it gets hit, what our maintenance implications are – that sort of thing,” he said.
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