| SKANSKA Stacy and Witbeck Bi-Weekly Project Update |
SSW Project Update:
Oct. 28 - Nov. 10, 2018
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Construction Progress:
October Baseball & the Tension is Building in LA
The Sixth Street Viaduct Project (Project) will feature incredible sweeping arches that span over the length of the viaduct, including the Los Angeles River, and construction of this intricate design is very demanding on its concrete. Not only will the concrete elements have to hold the changing weight of traffic, but they must support other elements of the bridge providing additional pressure. Fortunately, the concrete doesn't have to do all this all on its own. It has reinforcements.
Concrete does an excellent job of bearing weight. It's good at
compression
. Concrete is not as strong when it decompresses, or is pulled apart (known as
tension
). Steel, on the other hand, is very strong against tension forces. So how do you combine these two materials to get the best of both worlds?
Steel wire, in conjunction with traditional rebar, are wound together creating a
strand
. Metal ducts are used to create a hollow path for these strands after the concrete has been placed. Once the concrete is cured to the specified strength, a cluster of strands (called the
tendon
) are installed and tightened, or stressed, like the fibers of a muscle contracting. This compresses the concrete until it reaches the desired strength, then the steel tendon is anchored in place to keep the concrete compressed and offset tension.
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A Post-Tensioning tendon made up of several steel strands, installed in a concrete test block used to certify PT components.
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The process described above is known as
Post-Tensioning (PT)
, because the steel tendons are stressed after the concrete has been placed. It's a complex endeavor that must be performed throughout the entire Project. Because the bridge design is so dynamic, PT systems are being installed both longitudinally and transversely within each element.
The Skanska Stacy and Witbeck (SSW) subcontractor in charge of PT for the Project is DYWIDAG-Systems International (DSI). DSI installs all of the metal ducts and PT tendons within the concrete, then uses a specialized hydraulic Stressing Jack to tighten the steel strands. Four different sizes of tendons are being used: 4-strand, 19-strand, 27-strand, and 33-strand. A single strand is only 0.6 inches in diameter, but has a breaking strength of 41,000 pounds.
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The backside of a PT Coupler after testing. Couplers allow for multiple bridge frames to be continuously connected.
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But PT tendons are not the only reinforcements being deployed to help the concrete. Each concrete element is poured around a steel rebar frame for additional strength support. With so much going on inside the concrete, DSI must be extremely coordinated with fellow SSW subcontractor LA Steel, which is providing the Project's rebar. It's a challenging installation schedule to maintain, but these two subcontractors have used excellent communication to maximize their efficiency.
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Fabrication of rebar framing from LA Steel must coordinate with DSI's PT tendon installation before any concrete can be poured.
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On The Job: Michael Cardwell
Michael Cardwell grew up in Michigan, but his family took a vacation to Los Angeles when he was a kid. Recently, Michael watched an old home video of the trip where his parents, by an incredible coincidence, drove the family over the original Sixth Street Viaduct.
Now, Michael is a Quality Control Manager for SSW on the new replacement of the bridge he visited all those years ago. He manages Project inspections and is responsible for ensuring that the Project's Quality Control (QC) Plan is maintained. He's good at making plans.
"The most interesting or exciting element of the new viaduct," Michael says, "would have to be the Y-Bents." He was heavily involved in every phase of their installation, from the beginning location surveys to the fabrication of the steel frames and pouring of the concrete. From a QC standpoint, that also meant a lot of long hours. His team planned for the lengthy inspection activities and materials verification, but the schedule was still demanding (and may have led to Michael stress-snacking on his favorite guilty pleasure: gummy bears).
Michael says that one of the two most rewarding moments of the Project so far is the completion of those Y-Bents. What's the second most rewarding? Nearing the completion of the first Arch Rib Floor Beam. Why? Because that's what's next on the plan.
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