Texas Five-Pointed Star

June 19, 2002

Texans know how to win over people. Three U.S. presidents have done it on a mass scale over the past 35 years.

Texans know how to win over people. Three U.S. presidents have done it on a mass scale over the past 35 years.

But Lyndon Johnson, George Bush and George W. Bush never jumped in front of cars during morning and evening commutes asking for support. The Texas Department of Transportation (TxDOT) is trying to accomplish approval victory on a grand scale with the construction of the Dallas High Five interchange—which will sit in an area that attracts a bewildering 500,000 vehicles daily. Two major highways, U.S. 75 (North Central Expressway) and I-635 (LBJ Freeway), are the two main carriers, and effective traffic management during the creation of Dallas’ first five-level road structure has been the top promise since day one.

The numbers are big. The $260 million project (a TxDOT record) will involve 43 bridges covering 2.3 million sq ft, 630 columns and 591,000 sq ft of concrete paving. The longest bridge will be more than two miles and the height of the tallest column is set at 115 ft. Construction officially began back in January, but work has been going on since August 2001. The finish target is 2007.

Currently, the interchange operates on three levels. Eastbound I-635 is the lowest level, U.S. 75 north and southbound is level two and at the top is westbound I-635. The design is a modified cloverleaf, one that has two of the four exit-ramps. The problem with this system is motorists have to slow down dramatically when turning around the loop ramps, thus backing up traffic onto U.S. 75 and clogging the main lanes.

Forming the new five will be, from bottom to top, U.S. 75 main lanes, frontage road box carrying east, west, north and southbound cross streets, I-635 main lanes, direct connection bridges (two levels). The direct connection ramps will eliminate the modified cloverleaf.

TxDOT and prime contractor Zachry Construction Corp., San Antonio, Texas, will work in a total of three phases. Phase I includes work on TI Blvd.; Hillcrest Road and Park Central drive U-turns; Coit Road Bridge; U.S. 75 and I-635 frontage roads; direct connection ramps between freeways; Midpark Road and TI Blvd. access bridges; HOV wishbone ramps on U.S. 75 and the Greenville Ave. U-turn. During Phase II crews will be constructing I-635 main and HOV lanes, a frontage road box along U.S. 75 and HOV T-connection ramps at TI Blvd. U.S. 75 main lanes, the frontage road box along I-635 and a reversible HOV direct connection ramp will be worked on in Phase III.

“If you talk to the public it’s like, ‘How in the world are you going to build this five-level interchange and keep traffic flowing through these two highways?’” Wallace Heimer, director of structures for HNTB Corp., Plano, Texas, told Roads & Bridges. HNTB is the principal designer of the Dallas High Five.

Every day will be a holiday

The fight with traffic around the Dallas area can sometimes feel like the Battle of Gettysburg. In terms of time, there are many casualties. The U.S. 75/I-635 interchange has received an ‘F’ grade, which translates into the lowest possible level of service. Lane configuration is the main reason behind the poor showing. The north and south ends of U.S. 75 were recently widened to four lanes in each direction, but once you hit the interchange the number bottlenecks to two. The Dallas High Five will open U.S. 75 to a total of eight lanes (four in each direction) and I-635 will be improved to 10 lanes of travel and four dedicated barrier-separated HOV lanes.

“In the morning and evening peaks you’re looking at stop-and-go traffic in all four directions on most typical days,” said Heimer. “If there was a holiday you might have a little better shot of getting through there.”

To keep traffic advancing during construction Zachry is laying “detour” asphalt pavement. When work begins on a particular section, traffic is moved off to the left or right. When it’s complete, the flow moves to the new concrete road.

“That’s generally how it’s been accomplished, we’ll scoot everybody over to one side either completely on the existing pavement, a combination of existing and detour pavement or all on detour pavement,” said Heimer.

Five temporary bridges will be built to maintain the multi-level traffic configuration. The spans will be parallel to the existing bridges and “could last 35 to 40 years if they were left in place,” said Heimer. “They have the same engineering, utilize the same material, go through the same testing.” The only drawback is the cost to construct these short-term bridges is the same as those of the long-term variety. Two of the five carrying U.S. 75 north and south over eastbound I-635 are already in place but were not yet open to traffic as of May 24.

Adding to the importance of traffic flow is TxDOT’s lane rental charge. Zachry will be charged an hourly lane rental fee for every main freeway lane closure, with the fees varying by time of day. The contractor could be charged up to $110,000 an hour for a triple-lane closure during peak travel times.

To answer the call, Zachry plans to perform one-third of the work at night. The biggest closure so far happened back in January when half of an existing utility bridge was demolished and required I-635 to be completely shut down. Eastbound lanes were closed on Friday night, westbound lanes Saturday night. Work started around 9 p.m. and was completed by 5 a.m. the next day.

Heavy air traffic

If the Dallas High Five had an ego it would be the columns. And some are quite inflated. The tallest is 115 ft and will stand right in the middle of the interchange. The base will be next to U.S. 75, the lowest level, and the top will support one of the connectors.

Finding solid ground for the footings wasn’t a problem in Dallas. In some areas rock is only about 7 ft down, and the maximum depth was 30-35 ft.

“It made the design simpler,” said Heimer. “For the most part the footing was in the rock, it basically locked the bottom of the column.”

All 620 reinforced concrete columns will be cast in place using steel formwork, and as of press time 125 were complete.

Most of the spans on the projects will have prestressed concrete U-beams, which were the center of a minor debate during the planning stages. TxDOT and HNTB looked at three different structure alternatives—I-beam concrete girders, steel-plate girders and the U-beams. All three were evaluated on cost and aesthetics, and in the end TxDOT chose the U-beams.

“U-beams are more expensive per beam but you can space them further apart,” said Heimer. The U-beams will be precast in San Antonio, about 300 miles from the jobsite.

The chosen design also carries a lot of weight, about twice as much as the standard AASHTO I-beam. Smaller beams can be placed with one crane, but the larger ones will require two. Due to the tight space constraints of the interchange, crane placement could be a problem down the line. But so far the only tricky situation came in mid-May when crews were installing U-beams over Dallas’ DART rail line by TI Blvd. Lines had to be “de-energized” so work could be completed.

“It’s been smooth,” Pete Garza, project engineer manager for TxDOT, told Roads & Bridges. “We have to just communicate with each other as far as what we’re doing and what we’re planning to do.”

Positive progress has been made. U-beams have been installed on the Coit Road Bridge and on the south side of I-635 from TI Blvd. to Greenville Ave. Placement on the north side looked to be complete by June. Garza said crews were supposed to begin concrete deck pouring by mid-June.

U-beams, however, will not cover the entire Dallas High Five. Post-tensioned concrete segmental trapezoidal box spans—25 of them—will be used in the heart of the interchange, where spans stretch out to 300 ft. TxDOT acquired additional right-of-way so pieces could be precast in the southwest quadrant. Match-casting, where pieces are cast together and then separated, will take place, and the balanced cantilever method will be used during erection. The segmental bridges also will have an 8-ft overhang, or “wing.” Prestressing strands will act as reinforcement for the wings, and additional support will come from post-tensioned ducts, both transverse and longitudinal.

All structures will use high-performance concrete ranging from 4,000 to 6,500 psi.

The road surface is designed to be continuous reinforced concrete 14 in. thick.

TxDOT is using maturity testing by inserting an electrode that reads the temperature and strength of the concrete at any given time. Cylinders are used to test for compressive strength, and TxDOT will perform “cylinder breaks” all the way up to 28 days.

The frontage road system is being revamped. Under the old system frontage roads skirt around the perimeter of the interchange and motorists utilize cross streets in the four quadrants to get across the highway. The new frontage road box will create a “straight shot” through the area.

The Dallas High Five will have other quick alternatives. One of the major connector ramps will be for high-occupancy vehicles which will connect U.S. 75 north of the interchange to the HOV lane on I-635 west. This particular type of ramp is a first in the Dallas area.

Dressing up

When work is finished, everyone involved wants the Dallas High Five to look acceptable.

Large Texas stars will protrude from the tops of the columns. The stars are actually cast into the concrete using a form liner. The tallest will have brass stars with fiber-optic lighting. Three painted metal-accented stripes will run up the columns.

Scenic retaining walls will be installed throughout the project. Garza has already seen ones with a rolling creek. Other designs are yet to be unveiled.

Work on the interchange is about 10% complete and has gone without any major letdown, which doesn’t surprise Garza. TxDOT worked with Zachry on the U.S. 75 reconstruction project which was the largest job before the Dallas High Five.

Zachry could earn $32,000 a day up to a year for an early finish.

About The Author: Bill Wilson is editor of Roads & Bridges.

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