CTroyMathis

High Five construction: A game of inches
01/12/2003

By TONY HARTZEL / The Dallas Morning News
http://www.dallasnews.com/localnews...nner.557a3.html

Building bridges at the Dallas High Five requires the balance of a tightrope walker, the ingenuity of a handyman and the brains of a math professor.

In the northeast corner of the project at LBJ Freeway and Central Expressway, crews have postponed the connection of two long ramp sections. Their concern: a small bearing on which the final connecting segment will rest may be off by a few inches.

If the bearing doesn't work correctly, the bridge could suffer serious structural problems as the concrete and steel expand and contract during seasonal temperature variations. The solution is not as simple as building a bigger segment.

"We've got the segment made, and we're working on our geometry," said Greg Blackard, an assistant project manager for the contractor building the ramps, Rizzani de Eccher/Zachry Joint Venture. "We'd like for it to have been in place. But it's not going to affect us."

Workers also have used basic ingenuity to solve other potential hang-ups. Almost daily, crews hoist another 35- to 70-ton concrete ramp segment into place. Once the pieces are firmly connected, surveyors sometimes find that the massive segment has missed its predicted resting spot by one-eighth of an inch.

Left uncorrected, that slight shift could compound into a two-inch error as the ramp is extended – enough to cause major problems for the project.

But engineers have come up with their own version of the matchbook-under-the-short-leg-of-the-coffee-table trick. They use shims made of steel mesh between concrete segments, which will compensate for the small but significant flaws.

"It's very tightly controlled," said Brian Smith, an assistant project manager for the Texas Department of Transportation. "This is just a matchbook you can't see."

Keeping the growing ramps from crashing down under their own weight often requires extra steps, some of which motorists aren't likely to notice. Much of the planning involves a balancing act that accounts for the weight of the 101-ton Segment Erector.

The Segment Erector is a custom-built, $1 million machine that hoists ramp segments into place. Because it works in small spaces above traffic, it allows motorists to keep moving underneath without major detours.

But it also can act like the heavy kid on a seesaw as it travels farther out onto new bridge sections that are supported by a single bridge column in the middle.

On the ramp in the project's northeast corner, crews have installed a supporting tower that will be in place until the two sections are connected.

In other areas, workers have installed high-tension cables near the ramp support columns. Those cables pull on one side of the unfinished bridge to keep it from toppling over while the Segment Erector works.

The latest solution adds some heft to the opposite side of the ramp from where the Segment Erector is working. Workers are putting the finishing touches on a 145,000-pound steel counterweight that will shadow the Segment Erector's moves.

"The erector gets so far out on the ramp, we have to use a counterweight to balance it," said Alessandro Qualizza, a site engineer for the ramp contractor.

• The Fort Worth Transportation Authority will run shuttle bus service every Saturday from the Trinity Railway Express to the Southwestern Exposition Livestock Show and Rodeo. Passengers can catch the bus at the Intermodal Transportation Center on Jones Street in downtown Fort Worth. Buses will start running at 8:05 a.m., and the last will leave the stock show around 9:45 p.m. Cost is $2 round trip.

Tony Hartzel can be reached at P.O. Box 655237, Dallas, TX 75265.

gc

How funny that something so massive is really all about inches and centimeters!

Talk about an exact science.

__________________
“We shape our Cities, thereafter they shape us.”