Although vertical load testing is done routinely, “to our knowledge, no one has ever fully loaded a group footing that was all batter piles in a full-scale footing load test,” says Morvant of the LTRC. He explains that, because of inadequate training, the immaturity of proposed technologies and scarce funding, most research to date has been academic or exploratory.

“Now we have a known model that will help put to rest some doubts about actual loads,” adds the DOTD's D'Andrea. “What that test has done is to give us a sense of comfort that the bridge can withstand a barge collision.” The test also created a realistic base line against which other movement can be measured. “Since we applied a known horizontal load, we can tweak our model and say, when we are getting a wave load, what is the actual force that hits our bridge,” he says.

For the Long Run

Geocomp is refining the data acquisition system that will make it possible for DOTD to conduct ongoing, real-time monitoring of the large volume of data captured by the 340 sensors in the bridge.

Typical load data-acquisition systems—on steel trusses, for example—monitor once per minute, Marr explains. That rate yields a manageable 1,440 readings per sensor, per day. But the many kinds of loads monitored by the Twin Span Bridge's sensors, which also measure for traffic, wind and waves, means data comes in very fast. “We have to have a data collection system that can read rapidly to capture those loads,” Marr says. Consequently, the bridge's sensors deliver readings 20 times a second. “That's more than 15-million data points per day,” Marr says. “It would overwhelm a human being trying to digest that.”

Indeed, without intelligent culling, much of that data would not reveal useful information about heavy haul, wind or wave loads. “We developed a data collection system that would recognize when there was useful data and throw away all the stuff where nothing was happening,” Marr says. “We set certain limits on key sensors to say that, if the load measuring systems don't show any load above the threshold level, then the data are of no interest to us.”

Those specific parameters have not yet been set. When electrical service to the bridge is activated in October, Geocomp will spend a month establishing base-line readings. Then the DOTD will decide the trigger thresholds it wants to set for keeping data.

“We designed the monitoring system to feed the data by fiber-optic lines to the DOTD automatically,” Marr says. “If sensors show above-normal readings, messages will be sent to key staff. Someone can take out an iPhone and get a picture of what is happening to a component of the bridge at any time.”

After Jan. 1, LTRC will take over monitoring. “We will have multiple projects in the future to do all the analysis of the data that is coming in,” Morvant says.

DOTD's D'Andrea says the load data will provide important information for future bridge design as well as the bridge's operations. “We don't want to do law enforcement,” he says, but the bridge data will illuminate the problems overloaded trucks can create. “I want to understand the bridge's behavior and the effects of that load from top to bottom,” he says.

D'Andrea says the data will give the DOTD greater confidence in setting load restrictions and estimating maintenance. In one form or another, all engineers are trying to leverage technology to learn more, he says. “When there is a problem with this bridge, we can run some strain gauges to see what is wrong with it. It's like doing an X-ray before prescribing medicine.”

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