Workers look to reflectorless EDM technology after 9/11 attack.

Pentagon damage following 9/11.
Following the initial rescue and recovery efforts after the tragic September 11 attack on the Pentagon, workers soon began the daunting task of repairing and rebuilding the 58-year-old structure.

Prior to the attack, a portion of the damaged section of the Pentagon had been slated for occupancy by a division of the Department of the Navy. Following the attack, the decision was made to renovate a portion of the Pentagon basement for the Navy offices.

Without accurate structural drawings of the Pentagon basement, a detailed “as-built” survey of the 225 columns in the area to be developed was needed. The Gilford Corporation, the Pentagon’s General Contractor, called on Capital Development and Design Inc. (CDDI) of Alexandria, Va.

The existing basement floor elevation varied and had to be brought to a single elevation prior to the build-out. As a result, as much as two feet of concrete had to be added in certain sections to bring the existing finished floor to the elevation of the new finished floor. The addition of this much concrete required installation of auger piles to expand existing pile caps for structural support. The design of the new auger piles and pile cap extensions required an extremely precise location of all 225 columns in the section of the basement for the new build-out.

A baseline was established parallel to an existing column line to serve as horizontal control for the survey. A level datum line was marked around each column at an elevation of 4' above the proposed new finished floor elevation. Three corners of each column were to be located to ensure consistency throughout the project.

The survey was completed using a Leica TCRA1101 with reflectorless electronic distance measurement (EDM) technology (Leica Geosystems, Norcross, Ga.). The coaxial, reflectorless EDM was used to avoid alignment errors of a prism and to expedite the overall time to complete the survey.

The TCRA1101 is a motorized total station with automatic target recognition and extended-range reflectorless EDM. The Leica system uses an extremely narrow visible laser beam to mark the target precisely and measures the distance utilizing patented phase-shift techniques. The unique small size and bright visibility of the coaxial red laser dot make it easy to get accurate measurements under dark and difficult conditions without the need for looking through the telescope or even focusing. The TCRA1101 permits measurements at ranges up to 200 meters without the use of a prism or reflective tape.

“The basement was a dark, dusty and noisy environment,” says Kevin Fleet, LS, director of surveying for CDDI. Fleet was the on-site project manager and supervisor of all of the survey tasks on the job. A white card was held at each column corner to assist the instrument operator in accurately sighting the column. “The visible red laser in the Leica TCRA1101 total station allowed the person at the column to know when a shot had been taken and eliminated the need for much communication between the instrument operator and the person at the column. That feature alone saved us a lot of time.”

After the survey was completed, random field checks were made with a steel tape throughout the site to verify the reflectorless measurements. “We didn’t find any bad measurements,” Fleet says. “In fact, most measurements taken with the steel tape were identical to what we measured with the reflectorless EDM.”

Kevin Fleet, LS, with Leica reflectorless laser EDM.

Going Reflectorless

Fleet said that CDDI had its first experience with reflectorless EDM technology two years ago when asked to bid a deformation survey on a U.S. courthouse in the District of Columbia. “This was my first reflectorless survey,” he says. “In my recon of the job for bidding and estimation, I determined that it would need special equipment, so we rented a Leica TCRA1101 for the job. I never returned it. We bought it instead. Since then we’ve bought three more.”

According to Fleet, CDDI used the TCRA1101 to take a series of measurements on the underside of a 35 ft high overhang at the courthouse. “We shot a 10' x 10' grid on panel intersections on the underside of the overhang,” he says. To facilitate the process, a helper with a hand-held pen laser stood under the overhang to identify the measuring point for the total station operator. “Our measurements showed a 3" deflection of the canti-levered section.”

“We supplemented our reflectorless measurements with conventional leveling around the perimeter of the roof of the overhang, which confirmed the measurements taken below,” Fleet says.

“With the reflectorless EDM and the automatic targeting (ATR), we are able to make measurements to inaccessible points on a construction site accurately and safely, and monitor structures from a long distance without the need for a person with a prism at the point to be measured,” he notes.

CDDI has also used the total station with reflectorless EDM for a complex job that entailed monitoring 187 piles supporting timber lagging for a 70-ft deep excavation at Georgetown University, and for establishing ground control for the new Wilson Bridge across the Potomac River on I-95/495. For the Wilson Bridge project, CDDI used automatic target recognition with Leica’s precision circular prisms to take four sets of direct/reverse angles per point. “In addition to the control traverse, we monitored the existing bridge columns two times per day during pile driving operations using reflectorless EDM sighting on card targets mounted on each column,” Fleet says.

Kevin Fleet marks datum line on a column.

Productivity Gains

Fleet estimates that using the laser EDM can cut the time by up to 75 percent on certain jobs. It eliminates the need for conventional techniques, such as locating columns by using traverse legs and a steel tape, and manually reading cutoff dimensions to column faces. It also makes it easier and faster to plan and bid work more accurately. Moreover, lasers have allowed them to adopt new methods of monitoring that didn’t exist before.

“We wouldn’t be able to monitor some projects at all without the laser technology,” he says.

The combination of extended-range reflectorless EDM and automatic target recognition makes the TCRA1101 a powerful and accurate tool. “The closures I achieve with the TCRA1101 on traverses have exceeded all my expectations,” Fleet says. “We let the instrument point itself on high precision traverses. No one could convince me there is a better way to point to a target. The closures we see now are far superior to what we saw when a human was doing the pointing.

“I think the important thing for people to realize about this technology is that good field procedures are still required. The instrument is a machine and requires proper calibration and care. It has its limitations just like everything else, but it has proved to be a very valuable tool.”

Rebuilding the Pentagon

  • The Pentagon was constructed during World War II to provide a unified headquarters for the nation’s growing War Department. The building was intended to house upwards of 40,000 people. The site was mostly reclaimed swamps and dumps. The soil conditions necessitated a strong foundation. Some 5 million cubic yards of earth were needed, and the structure was supported by more than 41,000 concrete piles. The construction was completed in just 16 months at a total cost of about $83 million.

  • When completed, it was the world’s largest office building. It still retains that distinction. The massive structure, with a gross floor area of 6.6 million sq. ft., covers 29 acres. The center courtyard alone is 5 acres.

  • Ironically, the crash of American Airlines Flight 77 into the Pentagon on September 11, occurred just days away from the scheduled completion of a three-year $258 million renovation of that portion of the structure. Crews at the Pentagon were faced with the task of putting back together a section of the building that they had spent the last three years renovating. Repairs to the damaged area will take place simultaneously with the ongoing renovations in surrounding areas.

  • Experts stated that the recently completed renovations in Wedge 1, where the aircraft hit, probably helped to mitigate the impact of the explosion. As a part of the renovations, structural steel beams had been added through all five floors to strengthen the walls, and interior walls had been covered with a blast-resistant cloth material similar to Kevlar. In addition, blast resistant windows with two-inch thick glass had been installed. Photos showed that the windows on either side of the impact area remained intact.

  • The 11-year $1.2 billion renovation program is scheduled for completion in 2012. Construction costs for repairing the damaged portions of the building have been projected at up to $800 million.