A Twisted Tale to Tell
The History Channel has dubbed it a "Modern Marvel" in a special two-volume video set on skyscrapers. It is regarded as a structure of design excellence based on sustainable design and environmental responsibility. This noted structure, the new 13-story corporate headquarters of the California Department of Transportation (Caltrans) completed in 2004 in downtown Los Angeles, reflects both a radiance of impressive stature and the California sun. Reports and articles on the project honor a compendium of manufacturers and design groups including the architect Morphosis, a highly acclaimed architectural team credited with numerous other gallant structures in the city of Los Angeles. But one group is missing from every list: the necessary foundation of the surveying crew that laid out more than 5,000 work points to allow installation of the building's unique faÃ§ade. This team proved that setting points is not always straightforward.
A Twisted TaskJohn Walker and Steve Wells have stories to tell. The two men made up the team from Model Glass of Anaheim, Calif., that surveyed the Caltrans District 7 building faÃ§ade, and are credited with "monkeying around" on the job. In addition to Model Glass manufacturing the metal for the building, and coordinating and installing the curtain wall, perforated metal panel system and the insulated glass, Walker and Wells established control for every one of the pre-cut parts, which were in the hundreds of thousands. The company is tasked with installing the glass, aluminum and stone skin on many high-rise buildings. Projects on their roster include the Anaheim Convention Center in California, the Ronald Reagan Federal Building in California, the Hollywood & Highland replacement project (the location of the Academy Awards) and now the Caltrans District 7 Headquarters.
The unique building design for the Caltrans structure called for a double skin of glass behind perforated aluminum panels. The first skin acted as a typical protectant from the elements. The overlapping second skin of glass represented the true artistry of the architect-and a challenge to the workers who implemented it. "Many architects commission an artist to have a large piece of artwork that sits at the entrance of a building, something like a statue or fountain," Walker, the project's chief field engineer/surveyor, says. "The architect for this building, Morphosis, decided to make the skin of the building [as] the art. They designed the skin to twist and fold like a folded piece of origami paper. They also wanted the skin to be "alive' and ever-changing."
Alive and ever-changing meant that the panels of the faÃ§ade were created to mechanically open and close with the movement and timing of the sun through the use of pneumatic pumps. This shields the interior of the building from the sun, provides employees with a continuously altering view to the outside and supplies innovative energy efficiency. The two skin design of the building works much like a dual panel window, minimizing heat loss and cold loss by keeping direct sunlight and cold air off the interior skin. Two light wells (glass openings in the middle of the building about 30 ft square that span the whole height of the building) allow natural light to reach the interior sections of the building. This allows the offices in the interior to not depend on overhead lighting as much, thus cutting down on electricity.
Thus, the monkeying around. The points laid out were located about every three feet of the 716,200 sq ft building. To mark the points, Assistant Field Engineer/Surveyor Steve Wells, in a full safety harness, became a bit of a primate, shimmying down one pipe and up another and trying to hold on with his legs as he held a prism steady. "One of the things that many [people] forget is how hot steel can get during the summer," Wells says. "I complained many times that the steel was burning through my pants." Luckily, for Wells, the Leica Geosystems (Norcross, Ga.) TCR303 total station used on the job has a reflectorless feature, which meant he didn't have to climb out on the steel structures as often.
"With the reflectorless ability you can just aim the total station's crosshairs on the intended target and shoot," Walker says. "The real advantage of reflectorless is in doing as-built surveying of anything with a vertical surface, or on things that are hard for your rodman to get to with the prism. In the case of the Caltrans building we had both, lots of surveying on the vertical surfaces of the round pipes, and areas that were difficult to reach."
Sun-drenched SteelWhile the California sun has a reputation for beauty, it is often the nemesis of many construction workers. The glistening heat in the spring and summer months led the Model Glass team to apply some innovative techniques to the Caltrans job. Steel, Walker explains, is under the direct manipulation of the sun, causing continuous movement.
"As the sun rises, the building heats up and moves," Walker says. "This movement can play havoc on getting your control to stack from one floor to the next. To counter this effect we transfer control to each of the multiple floors before the sun rises. The building has zero'd itself overnight and you will get a consistent reading first thing in the morning. Once control is on each floor we set the total station on the desired floor, resection to the points on that floor, and set out on that floor. This way, as the building moves throughout the day, your instrument is moving right along with the building and the error effect is canceled out.
"We also keep an umbrella over the legs of the tripod when we are shooting out in the sun," Walker continues. "Even with the tilt compensator at work, I find my accuracy is higher if the sun doesn't have a chance to make the tripod legs expand."
Positioning Pieces of the PuzzleWalker notes that he and his team worked to a tolerance of 0.005 of a foot to make sure the 250,000-plus parts all fit together as designed. To begin the survey, Walker chose to do an as-built survey of the steel as it was installed. "I knew going into this project that the steel would be farther out of tolerances than would work for our system," Walker notes. "So we used the Leica TCR303 total station with both prism and reflectorless [mode] to survey all the installed steel and draw it in three dimensions in AutoCAD [Autodesk, San Rafael, Calif.]. Once in AutoCAD I overlaid our skin that I had drawn and checked the 25,000-plus connection points to see if the steel would work as designed. As expected many locations would not work as designed. We then modified the system, or had the steel moved as necessary to get all locations to an acceptable outcome. Once we had an area that was within our working capability, we set down work points that the installers could use to build."
After drawing the project in 3D, Walker identified the points and obtained the easting, northing and height of each point. "AutoCAD gives you this information in decimals of an inch," Walker explains. "I would import this information into Microsoft Excel and use Excel to convert to decimals of a foot. Then the information went into Leica Survey Office [software] to be uploaded to the [total station]. When doing as-builts I downloaded the shots from the machine in a DFX format that loaded as a block directly into AutoCAD."
A technique Walker prefers to minimize error is to use resectioning to orient an instrument as opposed to setting up over a specific point and using a backsight. "When you set up over a point you have the error in sighting down to that point-you have the error of your control point or the backsight could be off slightly," Walker notes. "When I resection I shoot from three to five points and use [the] Least Squares [method], which [considers] the residual error to see if there was a problem with my control points. I have found my error in closing out to be substantially less by this technique." Leica Survey Office software applies this least squares method.
On a typical surveying job, Model Glass sets out control on concrete floors and control is usually set out parallel or perpendicular to the XYZ coordinate system established for the job. This project differed in that the team had to conduct all of the control on the round steel pipes, many of which were elevated 20 to 100 (or more) feet. This disallowed any setup directly over a control point and backsighting to another. Walker says instrument locations were calculated using a resection of three to five points from the elevated control of a pipe. Control was set out skewed to the XYZ coordinate system, requiring much more work to calculate where these control points would be located.