On a Mission

How 3D modeling is helping to restore the mission at San Juan Capistrano

By Karen Moltenbrey

The swallows aren't the only visitors to the Great Stone Church at San Juan Capistrano. More than 550,000 tourists a year flock to this California coastal town to see one of the greatest ruins in North America, a 200-year-old mission that was the largest "modern" man-made stone structure west of the Mississippi River until an earthquake ended its brief splendor.

That catastrophe in 1812 failed to diminish the mission's ability to awe and amaze. But now, almost two centuries later, nature's elements are destroying the historical ruin at an alarming rate. In an effort to preserve the mission as it now stands, Thirtieth Street Architects of Newport Beach, California, is using VectorWorks, a computer-aided design program from Nemetschek North America (formerly Diehl Graphsoft; Columbia, MD) to document the existing structure and determine the most appropriate methods for protecting the old church from further damage.

The ruined mission at San Juan Capistrano remains open to tourists who visit each year. But during the current stage of the project to save the mission from further deterioration, access to certain areas is prohibited. (Images courtesy Thirtieth Street Ar

"This is a conservation project, not a restoration project. We are not rebuilding it," notes John Loomis, project architect and partner of Thirtieth Street Architects. "Our intent is to freeze this church in time as it is now and stabilize it, which we hope will slow its deterioration."

The Great Stone Church was built in nine years by the Juaneno Indians, without the use of metal or modern tools. When completed in 1806, the church was the crown jewel of the California mission chain of churches, made entirely of native stone with breathtaking domes, intricately carved rock cornices, and a 125-foot bell tower. Six years later, a tremendous earthquake toppled the bell tower and collapsed many of the domes and their arches, killing about 40 people attending services that day and leaving the remaining walls and interior exposed to the wind, rain, and other ravages of time.

"We have a saying that the mission has had a successful campaign of 200 years of deferred maintenance, and now it is virtually crumbling before our eyes," says Loomis. "The sandstone and volcanic tuff that was used to build the mission is soft and fragile, as are the plasters, paints, and stone carvings. And the deterioration is occurring along a logarithmic curve, where the rate of deterioration actually accelerates over time."

A 1991 conservation and restoration study by the University of Pennsylvania, which was conducted in the early stages of the project, documented the significant archeological value of the church's architecture and carvings as a vital historical source of the "materials, tools, construction, and level of skill in the period in which the church was built." It is ironic that 3D digital design tools of the 21st century are being used to analyze and, in some cases, copy these building methods from the past.

According to Loomis, the estimated $7 million mission conservation project that has spanned nearly a decade entails not one but three ambitious as pects-stabilization of the structure, conservation of the architectural finishes, and protection from elements that could cause further damage-all of which are being addressed through the use of CAD technology. Before any of these goals could be accomplished, the team (headed by Loomis and comprising an archaeologist, a Greek architect specializing in Byzantine structures, historians, a structural engineer, an eighth-generation stone mason, and University of Pennsylvania conservators) had to document the remains in painstaking detail.

Using 3D modeling made it possible to minimize the amount of resources devoted to materials and labor. The software's intuitive approach to layering helped the architects separately document the basic structure, the structural and cosmetic problems, t

When the project got under way in 1993, there was no record of what was actually built or what had been attempted during a repair effort more than 100 years ago. So the team's first task was to field-measure the structure, which proved especially challenging because of the irregular shapes of the individual stones as well as the architectural ruin as a whole. These unusual dimensions were input into VectorWorks, a Macintosh-based program chosen by Thirtieth Street Architects mainly for its freehand drafting ability that simplified the definition of the unusual shapes.

Modeling the two domes that remain standing was especially difficult because of their declining state. Re-creating irregularities due to holes, worn edges on the stone, missing stones, and so forth was accomplished by adding customized node points to the software's library of "real-world" object shapes such as columns and doors, and 3D model forms such as spheres and cones. To create precise measurements of the Stone Church, the architects devised a segmented spherical grid system for the dome and a 3-foot by 3-foot grid system for the walls. Three-dimensional measurements for the interior and rooftop dome were input into the computer to complete the model.

"Having this information enabled us to see how the walls were moving so the structural engineer could come up with solutions to prevent the dome from caving in," explains Elizabeth Sanchez, project manager at the architectural firm.

Preservation and repair of the domes is especially significant since the dome inside the church sanctuary contains original frescoes, although they contain "cracks so large you can put your whole arm through," Sanchez says. "With our documentation methods, we now have an idea of the problems with each individual stone and the plasters."

Using Adobe Systems' (San Jose, CA) Photoshop, Sanchez also superimposed photographic textures of the actual ruins onto the 3D model for an even more complete picture of the architecture.

"The most difficult aspect of this project was understanding the resource [the ruin], which in this case took a lot of time. But by using the computer model, we were able to cut sections and really get a good look at the church profile from all angles," says Loomis. "The second most difficult thing is the structural aspect, because it is so irregular. We have retrofitted hundreds of buildings, but this project is just off the chart."

VectorWorks' rendering and animation features enabled Thirtieth Street Architects to produce dynamic graphics and virtual walk-throughs of the mission for use during presentations to update subcontractors and historic committees as well as to secure permi

Since its completion, the 3D model has become an indispensable tool in the group's efforts to conduct a structural analysis and formulate a stabilization plan that involves as little impact to the church as possible. This was done simply by interrogating the 3D model instead of building a physical scale model that typically costs about $250,000, a still-popular retrofit technique. "But for this project, it would have been impossible to build a direct translation model out of the irregular-size stone and concrete," Loomis points out.

Using VectorWorks, the team can cut both 3D and 2D cross sections through the model. This helps the structural engineers better understand the thicknesses and shapes of the building, which in turn helps them in their efforts to design a structural support system for stabilizing the building.

Originally, the domes provided lateral stability for the top portion of the church walls. "Now there are freestanding walls where the domes collapsed. In those that remain, there are huge cracks, and one wall is falling down and pushing a section of the structure away from the dome," says Sanchez. "So we focused on pinning the walls together and keeping the roof intact by using high-grade steel rods."

Because of the model's accuracy, the engineers were able to use the CAD data to figure out the dome's center of gravity, from which they could determine the stresses that would occur during an earthquake, for instance, and then design appropriate structural restraints. The dome thickness varies tremendously (from six feet to two feet at its thinnest point), so it was critical that the engineers not penetrate the dome at the wrong place, "and the CAD model helped them verify placement," says Loomis. "Rather than having to build in a huge safety factor, which means using a greater amount of expensive materials, they can cut it much leaner without sacrificing safety."

Cost is not the only issue when it comes to using more materials than necessary to preserve the structure. "Our goal is to make the mission last for another 200 years, but with minimal changes," Loomis says. "We are striving for invisibility and reversibility, where the rods can be easily replaced if one snaps years from now."

Even with all the modern technology available today, the team is still limited in what it can do and how it can proceed. "Here we are on the cutting edge of technology, and we have found that it's still a slow process. There are no easy fixes or modern Band-Aids. Sometimes the best solution is to use traditional methods to deal with the traditional material," says Loomis. "What we have learned most is to respect the traditional methods and materials used by people coming out of the Stone Age in two prior generations."

While the project is about two-thirds finished, Loomis is reluctant to provide a projected completion date, since continuation of the work hinges on obtaining the needed funding. "But we know that at some point, probably less than 200 years down the road, someone else is going to have to go back and do some of the same work over again," adds Loomis. "Unlike with the previous attempt 100 years ago, this time we want to make sure they have the proper documentation to guide them."

VectorWorks, Nemetschek North America (www.nemetschek.net)