Anticipating the Big One
Southern California is expected to be hit by a massive earthquake (6.7) in the next three decades, according to a statewide study. While seismologists and citizens await the possible big one, the building industry is using new and unique methods to make structures safer in the event of a major jolt. The industry has been diligently working to make sure we don't have the collapse of buildings and structures, much like what happened when the 6.7-magnitude earthquake rocked Northridge, California, in 1994.
A little further south in San Diego County, the granite bedrock, found in some parts of the region, provides more resistance to damage from tremors, while other buildings are built on top of terrace deposit, which includes ocean sediments and red, silty sands that are softer and therefore amplify the rocking motion of an earthquake. However, government officials and industry leaders contend that even large structures in San Diego could weather a large temblor if built and secured properly.
For example, in an effort to seismically secure the new $260 million Rady Children's Hospital Acute Care Pavilion in San Diego (figure 1), McCarthy Building Companies, Inc., chose a Micro-Pile application of a Dywidag tie-down anchor rod system. Considered a novel approach in the healthcare design world, a tie-down prevents the building's foundations from being uplifted during an earthquake. The Uniform Building Code requires that the foundation of structures be designed to resist the upward force of a seismic event. The designers of the Rady project came up with the Micro-Pile technology because it required the least amount of redesign and would allow a relatively short resubmission time back to the Office of the Statewide Health Planning and Development for its final approval.
Construction site for new $260 million Rady Children's Hospital Acute Care Pavillion, being built by McCarthy Building Companies
It is not unusual for structural foundations to be supplemented with caissons or driven piles, should the soil conditions dictate the need to literally float the building with the assistance of these foundation elements and secure the foundation to the bedrock layer below. This insures the solid embedment of the foundation supports and makes sure that any building settling will be minimal and uniform.
At Rady, the problem was not how to float the building foundation, but how to hold it down in case of a seismic event. To complicate the problem, the need to add tie-down anchors was determined late in the preconstruction process, requiring the structural engineer to either redesign the entire foundation or come up with an acceptable hold-down alternative.
To keep the project on schedule, the project architect Anshen+Allen and the structural engineer KPFF, analyzed the problem and selected the tie-down technology—which is not commonly used in healthcare projects in California—as the most effective solution and means to keep the project on schedule. Steve Van Dyke, McCarthy Building Companies' senior project manager of the project, was very familiar with the Dywidag systems used on wall tie-backs, but had not encountered the Micro-Pile system on previous hospital construction. If the seismic safety requirement had been identified during the programming stage of the project, the structural engineer may have chosen a different method, such as beefing up the foundation members or using caissons to add additional anchorage to the building foundation.
The installation of the tie-downs required McCarthy's team to drill individual 60′-deep holes that were only eight inches in diameter for each tie-down rod (figure 2). Each rod was then picked up by a crane, since the typical rod is made of 2½″-diameter solid steel, and then lowered into the hole. Each hole was then filled with grout to secure the anchor and allowed to cure. An initial pull test was conducted on the anchor rod. Then a sleeve was placed over the exposed rod, and the foundation section was formed and poured with concrete. Once the foundation section was cured, the anchor rod was prepped and finally tested, and an anchor plate and a hex nut were then tightened down on the foundation section and torqued to specifications.
Workers from Anderson Drilling oversee machinery used in the drilling operation for the tie-downs used at Rady Children's Hospital's new Acute Care Pavilion
Testing the tie-down once the grout cured was a critical step in this process (figure 3). During testing, a relatively small hydraulic-ram testing rig was set up on the concrete surface near the tie-down rod, then attached to the rod. The rig was used to make sure the rod and the tie-down would perform as engineered.
Testing the tie-down once the grout cured was critical. A small hydraulic ram testing rig was set up on the concrete surface and tied to the testing rod. The rig was used to make sure the rod and the tie-down would perform as engineered
Making sure that the building is well-connected to the bedrock below the structure was also important. The bedrock gives the building the most secure connection to the earth possible. This keeps the building tied together and anchored to solid ground. The entire foundation system, structural steel and bracing system, and the interior sheer walls are designed to allow the building to resist the forces generated in an earthquake, but also to allow the structure to move as necessary.
The Rady Children's Hospital project included adding a new emergency generator facility to the campus, which will provide additional back-up power. Ensuring the strength and safety of this building, and the others on the Rady campus, is critical not only for emergency energy use but also for the safety of staff and patients.
There are now 144 of these anchor rod tie-downs installed in the foundation to meet the unique seismic hold down requirements for the new Rady Children's Hospital Acute Care Pavilion's four-story structure. This innovative solution not only kept the project on schedule, but it also provided all the seismic restraint required for the building. Most of all, this novel approach was used with the end user in mind: the patients and their families who rely on Rady Childen's Hospital for the best health care. HD
Ben Meyers is project director for the San Diego office of McCarthy Building Companies.
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