The world of academia is tackling key issues, including improving the seismic methodology of precast concrete diaphragms.
By Michael D. Cole
With precast concrete increasingly garnering recognition in the field for its technological advantages, quality, and ease of installation and maintenance, the synergies created by the worlds of industry and academia in solving important issues are proving to be valuable.
The skill sets contributed by professors and their graduate and undergraduate students – including their problem-solving abilities and proficiencies with the latest computer technologies – are helping meet the fast-paced demands of the precast concrete industry. It is a relationship that both parties alike are finding to be mutually rewarding.
“I have to say my own experience has been tremendous,” said Dr. Robert Fleischman, who, as assistant professor of the department of civil engineering and engineering mechanics at the University of Arizona, is in the midst of one of the most prominent university-driven research collaborations with the precast business.
In characterizing the complementary give-and-take between academia and industry, Fleischman said, “On our end, we have not spent our lives like they have, figuring out how precast structures go together and understanding what all the practical considerations are, such as getting designs accepted into codes. On the other hand, we know how to set up experiments and perform simulations.” Universities have critical expertise in computer skills and knowing how to design an experiment properly. And then when they get together with industry people, the meetings are extremely productive. “We’re not just sitting around. We’re getting a lot done.”
The research project Fleischman heads is affirming just how symbiotic the partnership can be. The current undertaking, funded by the National Science Foundation and other industry sources, is a collaboration of several industry experts, professors and students from three different universities: University of Arizona, University of California at San Diego (UCSD) and Lehigh University. Together they are developing a better seismic design methodology for floor systems utilizing precast concrete.
Fleischman said the questionable performance of precast concrete floors (or diaphragms) during some recent earthquakes underscores the importance of examining the entire precast structural system in those buildings. The most notable example of floor-system failure occurred in Northridge, Calif., during the Jan. 17, 1994, earthquake, when nine parking structures located at or near Northridge suffered severe earthquake damage, and six of them collapsed. Most of the damaged parking structures employed precast pretensioned gravity systems with cast-in-place concrete shear walls and topping slabs serving as the lateral load system.
Drift demand on the gravity system of the structures was based on the assumption that the floor system served as a rigid diaphragm between the shear walls, an assumption that has since come under scrutiny.
“We have come a long way (in terms of seismic-resistant designs) in the last 30 years,” Fleischman said. “But there is still work to be done” He explained that when designing a steel structure or a reinforced concrete structure, the seismic design specifications are very straightforward. But there are some special considerations for a precast structure.
Fleischman said the current research project is comparative to the Precast Seismic Structural Systems (PRESSS) initiative launched in the ’90s. That joint U.S.-Japan large-scale testing program for the seismic response of buildings was funded primarily by the National Science Foundation with two main purposes: to develop design recommendations needed for broader acceptance of precast concrete in different seismic zones, and to develop new materials and technologies accordingly. Conducted at various research universities throughout the country, the project tested a five-story precast/prestressed concrete building under seismic loading conditions during its culmination.
“That program mainly focused on walls and frames, vertical elements and how they would resist earthquakes,” Fleischmann noted. “We see this as the logical next step. It’s important for us to make sure that we have the proper design structure for the floor systems. Our goal is to make sure designers who otherwise might use precast in a high seismic zone will be able to take advantage of its excellent performance without worrying about its reliability.”
The current project has elicited the interest of several sponsor companies and organizations, including Blakeslee Prestress Inc., High Concrete Structures Inc., the Precast/Prestressed Concrete Manufacturers Association of California, Spancrete Inc., Tindall Virginia, Ivy Steel & Wire and Metromont Prestress. S.K. Ghosh, founder of S.K. Ghosh and Associates Inc., which provides structural seismic and code consulting services, has served the project team as its chief industry liaison.
“Our industry task group is very active in overseeing our work,” Fleischmann said. “It includes a dozen or more experts in precast and earthquake engineering, whom we meet on a frequent basis to make sure we are doing the proper research and exploring the proper issues.”
In divvying responsibilities among the multi-university research team, Fleischman said load tests and other analyses to understand joint and connection details of precast diaphragm systems are being performed at Lehigh, shaker table tests are being conducted at UCSD and computer simulations are being performed by Fleischman’s team at the University of Arizona. “It is really a computer simulation sandwich where the bread will be our experiments,” Fleischman said.
The project scope considers variables including topped and pretopped diaphragms, hollow core and double tees precast units, and low and high seismic zones. A challenge for the team, according to Fleischman, is to overcome some of the characteristic issues of precast diaphragms that make design methodology challenging, including their complex force paths and long floor spans. In addition, he noted that equivalent lateral force (ELF) design procedures currently in place with precast floor diaphragms may be significantly underestimating diaphragm inertial forces.
Ultimate research objectives are to determine design forces, limits on diaphragm flexibility and knowledge of detail capacities, as well as to establish a method for estimating internal forces.
While the planned three-year project has been underway for six months, Fleischman said research preparation prior to the project inception lasted for nearly two years.
Seismic testing performed
Detailed tests measuring shear response to tension are being performed at Lehigh, where a three-actuator load frame possesses the ability to provide proportional and non-proportional shear and tension loading.
Testing of seismic conditions is being performed at the largest shaking table in the United States, based at UCSD. The shaking table is a 25-by-50-foot steel platform with hydraulic actuators that are able to simulate earthquake-type conditions. “Opportunities like this don’t come around too often at the university level,” Fleischman said.
A half-scale model of a three-level precast concrete structure is being constructed and then tested at the shaker table. “We’ll subject it to increasing levels of (seismic activity) starting with small tremors and then working our way up to a large earthquake,” said Fleischman.
The shaker table testing at UCSD is being conducted by one of several students who are taking an active role in the project. “While I’m guiding them, they’re the ones who are getting on the computer and creating models and simulating an earthquake,” said Fleischman. “We have conference calls frequently and discuss the next steps. They’re the people who are doing the work.”
Advanced computer technologies and communications make the multi-university collaboration a more feasible undertaking. “The students are basically doing things that would take a person with a pencil and paper a few months to do,” said Fleischman. “They’re using Newton’s laws of physics to figure out how much the building would shake in the event of an earthquake. And simulations are allowing us to look on the screen to see what joints are being impacted by really high forces to help us focus our design efforts.”
University research is often conducted in a laboratory environment, but this goes beyond the traditional methods. “What makes this effort unique is that we are performing a lot of interesting experimental work,” said Fleischman. “We’re mixing big experiments with computer simulation at three different universities to answer some pretty important design issues.”
Bridging the gap
As Fleischman leads the team exploring improvements in the seismic response of precast floor systems, one of his team members, Dr. Clay Naito, assistant professor at Lehigh University’s Department of Civil and Environmental Engineering, is conducting additional research to enable the applicability of self-consolidating concrete (SCC) for use in precast bridge beam construction.
The project, funded by the state of Pennsylvania, has piqued the interest of the precast industry and the Federal Highway Administration, as well several state transportation departments, including those in Pennsylvania (a sponsor), Delaware, New Jersey and New York (participants). Design flexibility and the ability to construct beams off site are obvious appeals to constructing precast beams.
“You’re talking about a lot of cost savings and labor reduction,” said Naito. “We really wanted to look at this from a DOT point of view: Does it meet all the current specifications? Are there any problems, long-term or short-term?” He explained that this is truly an industry project, and the state of Pennsylvania helps with funding where it can. “This has been an effort to answer the problems of industry and make bridges more affordable and last longer. In that sense, this has been important work.”
With the relatively new introduction of self-consolidating concrete to the marketplace, Naito said the challenge is for DOTs to accept precast into their own design specifications. “Understandably,” he said, “they’ve been careful in sticking to the procedures that have been performed in the past because they’ve worked. They don’t like changing. You’re talking about bridge systems that need to be put out there for 40 to 100 years, and they don’t want to run into problems 20 or 30 years down the road (as a result of implementing less reliable methods). We’re making sure our studies are thorough.”
The experiment compared SCC and conventional high early strength concrete (HESC) through a series of plastic and hardened material tests and through the fabrication and examination of four full-scale SCC and HESC bulb tee beams. The project’s purpose has been to demonstrate that the SCC design compares favorably with a conventional mix in terms of strength gain, modulus of rupture, splitting tension, shrinkage, creep, hardened and plastic air, freeze-thaw resistance and chloride permeability.
Naito and a team of three students built full-scale beams 35 feet long and tested them using Lehigh’s loading fixture. “Measurements taken during release of prestress indicated that the beams have a transfer rate lower than recommendations,” said Naito. He added that the experiment results indicate that the SCC mix design meets requirements for DOT use in Pennsylvania, New Jersey, New York, Virginia and Massachusetts. The project, which began in March 2004, concludes this summer, according to Naito, who plans to send his findings to every state transportation department.
Naito praised the contributions of his three-student team. Graduate student researcher Greg Parent assisted in the fabrication and testing of the beams, while undergraduate Geoffrey Brunn served as a research assistant who performed material testing. Graduate student Tyler Tate is performing late-project experimentation on the performance of the SCC mix and its ability to bond to the steel in the beams.
Naito wants to continue in-depth work with precast concrete. “I want to get more involved because of the possibilities,” he said. “Precast concrete definitely has a lot of future growth potential. There are a lot of good things happening with precast concrete. It’s something I want to be a part of.”