AUTC addresses many research issues on a project-to-project basis. Other issues, however, are much more complex and multi-faceted, requiring a program of projects to address them. Updating national and state bridge seismic design codes is just such an issue.
Alaska’s engineering and transportation professionals have many questions about how bridge structures can better withstand earthquakes in a state that is both the most seismically active and frigid in the U.S. Through a program of nine projects with four research partners addressing structural, soil, and software assisted analyses issues, a research effort five years in the making is now bearing fruit. Leveraging a $1.7 million portfolio of match-funded projects (U.S. DOT RITA, Alaska DOT&PF, and AUTC), the collaborative effort enhances the predictive capabilities of designers, their software, and analysis. Its results are now being written into improved seismic design standards at the national and state level.
Alaska’s bridges are vital links to goods and services, often with no alternatives. Because removing a bridge from service is not an option, and replacement takes years, designing bridges to maintain service life after an earthquake is a necessity. This program of projects addresses the two parameters engineers consider when designing bridges for earthquakes: structural capacity and seismic demand.
The structural capacity, or resistance, is the structure’s ability to resist a seismic load. Four AUTC research projects examining structural capacity have focused on steel reinforced concrete columns and all-steel structures similar to those used in Alaska.
AUTC’s partners at North Carolina State University (NCSU), led by faculty member Dr. Mervyn Kowalsky, utilized NCSU’s Constructed Facilities Laboratory (CFL). They used this unique set of testing and instrumentation equipment to study the behavior of bridge structures during earthquakes, and better understand bridge seismic performance limit states—addressing the force a structure can withstand and still retain its strength after a seismic event—for seismic and temperature conditions typical of Alaska.
The NCSU team tested steel-reinforced concrete columns to measure how strength increases in the -40 degree temperatures common to Alaska. Using cyclic testing to mimic an earthquake, they pushed and pulled upon bridge columns to measure their maximum deformation capacity.
Researchers found these cold conditions increased concrete strength by 30-40 percent and steel by 10 percent. A similar project utilized advanced non-contact sensor technology placed directly on structures to measure how the specimen’s load history impacts the strain it can withstand. The results show the deformation a bridge pier can withstand and remain in service after an earthquake.
The team also studied all-steel bridge piers. In Alaska, these often serve as detour bridges, temporary work structures, bridges in remote locations, and marine structures like marginal piers and ferry terminals. Engineers are skeptical of their structural ductility (ability to withstand large deformations) and propensity to fail during seismic events. However, all-steel structures present an attractive construction option if built to reliably withstand large earthquake demands. This project tested structures built under current specs, and also tested reinforcement methods to improve ductility, helping refine Alaska standards for all-steel piers.
Alaska DOT&PF, consultants, and contractors benefit from the all-steel option. Testing identified previous designs problems and made required standards more reliable and cost-beneficial. New designs require minimal cast-in-place concrete, labor, and shipping costs—especially valuable in remote locations where these costs soar. Also, new designs exhibit quickened assembly time that reduces traffic delays. Most importantly, they are more reliable and ductile.
NCSU also looked at designs of steel pipe piles filled with reinforced concrete. Steel pipes can serve as the formwork and permanent casing for columns in a variety of ground conditions. They offer lower levels of environmental impact than conventional foundations. This type of bridge system also exhibits high ductility. This ongoing study is identifying materials strains at various performance limit states for more efficient and better performing designs.
Other projects investigate the seismic demand put on Alaska’s infrastructure. The demand, or load, is the force acting upon a structure. For example, what happens to a bridge foundation during an earthquake if it is built on a frozen crust of ground on a layer of liquefiable soil?
University of Alaska Fairbanks Civil Engineering Professor Dr. Leroy Hulsey, University of Alaska Anchorage Associate Professor of Engineering Dr. Joey Yang, and a team at UAA managed five overlapping projects addressing soil liquefaction (which occurs when an earthquake causes soil to lose its strength and behave like a liquid) and the frozen soil impact on bridge substructures. They also examined the mechanical properties of naturally frozen soils.
They looked at how temperature cycles, soil liquefaction, and frozen soil impact the seismic demand on structures. One study offers the first quantified evaluation of loads imposed on bridge foundations by a frozen crust with liquefaction and lateral spreading. The team validated a computer model that simulates a pile foundation’s response to seismic events in arctic conditions. Results show that pile performance is very sensitive to crust conditions, and the pile’s internal forces like bending moment and shear force vary by roughly 50% when the crust freezes. The team expanded these experiments with partners at China’s University of Science and Technology Beijing, utilizing a large-scale shake-table testing facility.
A previous project combined seismic data from bridge sites with computer models to assess the ground shaking intensity highway bridges experience during an earthquake—specifically bridges built on permanently and seasonally frozen ground. The study showed frozen soils, especially on permafrost, significantly change ground motion characteristics. Results indicate it is generally safe for designers to disregard the effects of seasonally frozen ground on site response. The study also shows it is imprudent to classify permafrost soil sites using only the seismic motion of the upper 30 meters of frozen or unfrozen soil, or to utilize code-defined site coefficients for seismic design.
These projects also advanced software-assisted analytical capabilities. Bridge designers use software-enabled moment curvature and “pushover” analysis to examine concrete filled steel pipe pile piers. Projects at UAA, NC State, and the University of Science and Technology Beijing had major software components. AUTC partners at Oregon State University also built upon previously developed structural earthquake engineering software to create and test new design software for cold-temperature tests on concrete and steel columns, enabling more accurate and time-efficient analysis.
Implementing the Results: National and State Standards
AASHTO’s seismic bridge design guide (2011 AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Edition) governs aspects of seismic bridge design. Findings from this suite of projects have been integrated into and sections 7 and 8, addressing structural steel and reinforced concrete components. They include language on several specific components, such as the mechanism for calculating the strength capacity of concrete filled steel pipes, and the design of column-to-beam joints. At the state level, Alaska’s seismic bridge design protocol is being rewritten to include these advances through Alaska DOT&PF.
By enhancing these design guides with research developed in the nation’s coldest and most seismic design environment—Alaska—DOT’s nationwide save potentially hundreds of millions through improved bridge design. These cost savings include prevented bridge failure, improved maintenance, replacement, retrofit, and other costs associated with bridge management. Most importantly, public safety will benefit as well.
This story originally appeared in AUTC’s most recent quarterly report. Read about this and other projects by visiting AUTC’s publication page, here.