Improving Bridge Performance with Finite Element Analysis
Finite element analysis enables better bridge performance from the ground up.
Historically, most large bridges are “overdesigned” with substantial margins of safety built in to compensate for unknown forces that could affect their integrity over time. For the reliability, maintenance, and economic viability of the bridges of the future, better performance from the ground up is critical.
Assistant professor of civil and environmental engineering Daniel Linzell and his research group at the Pennsylvania State Univ., University Park, employ advanced finite element analysis (FEA) to create computer models for studying the structural behavior of bridges. Using this technology, Linzell and his graduate students are able to focus on potential trouble spots in individual bridges, helping civil engineers anticipate problems and make adjustments before construction begins. The simulation results can also be used to make decisions about maintenance requirements.
A. This view from beneath a new bridge under construction shows pre-stressed concrete beams and deck at the abutments. B. The corner of bridge superstructure at abutment is modeled in the software. C. The detail of bridge deck, diaphragm, and beams at abutment are shown using Abaqus 3-D brick elements used to predict the structural response. The elements mathematically represent structural components and are linked together by nodes to form the model. Images: D. Linzell, Pennsylvania State Univ.
“Civil engineering has until very recently relied on linearly elastic, small deflection FEA methods used in software tools as the backbone of bridge analysis,” says Linzell. However, this method is, in many instances, an approximation and doesn’t capture the full range of real-world nonlinear responses in these increasingly larger and more complicated structures. Higher-order methods found in advanced FEA software are becoming more commonplace in the industry. They provide the capability to incorporate nonlinearities to account for realistic stresses and deformations that will influence the performance and service life of a bridge.
Aspects that can be incorporated into an advanced FEA model in addition to material and geometric nonlinearities to assess the structural integrity of a bridge include the response of concrete or steel to the weight of the bridge itself as well as to traffic, wind, water, temperature fluctuation, corrosion, and even time.
Economics is another driving force behind the need for more sophisticated analysis tools, Linzell says. “Extreme over-designing has become too expensive. There’s now a strong push to minimize material costs and simplify design to reduce labor.”
Lighter, stronger materials are being developed: steel that is available now has yield stresses of 100 ksi (100,000 psi), almost three times what it was just 10-15 years ago. But while stronger steel allows builders to use smaller sections to support the same bridge loads, the new materials may also be more flexible. “You need higher-order tools to better predict such nonlinear geometric deformation,” Linzell points out.
How to build a virtual bridge
Linzell’s group uses Abaqus/Standard software from SIMULIA, Providence, R.I., Dassault Systèmes’ brand for realistic simulation, to create these virtual bridges. First they build a numerical model based on an existing design. They can work with designs that are still on paper or in CAD (computer aided design) format, or use as-built measurements taken from a structure already under construction.
They next select elements, the tiny geometric shapes mathematically representing physical units that are linked by nodes to form a numerical model and then material models. “You choose elements depending on available material, (constitutive) models, and geometry, then select what’s best for the materials being used, such as concrete or steel,” says Linzell.
Next they set up the boundary conditions for the model. “We use Abaqus a lot in this stage,” he says. “We select how the bridge is going to be restrained, whether we are going to utilize a contact condition or a discrete restraint, and how friction will be represented, for example.”
Finally, Linzell’s group applies various loads to the parts of the bridge. “This is a fairly prescribed process, but it depends on what you are looking at–such as traffic loads or the weight of the structure itself,” he says. The Abaqus creep module is used for time-dependent factors. “Creep is a big issue with concrete, and similar time-dependant effects influence steel behavior as well. Thermal loads are important, too.”
Depending on where loads prove to be excessive, the bridge model and, ultimately, the performance of the actual bridge can be modified. The process can be repeated until the optimum configuration for the bridge is reached. “There’s a lot that goes into modeling how a massive, highly indeterminate structure like a bridge is going to respond,” Linzell notes. “Abaqus helps us get our bridge models as accurate as possible.”
Real-world applications
The software has other application potential in the field of bridge analysis besides designing and testing new structures, according to SIMULIA senior engineer Deepak Datye. “Abaqus can be used to evaluate the residual life of a damaged structure which is still standing but may be cracked. And it can also be used for forensic purposes, to help pinpoint the reason for a collapse.”
Linzell sees nonlinear FEA playing an increasingly important role in building better bridges for the future. “We are hoping to come up with unified FEA guidelines for bridges because our industry really doesn’t have a unified publication yet,” says Linzell. “Other disciplines like aerospace engineering, and to some extent mechanical engineering, already do, so we’re trying to initiate that process.”
—Martha Walz
Resources
• Pennsylvania State Univ., University Park,
814-865-4700, www.psu.edu
