Finite Element Analysis: A Powerful Tool for Structures
In today's world of high-end computers, the finite element method has
emerged as a powerful analysis tool for structural applications. The
method involves simulating the structure's behavior by building a computer
model and breaking down the structure into an assembly of finite-sized
elements. The behavior of the elements and the overall structure can
then be obtained by formulating a system of constitutive relationship
and algebraic equations that can be readily solved with computer processors.
"The power of the finite element analysis method lies in its versatility.
Its applications are unlimited," says Waider Wong of the Federal
Highway Administration's (FHWA) Eastern Resource Center (ERC).
The structural-related applications of the finite element analysis
(FEA) method include:
- Accurately assessing the reserve strength of structurally deficient
bridges.
- Evaluating significant cracking in concrete decks or steel girders.
- Performing analytical predictions of nonlinear bridge responses
and large deformations under loads.
- Refining analysis of live load distribution factors for the load
and resistance factor design method.
- Performing local stress analysis of skewed and curved bridges.
To define a good FEA model and generate useful results, experience,
good engineering judgment, and understanding of FEA computer software
capabilities are vital. The structural team at the ERC has specialized
training in the field of finite element modeling and now offers FEA
as a new service to its FHWA division offices and State highway agencies.
The ERC's structural team recently evaluated a series of bridge ramp
structures in Washington, DC, that had been built in the early 1970s.
Annual inspections had shown that numerous locations inside the box
cells had visual defects and discontinuities. Because of the complex
arrangement of the
structural components inside the multi-cell box girder, conventional
bridge analysis software was not able to capture the effects of localized
stress distribution. This analysis is important in determining the leading
cause of deficiencies that have been visually detected and revealing
problem areas that have not been discovered. The ERC team created a
finite element model of all of the components, using a program known
as MSC/NASTRAN. The analysis indicated that the bridge could experience
marginal stresses in certain components but not to the point that immediate
fractures or failures were anticipated. The stress zones were highlighted
in a full report, which recommended that the scope of repair could be
limited to these zones without compromising the functional safety of
the structure.
The ERC team was also called upon to assist with stress analysis of
fiber reinforced polymer (FRP) composite sandwich panels used on a deck
replacement project in Pennsylvania. The use of FRP composites, which
consist of glass fibers with thermoset resins, has been gaining acceptance
and popularity in the bridge construction industry due to the composites'
high strength, stiffness-to-weight ratio, and corrosion resistance.
Because of the complex behavior of the material properties, FEA is often
used to handle the stress analysis for FRP structures. Although in this
case the bridge manufacturer had produced an FEA report on the deck,
upon reviewing the report it was discovered that the longitudinal epoxy
joint used to connect two of the deck panels was missing from the original
FEA model. While failure of this joint would not cause immediate concern
with respect to the bridge's operational safety, its function does impact
live load distribution and deflection control on the bridge deck. The
ERC team was asked to create another full FEA model that included this
joint to study the joint's structural integrity and performance.
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| Finite element models, such as the ones shown above,
can simulate a structure's behavior. |
The team's concluding analysis indicated that the effectiveness of
this joint system significantly impacts the structure's load transfer
mechanism, which influences the deck panels' response to different loading
conditions. In a deck without any joint bonding, shear stresses come
close to reaching the design shear strength of the composite material
and total displacement is more than twice the desirable deflection limit.
In the case of the Pennsylvania bridge, stresses exceeded the design
strength but did not reach the failure strength of the joint. The ERC
team recommended that the State continue to monitor the joint performance
and look for any joint defects during future inspections.
The first nationwide FEA Applications in Infrastructure workshop is
currently being planned. For more information on the workshop, the ERC's
FEA services, or FEA in general, contact Waider Wong at the ERC, 410-962-9252
(fax: 410-962-4586; email: waider.wong@fhwa.dot.gov).
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Articles in this issue:
Avoiding Utility Delays: What Works
Finite Element Analysis: A Powerful Tool for Structures
FHWA Course Ushers in New Pavement Design Era
All HPC All the Time
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