By chance, the announcement
came as FEA and CAD vendors were
converging on SolidWorks World, a
major trade show. FEA experts at the
show had plenty to say about the federal
findings. Their conclusion: Had
FEA been around when the bridge
was built, it would have caught the
errors that seem to have lead to the
bridge’s collapse.
The I-35 bridge, a truss design, consisted
of steel beams connected to each other
at nodes or joints by so-called gussets, basically
steel plates bolted or riveted to hold the
beams together. Federal officials say several
gussets fractured, most likely because they
were too thin, only 0.5-in. thick when they
needed to be 1 in. The deck, though probably
designed with a certain safety factor, carried
tons of construction equipment on the day of the collapse. So loads were higher than normal.
Also, the structure had a nonredundant
design, which means a single component failure
could bring down the entire structure.
The original bridge calculations were evidently
lost, making it hard to tell whether the
undersized gussets resulted from a drafting
mistake or a calculation error. Experts claim
newer FEA programs help eliminate such mistakes. Structural and multiphysics
analyses take place before any concrete
is poured or beams get bolted,
thus catching design flaws early.
“The first goal of structural analysis
is to size beams correctly,” says
Suchit Jain, vice president of strategy at SolidWorks Corp., Concord,
Mass. “FEA software lets engineers
account for various loads affecting
the whole structure. For example,
the steel itself has weight, and so it is
a load. And there are direct mechanical
loads such as vehicles, side loads
from wind, and seismic loads from earthquakes. And on a smaller scale,
‘microseismic loads’ go into vibration
and dynamic analysis.”
After sizing the beams, engineers
analyze individual joints to predict if
they will withstand operating stresses,
says Jain. “Modern FEA software
lets users see how structures weaken
over time. Engineers can also ensure
resonances will not come into play, as
happened with the Tacoma Narrows
bridge failure.”
Bridges today are much more flexible
than those of the past, says Comsol
Branch Manager John Dunec.
“Better bridge materials let designers
make bridges longer and lighter, with
thinner cross sections, but they require
large deformation analysis,” he
says. “Fortunately, current software
performs nonlinear single and multiphysics
analyses, the kind of analyses
this task requires.”
For example, in this age of terrorist
attacks, it’s necessary to predict what
would happen should a large truck
or ship hit a bridge and spill corrosive
chemicals. “So, variables now include
static loading, impact loading, flow,
chemical diffusion, and reaction processes
weakening the bridge, among
many others. This is a problem multiphysics
software solves using partial
differential equations and matrix
math,” he says.
It’s possible to model an entire
bridge. And for some analyses, that
might be necessary, says Dunec.
“Needless to say, such models have
millions of degrees of freedom and
require lots of computational power.
But it lets analysts see where all the
loads are distributed, then hone in to
the component level to perform detailed
studies on parts of the bridge. If
the 1-35W designers had used modern
advanced FEA, they probably
would have found the gusset problem
before the bridge was built.”
Check out this Flickr link to see an
image of a real gusset plate: http://www.flickr.com/photos/80651083@N00/984752381/in/pool-35w-bridgedisaster