MDX Software - FAQ #20
Why do I get significantly different results using the same wheel distribution
on girders of a system that I use on a single girder of that system modeled as a
line girder project?
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The difference is due primarily to differences in ordinates of influence lines,
developed for the single girder, from ordinates of influence surfaces developed
for the system. A unit load is used in both cases, but the entire system across the
grid resists the unit load placed at a particular location on the system, whereas only
the single girder, modeled as a line girder project, resists the unit load placed at
a comparable location. Because multiple girders in the system are involved in the
resistance, influence ordinates over a girder in the system will be less than those
for that girder modeled as a line girder project.
In that respect it might then seem reasonable to apply a larger distribution to the
system for equivalent effects. This might be the case if only the girder for
which effects are being determined were the only girder loaded with the distribution
given for that girder. However, the influence surface for a particular effect
drapes over the entire grid, and there exists a counter argument that wheel distribution
given for all girders in the grid should be applied to the influence surface ordinates
at those other girders when determining a particular effect on the girder for which
live effects are being developed. This is the approach MDX employs by default.
Thus it would then seem that equivalent effects between a system model and a
line girder model would be obtained only by using a lower distribution value
for the system. But comparing system analysis modeling to line girder modeling has
a bit of the flavor of an apples and oranges comparison. Skew, curvature, and
short girder inserts are only a few of the factors that make it difficult to
determine an equivalent wheel distribution for a system which would produce results
comparable to that of a line girder model. Determining the equivalent wheel distribution
to be applied to the separate girders of a system is a task we have to leave to
the user exercising sound engineering judgement.
As an alternative for consideration, we have enabled a condition, LOCAL WHEEL
DISTRIBUTION ONLY, which causes only the girder for which live effects are being
determined to be loaded with the wheel distribution supplied for that girder. While
it is felt some rigor is sacrificed if this approach is used, it does reduce
considerably the compute time for a large system. In this case it would make sense
to use a larger distribution than that used for a line girder project because of
the differences in influence ordinates.
Whatever choice is made, the user can check results for a particular effect on
a particular girder by applying wheel distribution, for all girders loaded, to the
influence surface values at those loaded girders.