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Live Template 

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Modal timehistory
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analysis
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is
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wellsuited
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for
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the
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and
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analysis
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of
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humaninduced
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vibrations
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which
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result
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from
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such
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impacts
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as
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pedestrian
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footfalls.
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Guidelines
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are
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presented
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in
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this
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test
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problem.
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Please
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note
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that
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these
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parameters
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are
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not
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intended
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to
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be
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realistic.
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It
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is
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up
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to
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the
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engineer
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to
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determine
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load
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magnitude,
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stride
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length,
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load
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frequency,
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and
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other
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input
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values.
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Humaninduced
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vibrations
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may
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be
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modeled
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as
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follows:
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 Define
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 a
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 for
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 each
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 footfall
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 location,
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 and
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 for
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 convenience,
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 name
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 each
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 according
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 to
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 the
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 loading
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 sequence,
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 such
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 as
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 Step
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 1,
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 Step
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 2,
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 etc.
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 For
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 example,
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 100
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 footfall
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 locations
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 would
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 require
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 100
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 load
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 cases.
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 For each load case,
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 apply
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 a
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 point
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 load
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 in
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 the
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 location
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 of
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 the
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 corresponding
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 footfall.
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 It
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 may
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 be
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 convenient
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 to
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 use
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 a
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 unit
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 load,
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 then
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 adjust
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 magnitude
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 when
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 setting
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 the
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 scale
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 factor.
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 Define a single timehistory
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 function
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 which
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 will
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 represent
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 the
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 footfall
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 sequence.
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 If
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 a
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 unit
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 load
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 is
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 applied
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 to
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 each
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 load
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 case,
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 set
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 the
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 magnitude
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 in
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 the
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 function
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 definition.
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 Additional
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 functions
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 may
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 be
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 defined
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 to
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 consider
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 multiple
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 impulsefunction
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 shapes.
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 Define a timehistory
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 analysis
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 case
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 from
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 the
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 following
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 options:
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 Modal time history based on Eigen modes, where a sufficient number of modes should be captured for the given structure.
 Modal time history based on Ritz modes, which may be a better option as far as analysis goes, though Ritz formulation requires as many modes as there are footfall locations and load cases, since each position must be considered as a starting load vector. For example, 100 footfalls require 100 modes.
 Directintegration time history, which is not based on modes, but rather a stepbystep process. Direct integration typically demands more time, but if a large number of modes are necessary for modal analysis, direct integration may be more viable.
 Modal time history based on Eigen modes, where a sufficient number of modes should be captured for the given structure.
 Add each footfall load case to the Loads Applied section (Figure 1), and specify the impulse function, scale factor, and arrival time. The timing of load sequence should be carefully considered and calculated, since arrival time and the application of impact is critical to dynamic response. Ensure that sufficient time steps are provided in the Time Step Data section to cover the duration of the time history.
Figure 1  Loads Applied
See Also
 Powerspectraldensity FAQ article
Attachments
 SAP2000 V11.0.8 model (zipped .SDB file)