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General Description
Section 1
1-82
usually mass motion in other directions at some or all joints and
these mass directions (“loads” in weight units) must be entered to
be correct. Joint moments that are entered will be considered to be
weight moment of inertias (force-length
2
units).
Please enter selfweight, joint and element loadings in global
directions with the same sign as much as possible so that the
“masses” do not cancel each other.
Member/Element loadings may also be used to generate joint
translational masses. Member end joint moments that are
generated by the member loading (including concentrated
moments) are discarded as irrelevant to dynamics. Enter mass
moments of inertia, if needed, at the joints as joint moments.
STAAD uses a diagonal mass matrix of 6 lumped mass equations
per joint. The selfweight or uniformly loaded member is lumped
50% to each end joint without rotational mass moments of inertia.
The other element types are integrated but roughly speaking the
weight is distributed equally amongst the joints of the element.
The members/elements of finite element theory are simple
mathematical representations of deformation meant to apply over a
small region. The FEA procedures will converge if you subdivide
the elements and rerun; then subdivide the elements that have
significantly changed results and rerun; etc. until the key results
are converged to the accuracy needed.
An example of a simple beam problem that needs to subdivide real
members to better represent the mass distribution (and the dynamic
response and the force distribution response along members) is a
simple floor beam between 2 columns will put all of the mass on
the column joints. In this example, a vertical ground motion will
not bend the beam even if there is a concentrated force (mass) at
mid span.