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Re: Seismic Lateral Load

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Geoffrey G. Melchor wrote:
> I am designing a building and this is my first time to use a 3D Frame
> Analysis.  Is anybody wants to answer my question, Do I have to apply
> the Fx (determined node load)to all joints at the same level or divide
> the Fx by the number of joints on that same level.
> Thanks in advance
> Geoffrey Melchor

Geoffrey, you mentioned Fx being the lateral load at the specific node,
but also mention dividing the load by the number of nodes at that level.
It sounds like Fx is the load at that level if so.  This is a method I
use sometimes. Please verify the results to feel confident about the

Assuming that your force Fx is the seismic force applied at level x and
further more that your 3-D model consists of frame elements located
geometrically correct.  For a diaphragm that lies in a plane, you can
assign a node representing the center of mass of level x in the plane of
the diaphragm.  Release the node so that it can spin in the plane of the
diaphragm.  From this node, extend members (spokes) to the corners or
joints of each frame at the level x diaphragm and frame intersection. 
For example; an "x"-brace would have a spoke connected at each
brace/floor beam joint namely 2 spokes each frame at each level.  The
spokes must have the following section properties; a large area, a very
large moment of inertia about a line perpendicular to the diaphragm, a
medium to low value of torsional rigidity and a medium moment of inertia
about the plane of the diaphragm.  Assign a low density if you have
"gravity loads toggled on", and a high modulus of elasticity to the
spokes.  Release the spoke ends at the frame joints to allow no moment
normal to the plane of the diaphragm.

	Apply Fx to the node and a moment representing the accidental torsion. 
If you would like, you can model the edge of the building in the same
manner with spokes and a frame on sliders to easier determine the
translation and rotation of the building corners when checking torsional

	You can then adjust the framing member sizes to theoretically better
the balance of the torsional rigidity if you desire. This method of
spokes gives a fairly good representation of a rigid diphragm and can be
used on fairly square semi-rigid diaphragms.  Be aware that large
openings in the diaphragm may have effects not represented by this

I Hope this helps,

Kenneth G. Penney PE