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• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: Seismic Lateral Load
• From: "Geoffrey G. Melchor" <melbec(--nospam--at)saipan.com>
• Date: Sat, 02 May 1998 15:51:55 -0700

```Ken and Jane Kukes Penney wrote:
>
Thanks Ken and Jane,  this will helps me and some of my future works.  I
appreciated it.

Geoffrey

> 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
> model.
>
> 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
> irregularity.
>
>         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
> method.
>
> I Hope this helps,
>
> Kenneth G. Penney PE
>

```