Facts and Forces

Lateral forces are generated in stationary buildings by accelerations resulting from ground movement and by external forces like wind and waves. Once these forces are determined, building designers track their path(s) through a structure until they are safely delivered to the ground at the bottom of the foundations. Building elements, both structural and non-structural, that are oriented parallel to the applied force can be subject to shear forces.

If we were to look at only one portion of a building and only one wall in that portion we would see that forces are delivered to the top of a shear wall and transferred to the bottom by means of:

  1. the shear-resisting material;
  2. the supporting framing;
  3. the connection between (1) and (2);
  4. and, if necessary, the anchorage of the support framing to other elements/framing.

These four components are consistent throughout all shear wall design.

The first step in understanding shear walls is identifying the forces that act on them.  Each force will require a different response on the designer's part to make sure the shear wall functions as expected.

The First Force Pair
Looking at our wall we see forces applied at the top
and resisted at the bottom. The applied force resulting
from wind or seismic activity is called "FORCE".  The
wall materials react to this applied force generating a
"RESISTANCE". Note the direction of the arrows.

Net Rotation
If I were to stick a pin in the middle of the wall, the force
arrows would cause it to spin in one direction. The
tendency of a shear wall to "spin" in this way under applied
loads is called overturning. The overturning moment
is the applied load times the distance from the bottom of
the wall.

Counterbalancing Forces
Statics requires that in order for a system such as this wall to be in balance,
opposite forces must be developed to oppose the first set of forces.

No Net Rotation
Once the counterbalancing forces are applied the
wall remains stationary. Shear wall design starts with
lateral forces and ends when all resulting forces are
accounted for by appropriate connections and members.

Unit Shear Forces

If we take the total lateral load (FORCE) and apply it over the width of the shear-resisting material we can determine the "unit" shear in the panel. Once the unit shear is determined, we can eliminate those materials that will not be adequate to the task. The table of allowable shears presented previously can be helpful here.

To explain the next topic we'll have to move into some schematics of shear walls. Please pardon the math.