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:
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.
|If I were to stick a pin in the middle of the wall,
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
|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.