# Forces

In the analysis of the structure and function of a building, forces are studied to achieve equilibrium. Magnitude or size, direction, and point of application of forces are all equalized data of essential balance. Forces can be uniformly applied like a wind load on a wall or a snow load on a roof. They can also be applied at points like columns on concrete foundations. Forces may be parallel or axial to the structure like the load from a beam from above onto a column or parallel and separated by a distance such as the central loading of a beam on two supports. Forces all subject materials to a wide range of stresses which in turn are factors in the selection of appropriate structural systems and building materials.

## Activity 1 – Three Laws of Statics

Statics is defined as the balance of internal and external forces in a structure such that there is no change in momentum; the opposite study of changes in momentum is known as dynamics.
Under constant force, a body moves in a constant direction relative to the force applied. When force is applied, the greater the force, the greater the speed. If a body is in equilibrium, the force of gravity equals the force holding it at rest.

Draw a body with gravity applied and the equal and opposite force supporting it at rest.

## Activity 2 – Laws of Equilibrium

Three Laws of Equilibrium Equilibrium is defined as a state of balance of forces. The algebraic sum of all horizontal forces must equal zero. The algebraic sum of all vertical forces must equal zero. The algebraic sum of all the moments of forces about any point must equal zero.

With balance and statics in mind, let’s take a look at some key forces.

## Activity 3 – Types of Forces

TENSION Tension is the force of pulling or stretching apart materials. Cables in suspension bridges are good examples of material in tension.

COMPRESSION Compression is the opposite of tension in that it involves pushing or squeezing a material together. Load-bearing columns are in compression.

BENDING Horizontal load-bearing members tend to deflect, or bend. The top of the member shortens or compresses while the lower surface of the member pulls apart of lengthens in tension. This oppositional stressing of the top and bottom of beams is common.

SHEAR A slipping tendency along a plane in a material caused by parallel but opposite forces applied simultaneously to two sides of a plane is known as shear. Both tensile and compressive stresses are produced at right angles to each other cracking or splitting the material. Loads tend to cause shear at the intersection of a beam downward at its supporting column.

DEFORMATION Every material lengthens or shortens (deforms) with respect to the load being applied. The deformation or strain is directly related to the amount of stress. All buildings (and materials, are subject to deformation but are designed to allow for it.

Draw sectional views of a simple beam on two columns and record and label the different types of forces and their location when loads are applied and upload it to the gallery!

## Activity 4 – Building Loads

Every building supports two types of Loads- Static Loads and Dynamic loads. Static loads are stationary and can be easily anticipated (machinery, occupants, materials). Dynamic loads are moving and can vary in size and duration (snow, wind, earthquake). To simplify the structural analysis, these forces are averaged and converted to a static equivalent. Building codes and national manuals are responsible for minimum figures of structural design based on the net effect of the maximum expected combination of static and dynamic forces. Make a chart that lists Live Loads and Dead Loads.

## Activity 6 – Live Loads

Live loads include all loads other than the actual weight of the building structure. They vary as the loads applied change over time, with use, and from situation to situation. Snow, rain, wind, earthquakes, occupants, machinery, and furniture are examples of live loads. Movable loads such as people, equipment and partitions are uniformly distributed on the structure. Wind load is unusually difficult to calculate because it involves wind velocity, building orientation, surface texture, height, and shape; It creates both pressure and suction on any structure. A building’s height and weight are a few of the variables in seismic design which makes designing structures to resist damage to earthquakes so difficult. In which city are tall buildings built on a ball foundation to allow to move in an earthquake?

## Activity 7 – Dead and Live Load and Under, In, On and Above Construction

Dead and Live Loads of the most extreme are totaled in the design of the support system of a building. The material(s) chosen, the type(s) of structural systems utilized, and the conditions of the site determine the structural configuration of each building. This is why the location of a building- its biome, climate, precipitation, wind force, susceptibility to earth quake, floods, snow loads, etc. became very instrumental in dictating how the building is designed and connected to the land. Which leaning tower in Italy is a classic example of the land not being able to hold the weight of the building in equilibrium. Building collapses, are devastating occurrences of failure of building equilibrium.

## Review

• **Elastic Behavior** is the tendency for a material to return to its original shape and size, without loss of dimension when load is removed. (Think like a rubber band!)
• **Plastic Behavior** is the tendency for a material to become permanently deformed in shape and size when loading is removed.
• The **Yield Point** is the level of stress at which a material crushes, snaps, or loses its elasticity.
• **Tension** is greatest at the top face of the beam and decreases toward the neutral or central axis.
• **Compression** is greatest on the bottom face of the beam and decreases toward the neutral or central axis.