2. Compression and Tension Shear Tension Compression Bending Torsion

There are five stresses or ways in which materials react to the forces or loads placed upon them. They are compression, tension, shear, torsion, and bending. These stresses may also act in combination with each other.

Compression is the tendency to condense or squash a material Compression is the tendency to condense or squash a material. Pushing is the essential feature of compression.

Consider a brick wall: each brick is compressed by the bricks above pushing down under their own weight. If the wall holds up a roof, then the roof is also pushing down on the bricks.

Tension is the tendency for a material to stretch, or to be pulled apart. Pulling is the essential feature of tension.

A crane lifts heavy loads by using a steel cable under tension A crane lifts heavy loads by using a steel cable under tension. The cable lifts the load up by pulling on its support.

Compression and tension can work in opposite ways to achieve the same result. For example, a plant pot can be hung from a rope in tension or it can be propped up by a column in compression. The result is the same, the plant is supported.

However there are differences However there are differences. The system using tension has the advantage of weighting less. A lighter structure does less work holding itself up. But nothing is free. A structure in tension is less stable than a structure in compression. For example, tension structures move more readily in a strong wind. As structure in tension can be affected more by live loads and in a more dramatic way.

The part of the structure that carries the load in tension is called a tie… …and the part that carries the load in compression is called a strut.

Shear The third phenomenon is shear. Shear is the tendency for a material to be divided by two opposing forces. In shear, one part of the solid slides past the other part.

Sliding is the essential feature of shear Sliding is the essential feature of shear. Shear can also be described as compression and tension at right angles to each other.

It is important to understand how the internal components of a structure resist shear. Shear forces are common in all structural systems.

Bolts, rivets, and welds are usually subject to shear forces Bolts, rivets, and welds are usually subject to shear forces. A bolt in double shear is stronger than a bolt in single shear.

Torsion or torque is another result of the forces acting on a structure. It is the tendency of a material to be twisted. Torsion too can be explained as compression and tension. If a grid of squares is drawn on a cylinder and the cylinder is twisted, the squares will be distorted into parallelograms.

Torsion is often the most difficult stress to deal with Torsion is often the most difficult stress to deal with. For example, it took a great amount of time, research, and testing to develop a safe monoplane near the end of World War I. The wings of the early designs had a tendency to twist off when the plane came out of a dive.

The last stress to be considered is deflection or bending. When a load is placed on the center of a horizontal beam resting on two supports, the beam will bend downwards. This is called sagging. The top surface of the beam is compressed and the bottom surface is stretched in tension.

Hogging occurs when the support is in the center of the beam and the loads are placed on the ends. Now the top of the beam is in tension and the bottom is in compression.

Bending can cause a side effect that creates an additional force called bending moment or lever arm.

Structural Failures The five ways in which structural members react to the loads placed upon them are compression, tension, shear, torsion and bending.

These stresses may also act in combination with each other These stresses may also act in combination with each other. When the limits of stress and strain are surpassed within a structure, it will fail. Let’s examine some of the ways in which bridges may fail if they are not designed to withstand the loads placed upon them.

First, bridges are subject to bending First, bridges are subject to bending. Usually, bending creates compression on the top and tension on the bottom.

The appearance of the break often shows whether the failure was because of compression or tension. Bridges are subject to shear especially at or near the supports.

Sometimes the bridge piers will crumble due to the compression forces applied to them.

Bridges that use cables will sag when the cables stretch due to the tension forces within them.

Uneven traffic on both sides of a bridge subjects the bridge to torsion or twisting.

How tall can a structure be made before it tips over How tall can a structure be made before it tips over? Consider the Tower of Babel. A tower with vertical walls can be built about two km. high before the weight of the stones above crush the stones at the bottom.

A tower with battered (tapered) walls can be built much higher A tower with battered (tapered) walls can be built much higher. The Egyptians built their towers (pyramids) with battered walls.

Mt. Everest ( a natural tapered wall pyramid) is about eight km Mt. Everest ( a natural tapered wall pyramid) is about eight km. high and shows no sign of collapsing.

If using battered walls is the key to building tall structures, the builders of ancient times would run out of oxygen long before the Tower of Babel ever collapsed. Why then did the ancients fail in their quest for “a tower with its top in the heavens?”

Why is it that even today the tallest buildings are just over ½ km tall? They are even built from steel and concrete which is vastly superior to the stone structures of the past.

You knew this as a child playing with wooden building blocks You knew this as a child playing with wooden building blocks. The structure always tipped over long before the compression strength of the blocks was reached. You are about to learn why.

Thrust lines Engineers must contend with forces other than just the weight of the blocks above the ones below. The problem with tall structures is they are not stable.

Thrust lines are the imaginary lines that forces, caused by loads follow, as they are transmitted through the system to the ground.

For example, the thrust line passes straight down the center of a pillar with vertical walls. The thrust lines may be moved off center by live loads such as wind, or by not having the weight of the structure evenly distributed.

When the thrust lines wanders outside the middle of a tower it becomes unstable. In fact, if the thrust line wanders away from the middle 1/3, the structure tips over.

Tall buildings such as the CN Tower, will sway more than one meter in a strong wind. The limiting factor in the height of a structure is the location of the thrust line, not the strength of the material.

Many structures use beams as the components that transmit these forces to the ground … … other structures carry the load in tension.

By understanding compression, tension, shear, torsion and bending one can begin to visualize how a structure will react under load.

Last card on the Compression and Tension tour. You have completed the Compression and Tension tour. You can go over the information again and study it some more or if you feel you know the information well enough you can go and get the Compression and Tension Quiz from Mr. Leidl.