A structure is a group of elements somehow united to support a load with stability.
Everything has a structure, otherwise it would collapse under the force of gravity, the force of wind, or any other force.
For example, humans have a skeleton, bicycles have a frame, houses have columns, beams and a roof.
Types of Structures
Frame Structures
The frame structure is the most common choice of structure.
These structures are composed of long elements (bars, tubes...) joined to each other at the ends.
The elements of frame structures are made of strong materials.
The space between the elements is empty space.
Advantages of frame structures: they are relatively easy to design and build, inexpensive to manufacture, lightweight and do not require much material.
Disadvantages: They are not good for very big loads or strong impacts.
Shell Structures
Shell structures are made of a thin outer layer of material that surrounds a volume. That volume can be empty space or can be filled with something that has no structural relevance.
Examples of natural shell structures: eggs, shellfish. . .
Examples of artificial shell structures: boxes, bottles, balloons, aeroplanes. . .
Arch
It was invented by the Romans.
There are many types, but the semicircular arch is the most used.
In Roman times, arches were made of stones. The stone at the very top is called the "keystone'' Its own weight is the main load and the geometry of the arch displaces the forces to the sides, so that a large free space is left under the arch.
This makes it suitable for bridges since the large space underneath can span a river or other obstacles.
Suspension structures
A suspension structure holds an element (often a beam) by cables that are held from the top of a tall column.
The space (distance) between one column and the next is called the "span''. This type of structure has the longest span of all structures.
Because of this, suspension structures are often best for bridges that need to cover big rivers or even small portions of the sea.
Advantage: good for covering a very large span.
Disadvantage: expensive to build.
Structural Elements
An element can be defined as "the simplest part of the whole".
In a structure, the forces for every element are calculated and the element (i.e. beam or column) is designed to withstand them.
In the drawing shown, the foundation is an element joined to the ground by friction.
The foundation is also joined to the column by friction.
The column is another element, joined to the foundation and to the primary beams.
The beams and the column are joined by welding if they are metallic.
In some cases the beam can be laid on the column (held by gravity).
Types of Unions
All structural elements must be somehow united to the other elements and the whole structure is often united to the floor/ground as well.
Permanent Unions
Only for structures that will not need to be disassembled. Examples: welding, rivets, glue...
Non-permanent Unions
Non-permanent unions are those designed to assemble and disassemble the elements of the structure as many times as necessary. Examples: nuts and bolts, clamps
Forces on structural elements
A successful structure must be able to withstand all the forces that it will experience without falling over or collapsing.
It is important to understand the kinds of forces that can act on and within the structure.
Each of the elements in the structure will withstand a particular force, which is a result of the load that the structure is supporting.
Tensional forces: Is the strain that causes an element to stretch. It can be due to forces that pull an element from its ends.
Compression forces: They are those forces which cause an element to get squeezed or buckled.
When you "squeeze'' an element, it becomes shorter and thicker.
Shear strain: Shear forces act across a material in such a way that they can force one part of an element to slide over another.
A good example is when you cut paper with a pair of scissors.
Torsion forces: If you apply a turning force (called "torque'') at one end of an element and, if the element is fixed to a support at the other end, the element twists.
Bending forces: Bending forces act at an angle to a member (at 90º from the axis line in the drawing), making it bend.
ACTIVITIES
Activity 1: Can you think of some famous structures? Name, draw and describe two famous structures. Think about a big structure near your town.
Activity 2: Go to the following web page and do some of the lesson starters, such as lc, 6, 10, 13 and 18.
Activity 3:Built a main structure with paper sticks that support a weight of 2 kg.
VIDEO
How to build a bridge in the sea:
VIDEO
How to build a bridge in the sea:
