Structure – Lesson 1
Fundamentals of Structure
Objectives of today’s lesson. What is a Structure? Examples? Why is there a need for manufactured structures? What are natural and man-made structures? What are the four functions of structures? What are the types of forces that a structure might experience? What are the effects of forces on structures?
Structures are all around us •Structures come in all shapes and sizes. •Structures can either be man-made or natural •We need to manufacture structures to satisfy many functions.
On 15 Mar 86 at around 11.25 a.m, Hotel New World, situated at the junction of Serangoon Road and Owen Road, collapsed. By 11.26 a.m, when the police received a call from the public, the Hotel was already a twisted pile of rubble. The collapse of Hotel New World, was Singapore's worst post-war disaster.
WHY DID THE HOTEL COLLAPSE? A Commission of Inquiry was set up to investigate the cause of the unprecedented collapse. It was believed that underdesign, shoddy workmanship and corner cutting of the structures by the contractors were responsible for the sudden collapse of the building. But it was later discovered that overloading of the building was the reason for the disaster.
Structures
Many people only consider buildings, bridges and other such frames as structures.
However there are many simple objects which can be classed as structures such as a cabinet or even the humble school chair. So what is the definition of a structure? Anything that supports a load is considered as a structure. These load include the structure’s self-weight as well as other weight imposed on it.
Structures ďƒ˜
Classified into two main types
Structures
Frame structures
Shell structures
More examples of frame and shell structures Natural Structures
More examples of frame and shell structures Engineered Structures
More examples of frame and shell structures Frame structures
Shell structures
Functions of Structures What is the purpose of having a structure?
Supporting
Spanning
Basic functions of structure:
Containing
Protecting
Supporting
Containing
Spanning
Protecting
Structures ďƒ˜
Structures are designed to resist the loads and forces that they are subjected to. Structures must be able to support or carry the loads (forces) for which they are designed; 1. They must not fail or collapse during use or under extreme circumstances, such as earthquakes (strength, stability). 2. The structural components of structures must maintain the correct relative positions during use (stable). (e.g., minimal deflection of floors in multistory buildings — a floor can substantially deflect while maintaining its strength).
LOADS I)
Static loads - A force that has a constant magnitude, position and direction on or within a structure.
II)
Dynamic Loads - A force that changes magnitude, position or direction on a structure.
Forces ďƒ˜ A structure must be able to withstand all the force that it is subjected to when loaded. ďƒ˜ The five main types of forces are : Compression, Tension, Bending, Shear and Torsion - Lets find out more about them from the website below. Link 2
http://www.pbs.org/wgbh/buildingbig/lab/forces. html
Effects of forces on structures ďƒ˜ Tensional Forces In many structures thin members or cables are used to resist tension. An example of this are the ropes or chains used on a children's swing in the park.
ďƒ˜ Compressive Force The inspection platform which is used in many garages allows the mechanic to inspect the under-side of a vehicle The weight of the vehicle on the platform acts on the ram causing it to be subjected to a compressive force. If the member is too thin it may buckle and break.
ďƒ˜ Wires, cables and thin (slender) sections are used for members in a structure that are subjected to tension.
ďƒ˜ Thick, stocky or special sections are used for members in a structure that are subjected to compression. A design that went wrong!!!
Summary 1. Structures are all around us. 2. Structures can be natural or man-made. 3. Structures can be classified as frame structures or shell structures. 4. The four functions of a structure. 5. The type of forces that effects a structure. Enjoy this!!
Strengthening structures. A diagram consisting of frames is set up as shown. When a horizontal force is applied at d can you see what happens? Do you think the structure is stable? Can you think of some ways to strengthen the structure.
Lets look at two ways of strengthening the structure. ďƒ˜ 1st way (Bracing) d
c
When a third member is added across ac, the square is split into two triangles.
Now if the same force is applied at d, the structure becomes rigid. The bracing member prevents a and c from moving apart as in the first case.
ďƒ˜ 2nd Way Add a gusset plates to the structure as indicated. The gusset plates strengthens the corners of the frame. When a pushing force is applied, the frame again remains rigid as the gusset plates prevent a and c from moving.
Strength of structures ďƒ˜ Shape of a structure can affect the strength of the structure. Lets take a look at three different shapes: a rectangle, arch and a triangle. ďƒ˜From the weblink given below, lets find out: I) How forces acting on them affects the structure and II) How to strengthen the structures. Link 3 http://www.pbs.org/wgbh/buildingbig/lab/shapes.html
Type of material ďƒ˜The materials used in the construction of a structure largely determine its performance. ďƒ˜ So it is very important for you to know how materials react when they undergo forces.
Lets find out from the weblink below. Link 4 http://www.pbs.org/wgbh/buildingbig/lab/materials.html