S2 Unit D Structure & Forces Presentation PDF

2.1 Measuring Forces The actual effect of a force on a structure depends on three things: the magnitude, or size, of the force the direction of the force the location where the force is applied 1 THE NEWTON (N) NEWTON: Unit of Force is referred to as a NEWTON (N) 1 Newton (N) = amount of force neede...

2.1 Measuring Forces The actual effect of a force on a structure depends on three things: the magnitude, or size, of the force the direction of the force the location where the force is applied 1 THE NEWTON (N) NEWTON: Unit of Force is referred to as a NEWTON (N) 1 Newton (N) = amount of force needed to hold up a mass of 100 grams (g) Named after Isaac Newton. 2 2.2 EXTERNAL FORCES ACTING ON STRUCTURES EXTERNAL FORCE: A force is a push or pull that tends to cause an object to change its movement or shape. In order for an object to remain standing it must resist gravity. MASS: Mass is the amount of matter in an object. The more mass an object has, the greater the gravitational force. Weight is the pull of gravity on mass. 3 An external force is a force that is applied on a structure by something else. Center of Gravity Scientists have discovered that even though gravity acts on all parts of a structure, there is a point where we can think of the downward force of gravity acting on a structure. That imaginary point is called the centre of gravity. The main method of increasing a structure’s stability is to increase the width of its base relative to its height. One way to do this is to place most of the mass of the structure close to the ground. This lowers the centre of gravity. 4 CENTER of GRAVITY: Scientists have discovered that even though gravity acts on all parts of a structure, there is a point where we can think of the downward force of gravity acting on a structure. The main method of increasing a structure’s stability is to increase the width of its base relative to its height. One way to do this is to place most of the mass of the structure close to the ground. This lowers the centre of When a structure is supported at its centre of gravity gravity, it will stay balanced. Therefore, the location of the centre of gravity of a structure determines the structure’s stability. Center of gravity simulator 5 SYMMETRY Symmetry is a balanced arrangement of mass that occurs on opposite sides of a line or plane, or around a centre or axis. The force of gravity on either side of the centre point of the ruler (where the finger is supporting it) is the same. For a symmetrical structure to be stable, its mass must be distributed equally around the centre of the structure’s base. This means that the force of gravity around the centre is also equal, making the structure stable. 6 For a symmetrical structure to be stable, the mass of that structure must be distributed equally around the centre of the structure’s base, meaning force of gravity around the centre is also equal. 7 Load A load is an external force on a structure. The weight of a structure—and the non-moving load it supports—is called the static load. A dynamic load is an external force that moves or changes with time. Types of dynamic load include people, traffic, earthquakes, wind, waves, and blasts. Any structure can be subjected to dynamic loading and the changes that come with a dynamic load can be random, periodic or a combination of the two. 8 An elevator is an example where static loading occurs. When ten people stand in an elevator waiting for the doors to close, they are exerting a load on it that is static because the people and the elevator are not moving relative to each other. STATIC and DYNAMIC LOADS When structures are built the idea of having weight put on them is something every engineer or designer must think about. LOAD: External force on a structure. 9 STATIC and DYNAMIC LOADS 1. STATIC Weight of a structure and the non-moving load it supports. Example: books on a bookshelf 2. DYNAMIC External force that moves or changes with time. Example: moving students on a staircase at school 10 REQUIREMENTS FOR A STRUCTURE How well will a structure hold up under the load it was designed to carry out? Important for safety, cost and efficiency-Performance Requirements should always be expressed in Maximum Weight 11 SUPPORTING THE LOAD Engineers need two conditions to decide what type of bridge is suitable: 1. What the bridge is crossing (water/land) 2. What kinds of loads the bridge will be supporting 12 TYPES OF BRIDGES 1. Truss Bridge Is a lightweight but strong bridge, made of trusses (triangle-shaped frames) along its sides. 13 TYPES OF BRIDGES 2. Suspension Bridge 14 2.3 INTERNAL FORCES WITHIN STRUCTURES Internal Force: An internal force is a force that one part of a structure exerts on other parts of the same structure. In other words, internal forces are forces that act within a structure. 15 Compression: Is a force that acts to squeeze an object or push parts within an object together. Tension: Is a force that acts to stretch and pull apart something. Shear: Is a force that acts to push parts that are in contact with each other in opposite directions 16 Complementary Forces: When different kinds of internal forces act on a structure at the same time. Bending is one example. 17 2.4 Designing Structures to Resist Forces and Maintain Stability 18 Arch An arch is a common shape in structures such as bridges. The arch can support a large load because the force of the load is carried down through the arch to the foundation. This spreads out the load. 19 Beam A beam is a flat structure supported at both ends. I-Beam: Have less mass and make it stronger. Girder (box beam): are long beams in the shape of hollow rectangular prisms 20 Truss A truss is a framework of beams joined together. Trusses are usually in the form of interlocking triangles Cantilever A cantilever is a beam that is supported only at one end. When weight is placed on the beam, the beam bends in an N-shape to resist the load. 21 Column A column is a solid structure that can stand by itself. Columns can be used to support beams. 22 Structural Stress, Fatigue or Failure Sometimes too great a combination of external and internal forces acting on a structure can weaken it. The result can be structural stress. A strong, stable structure is designed and built to be able to resist stress without any damage happening. However, repeated abnormal use of the structure could cause structural fatigue. This is a permanent change in a structure caused by internal forces such as compression, tension, and shear. Structural failure, such as the collapse of a bridge, occurs when a structure can no longer stand up to the forces acting on it. 23