Bridges bridge, span a structure that allows people or vehicles to cross an obstacle such as a river or canal or railway etc.span physics, the physical influence that produces a change in a physical quantity; (Force=mass x acceleration) Disaster, Something that may occur when the laws of physics are ignored.

Your Mission Over the next week you will have a chance to experiment with bridge designs in an attempt to span a gap and overcome the force of gravity while carrying a load. But first…let’s make some considerations.

Static vs. Dynamic: Static: Not moving. Static Load: Forces on a nonmoving structure. Dynamic: In motion or changing. Dynamic Load: Forces changing due to movement of structure. force

A little quiz: Identify the static and dynamic system.

Types of Forces continued Tension : A stretching force. Pulls molecules apart. Two people pulling put the rope in tension.

Types of Forces continued Compression : a pushing force. Compacts molecules. Ex: The weight lifter’s body is compressed by the barbell.

A little quiz: Tension or compression?

Types of Forces continued Bending : A structure subjected to bending is being stretched and compressed at the same time. Where are the tension and compression forces on this member? Tension Compression

Types of Forces continued Shear: A force resulting from two forces acting in opposite directions. A shear force is created where two opposite forces try to cut, tear or rip something.

Types of Forces continued Scissors, Another Example: The two handles put force in different directions on the pin. The force applied to the pin is a shear force.

Types of Forces continued Torsion : A turning or twisting force. EX: As ends of the plastic ruler are turned opposite directions, the ruler is said to be in a state of torsion.

Frame Structures A bridge is a type of frame structure. Frame structures are made from many small parts, joined together. Bridges cranes and parts of this oil rig are just a few examples.

Frame Sructures The different parts of a frame structure are called members. Each type of member has a different job to do in supporting the structure.

Structural Forces & Members Beam: A Beam is a piece of material supported at both ends. When a beam is loaded the top is compressed and the bottom is in tension. Compression Tension

Members Beams used in larger structures take many different forms, some are simply solid, some are hollow, and others have special cross- sections to provide strength and rigidity

Members Columns: The vertical pieces supporting a beam are called columns.

Members Cantilever : A cantilever is a beam which is supported at one end only. Cantilevers are used where it is not possible to have a support at both ends (a diving board for instance).

When a cantilever is loaded, the top surface is in tension and the bottom is in compression. Compression Tension

Structural Forces & Members Using a computer desk as an example different forces can be seen working PART A: Is in tension because the weight of the computer is stretching it. PART B: Is under compression because the weight above it is pushing downwards and compressing it. PART C and D: This is the same member but on the inside compression is taking place and on the outside it is being stretched (under tension).

Structural Forces & Members Tie: A member mostly under tension forces is called a tie.

Structural Forces & Members Strut: A member mostly experiencing compression is called a strut.

Structural Forces & Members The vertical column experiences both tension and compression.

More examples of struts and ties (WALL) The beam is held in position by a steel rod. The weight of the beam is stretching the rod (tensile force).

More examples of struts and ties (ROOF) The roof beams are under pressure from the weight of the tiles on the roof (compressive force). The floor beam is being stretched (tensile force).

More examples of struts and ties (FLAGPOLE) The wires on either side of the flagpole are being stretched (tensile force). Why is the pole under a compressive force ?

Making Structures Rigid When forces are applied to a simple four-sided structure it can be forced out of shape quite easily. A structure which behaves in this way is said to be non- rigid.

Making Structures Rigid By adding an extra member the corners are kept from moving apart. The structure cannot be forced out of shape, and is said to be rigid. Notice that the additional member has formed two triangles in the structure.

Making Structures Rigid Most frameworks are built using a combination of struts and ties to make triangles. Triangles make very strong and rigid structures. This is called triangulation

Making Structures Rigid An alternative to triangulation is to use a gusset plate. A gusset is a piece of material used to brace and join the members in a structure. A triangular gusset plate has been used here but they can be made in a variety of shapes.

A Bridge Example The bridge below is a common type called a Box Girder Bridge.

A Bridge Example Triangulation distributes the weight of any vehicle or pedestrian crossing the bridge. The weight is distributed through all the members which increases the load the bridge can hold.

A Bridge Example This type of bridge is favored by the Army. The Army Engineers have transportable bridges like the one that can be dismantled and transported anywhere in the world and reassembled. They are bolted together and are semi-permanent structures. They are useful for short spans.

Aspect Ratios Aspect Ratios are used to describe everything from television sets, to photographs, to aircraft wings, to movie projection formats, to tires, to basically anything that can be described as having a length and a width. Let’s take a look.

Aspect Ratio Graphics and photos are described in terms of aspect ratios. If a graphic has an aspect ratio of 2:1, it means that the length is twice as large as the height. The term is also used to describe the dimensions of a display resolution. For example, a resolution of 800x600 has an aspect ratio of 4:3 (or 1.33 to 1).

Aspect Ratios Basically, aspect ratio is a measure of how square something is. It is calculated by dividing length by width. 10 cm 2 cm AR=10:2 = 5:1 or just “5”

How can Aspect Ratio be applied to Bridges? Let’s do a demo shall we? What we should have found out. Short and Wide is better than… Long and Narrow (at least for bridges) A piece of spaghetti is stronger if it is pulled than if it is pushed. In fact, an aspect ratio of 20:1 is ideal for bridge supporting members.

Pulling vs. Pushing Ok. So…describe the pulling and pushing forces on this bridge.

Forces Involved in Bridge Building

Some bridge types that you might want to think about. Trestle Trestle and Arch

The Beam Bridge

The Forces on the Beam Bridge Beam Bridge: Forces When something pushes down on the beam, the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.beambends

Types of Beam Bridges Continuous span Swing Bridge

The Truss Bridge Consists of an assembly of triangles.

The Forces on the Truss Bridge Every bar in this cantilever bridge experiences either a pushing or pulling force. The bars rarely bend. This is why cantilever bridges can span farther than beam bridges. cantilever forcebendspanbeam bridges

Suspension Bridges Can span 2,000 to 7,000 feet -- way farther than any other type of bridge!

The Forces on the Suspension Bridge In all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight. steelcablestowersconcreteload compression

Cable-stayed Suspension Bridge Cable-Stayed Bridge The cable-stayed bridge, like the suspension bridge, supports the roadway with massive steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique "A" shape.suspension bridge steelcables tower

The Arch Bridge

The Forces on the Arch Bridge The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.force

(Back to Flight- ) Wing Aspect Ratio: Describe the diagram using the forces drag, lift, and weight. Why does a high aspect ratio work well for this low speed glider? The aspect ratio is a measure of how square the wing is. It is calculated by dividing the wing length from tip to tip by the wing width. Generally, higher aspect ratios are more efficient during low speed flying.