Bridges new

BridgesTech Ed IMr. Benich

Objectives Identify the loads acting on

structures Analyze the forces acting on a

structure Demonstrate how structures can

be designed to withstand loads. Design and make a product that

incorporates structural principles.

History of Bridges The function of bridges is essential to

modern America. Without adequate bridges in place, the

pioneers would sometimes travel many miles out of the way to find a safe place to cross a river.

Other times they waited for days or weeks for the flooded rivers to recede until it was safe to cross.

Consider how the movement of people, goods, and services would be affected without highway systems, which include bridges.

Factors in Bridge Design A number of factors must be

taken into account when an engineer is designing a bridge.

1. What is the span of the bridge? Span means the length of the bridge from one side of the river to the other (banks).

2. What will be the load on the bridge? Load means how much weight will be on the bridge.

The engineer considers two types of load.Live load- weight of the car, trucks, or people expected on the bridgeDead load- weight of the bridge itself

3. What environmental factors will effect the bridge? Examples: strong winds, ice, snow, extreme cold or heat

4. What is the budget allowance for the bridge?

5. What are the soil characteristics of the river banks? Must have a firm foundation to withstand the weights which will be placed on them.

6. What are the soil characteristics of the bottom of the river if supporting columns are required in the bridge design?

7. What is the time frame in which the bridge must be built?

After considering these and other factors, including the attractiveness of the bridge, the engineer is ready to begin the design.

Other Types of Loads Static Loads – loads that are

unchanging or change slowly.They maybe caused by the weight of the structure itself. Columns, beams, floors, and roofs are part of this load. Dynamic Loads – load that

is always moving and changing on a given structure. A truck moving across a bridge.

Forces Acting on Structures

Both static and dynamic loads create forces.

Understanding these forces….. Compression – being pushed

together, squeezed. Tension – being pulled apart,

stretched The design and construction of

structures must minimize the effects of bending. Parts must be shaped so the forces of tension and compression are balanced.

Designing to Withstand Loads

After members have been shaped to resist compression and tension, they must be connected in a way that minimizes bending.

Square example… A rigid member in tension is called

a tie. A rigid member in compression is

called a strut. A triangle is the strongest shape.

Types of Bridges A major problem with bridges is

that they bend. One common way to prevent a Beam Bridge from bending is to support the center with a Pier. (not always possible to build piers under a bridge)

Truss Bridges Truss Bridges – make use of the

triangle in their design. Many triangles are used to construct most truss bridges.

Suspension Bridges – a bridge deck can be supported from above by cables called stays.

Suspension bridges are the longest bridges.The bridge deck is suspended from hangers attached to a continuous cable.the cable is securely anchored into the ground at both ends.The cables transfer the mass of the deck to the top of the towers, where compression transfers the mass to the ground.

Arch bridges – the compressive stress created by the load is spread over the arch as a whole.The mass is transferred outward along two curving paths.The supports where the arch meets the ground are called abutments.In a stone arch bridge, the middle stone is called a keystone.