Structural Engineering Weight Bearing Structures

What is a structure? things with a definite size and shape, which serve a definite purpose or function. To perform its function, every part of the structure must resist forces (stresses such as pushes or pulls) that could change its shape or size.

STRESS (Force) Forces acting upon a structure. Actually, stress is a force or pressure when one member of part of a member presses on, pulls on or pushes against another member or part of a member.  It can also be the pressure that tends to compress or twist another member or part of a member.

FORCE & LOAD How are these terms used? For example, a brick wall must be able to stand up to the force of the wind. The bricks at the bottom must support the weight of the bricks above. If a person climbs on top of the wall, the bricks must support that load (the weight carried or supported by a structure) as well.

Types of Forces Static Compression Dynamic Torsion Tension Shear

Static Load A constant force applied to a stationary object. A good example of this is the person on the right. He is holding a stack of books but he is not moving. The force downwards is static.

Dynamic Load A good example of a dynamic load is the person on the right. He is carrying a weight of books but walking. The force is moving or dynamic.

Tension Tension forces try to pull things apart

Compression Compression forces try to crush or buckle (bend) things.

Torsion Torsion is a twisting force. Think of a ruler being twisted between both hands. The ruler is said to be in a state of torsion.

Shear A good example of sheer force is seen with simple scissors. The two handles put force in different directions on the pin that holds the two parts together. The force applied the the pin is called shear force.

Structural Components

Brace A piece of the structure that is designed to resist pressure (or stress) on the structure. Braces typically run between columns and are less than vertical.

Column A full height vertical piece of the structure that has the primary purpose of supporting weight. Since columns primary purpose is to hold weight it is important that the bottoms of the columns contact the ground and the tops of the columns contact the surfaces they are supporting.

Truss A truss is essentially a triangulated system of straight interconnected structural elements. The most common use of trusses is in buildings, where support to roofs, the floors and internal loading such as services and suspended ceilings, are readily provided.

Lap Joint A lap joint is a joint made by placing one member over another and fastening together. Generally the pieces will lay flush, but there are many variations of this type of joint.

Miter Joint A miter joint is made by beveling each of the two pats to be joined. Each piece is usually cut at a 45° angle, to form a 90° corner.

A butt joint is where one piece is butted against another. Butt joints are the most basic method for connecting two pieces of wood, and while it isn’t the strongest of joints, it is very useful in some situations.

Safety General Safety Weight Placement No horseplay When cutting, ALWAYS direct sharp edges away from your body DO NOT cut in your hand If you can not use a tool safely or do not know how to use it THEN DO NOT USE IT. Ask for help. DO NOT misuse or abuse glue Lift weights with your legs, NOT your back Be aware of fingers when working with weights When placing weights or testing structure, be sure area is completely clear in case of structure failure.

Cuts should be made at a 45 degree angle

Requirements Complete planning worksheet You will have 2 class blocks to build your structure 1 block will be used to test your structure Complete group evaluation Height: minimum of 14” Weight: Must hold 25lbs Do not use glue as filler Measure twice, cut once Build symmetrically Do not double up materials

Additional Vocabulary Buckling Load: The amount of weight required to cause a structure to fail. Tensile Strength: The greatest stress a substance can bear without tearing apart

Basic Structural Shapes Most efficient shapes and bracing Triangles: Rigidity of the triangular shape can support weight securely Squares: Can support weight, but can twist and collapse if not supported Polygons: Like squares, can support weight but will twist and collapse if not supported. X Brace Y Brace