Topic 9: Structures (10 hours)

9.1 Young’s modulus—stress and strain

	Assessment statement	Obj	Notes	References
9.1.1	Define Young’s modulus.	1		Slide Show 1
9.1.2	State that stress (load) is force per unit area acting on a body or system.	1		Slide Show 1
9.1.3	State that strain is the ratio of a change in dimension to the original value of that dimension.	1		Slide Show 1
9.1.4	Draw and describe a stress/strain graph and identify the elastic region, plastic flow region, yield stress and ultimate tensile strength (UTS).	2	For most materials the elastic region is a straight line, which changes to a curved line (plastic region). Quantitative details of specific materials are not required.	Slide Show 1
9.1.5	Outline the importance of yield stress in materials.	2	This is the stress at the yield point on the stress/strain graph. Beyond the yield point, the material undergoes plastic deformation	Slide Show 2
9.1.6	Explain the difference between plastic and elastic strains.	3	When a material behaves elastically, if the stress on the material is released before it breaks, the extension (strain) relaxes and the material returns to its original length. Beyond the yield point, the material deforms plastically and does not return to its original length or shape.	Plastic Stress University of Cambridge
9.1.7	Calculate the Young’s modulus of a range of materials.	2		Slide Show 2
9.1.8	Explain how knowledge of the Young’s modulus of a material affects the selection of materials for particular design contexts.	3	Young’s modulus provides quantitative data relating to the relationship of strength and stiffness in structures.	Slide Show 1 Slide Show 2

9.2 Forces

2 hours

	Assessment statement	Obj	Notes	References
9.2.1	Describe what is meant by an external load acting on a structure.	2	This involves loads where physical contact is made.	Forces
9.2.2	Describe what is meant by body load.	2	This is a load without physical contact, for example, a structure’s own weight.	Forces
9.2.3	Describe the difference between weight and mass.	2	Refer to the effect of gravity and how commonly people refer to the weight of an object when they should refer to its mass.	Forces
9.2.4	State the units of weight and mass.	1		Forces
9.2.5	Explain the relationship of external loads to internal forces and the concept of the balance of equilibrium of forces within a structure.	3		Forces
9.2.6	Explain how a structure “works” by interpreting how external loads give rise to internal forces within the structural members.	3	A static structure is in equilibrium, otherwise it would move (the forces acting upon it are equal in size and opposite in direction).	Forces
9.2.7	Explain the differences between tensile and compressive forces and how they affect equilibrium within a structure.	3	Tensile loads tend to extend or stretch a structural member. Compressive loads tend to compress or shorten a structural member. Tensile and compressive forces must balance if the structure is to maintain equilibrium. Only forces that are parallel or perpendicular need to be considered. Knowledge of trigonometry or quantitative resolution of vectors into components is not required.	Forces
9.2.8	Calculate a tensile or compressive stress, given values of force and area.	2		Forces
9.2.9	Calculate a tensile or compressive strain, given values of the original dimension and the change in dimension.	2		Forces
9.2.10	Evaluate the importance of forces in a design context.	3

9.3 The strength and stiffness of structures

3 hours

	Assessment statement	Obj	Notes	References
9.3.1	Explain the relationship between deflection and stiffness in structures.	3	If an external load is applied to some part of a structure, that part will be deflected to an extent that depends on the size of the load and the stiffness of the structure.	Forces
9.3.2	Calculate the stiffness of a structure.	2		Forces
9.3.3	Outline what is meant by bending moment in relation to structures.	2	This is the moment that a beam has to resist in bending at a particular section.	Forces
9.3.4	Outline what is meant by moment arm.	2	The load × distance from the pivot is called the moment about the pivot. The distance between the load and the pivot is called the moment arm.	Forces
9.3.5	Explain the need for a factor of safety in structural design.	3	Structures are designed to take higher loads than those they are normally expected to support.	Forces
9.3.6	Calculate the factor of safety for a structure.	2		Forces
9.3.7	Apply the concept of factor of safety to other areas of design.	2	A factor of safety is simply the ratio of the quantitative value of a design (factor) divided by the normal maximum expected value.	Forces
9.3.8	Evaluate the importance of strength and stiffness in a design context.	3		University of Cambridge

9.4 Beams

3 hours

	Assessment statement	Obj	Notes	References
9.4.1	Describe a beam.	2	Beams are structural members that are subject to loads acting normally to their longitudinal axis. The loads create shear stresses and bending moments and cause the beam to bend or flex. Beams are classified according to the way they are supported; for example, cantilever beams are rigidly supported at one end with the other end free.
9.4.2	Describe how beams are designed to transfer forces and distribute loads through the beams.	2
9.4.3	Describe the historical development of the materials used to manufacture beams.	2	Solid wood beams—high bulk. Concrete beams with metal. Metal sectional beams. Reduction in the amount of material in the beam.
9.4.4	Identify a variety of shapes for sectional members of a structure.	2	Consider rectangular, circular, L-shaped, -shaped, castle-shaped
9.4.5	Describe how the shape of sectional members of a structure makes the most effective and economic use of materials.	2
9.4.6	Explain that sectional members of a structure may be manufactured in sheet material.	3	For example, laminated veneer lumbar (LVL).
9.4.7	Outline the benefits of using LVL beams in the construction industry.	2	LVL is used in place of more expensive wooden beams where the finished product is hidden by other forms of cladding.
9.4.8	Explain the importance of factor of safety in the design of beams.	3