Knowledge of structures CMT 7-5
Achievement standard 2.24 AS91348
A structure refers to framework that is used to support a load(s). A framework is comprised of structural members that are assembled using pin or fixed joints. The integrity of a framework is reliant on the strength, weight, material and profile of its structural members; the combination and means of joining structural members; and the safety factors applied to the structure.
Knowledge within this component includes understanding of how pin jointed structural members transfer forces when a framework is subjected to gravitational loads; how safety factors are applied to ensure a framework's integrity; and calculating using vector diagrams the magnitude, direction, and type of force acting on pin jointed structural members in a framework.
Achievement objective: CMT 7-5
Students will:
- demonstrate understandings of advanced structures.
Indicators
Students can:
- describe where pin and moving joints are used on frameworks
- describe the effects of loads when fixed joints are used in frameworks
- describe the effects of load on pin jointed frameworks using vector diagrams
- explain the types of forces that can act on pin jointed structural members when a framework is placed under known gravitational loads
- explain how structural members combine to resist loads and transfer forces within a pin jointed framework
- explain structural member profiles and forms and why they are used in a framework
- explain how structural members combine to resist loads and transfer forces within pin jointed framework
- explain how safety factors are determined, and discuss how they have been applied to ensure the integrity of a framework.
Progression
At level 6, students investigate basic structures limited to pin jointed columns and beams. Students learn:
- what is meant by tension, compression, shear and torsion
- to identify types of structural members and joints
- how safety factors are applied in the design of these basic structures
- how structural members and pin joints transfer forces
- how the integrity of a structure is established.
This should progress at level 7 to students learning how to understand structural frameworks in terms of the loads on fixed joints and the forces within framework members, and how safety factors have been applied to ensure the integrity of the structural framework. At level 7, students learn to calculate the forces using vector diagrams and include consideration of dynamic loading when considering safety factors.
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Teacher guidance
To support students to understanding advanced structures at level 7, teachers could provide opportunity for students to:
- understand how, where, and why pin and moving joints are used in frameworks across different framework structure contexts (for example, bridges, cranes, trusses)
- explain, using vector diagrams, the magnitude, direction and type of force acting on pin jointed structural members when a framework is subjected to known gravitational loads
- explain how structural members combine to resist loads and transfer forces within a pin jointed framework to ensure the framework is maintained in equilibrium
- understand how "safety factor" is applied across different framework structure contexts (for example, bridges, cranes, trusses) to ensure a framework's integrity is maintained.
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Contexts for teaching and learning
This learning objective is a knowledge objective and hence there is no requirement for students to actually construct the structural framework(s) they are studying. However, within a programme of work it would be possible to tie the knowledge related to this learning objective with a project where students are actually designing and/or constructing an outcome that involves a structural framework. Knowledge of structural frameworks could be covered before students design and make their own technological outcome that includes a structural framework of some description.
It would be beneficial if during the teaching and learning, students look at a range of structural frameworks even if they are only going to study one example in detail for assessment. Students should be provided with the opportunity to explore a range of structures to identify where, how, and why pin and moving joints are used, and to fully understand how these structures derive their structural integrity. Examples of suitable structural frameworks include such things as: bridges, cranes, trusses, climbing frames and other playground equipment, piers and jetties, gazebos, geodesic domes, scaffolding or ladders, roofing, towers, or a furniture item.
As part of this teaching and learning, students should learn:
- how to draw vector diagrams that explain the forces acting in members of a pin jointed structural framework
- what is meant by the terms: forces, structural members, and safety factor as they relate to structural frameworks
- the different types of structural members and joining methods (fixed and moving) used in structural frameworks
- how different structural components transfer forces, resist loads, and contribute to ensuring the integrity of frameworks.
Students need to understand the importance of diagrams in this topic. Vector diagrams will be used to show forces, but other annotated diagrams will be an important part of demonstrating understanding of structural frameworks. For example, a diagram to show members and types of joints.
Although vector diagrams and the transfer of forces are critical aspects of this learning objective it is also important that the topic is "bought to life" by looking at real examples of structural frameworks. These could be from a historical and/or contemporary perspective, and could include local, national, or international examples. For example, students could look at iconic bridges from around the world, or research scaffolding in different countries (an internet search of scaffolding in the third world will yield fascinating results!). Alternatively, they may focus their study on a well-known local structure.
The topic can be further enriched with visiting speakers (for example, a Futureintech ambassador who is a structural engineer) or a visit to a local structure of interest.
When exploring the concept of the integrity of a structural framework, an interesting angle can be to look at well known cases where the structure has failed. An internet search of bridge failure or similar would be a possible starter, then go on and explore structural integrity.
The focus at this level is static loads, however, when exploring safety factors the concept of dynamic loading is introduced, so within the learning programme students could explore such things as how wind and earthquake loads are different in different parts of the country. Without being a requirement as such at this level, students could be introduced to the building code and various NZ standards.
Literacy considerations
Teachers need to ensure students understand the specialist language related to structures. There is a large list of specialist terms such as structural frameworks, pin joints, moving joints, static load, fixed joints, framework members, forces, safety factors, framework member profiles, framework member forms, framework integrity, vector diagrams, tension, compression, torsion, shear, dynamic loadings, and framework integrity. Teachers will need to support students to develop an understanding of all these terms. Just providing a list of definitions will not be sufficient.
In addition, students must understand the language necessary to prepare evidence to demonstrate their understanding. Students need to understand words such as describe, explain, and discuss. Teachers need to give students strategies to understand what is expected for these different words – for example, what does it mean to explain framework member profiles, or discuss how safety factors have been applied?
Resources to support teaching and learning
Assessment for qualifications
The following achievement standard(s) could assess learning outcomes from this learning objective:
- AS91348 Construction and mechanical technologies 2.24: Demonstrate understanding of advanced concepts related to structural frameworks
Key messages from the standard
A structural framework will be made up of combinations of pin jointed members acting as struts and/or ties. Refer explanatory note 3. Students may focus in detail on one structural framework or may base their evidence on a number of different structural frameworks. Examples of suitable structural frameworks include such things as: bridges, cranes, trusses, climbing frames and other playground equipment, piers and jetties, gazebos, geodesic domes, scaffolding or ladders, roofing, towers, or a furniture item.
Students need to understand the different structural members and joining methods used in the framework.
When discussing the framework members, students need to explain the profile and form of the members. This is defined in explanatory note 4 and 5. Framework member profiles may include but are not limited to: I-beam, channel, round, and rectangular. Framework member forms may include but are not limited to: solid, tube, linked, and multi-strand cable.
Students need to understand pin joints (such as in a folding chair or collapsible gazebo), moving joints (such as bearing blocks on a draw bridge or a floating jetty), and fixed joints (such as a welded joint).
Students need to understand load in terms of static point loads only, and forces in terms of tension, compression, torsion, and shear forces. Refer to explanatory notes 6 and 7.
It is expected that when explaining forces, students will include calculation of the forces acting in members using vector diagrams. Refer to explanatory note 8.
When addressing safety factors, students could consider the implications of such things as static and dynamic loadings and the effects of wind and earthquake. Refer to explanatory note 9. Dynamic loads acting on a structural framework may include those resulting from changes in heat, wind, velocity, g-force, tension, and earth movement.
For excellence, students need to discuss how safety factors have been applied to ensure framework integrity. Explanatory note 10 clarifies the concept of framework integrity.
Evidence for this standard may be generated as students make outcomes with strong structural integrity requirements or may be generated by stand-alone activities. So the evidence could be embedded in a student’s portfolio associated with a practical project, or could be produced as a separate report or presentation related to a separate research investigation or other stand-alone activity. Refer to the conditions of assessment for further details.
Resources to support student achievement
Last updated June 10, 2022
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