Introduction to Structures
Lecture # 1
Structures are everywhere, if you care to notice them.
1 What is a structure?
- Assembly of different types of (structural) members
- Carries loads & transmits forces through its members
- Functional
- Buildings
- Bridges
- Dams
- Tunnels
- Rockets……
Structural design is about the selection of safe and effective structural forms. The use of materials is determined by stresses and deformations under design loads. It’s analysed through structural mechanics and structural analysis.
2 Structural Members
2.1 Struts and ties
Only transmit axial forces (compression and tension), do not transmit bending forces.
Of course everything acts in bending, etc. In real life, nothing is only submitted to axial forces. But we take the situation where the main function of the member is to resist axial forces.
Note that beams are generally stronger in tension than they are in compression: compressive forces have a tendency to cause buckling.
Struts are drawn with a single thin line:
2.2 Trusses
An assembly of struts and ties (bars) providing a stiff frame with efficient use of material.
Efficient in use of material, strong and stiff. Many bridges are truss bridges; see the Forth road bridge.
2.3 Beams
Members also subjected to bending moments and shear forces. Drawn with a slightly thicker line.
2.4 Columns
Resist compressive axial force. Often found in buildings.
2.5 Shafts
Only subject to torsion, not compression or tension.
2.6 Arches
Structures curved in the vertical plane. Any load is transmitted directly into the ground; the whole structure is in compression (very strong). May be built from loose bricks.
Domes are 3D arches. (solid of revolution)
2.7 Plates and slabs
A two dimensional beam which gives a complete floor. Plates are metal, slabs are concrete.
2.8 Shells and pressure vessels
3D curved thin structures, usuallyl for storing things and building stiff and strong structures. The airframe of a rocket is an example of a shell.
2.9 Supports
Set the boundary conditions for the structures. When drawing a support, don’t draw a continuous floor if it’s not necessary; draw floor only where the supports connect to something fixed. It’s often a good idea to draw the minimum necessary support to make a structure stand.
2.9.1 Pinned support
1 degree of freedom. Does not support translations; only rotations are allowed.
2.9.2 Roller support
2 degrees of freedom. Supports translation in the direction of the roller as well as spin.
2.9.3 Fixed support
No degrees of freedom.
3 Representing structures
Done through structural diagrams.
Relevant information:
- Geometry
- Type of member, type of material
- Sources of load
Joints are free to rotate.
Forces would cause translations in a system outside of equilibrium. Act on a point.
Moments would cause rotations in a system out of equilibrium. Act on a point.
Forces spread out across an area.
4 Assignment
Structure Photography Assignment
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The difference between a structure and a mechanism would be the number of reaction forces supplied to the fixed reference; a mechanism has degrees of freedom (used in slightly different way in structural mechanics). A support may have “degrees of freedom” - any part of the structure that can theoretically move, which prevents it from creating force in that direction. So the actual difference of the structure is that that motion is resisted by the structure, whereas the motion is not resisted by the mechanism. It can’t move horizontally, vertically, and it can’t rotate: there are horizontal reaction forces, vertical reaction forces, and rotational reaction forces.
The difference is - if you can push on it with a small force to cause it to move, it’s a mechanism not a structure.