O Level Revision : Integrated Science - Science in Structures and Mechanical Systems - Structures

Trusses

• A truss is a structure made up of many beams joined together. It performs the function of a beam.
• The roof (asbestos sheets) of a classroom block is supported by many parallel wooden triangular trusses. Each truss is made up of many beams joined together forming smaller triangles.

Trusses

• Trusses are used to support large structures such as tower lights, bridges, roofs, and scaffolds.
• The triangle is the most stable shape hence its use in trusses to withstand shear.

Advantages of trusses over beams

1. Economy - trusses use less materials than beams as material in the neutral zones is removed so trusses are cheaper.
2. Strength - trusses are just as strong as solid beams.
3. Strength/mass ratio - trusses are lighter than solid beams but are equally strong hence have a higher strength/mass ratio than beams. Trusses thus have a smaller self-load.
4. Trusses are more rigid (unlikely to change shape) due to triangulation.

• A beam under compression is called a STRUT (→←).
• With BD removed loading the truss results in gap BD widening hence beam BD is pulled apart when the structure is loaded thus BD is under tension. A beam under tension is called a TIE (←→).
• In exams imagine removing a beam then loading the structure. If the structure collapses inwards (closes the gap) the ‘removed’ member is a strut. If the truss collapses outwards (the gap opens) then the member is a tie.

 Joining materials

• The method of jointing depends on the type of beam material available.
• There are two classifications of jointing methods namely:

1. Pinning method.                 2.   Surface contact.

• A joint is the weakest point on a structure.
• The strength of a joint depends on:
  • size of contact area-area is proportional to strength; and
  • number and position of pins-more pins means more strength.

Large structures

• The following materials are used in large structures:

(i)  wood                      (ii) metal                             (iii) concrete                   (iv) stones

Comparison of properties of construction materials

• Durability is the ability of a material to withstand deterioration by organisms, elements of weather and chemicals (resistance to decay, corrosion and rusting).

Bridges

• A bridge is any structure across a gap used by pedestrians, animals or vehicles to cross the gap.
• It has two parts namely

(i)   the deck, on which traffic moves, and

(ii)  the pier and abutments, which support the load (traffic).

Types of bridges

(i)   Pier and beam bridge                (ii)    Arch bridge                     (iii) Suspension bridge

Pier and beam bridges

Pier bridge

Suitable materials

Piers: materials strong in compression and resistant to corrosion by moisture e.g. concrete, metals, stones.

Beams: durable materials, that are strong under both tension and compression e.g. metal, reinforced concrete.

Reinforcements are placed in regions where tension is experienced and not all over the beam!

Arch bridges

Arch bridge

• The function of the arch is to support the load by changing the downward force into sideways thrusts.
• The arch is very strong in compression and only experiences compression stresses with forces on abutments being very large hence foundations for these are built on solid rock.

To demonstrate the strength of arches

1. Load both bridges with masses until each one gives in.
2. Compare number of masses needed before each one gives in.

The arch ‘bridge’ withstands a much bigger load than the beam bridge.

 Suitable materials for arches

(i) steel (metals)                    (ii)   stone                         (iii)  concrete                   (iv)  bricks

Suspension bridges

Suspension bridge

• Cables experience only tension.
• Towers experience compression.
• Cables take up the best shape to support the load.
• Suspension bridge is an inverted arch (upside down arch) hence reversal of forces in the arch (tension in place of compression).

Suitable materials

Towers:                                       –   concrete, metal, reinforced concrete.

Arches and vertical supports:  –   steelcablesmade from many wires so thatif one wire failsothers will still support the load.

Dam walls

• A dam is a structure built to hold back water in a reservoir.

To show that liquid pressure increases with increase in depth

• Water shoots furthest from hole C (bottom) and least from A (hole at top).
• This shows that pressure increases with depth, hence dam walls must be thicker at the bottom to withstand higher pressures.

There are two types of dam walls: (i)   concrete dam walls; and

(ii)  earth dam walls

(i)   Concrete dam walls

Concrete dam walls

2. Usually arch shaped so that the arch collects the horizontal water forces and changes them into lateral forces.
3. Are expensive to construct.
4. Usually built across high, narrow gorges.

Advantages of concrete dam walls

1. Use of wall as a bridge.
2. Generators (hydro-electric) fitted on the dam walls.

(ii)  Earth dam walls

  Earth dam wall

• Usually straight.
• Cheaper to construct than concrete walls.
• Suitable for wider gorges.

semi-impervious layer (gravel and earth soil

• Grass prevents erosion as roots hold soil particles together.
• Stones/rocks prevent erosion by water.