Loads under gravity (wk 1)

Define load

a force acting on a structure

• Come from the mass of an object

  • vertical force

  • E.g. beams, walls, floors, objects on the floor

• Also comes from

  • fluid pressure (water, wind/air)

  • dynamic events (e.g. earthquakes, winds, storms)

  • not always vertical

Different equations to calculate weight

Densities of common engineering materials

• water

• Steel

• concrete

Water: 1000 kg/m3 = 1 g/cm3

Steel: 7800 kg/m3

Concrete: 2400 kg/m3

What is density

mass per unit volume

p=m/V

1kN/m²

1 kPa

1 N/mm²

1 MPa

1N/m²

1 Pa

How do we measure the volume of irregular objects

Archimedes Principle

1. When we suspend a mass and put it in fluid, we can measure the increase in mass of the water

2. we can calculate the volume of the object

   1. Volume of water displaced (∆ water volume)= volume of the object

   2. Volume = density x mass

   3. Density = density of water = 1000kg/m³

   4. Mass = ∆ in mass of the water

If the object floats on water, like timber, attach it to a heavy mass of which we already know the volume

what is Archimedes Principle

any objects submerged in a fluid experiences an upward buoyant force equal to weight of fluid displaced

Common loads in structures

Dead load - permanent loads from self-weight, permanent equipment

• Physically part of the building

• e.g. columns, bricks, etc..

Live load - temporary load from the structure

• Anything that is moveable

  • e.g. chairs,

• Building codes have different estimates for live load depending on function (e.g. office building, factory)

• So we can safely accomodate for the live load

Three main types of loads

• Point load - a load applied on a concentrated area at a point

  • Force (kN)

• Distributed load - load distributed over a length

  • Can be uniform or varying

  • Force per unit length (kN/m)

• Pressure - load distributed over an area

  • Force per unit area (kN/m² or kPa)

Converting loads

• Pressure to point load

• Point load to pressure

Converting loads

• Point load to distributed load

• Distributed load to point load

Converting loads

• Pressure to distributed load

• Distributed load to pressure

St Venant’s principle

At points far away from the load, it doesn't matter what kind of load it is (e.g. pressure, distributed point) since long as they are statically equivalent, the result will be the same

Why do we have a safety factor when we consider different load combinations

• Different combinations of loads must be considered to ensure safety

• Due to uncertainties in load, estimated loads are scaled up

  • Provides a factor of safety for load magnitudes (risk)

why do structures fail