Units (about)

Categories: Support & Terminology

Introduction
The construction industry uses the metric system of measurement.  This comprises base units of kilograms, metres and seconds and units derived from them.  However, the industry is not consistent in its use of units and a wide number are used in different sectors of the industry. For instance Pa are used by the window industry but kN/m² are used by structural engineers.  Furthermore UK practice sometimes differs from that in other European countries, the use of N/mm² and MPa for stress being an example of this.

Although metric units are used the UK used to use imperial units of measurement and many existing buildings were built using units of inches and feet.  To enable dimensional co-ordination, as most building material may end up in either new build or refurbishment projects, materials are still supplied to imperial sizes or similar.  This means that timber joists are 100, 125, 150mm deep (4", 5", 6" deep) and panels are 1200mm x 2400mm (4' x 8').

This page lists and describes units commonly used in the construction industry.
 

Metric units of measurement
The metric system of measurement is based on the three units:

• m - metre - the fundamental unit of length,
• kg - kilogram - the fundamental unit of mass,
• s - second - the fundamental unit of time.

These units do not enable us to measure all physical things and so there are secondary and derived units.
 

Secondary units
Secondary units are:

• K - degrees Kelvin - temperature
• ^o - degrees - unit of angular measurement
• V - volt - unit of electrical potential
• A - Ampere - unit of electrical current

Suffixes
The basic units of measurement are not always convenient to use.  It is difficult to measure either a road journey or a paint thickness using the metre as a unit of length.  For this reason prefixes are used instead of a long string of noughts.  The suffixes are:
 

Note that the prefixes 'Deci-' and 'Centi-' are not preferred units and so, for instance, metres or millimetres should be used in preference to centimetres.
 

Derived units
Derived units are combinations of the fundamental and secondary units given above.  They are used to measure other quantities such as volume (m³ ,spoken as metre cubed) and velocity (m/s, spoken as metre/second).  The fundamental, secondary and derived units are described below.
 

Dimensional similitude
All units of measurement, including derived units, are associated with dimensions and quantities can be written in a dimensional form irrespective of actual units.  For instance, area has dimensions of length squared and can always be written as [L]².  Velocity always has dimensions of length divided by time and can be written as [L] / [T].

When using equations with an equality sign (=), the equation should still hold true when written in dimensional form.  For instance when dealing with energy:

Potential energy is    P.E. = m  x g x h  (mass x gravity x height)

where:

• mass (m) has dimension of mass [M]
• gravity (g) is an acceleration and has dimensions [L] / [T]² or [L] [T]^-2
• height (h) has dimension of length [L]

Potential energy then has dimensions of [M] x [L] [T]^-2 x [L],  or  [M] [L]² [T]^-2

All forms of energy lead to the same dimensional form for energy;

Kinetic energy is     K.E. = 0.5 m x v²  ( 0.5 x mass x velocity squared)

where:

• velocity (v) has dimension of length divided by time [L] / [T] or [L] [T]^-1

Kinetic energy then has dimensions of [M] x { [L] [T]^-1}² ,  or  [M] [L]² [T]^-2 , and in this case the factor 0.5 has no dimensions but is dimensionless.  In this case the formula given for kinetic energy is true whatever consistent units are used.

Energy is measured in Joules and the consistent units of mass, length and time are kilogram, metre and second.  Using centimetres instead of metres would clearly give an answer that is too big by a factor of 100.

Not all constants in formulae are dimensionless, for instance the coefficient (0.613) in the wind loading formula:

pressure (p) = 0.613 v²  (0.613 x velocity squared)

where:

• pressure (p) has dimensions [M] [L]^-1 [T]^-2
• velocity (v) has dimensions [L] [T]^-1

In this case the constant has to have dimensions of [M] [L]^-3 to give the dimensional equality implied.  In fact the factor 0.613 has the same dimensions as mass density.  In this case the factor of 0.613 only applies when velocity is in m / s (metres per second) and the pressure is in kN / m² .  When working in units of feet per second and pounds per square foot the constant has a different value.
 

Quantities

Length [L] is measured in metres.
 

Area [L]²is measured in square metres.  Other units are square millimetres.
 

Volume [L]³is measured in cubic metres.  Other units are litres (l) and cubic centimetres (cc).
 

Mass [M] is measured in kilogrammes (kg).  Other units are tonnes and grammes (g).  A tonne is somtimes called a metric tonne to distinguish it from the imperial ton.
 

Weight is commonly used to mean mass.  It is actually the force due to gravity at the earth's surface acting on that mass.  The unit of kilogram force (kgf) is sometimes used.  It is the force acting on a mass of 1 kg at the earth's surface and 1 kgf = 9.81 N.  A 1 kg exerts a force (weight) of 9.81 N at the earths surface.
 

Force [M] [L] [T]^-2is measured in Newtons (N).  Other units are kilo Newtons (kN).  Imperial units are pounds force (lbf), commonly called pounds. The force of gravity acting on a mass of 1 pound or its weight.  1 Newton is the weight of an average apple!
 

Pressure [M] [T]^-2 [L]^-1 is measured in Newton per square metre (N/m² ).  Other units are kN/m².  The Pascal (Pa) is widely used in the cladding industry as is the kilopascal (kPa).
 

Stress [M] [T]^-2 [L]^-1 is measured in Newton per square millimetre (N/mm² ).  Other units are megapascal (MPa).
 

Energy [M]  [L]² [T]^-2 is measured in Joules (J).  Other units are kilojoule (kJ) and kilowatt hour (kWh).
 

Power [M]  [L]² [T]^-3 is measured in Watts (W).  Other units are kilowatt (kW).
 

Temperature is measured in degrees Centigrade (^oC) or degrees Kelvin (K).

A change of temperature of 1^oC is the same as a change of temperature of 1K.  Degrees Centigrade and degrees Kelvin are measured on the following scales: