SI Units for Structural Engineers

The international system of units SI (System Internationale units) Units for Structural Engineers, com­monly called SI, is being adopted allover the world as a uniform meas­urement system. While the complete transition from customary units to the SI system may take years, the use of SI (System Internationale units) Units for Structural Engineers units in the fields of en­gineering and science is proceeding rather rapidly, and it will soon be­ come necessary for the modern civil engineer to gain experience in using the SI system. Fortunately, the cl1~ngeover from the now common MKS units to SI units is quite simple, unlike the changeover from FPS to MKS units. In this book, SI units have been used throughout, with only mi.nor modifications, to suit the requirements of the engineering world.

The basic and derived units for various categories of measurement are discussed in the following sections.



The basic unit of length is the metre (m), which together with the mil­limetre (mm) is used exclusively for geometrical quantities. Although the centimetre (cm) is a convenient quantity, its use is generally avoided in the SI system. The use of mm for section modulus and moment of inertia involves large numbers for the majority of common flexural members. This problem is met by listing steel sections properties as section modulus x 103 mm3 and moment of inertia x 106 mm4. Very small sections, such as light gauge steel shapes may be listed as section modulus x mm3 and moment of inertia x 103 mm4.

 Mass and Density

Mass is a basic quantity in the system. The base unit of mass is the kilogram (kg). The use of kg should not be confused with the old metric force called kgf.

Material quantities are measured in mass units rather than in weight or force· units. Thus, the mass per length of a steel beam is expressed in kg/m, gravity floor loading in kg/m2 and the mass of an object in kg. Mass density is given in kg/m3. In contrast to weight units, these quan­tities do not depend upon the acceleration due to gravity. Weight is not used directly in the 51 system, but force is obviously caused by gravity acting on mass.

 Force, Moment and Stress

The unit of force is the newton (N), which is the force required to give 1 kg mass 1 m/s2 acceleration. Thus 1 N is 1 kg.m/s2, The newton is a derived unit that is independent of the acceleration due to gravity. A kilo­newton (1000 newtons) or kN, which is about 100 kgf, is a convenient quantity in structural analysis and design. Approximating the acceleration due to gravity as 9.81 m/s2, a kg of mass exerts a force of 9.81 N on its support point.

The stress unit is newton per square metre (N/m2),.called.pascal (Pa). This is a very small unit (1 kg/cm2 appr,oximates to 98100 Pa) and be­c.omes practical only when used with a, prefix (k or M). The most convenient SI stress unit for structures is 1,000,000 Pa; the mega pascal or MPa, which is identical to MN/m20rN’/mm2. Themodulusc of steel is about 200,000 MPa in SI units.

Surface loadings and allowable soil pressures have the units of pressure or stress and thus may be expressed in Pascals, but common· usage will dictate their expression in kN/mf or similar units. Surface loads in par­ticular are well expressed in kN/m2 because their effects must be con­verted into kN during structural analysis.

Moment is expressed in N.m or kN.m. These units are convenientsinc;e 1 N.m is close to 10 and 1 kN.m is close to 1/10 t.m.

 Angle, Temperature, Energy and Power

Plane angles are measured in radians (rad), but degrees are also used. Temperature in the SI system should be expressed in Kelvin (K) but the use of degrees Celsius (ºC), formerly called centigrade, is also permis­sible. Kelvin and Celsius are equal for temperature changes since an in­crement of 1°C equals an increment of 1 K. Energy is expressed in joules (J), where 1 J is I N.m. The unit of power is the watt CyV) which 1S equal to one joule per second (J/s).

Some Simple Rules to be Observed in Using  SI Units

Prefixes are to be selected from the following table, in which each prefix is a multiple of 1000.

Prefix Symbol Multlplying factor
giga G 109
mega M 106
kilo k 103
milli m 10-3
micro Il 10-6
nano n 10-9

Compound units, such as for moments, are written with a dot to indi­cate multiplication, such as kN.m (kilonewton-metre).