BASIC UNITS OF STANDARD INTERNATIONAL SYSTEM

There are seven basic units in the SI system:

Mass (kg): The kilogram is the metric unit of mass. It's the mass of the international a prototype of the kilogram: a standard platinum/iridium 1 kg mass housed near Paris at the International Bureau of Weights and Measures (BIPM).

Length (m): The meter is the metric unit of length. It's defined as the length of the path light travels in a vacuum during 1/299,792,458 of a second.

Time (s): The basic unit of time is the second. The second is defined as the duration of 9,192,631,770 oscillations of the radiation corresponding to the transition between the two hyperfine levels of cesium-133.

Temperature (K): The Kelvin is the unit of thermodynamic temperature. It is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The Kelvin scale is an absolute scale, so there is no degree.​

Electric Current (A): The basic unit of electric current is the ampere. The ampere is defined as the constant current that, if maintained in two infinitely long straight parallel conductors with a negligible circular cross-section and placed 1 m apart in a vacuum would produce a force between the conductors equal to 2 x 10-7 newtons per meter of length.

Amount of a Substance (mol): The mole is defined as the amount of a substance that contains as many entities as there are atoms in 0.012 kilograms of carbon-12. When the mole unit is used, the entities must be specified. For example, the entities may be atoms, molecules, ions, electrons, cows, houses, or anything else.

Luminous Intensity (cd): The unit of luminous intensity, or light, is the candela. The candela is the luminous intensity, in a given direction, of a source emitting monochromatic radiation of frequency 540 x 1012 hertz with radiant intensity in that direction of 1/683 watt per steradian.

Why the first alphabet of only Kelvin & Ampere are in uppercase?

Because these are the names of Engineers who had invented these units;

• Temperature (Kelvin): It is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin.
• Electric Current (Ampere): It is named after André-Marie Ampère, French mathematician and physicist, considered the father of electrodynamics.

BALANCED, UNDER REINFROCED AND OVER REINFORCED SECTIONS

UNDER REINFORCED SECTION:

In this section, the quantity of steel is not adequate to make the extreme concrete fibers in the compression area to get compressed to their highest permissible stress.

In this section, the quantity of steel is not adequate to make the concrete to get compressed in compression area to their highest permissible value. That means the steel is provided less than that a balanced section is required. In under reinforced section, the depth of actual Na is less than the critical Na.

i.e; Na<Nc

BALANCED SECTION:

The section in which the quantity of steel is just sufficiently provided that the concrete in compression zone and steel in tension zone reaches to their permissible stresses simultaneously is called balanced section.

In this section, the critical depth is equal to its actual depth.

i.e n = Na = Nc

OVER REINFORCED SECTION:

In this section, the quantity of steel in tension zone is greater than the quantity of steel required to make compressive zone concrete to get compressed to their most extreme admissible value. In other words, when the extreme compressive stress in concrete achieves its allowable limit, the comparing tensile stress in steel will be not as much as its permissible value.

So in over reinforced section, the depth of actual Na is greater than the critical Na.

i.e; Na > Nc