volcanoes by Mr. Charles Dyson

Types of Volcanoes & Eruptions

 

Volcanic Fields

Volcanic fields, such as Auckland and Northland, are where small eruptions occur over a wide geographic area, and are spaced over long periods of time (thousands of years). Each eruption builds a new single new volcano, which does not erupt again. Mount Eden and Rangitoto Island are examples in Auckland.

Cone Volcanoes

Cone volcanoes (also called composite cone or stratovolcanoes) such as Ruapehu, Taranaki / Egmont and Ngauruhoe, are characterised by a succession of small-moderate eruptions from one location. The products from the successive eruptions over thousands of years build the cones.

Caldera Volcanoes

Caldera volcanoes, such as Taupo and Okataina (which includes Mt Tarawera), have a history of infrequent but moderate-large eruptions. The caldera forming eruptions create super craters 10-25 km in diameter and deposit cubic kilometres of ash and pumice.

Types of eruptions

Multiple types of eruptions can occur at each of New Zealand’s volcanoes - the eruption type can vary minute to minute. The style of eruption depends on a number of factors, including the magma chemistry and content, temperature, viscosity (how runny the magma is), volume and how much water and gas is in it, the presence of groundwater, and the plumbing of the volcano. For information on volcanic hazards which can be produced by our volcanoes.

Hydrothermal eruption

An eruption driven by the heat in a hydrothermal systems. Hydrothermal eruptions pulverise surrounding rocks and can produce ash, but do not include magma. These are typically very small eruptions

Phreatic eruption

An eruption driven by the heat from magma interacting with water. The water can be from groundwater, hydrothermal systems, surface runoff, a lake or the sea. Phreatic eruptions pulverise surrounding rocks and can produce ash, but do not include new magma.

Phreatomagmatic eruption

An eruption resulting from the interaction of new magma or lava with water and can be very explosive. The water can be from groundwater, hydrothermal systems, surface runoff, a lake or the sea.

Lava

Lava is molten rock erupted at the ground surface. When molten rock is beneath the ground, it is called magma.

·        Lava flows are the effusive (non-explosive) outpourings of lava, and usually flow slower than walking pace. Lava flow types include a’a, blocky and pahoehoe.

·        Lava fountains are a fountain of runny lava fragments from a vent or line of vents (a fissure). They can form spatter piles, and if the fragments accumulate fast enough, they can form lava flows.

·        Lava domes are mounds that form when viscous lava is erupted slowly and piles up over the vent, rather than moving away as a lava flow. They are generally caused by viscous, thick, sticky lava that has lost most of its gas. They can range in volume from a few cubic metres to cubic kilometres.

Strombolian and Hawaiian eruptions

These are the least violent types of explosive eruptions. Hawaiian eruptions have fire fountains and lava flows, whereas Strombolian eruptions have explosions causing a shower of lava fragments.

Vulcanian eruptions

Vulcanian eruptions are small to moderate explosive eruptions, lasting seconds to minutes. Ash columns can be up to 20 km in height, and lava blocks and bombs may be ejected from the vent.

Subplinian and Plinian eruptions

Eruptions with a high rate of magma discharge, sustained for minutes to hours. They form a tall, convective eruption column of a mixture of gas and rock particles, and can cause wide dispersion of ash. Subplinian eruption columns are up to 20 km high, and are relatively unsteady, whereas Plinian eruptions have 20 to 35 km tall columns which may collapse to form pyroclastic density currents (PDC’s). Very rare Ultraplinian eruptions are even larger and have a higher magma discharge rate than Plinian eruptions.

Volcanoes and types of volcanic eruptions

Environmental impact of modern agriculture by Dr.S.Charles Ruskin Kumar

 Stress and Strain

Stress

Stress is the ratio of applied force F to a cross section area - defined as "force per unit area".

                                                          

• 
  tensile stress - stress that tends to stretch or lengthen the material - acts normal to the stressed area
• compressive stress - stress that tends to compress or shorten the material - acts normal to the stressed area
• shearing stress - stress that tends to shear the material - acts in plane to the stressed area at right-angles to compressive or tensile stress

Tensile or Compressive Stress - Normal Stress

Tensile or compressive stress normal to the plane is usually denoted "normal stress" or "direct stress" and can be expressed as

σ = F_n / A                                    

where

σ = normal stress (Pa (N/m²), psi (lb_f/in²))

F_n = normal force acting perpendicular to the area (N, lb_f)

A = area (m², in²)

• a kip is an imperial unit of force - it equals 1000 lb_f (pounds-force)
• 1 kip = 4448.2216 Newtons (N) = 4.4482216 kilo Newtons (kN)

A normal force acts perpendicular to area and is developed whenever external loads tends to push or pull the two segments of a body.

Shear Stress

Stress parallel to a plane is usually denoted as "shear stress" and can be expressed as

τ = F_p / A                              

where

τ = shear stress (Pa (N/m²), psi (lb_f/in²))

F_p = shear force in the plane of the area (N, lb_f)

A = area (m², in²)

A shear force lies in the plane of an area and is developed when external loads tend to cause the two segments of a body to slide over one another.

Strain (Deformation)

Strain is defined as "deformation of a solid due to stress". 

• Normal strain - elongation or contraction of a line segment
• Shear strain - change in angle between two line segments originally perpendicular

Normal strain and can be expressed as

ε = dl / l_o

   = σ / E                             

where

dl = change of length (m, in)

l_o = initial length (m, in)

ε = strain - unit-less

E = Youngs Modulus (Modulus of Elasticity) (Pa , (N/m²), psi (lb_f/in²))

• Young's modulus can be used to predict the elongation or compression of an object when exposed to a force

Note that strain is a dimensionless unit since it is the ratio of two lengths. But it also common practice to state it as the ratio of two length units - like m/m or in/in.

• Poisson's ratio is the ratio of relative contraction strain

Strain Energy

Stressing an object stores energy in it. For an axial load the energy stored can be expressed as

U = 1/2 F_n dl

where

U = deformation energy (J (N m), ft lb)

 

Deflection 

Deflection is the degree to which a structural element is displaced under a load (due to its deformation). It may refer to an angle or a distance. The deflection distance of a member under a load can be calculated by integrating the function that mathematically describes the slope of the deflected shape of the member under that load. Standard formulas exist for the deflection of common beam configurations and load cases at discrete locations. Otherwise methods such as virtual work, direct integration, Castigliano's method, Macaulay's method or the direct stiffness method are used. The deflection of beam elements is usually calculated on the basis of the Euler–Bernoulli beam equation while that of a plate or shell element is calculated using plate or shell theory. An example of the use of deflection in this context is in building construction. Architects and engineers select materials for various applications.

The deformation of a beam

The deformation of a beam is usually expressed in terms of its deflection from its original unloaded position. The deflection is measured from the original neutral surface of the beam to the neutral surface of the deformed beam

Thermal Stress and strain

Thermal stress is mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include material types and constraints.[1] Temperature gradients, thermal expansion or contraction and thermal shocks are things that can lead to thermal stress. This type of stress is highly dependent on the thermal expansion coefficient which varies from material to material. In general, the greater the temperature change, the higher the level of stress that can occur. Thermal shock can result from a rapid change in temperature, resulting in cracking or shattering.

 

Thermal strains are strains that develop when a material is heated or cooled, they can be the bane of an engineer’s existence if they are not considered Materials that fit perfectly at one temperature can rupture or fall out when their environmental temperature changes.

TRUSSES

 

Trusses are structures made of up slender members, connected at joints which can be approximated to pinned connections. It is typically assumed that loads are applied to the joints of the truss, not directly to the members. Because of this, the members only carry axial forces - they do not carry bending moments. The internal force in each member is constant, and a member can be either in tension or in compression. It is important for engineers to be able to determine the axial force in the members of a truss, so that they can be designed appropriately. In this video I cover the two main methods for doing this, which are the Method of Joints and the Method of Sections. The Method of Joints involves applying the equilibrium equations to solve the internal forces acting on every joint within the truss. The Method of Sections involves creating an imaginary cut through the members of interest, and applying the equilibrium equations to the external and internal forces.

BUCKLING IN TRUSSES

A vision towards sustainable concrete prepared by Mr. G.Jein Jenish 

Concrete is the most used man-made material on earth. It forms the foundations of cities and connects communities. Without it, many of the elements of modern life we take for granted today wouldn’t be possible – safe buildings and homes, roads, tunnels and bridges, clean water, and clean energy. Concrete will also play a vital role in providing solutions to the challenges of the future and in building a sustainable world of tomorrow. Concrete has amazing sustainability benefits and we are working hard to make it even more sustainable.

Three-phase soil system

Three-phase soil system Prepared by Mr.Bright Winsley

Soil is a three-phase system consisting of solid particles (called soil grains), water, and air. The void space between the soil grains is filled partly with water and partly with air. However, if we take a dry soil mass, the voids are filled with air only. In the case of perfectly saturated soil, the voids are filled completely with water.

  

 

wind rose diagram

A wind rose diagram is a tool which graphically displays wind speed and wind direction at a particular location over a period of time. The diagrams normally comprises of 8, 16 or 32 radiating spokes, which represent wind directions in terms of the cardinal wind directions (North East South West) and their intermediate directions. Meteorologist use wind rose diagrams to summarise the distribution of wind speed and direction over a defined observation period. Data is taken from a weather station, which should be a near as possible to your project /site.

What do the Spokes Represent?

Each ‘Spoke’ shows:

1. How often the wind blows from each direction
2. How often the wind blows within each pre-defined wind speed range (bins). This is shown by the colour bands on each spoke.

A wind rose diagram uses a polar coordinate system, whereby data is plotted a certain distance away from the origin at an angle relative to north.

It is possible to create these plots using costly purpose-made software or, alternatively, very basic wind rose diagrams can be made using Microsoft Excel. Wind Rose Excel also provides 2 paid options:

1. WRE Web App – allows users to fetch data from 30,000 different weather stations and create wind rose diagrams within a web application.
2. WRE v1.7 – allow users to copy their own data into a macro-enabled excel workbook in order to produce wind rose diagrams.

 

Interpreting an Example Wind Rose Diagram

 

In Diagram 1 the westerly spoke and its colour bands provides the following information:

• The wind blows from the west (a “westerly”) 12% of the time at location x
• Westerly wind speed distribution for location x:
• 1% of total time the wind blows at 0-2 knots from the west
• 1% of total time the wind blows at 2-3 knots from the west
• 1.5% of total time the wind blows at 4-6 knots from the west
• 3.5% of total time the wind blows at 7-10 knots from the west
• 3% of total time the wind blows at 11-66 knots from the west
• 1% of total time the wind blows at 17-22 knots from the west
• 1% of total time the wind blows at >23 knots from the west

What is Possible using Excel?