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Strength vs. Stability

Strength vs. Stability

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There are two issues that will be emphasized throughout this course that are crucial to the understanding of Architectonics:

• Strength
  the capacity of the individual elements, which together make up a structural system, to withstand the load that is applied to it.
• Stability
  the capability of a structural system to transmit various loadings safely to the ground.

These two critical issues are experienced daily from the moment that an individual is born. A newborn baby cannot even hold its own head upright. The large mass of the head requires a support system that has sufficient strength to enable the head to maintain its stability. This steadily increases as the bones, muscles and tendons of the skeletal and muscular systems increase in strength. Eventually the extra support provided by the arm or hand is no longer needed. The first challenge posed by gravity is overcome.

Crawling on four points of support proves to be a very stabile situation for quite a long time. The "leap" to the unstable two point stance is the next development in our understanding of the influence of gravity. Again, the structural system must develop to the point that the individual elements of the system have acquired sufficient strength. The first steps are made: an action of supreme coordination of hundreds of elements that becomes second nature to homo sapiens.

The list can be extrapolated to touch on many aspects of the human experience; riding tricycles and bicycles, jumping on trampolines, exercising on parallel bars, sliding on ice skates, sailing in a heavy wind, rocking a small boat, . . . . the list is endless. These are part of the human experience and each and every one rely on an inherent understanding of strength and stability.

How many times has a parent scolded a child to "put four on the floor!!!"? What the parent really means to say is, "if you do not put all of the legs of your chair on the ground, you are going to tip over!" Both strength and stability issues are addressed in this simple exclamation. Under normal conditions, the elements which make up the chair (its legs, bracing and seat) can easily resist the implied vertical loads. The strength of the individual elements of the chair have been designed for this type of static load. The seat (as a horizontal load-bearing element) must transfer its load through a connection to the legs (vertical load-bearing elements). Granted, some chairs will withstand a greater load than others, but they all resist the pull of gravity on the person sitting in them. If the legs cannot support the applied load they will fracture or break. These are examples of strength failure.

The stability of the system of elements depends upon the orientation of the chair in space. When it stands upright, on all four legs, it is a stable stystem. If it is on it's side, the chair might not be able to resist the loads for which it was designed. As it is tilted onto the back two legs, the structural system loses its equilibrium. At a certain point the chair as a system becomes unstable, fails and gravity pulls the supported load to the ground. This is a stability failure. In this type of failure, the individual elements retain their strength even as the system fails. The chair (system) could also have failed if the two supporting legs had experienced a strength failure (broken).

In each of these situations the chair, as a structural system, has reached the limit of its strength. As the saying goes, a chain (structural system) is only as strong as the weakest link (element)!

Any structural system can be studied in light of these two issues. For example, the column of the Greek temple shown above is an element that can experience a strength (crushing) failure, or a system (buckling) failure. It is/was part of a larger structural system.

Questions for Thought

What are some structural systems that you can see around you as you sit? How could they fail? How would one of Marcel Breuer's stainless steel tube chairs be discussed in relation to the issues of strength and stability? How would you describe the working of the support systems of your body in relation to the issues of strength and stability? How would you describe the basketball backboard and supporting structure shown in terms of strength and stability?

Factors that affect stability

Factors that affect stability.


1. The position of the centre of gravity. 
     A lower centre of gravity gives more stability to an object.

2. The size of the base area. 
    An object with a large base has better support and more stability compared to an object with a smaller
    base.

3. The weight of the object. 
    A heavier object is more stable than a lighter one. If an object has different densities, the heavier part of it    
     will have a lower centre of gravity.

The Importance of Stability In Our Daily Life

1. Racing cars are made more stable by having most of their weight as low down as possible. This ensures a
    low centre of gravity for the cars. Their wheels are also kept far apart to give them a wide base.

2. A weight lifter bends his leg and keeps them wide apart.

3. The passengers of a double-decker bus are not allowed to stand on the upper deck.

• other reference books add one more factor that  affect the stability is the weight of the object. 

Relationship Between Centre of Gravity and Stability

1. When an object is in equilibrium, its supported either at its centre of gravity or at a point vertically above or below its centre of gravity.

2. Stability refers to an object’s ability to remain in its original position.

3. The stability of an object is its ability to return to its original position when the object is moved or tilted
slightly.

4. Its is unstable equilibrium if it continues to move further from its original position after being displaced and then released.

5. Its in neutral equilibrium if remains in its displaced
                                         
6. Figure below shows three type of equilibrium;
       

Strength of Structure

      The strength of a structure is the ability to resist stress and strength put on the structure. Bending, compression, tension, vibration and turbulence are some of the stresses that structures must withstand. Factors that affect the strength of a structure include the types of materials used, its length, the cross sectional area or shape, how the structure is placed, weathering environment such as high or low temperature, humidity and others.

 Wood, brick, stone, iron, steel and aluminium are examples of some of the materials available for building structures, We can combine materials in order to use their best properties for examples fiberglass or glass reinforced plastic. So does reinforced concrete which enables concrete beams to withstand tension.

Aluminium

Steel

Brick

Wood

Stone

      

Rusted Steel

Steel is material created for high strength, corrosion resistant and elasticity. Steel is very strong because of its weight, strength and is relatively inexpensive. Steel is one of the strongest materials in construction, strong in compression and tension. The weakness of steel is it rusts and loses strength in extremely high temperatures.

The use of concrete in construction dates back to Roman era, but the modern practice of using reinforced concrete in construction is new to this century. Using steel embedded within a concrete beam, column or slab utilizes the strength of the steel in conjunction with the compressed strength of the concrete to make a stronger and safer structure called reinforced concrete. An example is in the move towards more and more reinforced concrete in the construction of long span bridges.

Long Span Bridge

       

Wood is quite strong in compression. This is one of the reasons why people build houses from wood. Wood is not easy to break because it is strong when pulled in the direction of its fibres. It is three times easier to break a block of wood if it is stretched from top to bottom, across the direction of its fibres.

Wood House

       

Just as important as the type of materials used in building a structure, the way in which the structure is placed is also important. Architects and engineers need to be aware of the loads and stresses on structures. Arches for instance, have been used in stable constructions, such as the main support structure most often found in bridges.

Stability of Objects

Stability can be defined as the ability of objects to return to its original state if disturbed. If an object is more stable, it can be able to resist larger forces. Objects that are stable will not topple over because they have their weight concentrated low down. This point is called the centre of gravity and the lower it is, the more stable is the object.

       An object is stable when its centre of gravity is located over its base. The lower an object’s centre og gravity is, complete to the height, the less likely it is to fall. The higher the object, the less stable it is. It means, the taller a structure is, the more it moves when forces like wind acts on it. The other factor that affects stability is based area. The wider the base area, the more stable the object is. The wider the bass of support, the easier it is to maintain balance.

Here are some examples of everyday life situations used to explain how base area and height affect the stability of a structure.

Based area	Height
* A heavy weight lifter spreads his legs to add stability.	* Big animals such as elephant and rhinoceros have short legs to lower the centre of gravity for stability.
* The wide distance between the wheels of a racing car is to increase the base area of the car in order to maintain its stability when it is moving fast.	* Racing cars are designed with low bodies to lower the centre of gravity.
* Laboratory apparatus such as a conical flask and tripod stand has a wide base for the purpose of stability.	* Boat passengers are advised to sit when the boats for stability.
The cross sectional * A raft is more stable than a kayak because a kayak has less base area

The shapes of objects in structure

• Many objects around us are made up of basic shapes.
• The following diagrams are some of the basic shapes that can be found around us.

1. Sphere

2. Hemisphere

3. Cylinder

4. Cone

5. Cube

6. Pyramid

7. Cuboid

Chapter 12-Strength and Stability

The two most important things that engineers or architects need to look at when building something is its strength and stability. The bigger the objegt that they want to build, the more important it is for the object to be stranger and more stable. For example, stronger materials such as steel and concrete are need when building a big bridge that allows thousands of vehicles to travel on it at the same time. However, less stronger materials such as wood can be used to build smaller bridges that are used by people or fewer vehicles. Apart from the materials used, the shape of the object is important as it provides stability for the object. This is especially important in the construction of tall buildings such as the Petronas Twin Towers ( KLCC ) and the Kuala Lumpur Tower.

Moon Phase

The Phases of the Moon

The Phases of the Moon

The revolution of the Moon around the Earth makes the Moon seems to change its shape in the night sky. This is caused by the different angles we see from the bright part of the Moon’s surface. This is called “phases” of the Moon. Of course, the Moon does not generate any light itself; it just reflects the light of the Sun. The Moon goes through four major shapes during a cycle that repeats itself every 28 days. These phases follow the sequence of their occurrence,

      

Moon Phase Views...

For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of the location on the Earth from where the Moon is observed. That is, all the phases occur at the same time regardless of the observer's position. New Moon, First Quarter, Full Moon, and Last Quarter phases are considered to be primary phases and their dates and times are published in almanacs and on calendars. The two crescent and two gibbous phases are intermediate phases, each of which lasts for about a week between the primary phases, during which time the exact fraction of the Moon's disk that is illuminated gradually changes.

New Moon - The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).The lighted side of the Moon faces away from the Earth.  This means that the Sun, Earth, and Moon are almost in a straight line, with the Moon in between the Sun and the Earth.  The Moon that we see looks very dark. Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing. This Moon can be seen after the New Moon, but before the First Quarter Moon.  The crescent will grow larger and larger every day, until the Moon looks like the First Quarter Moon. First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing. The right half of the Moon appears lighted and the left side of the Moon appears dark.  During the time between the New Moon and the First Quarter Moon, the part of the Moon that appears lighted gets larger and larger every day, and will continue to grow until the Full Moon. Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing. This Moon can be seen after the First Quarter Moon, but before the Full Moon.  The amount of the Moon that we can see will grow larger and larger every day.  ("Waxing" means increasing, or growing larger.) Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight. The lighted side of the Moon faces the Earth.  This means that the Earth, Sun, and Moon are nearly in a straight line, with the Earth in the middle.  The Moon that we see is very bright from the sunlight reflecting off it. Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing. This Moon can be seen after the Full Moon, but before the Last Quarter Moon.  The amount of the Moon that we can see will grow smaller and smaller every day. ("Waning" means decreasing, or growing smaller.) Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing. Sometimes called Third Quarter.  The left half of the Moon appears lighted, and the right side of the Moon appears dark.  During the time between the Full Moon and the Last Quarter Moon, the part of the Moon that appears lighted gets smaller and smaller every day. It will continue to shrink until the New Moon. Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing. This Moon can be seen after the Last Quarter Moon and before the New Moon.  The crescent will grow smaller and smaller every day, until the Moon looks like the New Moon.

Days and Night

The Discovery of Day and Night

Astronomy was first studied by ancient Egyptians in their exploration of the Moon, stars and other objects in space. The ancient Egyptians were the first explorers to find out what causes day and night. They studied how the Sun and the Moon travelled across the sky by observing their different movements. The early astronomers discovered that the Earth contains an imaginary line, called an axis, that passes through the centre of the Earth. This axis also passed between the North and South Poles.

Occurrence of Day and Night

The Earth rotates on its axis. This rotation causes day and night. When the Earth faces the Sun, the part of the Earth experiences daytime. On the other hand, when the Earth faces away from the Sun, the part of the Earth experiences night-time. Thus, day being on one side of the Earth and night is on the other side of the Earth. Each rotation on the Earth’s axis takes about 24 hours to complete. This 24 hour cycle includes both day and night, and makes one day.

The Solar System

Solar System

Solar System

       Our Solar System consists of nine planets. The Earth is the third planet from the Sun. The Earth is always rotating on its axis from the west to the east. An axis is an imaginary line that connects the North and South Poles. It takes 24 hours or one day to complete one rotation. The Earth also moves around the Sun at the same time. It revolves around the Sun in 365¼ days or one year.

         The Moon is called a natural satellite of the Earth. The Moon rotates on its axis. It takes about 28 days to complete one rotation. At the same time, it also moves round the Earth. It takes about 28 days to complete one movement. The Earth moves around the Sun and the Moon moves round the Earth simultaneously.

         The sunlight travels in a straight line. It cannot go around things. That is why there are dark shadows behind objects that stand in its way. In early mornings and evenings, the Sun rises on the horizon and makes long shadows. At noon, the Sun is directly overhead and makes short shadows. When the Earth rotates eastward, the Sun looks as though it is moving to the west. The rotation of the Earth on its axis from the west in the east changes the length and the position of the shadow throughout the day.