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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.