Analysis of structures is very important for Structural engineers as it provides a basis for the design and also it evaluates the performance of a structure. This post is more like a pre-introduction than the introduction to analysis.
A structure is something which is made up of different elements connected in a way that it can withstand different loads that are applied to it. These loads may be Gravity Loads, Wind Loads, Seismic Loads, Temperature and other. Examples of structures are Buildings, Bridges, Dams etc.
Civil engineers, in particular Structural Engineers, design various structures for different types of loads that are expected to act on that structure. However, the primary responsibility of a Structural Engineer is to ensure that the structure does not collapse when it is subjected to loading.
Thus, determining the loads that the structure may experience in its design life is very important. We shall learn more about the design loads later.
In general, a structure deforms when loaded. It may also move as a rigid body if it is not properly restrained. As for as Civil Structures are concerned, Rigid body motion is to be avoided and the deformations should be within the limits.
Before we go for Introduction to analysis, a Structural Engineer should know about the possible failures and some major concerns with respect to Structures.
One of the major reasons for failure is the loss of stability. When a structure becomes unstable, it immediately collapses or loses its load-carrying ability. A structure can become unstable for any of the reasons mentioned below.
1. INITIAL INSTABILITY
A structural engineer has to check for the Initial Instability because if a structure is initially unstable then it fails under a very small load. There are two major reasons for this to happen.
a. LACK OF SUPPORTS
If the number of support reactions is insufficient to satisfy the equilibrium conditions, then the structure is said to be unstable. This type of instability can be corrected by changing or adding support to the structure.
If the number of support reactions is equal to that of equilibrium conditions, such structures are termed Statically Determinate whereas if it is more than that of equilibrium conditions, such structures are termed Statically Indeterminate.
Both Statically Determinate and Statically Indeterminate are Stable Structures.
b. IMPROPER/INADEQUATE ARRANGEMENT OF MEMBERS
Even if the structure is properly supported, it may still be unstable. This can happen if the members are not properly connected together to provide sufficient internal forces to resist the applied loads.
2. FAILURE OF STRUCTURAL ELEMENTS
Assuming the structure is initially stable, a structure can still lose its ability to carry the load, if some structural elements fail under extreme loading. Possible reasons for the failure of Structural Elements are:
a. BUCKLING FAILURE
Although there are various kinds of member deformation failures like Buckling, Torsional and Flexural failures. Buckling failure needs to be controlled as it causes Instability and fails suddenly.
It is associated with slender members subjected to compressive loading. In General, Columns undergo buckling.
As long as the compressive load on the member is less than the critical load, the member remains straight and as soon as the load crosses the critical value, the member starts to buckle. Euler’s load is considered as Critical load for Columns.
This Load deflection behaviour is very similar to the Inelastic behaviour in the sense that the member experiences a large deflection with no increase in load.
b. MATERIAL FAILURE
This failure depends on various properties of the material. Materials are classified into Brittle and Ductile based on their behaviour.
For Brittle materials, stress is linear to strain up to a Peak Stress, at which point, the material fractures and loses its load-carrying ability. This is not desirable.
Brittle behaviour must be avoided as it can result in sudden failure.
For Ductile materials, stress is linear to strain up to a certain value called Yield Stress.
For further straining, stress remains constant. Hence, the material stiffens and fails at a strain which is much greater than Yield Strain.
This is more desirable since the member does not lose its load-carrying ability when yielding occurs.
FINAL WORDS
A structural engineer should ensure that the structure is stable and do not collapse on loading.
1. Of all the reasons mentioned, a structural engineer should focus more on Initial Instability ensuring that the structure is initially stable . The structure fails under a very small load if it is unstable.
2. Next is to avoid Buckling of the members. As it results in large deformation and creates a huge loss in the load capacity of the member.
3. And the final is Loss of Stability due to Material Failure. There is no loss in load capacity from Ductile material failure, but still, the inelastic deformation of the members needs to be limited under extreme design loading.