Structural analysis is the process of finding out the load or moment that a structure can carry under specific loading conditions. Thus, structural analysis is depicting the behaviour of structure, i.e. its response under any type of threat like loading, weather action, etc.
Structural analysis forms an important part of structural engineering. The given structure is analyzed for its strength and stability in structural analysis. The strength and stability are relative concepts. It depends upon the threat a structure is exposed to.
For example, if the structure is located in earthquake Zone I, then seismic considerations are not that important. However, for a structure located in seismic Zone III or IV has to be designed taking in view seismic concepts, else the structure would not be stable enough.
Structural engineering is even applied in our day-to-day life. The container for food parcels applies the principle of structural engineering. To know how, read out Structural Engineering in day-to-day Life.
Importance of Structural Analysis (Why Structural Analysis is carried Out?)
Structural analysis holds an important place in structural engineering as explained below:
- Structural analysis helps in knowing the maximum load that a structure can carry under actual load conditions before failure
- Structural analysis helps to know the deflection and deformation of the structure under the load; to know whether it is under the permissible limit or not
- Structural analysis helps to know the behavioural pattern of the structure under load
The structural model is prepared by the architecture. But, it has to be analyzed for its behaviour when exposed to the climate.
Preliminary Data Required for Structural Analysis
To carry out structural analysis, the following data should be known:
- Type of material and its properties
- Modulus of elasticity (Young’s modulus can be known from the properties of the material)
- Dimensions of the structural member
- Grade of concrete and steel
- Support conditions
Results Obtained as a Result of Structural Analysis
After carrying out structural analysis, we will have following data:
- Safe load and moment that the member can be subjected to
- Stress in the member
- Reactions at support
- Allowable deflection in the member
Basic Understanding and Procedure
A structure is taken as a physical system for analysis purpose. All the structures are 3-D in real life. But, we assume them to be 1-D or 2-D for the purpose of simplifying the analysis process. There are some of the basic concepts to be understood for the analysis as explained below:
Understanding Representation of Structure
A structure needs to be idealised else it would be too complicated to analyse. Hence, assumptions are made. Idealizations should be made in such a way that the assumed structure is as close to the actual structure as possible.
Idealization of Structure
Let us understand this with the help of an example.
A chimney can be idealized as a line so that it becomes a 1-D structure. Also, the chimney is fixed at the base, so it acts as a fixed support.
Another example can be taken of an elevated water tank. It can also be idealized as line as the length is much greater than the other two dimensions. And there is water tank at the top which has a considerable mass that cannot be neglected.
A truss or frame is idealized as a 2-D structure as shown below. On noticing carefully, you may find that the truss or frame is composed numerous identical elements. Therefore, concept can be applied on one element and then applied to all the other similar elements of the structure.
Frame is fixed to the ground and thus the support is also idealised as fixed support.
Understanding Threats on Structure
First let us see what are the threats to the structure? Threat is something that threatens the stability of the structure. These are loads. as the structure is erected, various loads act on it during its lifetime.
Load also needs to be idealised as different types of loads act on a structure. The self-weight of the structure, weight of fixed furniture, etc. are dead loads. Apart from DL, there is live load comprising load of occupants, etc. Earthquake load, wind pressure, earth pressure, water pressure, snow load, impact load, wave load, etc. are some of the other loads that may be considered.
To decide whether a threat is significant or not depends on the purpose and location of structure and foundation settlement.
| Threat | Condition when the threat is significant |
| Earthquake Load | If the structure lies in earthquake prone area |
| Wind Load | For cities where basic wind speed is high For tall buildings |
| Earth Pressure | For structure supporting soil like – retaining walls – underground water tank |
| Water Pressure | For structure holding water like – dam – water tanks |
| Snow Load | If the structure is located in an area where snowfall is frequent and snow gets accumulated on the roof having considerable weight |
| Wave Load | For offshore structures where the waves may strike against the structure and exert load |
| Impact Load | If the structure lies at the bottom of the hill and there are occurrences of stone rolling down |
Idealization of Threats
The loads are also idealised. The self weight in a beam is considered to act over the span of beam uniformly. This is taken as uniformly distributed load.
Similarly, if the point where the column of upper storey rests on the beam is considered to have point load.
Note that there are dimensions of column and it is not ‘actually point load’. But it is idealised as point load as the space where this load acts is much less than span of beam.
Understanding Response of Structure
Upon the action of threats, the response of structure is also idealised.
There are three criteria considered under the idealisation of structure
Safety requirement
The structure should not fail on exposure to threats. The loads should not cause the structure to collapse.
Serviceability requirement
Besides collapse, the structure should not exhibit deflection above a certain limit. Otherwise though the structure will not collapse, but it would not be able to fulfil its purpose. Excessive deflection, formation of large cracks, etc renders the structure devoid of serviceability.
Economy requirement
The structure should not be too expensive.
Idealization of Response
The response of the structure against the threat of load is in the form of internal forces in the structure as listed below:
- Axial force
- Shear force
- Moment
Basic Concept of Structural Analysis
Analysis of Structures
The analysis of structures as discussed above is carried out to determine the reactions caused by the external loads on a particular structure.
The analysis of indeterminate structures is more complex as compared to that of the determinate structures.
Analysis of Determinate Structure
The analysis of determinate structure can be attained just by applying the equilibrium equations. Different types of methods used for analysis of determinate structure are as follows:
- Analysis of Truss or Frame
- Method of Joints
- Method of Sections
- Unit Load Method
- Influence Line Diagram
- Analysis of Beam
- Macaulay Method
- Moment Area Method
- Conjugate Beam Method
- Method of Consistent Deformation
- Unit Load Method
- Influence Line Diagram
Analysis of Indeterminate Structure
The analysis of indeterminate structures cannot be solved just by equilibrium equations. Other equations such as compatibility equations are needed to be applied to reach the solution.
- Direct Stiffness Method
- Force Method
- Displacement Method
- Slope Deflection Method
- Moment Distribution Method
- Castigliano’s Theorem
- Influence Lines Method- For moving loads
- Matrix Stiffness Method
- Matrix Flexibility Method
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