M.E. Structural Engineer / Content Writer
Every object is a structure. Structure is made by connecting various member to make it whole to perform the intended function. Natural structures vary from the very smallest part of an atom to the entire creation of the universe. Man-made structures include buildings, bridges, dams, ships, airplanes, rockets, trains, cars, and even large artistic sculptures. Structural engineers design and access structure to ensure that they are functionally efficient and stable.Structural analysis deals with a calculation of load coming on the members and analyze them whereas, structural design decides the dimension (shape and size) of the structural members on the basis of calculated load from structural analysis.Structural engineers are responsible for both “structural analysis” as well as “structural design”. Both are an important part of civil engineering. So, it is essential that one must have clear understanding of structural analysis and design both the terms. Here we have explained the differences between structural analysis and structural design.
- The process to determine the response or behavior of a structure under some specified loads or combinations of loads is known as structural analysis.
- The process to find out the safe, durable and economical specifications of the structure including materials, technology, geometry, the size of structural members sufficient to carry loads of structure during the life period of the structure is known as structural design.
- The analysis process is done by making some assumptions. In the analysis, i.e. we assume the dimensions of the all the structural elements, loads, material properties, and support conditions. The sizes are generally assumed either from experience or by thumb rule which have arrived at empirically historically.
- The design process of any structure starts with the selection of materials vis a vis construction technology, and of course the structural system, which is core and most important. The choice of material depends on local availability and cost, while technology will depend on the availability of skilled manpower, and plant and machinery needed.
- Analysis of a structure involves its study from the viewpoint of its strength, stiffness, stability, and vibration and response of all elements.
- Design of structure involves its study from the viewpoint of its serviceability (it means that structure is in that condition in which it is still considered as useful and safe for occupants), safety, and durability.
- Response means to find out support reactions, shear forces, bending moment, torsion, deflection, stress, strains etc., that the particular member would undergo due to the applications of loads.
- It is process to find out geometry, size and shape of all the members of structure. In structural design individual elements as well as joints are designed.
- For structural analysis, one must have knowledge about the behavior of structural elements, and hence basic mathematics, and basically physics i.e. structure mechanics. For assumed section one has to find reactions, moments so that resulting stress, strain etc. can be checked.
- The process of design requires not only imagination and conceptual thinking but it basically requires the knowledge of basic mechanics including physics and chemistry of materials. And also, sound knowledge of practical aspects such as basic mechanics, recent design codes, and bye-laws which provide the guideline for the safe structural design.
- It is important because it evaluates the different loads on structures, and their effects. It is an accurate method to ensure the capability of the structures to resist the different expected loads, and assist in designing the structures accordingly. Structural analysis is important because it can check out whether a specific structural design will be able to withstand external and internal stresses and forces expected for the design. A basic reason that structural analysis is beneficial is to determine the cause of a structural failure.
- The structural design is important as structures are intended for human habitat and related uses for different human activities with structural safety not only for its own sake but for safety and security of human life, as it all depends on the adequacy of the structure. Structural design is important as it provide safety to the structure as well as occupants during its intended life without failure, with minimum repair and maintenance.
- When conducting analysis, civil/structural engineer investigates magnitude of force applied on the structure, direction of force, and the position on which the force acts.
Steps for Analysis of Structure
- Structural Idealization
- Applying Loads
- Calculating Reactions
- Calculating Internal Forces
- Calculating Internal Stresses
- Evaluating Efficiency and safety
- In the structural design once reactions are known which we get from the analysis we start the design process. We check whether the selected geometry and assumed dimension and size fulfills the design load criteria or not. If not then, we change the dimensions of the element and redesign it. The process is continued till the design criteria are totally fulfilled i.e. stresses in member do not exceed permissible limits and permissible deflection etc. All the elements are checked for its load resisting capacity i.e. stress of different types i.e. flexure, direct shear and stress etc., strain, deflection, rotation etc. If it exceeds permissible limits it has to be redone again and again until it falls within the permissible limits as per codes of design.
- The geometry is assumed to find out reactions.
- For given loading, geometry is checked, assured and ascertained.
- The structure we are going to build has assumed some specific lifespan. We are going to design it in such a way that it must be capable of taking all applied load (dead as well as live) without failure during its lifespan. So, before the design, we have to determine the behavior of structure under different load conditions. The structure is analyzed for different load combination like gravity load, live load, wind load, earth quake load.
- The structure must be capable of giving safety to the occupants (human) from heat, rain, earthquake, fire, flood, animals. It means that it must be capable of giving safety to the structure as well as occupants from the all the aspects. So, the main purpose of design is to produce a structure which is capable of resisting all applied loads and give safety to the occupants without fear of failure during its intended life.
- Methods of Structural Analysis
01. Analytical Method
In analytical method there are three approaches:
- The mechanics of material approach also known as strength of materials.
- The elasticity theory approach.
- And the finite element approach.
02. Method using Numerical Approximation
03. Finite element method
Methods of Structural Design
01. For RCC and Steel Structure
- Working stress method
- Ultimate load method
- Limit state method
02. For Wooden Structure
- Allowable stress design
- Load and resistance factor design
- There is only one specific solution, once system is frozen.
- There can be many possible solutions, with variation in materials and its properties.
- From the analysis we can get bending moment diagram, shear force diagram, deflection curve, shear stress and strain etc.
- From the design we can prepare detailed drawing for execution with details of reinforcement in case of RCC structures, details of weld or bolts in case of steel structure, and details of joints in case of wooden framed structure.
- Result that we get from analysis always remains with structural engineer.
- Structural engineer studies the data we get from analysis, and design and finalizes the design of member and prepare the drawing for execution.
Software Used for Structural Analysis
- MasterFrame FEA
- RISA-3D etc
Software Used for Structural Design
- STAAD Foundation Advanced etc