Engineering structures must carry load with small deformation. Examples of common engineering structures are: ** building, bridges, dams, offshore oil platforms, water & gas tanks, piping system, nuclear & chemical reactor vessels , ships, air planes, buses & other transportation systems**.

** From engineering point of view**:

- A framed structure is an assembly of members inter connected by joints. Other types of structural systems are shells, domes, dams, walls e.t.c

2 . structural behavior is the response of a structure to applied loads & environmental effects(wind, earthquake & temperature changes)

3. Structural analysis is the the determination of reactions, member forces, deformation of the structure & deflection of the joints due to applied loads & environmental effects.

** RELATION BETWEEN ANALYSIS & DESIGN**: The primary work of an engineer is to design. But analysis of a structure is required to check whether the design is safe either of a new or old structure. Usually design consists of preliminary design & checking by analysis, redesign with suitable modifications which which is repeated a number of times till final design is obtained. At this stage, all the design constraints are fulfilled, that’s, stresses & deflections are within permissible limits & cost is usually minimum.

** STRUCTURAL IDEALIZATION**: Structural analysis is conducted on an analytical (mathematical) model which is an idealization of the actual structure. Engineering judgement must be used in defining the idealized structure such that it represents the actual structural behavior as accurately as is practically possible. Depending on the problem in hand, structures are idealized using line, plate or brick elements. Line elements are used to model the structural components that have one dimension much greater than other two. Typical examples are truss members, beams, columns, etc. Plate elements are used to model structural components such as slabs & shells of which two dimensions are much greater than the third . Brick elements are used to model structures such as thick slabs & machine components where the structural action is truly three dimensional. Assembling the elements in accordance with the geometry of the structure produces a model of the complete structure that can be used to investigate the effect of the applied loading.

*The following points can be observed:*

- The actual structure is a welded frame of I section. The section is variably I.e it’s thicker at B & D & thinner at A, C & E.
- It’s supported at the ground level by means of concrete foundation. Hence in the idealized model represented by the line diagram we should have variable I & Area, large I & area under near B & while small I & Area near the supports A & E. And mid region C. If the supports allow rotations it should be pinned, if no rotation occurs it should be considered as fixed support.

** CLASSIFICATION OF STRUCTURES**: Structures can be classified in many different ways. One way of classification is based on dimensions, such as

- One dimensional , for example , trusses, beams, frames, grids,arch ribs & cable structures. Here one dimension, say length , is very large compared to other two dimensions for the components of the structure.
- Two dimensional or surface structure e.g slab, deep beams, shell or walls. Here length & breadth are large in comparison to thickness.
- Three dimensional or solid structure where all the three dimensions of length, breadth or height are equally prominent e.g foundations, retaining walls, gravity dams.

Structure can also be classified from the way they carry load. For example,

a) Beams carry their loads by developing bending moment & shear force at different sections.

b) Trusses carry their loads by developing axial force in the bars

C) frames carry their by axial force, bending moment & shear.

D) Arches carry load by compression & bending.

E) cables carry loads by developing axial tension.

F) Two way grids are subjected to both bending & twisting.

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