Mega-Structure Construction's of Engineers

 

Whatever may be the type of structure, one who is connected with architecture should know the forces for which structural analysis is essential. Stability, i.e., equilibrium, strength, stiffness and factor of safety are essential conditions to be checked in structural analysis. A structure should satisfy these conditions. 

Stresses and strains are due to loads, structural behavior and the nature of materials. 

1. Stress Due to Loads : Gravity loads and horizontal forces due to external agencies act on a structure. A gravity load consists of self-weight and superimposed load, i.e., live load and dead load. Horizontal forces are due to wind and earthquakes. The pattern of airflow depends upon the shape of building, roof, its angle to the wind, surroundings building and their height, width and length. Dynamic effect is caused due to the movement of vehicles or cranes, or people jumping on the floor etc. Impact and vibration effects are caused due to stationary machinery such as cranes. Rubber blocks and springs help in absorbing vibrations. Basement walls are supposed to resist earth or water pressure. 

2. Stress due to Structural behavior and Nature of Materials : Stress is also caused due to the nature of the materials, behavior of the structure and connections between the structural members.

Elastic strain causes strengthening or lengthening of the materials under load. Plastic deformation causes flow in concrete and non-recoverable changes in the shape of wood i.e., sag. Thermal expansions and contraction set up movements in metal and concrete. Fire and corrosion create destructive forces. Hence prior to the design, it becomes necessary to know the behavior of steel, wood, concrete, stone and brick under the above mentioned forces when acting independently and/or in combination.

An uneven settlement for the foundation, buckling of metal and wooden columns, splitting of concrete and wood, tipping over of walls, shear and tension in beams, excessive deflection in cantilevers, torsion, punching shear around columns, and tension in corners in slabs, all adversely effect the structure. 

3. Stability: Stability, i.e., equilibrium is the first essential condition which is to be checked for the two conditions, F = 0 and M = 0, where F is the force and M the moment. A structure is considered with all forces and moments.

Horizontal and vertical forces should be ascertained so that the structure does not move or rotate. Hence, it should be ascertained that the building as a whole is in equilibrium and so is every element. This can be done by showing a free body diagram showing all forces and moments. 

4. Strength: After checking and satisfying the condition of equilibrium, the size of the section, the distribution of stress is checked for every member to verify the strength of the materials. The section should be adequate to resist stress (force per unit area). Stress causes strain which is defined as a unit deformation. There are two types of structural members. One type carries axial forces causing direct stress and the other carries couples of shears, causing shearing stress, bending stress and torsional stress on sections normal to the area of the members. 

5. Stiffness: It should be remembered that although a structure may be in equilibrium or may have adequate strength, it may fail if it is not stiff. Deflections of slabs, swaying of frames, cracking of plaster due to varying directions, vibrations and buckling of columns are caused due to insufficient stiffness. A column should be checked for buckling in xx,yy axis, of its cross-sections and also about the z-axis. Model analysis assists in studying vibrations of and in complicated structures. 

6. Factor of Safety: This can be defined as the ratio of failure load to the expected load. Factor of safety can be found for the structure as a whole or for each part of the structure. The load-bearing capacity at the at failure differs for different materials. The quality of steel can be controlled as compared to concrete, while the load bearing capacity of the timber varies according to its quality. Hence, the factor of safety changes from material to material. 

Failure also depends upon the type of loading (static/dynamic) and the workmanship during the construction. Failure load can be defined as the load which will cause the stress to reach a certain allowable level, i.e., working stress for the material. Failure load can also be defined as that load which causes excessive deflections or vibrations, and ultimately complete collapse. Structural analysis clearly shows that in every structure and design, checking of every member is important. What is equally important is the design of the connections between the various members. A structure should be a symbol of unity of art and unity of technology.