- A structure such as a building withstands various loads. In doing so, it gets slightly distorted. Its various parts carry tension or compression. Imagine effect of a heavy ball on a simple structure. Imagine the ball is suspended from a horizontal plank with a vertical wire. Imagine the plank rests at ends on 2 pillars. The wire extends (distorts) carrying tension. The plank bends (distorts) carrying compression in its top fibres which shrink (distort) and tension in its bottom fibres which extend (distort). The pillars shrink (distort) in height carrying compression. Internal forces produced in materials of structures are called stresses.
- Components of ordinary structures may be selected by experience. But for important structures, a designer uses mathematics, reference manuals and computers for proper analysis. The designer checks that the distortions are within limits, and the various components have enough cross sectional areas to carry safely tension or compression produced under different combinations of loads. Loads are of various kinds. Occupants, furniture, vehicles have weight. The structure itself has some weight. These are downward loads of gravity. Wind and earthquakes produce loads in lateral and vertical directions. Moreover, earthquake loads are sudden and fluctuating. Water in a tank has weight and it exerts horizontal load on vertical walls. Changes in climatic temperature produce loads in certain cases. The structure is also subjected to the balancing action produced by ground supports.
- Construction materials are elastic upto a limit. Distortion of an elastic material disappears when the distorting load is removed. Beyond elastic limit, there is a non-elastic range. Material in this range does not break, but its distortion is substantial and part of it remains permanent on removal of the load. Finally, there is the breaking point. Structures are usually designed for elastic range. Imagine a component has elastic limit of 15 tons and breaking point of 25 tons. The designer may allow in it a stress of 10 tons under normal loads, and not more than 12 tons stress under a severe earthquake or hurricane which may occur just for a short time, once in 50 years. In above example, the factor of safety is 15/10 = one and a half, for normal loads.
- A structure, like a spring, stores energy when loaded and releases that energy when loads are removed, if it is elastic. When a structure gains energy, something else loses energy. In the above example, the ball loses energy. It moves down due to distortion of wire, plank and pillars. This movement multiplied by weight of the ball is its loss of potential energy, with respect to earth. A bomb-shelter may be designed on non-elastic basis. Under impact, it would distort and absorb substantial amount of energy. Distortion is then permanent but the occupants can be saved.
- Stability of structures is very important. The wire, plank and pillars may be strong, but they would collapse if the supporting ground is weak and sinks. A wire (slender rod) may take some tension but it would buckle (bend) if pushed inwards from both ends. Hence the compression-carrying components are made with stout cross sections. A ball suspended with a vertical wire is a pendulum, which swings on applying a small push. An isolated wire is not a good structure. It was considered to explain a tension-carrying component. A structure is 3-dimensional and its various components are interconnected at intervals, ensuring some stability. Imagine a tall, lightweight stool standing on 4 legs. A horizontal push at its top may tilt the stool, lifting 2 legs above floor. A horizontal push at mid-height may slide the stool along the floor. A tall building should not tilt or slide under horizontal wind /earthquake load. Necessary provisions are made.
- Actual structures are not simple like a plank resting on pillars. They are complex. They have various shapes and big dimensions. They contain many constraints, which reduce distortions, ensure economy and improve stability. The above plank would be blown away by a high wind. For stability it has to be anchored to the pillars. These anchors partially restrain bending of the plank under gravity loads. Bending at mid-span is reduced, while some reverse bending occurs in the plank near the pillars, with compression in bottom fibres and tension in top fibres. In turn, the anchors produce some bending action on the pillars.
- A tall building has several columns and horizontal beams at various levels. Generally joints of columns and beams are made rigid, and hence a building may have hundreds of constraints and the structural analysis would be intricate. A plank bends in one direction. But a horizontal rectangular slab supported at all edges bends in both short and long directions when loaded. The slab too is a complex structure with constraints. For analysis, a designer takes into account sizes and materials of various components and matches them with distortions (movements and rotations). Take a simple example. Imagine a ball is suspended with two vertical wires, one thin made of steel, and other thick made of copper. Both wires together carry tension equal to weight of the ball. Both wires extend equally (matching of distortions) which gives a relationship how the load is shared by the wires. In practice, a designer may use some short cuts for design.
- There are many other considerations. Architects may specify particular sizes of components for a better appearance. Trucks and cranes may not be adequate to handle long, heavy steel girders, which are then made in parts. These parts are assembled at site with connecting bolts strong enough to transmit stress from one part to another. Bolts require holes, which reduce the stress-carrying capacity of a component. Climate may be corrosive, and it may not be easy to paint a steel structure frequently. Larger sizes are then used to allow for the rusting losses. A size suitable for 5 tons stress may be adopted for all parts in 1 to 5 tons stress range, another size may be adopted for 6 to 10 stress range and so on. Sizes may not be varied for intermediate values. This reduces excessive variety and simplifies construction. An opening may be required in a slab. Suitable arrangement is then made to divert stress around the opening.
- Structures are made in several forms. A plank is stronger in bending under gravity loads if its broad face is kept vertical and not horizontal. A roof frame may look like a series of bars forming triangles. Every bar carries tension or compression when the frame is loaded. A pipe carrying pressurised water expands in circumference, with tension in its shell. Strong supports at ends are required to prevent flattening of an arch. Concrete is good in compression. Expensive steel bars are added in it mainly to take tension. Combination is reinforced concrete. In prestressed concrete, some wires are stretched. They are released, forcing nearby concrete to carry compression stress. This concrete withstands tension stress caused by loads. (Concept of reinforced and prestressed concrete is over-simplified here).
- The site engineer translates a design into reality, with proper techniques. For example, the exposed surfaces of freshly laid concrete are watered for a number of days, or coated with an anti-evaporation chemical. This ensures presence of water inside concrete for initial few days to complete hardening of cement. Welding of steelwork is done in stages, some welding at top, some at bottom, some at left, some at right, followed by further welding in the same fashion. If full welding is carried out in one stretch at one end, the job is unevenly heated and gets warped. A foundation may need inclined cuts and not vertical cuts in deep soft soil. Moreover, pumps may be required to remove any water, which oozes out.
- The site engineer is like a manager taking many decisions, such as --- When to order particular materials? Where to store them on the site? How many workers and machines may be engaged to complete a particular item of construction in a particular time? How to attend to accidents at the site? and so on. Repairs and dismantling of structures too require proper planning and execution. Some temporary props may be required to support a structure under construction. These props should not be removed, until the regular structure is formed and is capable to stand on its own. Until it hardens, the wet concrete is like a liquid, which exerts lateral pressure too on supports. This factor has to be accounted.
- Structural engineering is a creative and vast subject. This note is just elementary. Design and construction of skyscrapers, bridges, dams, domes, etc are challenging tasks demanding great skills. Engineering progresses with inventions and innovations. Load values, material strength values, graphs, tables, formulas, charts etc in textbooks, codes and manuals are helpful. But sometimes logical changes, some innovations have to be made in design and construction, looking to peculiarities of job. Sometimes, it is necessary to test structures under particular excess loads. Sometimes, non-destructive tests are made on structures, with instruments based on supersonics, x-rays and other principles of science.
- The engineer should have clear conceptual knowledge of how a structure would behave under various loads. He/she may make small models to test new ideas. He/she should not take improper risks. He/she should be modest and open to discuss any scheme with other experts, to evolve solutions to give both safety (which is foremost) and economy. He/she should improve talent by reading technical books and journals, attending seminars, visiting exhibitions of new products, and so on. In conclusion, Structual Engineering is a very responsible, money-rewarding and delightful art and science in service of mankind.
Earthquake Resistant Buildings
- A building should withstand loads (mainly horizontal) caused by wind storms and major earthquakes. Various standards and regulations of public authorities should be followed. Here a few concepts are presented in layman's language. Usually, an earthquake and a windstorm do not occur at the same time. Their effects are considered separately. For stability of a building, as a whole, the ground should have enough bearing capacity and frictional strength. Moreover, a multi-floor building should be heavier at lower floors. Its height should not exceed about three times its width.
- Wind load depends on the exposed area (height multiplied by the wind-facing width) of a building, and the speed of wind. Suction (negative loads) may be set up on other surfaces. Wind load increases and decreases gradually. On the other hand, the earthquake load is sudden with several quick vibrations. It depends on many factors, such as geographical location, number of floors, flexibility of structure, friction among various components, weight of structure etc. During earthquake, the upper floors of a building are subjected to greater horizontal loads due to inertia (a concept in physics, whereby the head of a person, sitting in a fast moving bus, leans forward when the bus suddenly stops)
- A building has several columns, beams, slabs, walls etc. These components carry earthquake-generated fluctuating stresses. Usual gravity loads too create stresses. Combined stresses in all components should not exceed safe limits. Earthquake shock should be absorbed elastically, and, if very necessary, non-elastically. (See points 3 & 4). Non-elastic behavior implies permanent distortion of the building, or of some components, but the occupants can be rescued. Concrete is brittle, while steel is ductile (elongates significantly before breaking). So optimum amount of steel is provided in certain portions to harness its ductility for non-elastic behaviour.
- Principal horizontal earthquake loads may act, say, in east-west direction. At the same time, there may be secondary loads of smaller magnitude in north-south direction, and additionally there would be some up-down loads. Deflection of a building should be within safe limits. Excessive sway may cause cracking of windows, frightful oscillation of water in toilet bowls, short-circuit fires etc.
- Building components may be compared with some models. Imagine 4 cardboard strips forming a rectangular frame (of 2 columns, 2 beams) with 4 pins passing through tiny holes at corners. The frame will distort easily, but it will be stable if the joints are stapled. It is beneficial to provide rigidity at many joints of beams and columns. A card cannot stand on edge, but a card folded at right angle can do so. It is beneficial to provide good joints between load-bearing walls, cross-walls, and slab somewhat like rigid edges of a metal box. A triangular frame of 3 strips, with even pins at corners, is stable. This principle may be used in some buildings, having some triangle like frames and diagonal members in vertical plane.
- It is beneficial to provide many columns in rows in east-west and north-south directions, with interconnecting beams, at all floors, to create strong frames in both directions. It is desirable to provide continuous columns from bottom to top. Some tall buildings are stiffened with full-height structural walls, called shear walls. Large cantilevers (projections) like balconies may be problematic in respect of the up-down loads. Generally speaking, it is beneficial to make a building with nearly square and symmetrical layout (viewed from top). Directions east-west etc are mentioned here for ease of description. Loads and building axes may be actually in various directions.
- Based on various factors, cost of making a multistory building may rise by say 20%, to ensure safety against earthquakes. Based on various factors, cost of building may be say 25% project cost (cost of building, cost of land, cost of service roads etc added together) The increment in project cost is about 20% x 25% = 5%. For achieving this so-called saving, one should not make risky buildings, putting occupants to great peril.
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Updated on :
$ December 22, 2004 $
Author : Madhukar N Gogate