Q.
- a. High-rise buildings
- b. Long-span bridges
- c. Structures located on soft ground
- d. Structures located in highly seismic areas where forces acting on the structure due to an earthquake are in general proportional to the weight of the structure.
2. Ductility. As discussed in the previous section, steel can undergo large plastic deformation before failure, thus providing large reserve strength. This property is referred to as ductility. Properly designed steel structures can have high ductility, which is an important characteristic for resisting shock loading such as blasts or earthquakes. A ductile structure has energy- absorbing capacity and will not incur sudden failure. It usually shows large visible deflections before failure or collapse.
3. Predictable material properties. Properties of steel can be predicted with a high degree of certainty. Steel in fact shows elastic behavior up to a relatively high and usually well-defined stress level. Also, in contrast to reinforced concrete, steel properties do not change considerably with time.
4. Speed of erection. Steel structures can be erected quite rapidly. This normally results in quicker economic payoff.
5. Quality of construction. Steel structures can be built with high-quality workmanship and narrow tolerances.
6. Ease of repair. Steel structures in general can be repaired quickly and easily.
7. Adaptation of prefabrication. Steel is highly suitable for prefabrication and mass production.
8. Repetitive use. Steel can be reused after a structure is disassembled.
9. Expanding existing structures. Steel buildings can be easily expanded by adding new bays or wings. Steel bridges may be widened.
10. Fatigue strength. Steel structures have relatively good fatigue strength.
DISADVANTAGES OF STEEL
1. General cost. Steel structures may be more costly than other types of structures.
2. Fireproofing. The strength of steel is reduced substantially when heated at temperatures commonly observed in building fires. Also, steel conducts and transmits heat from a burning portion of the building quite fast. Consequently, steel frames in buildings must have adequate fireproofing.
3. Maintenance. Steel structures exposed to air and water, such as bridges, are susceptible to corrosion and should be painted regularly. Application of weathering and corrosion-resistant steels may eliminate this problem.
4. Susceptibility to buckling. Due to high strength/weight ratio, steel compression members are in general more slender and consequently more susceptible to buckling than, say, reinforced concrete compression members. As a result, considerable materials may have to be used just to improve the buckling resistance of slender steel compression members.
ADVANTAGES OF REINFORCED CONCRETE
1) Reinforced concrete has a high compressive strength compared to other building materials.
2) Due to the provided reinforcement, reinforced concrete can also withstand a good amount tensile stress.
3) Fire and weather resistance of reinforced concrete is fair.
4) The reinforced concrete building system is more durable than any other building system.
5) Reinforced concrete,as a fluid material in the beginning, can be economically molded into a nearly limitless range of shapes.
6) The maintenance cost of reinforced concrete is very low.
7) In structure like footings, dams, piers etc. reinforced concrete is the most economical construction material.
8) It acts like a rigid member with minimum deflection.
9) As reinforced concrete can be molded to any shape required, it is widely used in precast structural components. It yields rigid members with minimum apparent deflection.
10) Compared to the use of steel in structure, reinforced concrete requires less skilled labor for the erection of structure.
DISADVANTAGES OF REINFORCED CONCRETE
1. The tensile strength of reinforced concrete is about one-tenth of its compressive strength.
2. The main steps of using reinforced concrete are mixing, casting, and curing. All of this affects the final strength.
3. The cost of the forms used for casting RC is relatively higher.
4. For multistoried building the rcc column section for is larger than steel section as the compressive strength is lower in the case of RCC.
5. Shrinkage causes crack development and strength loss.
ADVANTAGES AND DISADVANTAGES OF TIMBER STRUCTURE
INTRODUCTION
Wood is an organic, hygroscopic and anisotropic material. Its thermal, acoustic, electrical, mechanical, aesthetic, working, etc. properties are very suitable to use it is possible to build a comfortable house using only wooden products. With other materials, it is almost impossible. But wood has some disadvantages too. Following is some very short information about this subject.
Thermal Properties:
Wood does not practically expand against heat. On the contrary, by the effect of heat, it dries out and gains strength. The only time wood expands a little is when the humidity level is below 0%, and this is only scientifically significant. In practice, the humidity level of wood does not drop under 5% even in the driest climate.
- The coefficient of thermal conductivity of the wood is very low. Specific heat of wood is high.
- Wood is 400 times better as a thermal insulator than steel and 14 times better than concrete.
- In solid form, wood also has significant thermal mass properties, retaining heat from the day and
- releasing it at night.
- Wood also acts as a humidity regulator, absorbing moisture when humid and desorbing moisture when the air is dry.
Acoustic Properties:
Sound isolation is based on the mass of the surface. Wood, as a light material, is not very perfect for sound isolation; but it is ideal for sound absorption. Wood prevents echo and noise by absorbing sound. For this reason it is extensively used in concert halls.
Electrical Properties:
Resistance to electrical current of a completely dry wood is equal to that of phenol formaldehyde. An oven dried wood is a very good electrical insulator. To some extent air dried wood is the same. Unfortunately electrical resistance of wood is lowered by increasing the moisture content. The resistance of wood saturated with water. Static electricity that is dangerous for human health is not observed in wood unlike metal, plastic and other materials. For this reason wood is preferred as a healthy material.
Mechanical Properties:
Although wood is a light material, its strength is quite high. For instance, while the tensile strength of wood with 0.6/cm3 specific gravity is 100 N/mm2, the tensile strength of steel with 7.89/cm3 specific gravity is 500 N/mm2. Dividing tensile strength by specific gravity gives the breaking length and quality of material. This figure means the breaking length of the material, when hung as a result of its own weight. While the breaking length of steel is used for construction is 5.4 km, chrome mobile steel is 6.8 km, hardened bow steel is 17.5 km, breaking
4. 4 length of spruce wood is 19.8 km, and laminated wood made of beech is 28.3 km. For this kind of properties, wood and laminated wood is used in wide-gap constructions like health centers and sport halls.
Aesthetic Properties:
Wood is a decorative material when considered as an aesthetic material. Each tree has its own color, design and smell the design of a tree does change according to the way it is sliced. It is possible to find different wooden materials according to color and design preference. It can be painted to darker colors of varnished, and can be given bright or mat touches.
Oxidation Properties:
Although wood has oxidation characteristics in some way, it is not the kind of oxidation seen in metals. Metals get rust, wood doesn’t. For such characteristics, use of wood is preferred to avoid rust when necessary.
Working Properties:
It is easy to repair and maintain wood. While old woods can be renewed by special touches other materials are highly difficult and costly to maintain and to repair. Therefore they are usually disposed of.
Variation:
There are more than 5000 kinds of woods in the world. Their specific gravity, macroscopic and microscopic structures are different. Accordingly, their physical, thermal, acoustic, electrical and mechanical properties are also different. Because of this variety, it is possible to find wood suitable for needs. For instance, for heat isolation and sound absorption woods in lightweight are used. Similarly, heavy ones are used for construction purposes.
DISADVANTAGES OF WOOD
Shrinkage and Swelling of Wood:
Wood is a hygroscopic material. This means that it will adsorb surrounding condensable vapors and loses moisture to air below the fiber saturation point.
Deterioration of Wood:
The agents causing the deterioration and destruction of wood fall into two categories: Biotic (biological) and a biotic (non-biological). Biotic agents include decay and mold fungi, bacteria and insects. Abiotic agents include sun, wind, water, certain chemicals and fire.
Biotic Deterioration of Wood:
Woods are organic goods. Like any organic good, wood is a nutritional product for some plants and animals. Humans cannot digest cellulose and the other fiber ingredients of wood, but some fungi and insects can digest it, and use it as a nutritional product. Insects drill holes and drive lines into wood. Even more dangerously, fungi cause the wood to decay partially and even completely. Biological deterioration of wood due to attack by decay fungi, wood boring insects and marine borers during its processing and in service has technical and economical importance.
Insects:
Insects are only second to decay fungi in the economic loss they cause to lumber and wood in service. Insects can be separated into four categories: Termites, powder post beetles, carpenter ants and marine borers.
Carpenter ants
Carpenter ants do not feed on wood. They tunnel through the wood and create shelter. They attract most often wood in ground contact or wood that is intermittently wetted.
Carpenter bees
They cause damage primarily to unpainted wood by creating large tunnel in order to lay eggs.
Marine borers
They attack and can rapidly destroy wood in salt water and brackish water.
COMPARISON STEEL VS CONCRETE
SAFETY
Steel - Experts acknowledge that steel can soften and melt with exposure to extremely high temperatures. However, with the addition of passive fire protection, such as spray-on fireproofing, buildings built of structural steel can sustain greater temperatures and, therefore, provide additional safety.
Concrete - is safer. The building’s core (where elevators, stairs, and power systems are located) will be encased in 2-foot-thick concrete for protection in the event of a fire or terrorist attack. Moreover, it can endure very high temperatures from fire for a long time without loss of structural integrity. Concrete requires no additional fireproofing treatments to meet stringent fire codes, and performs well during both natural and man made disasters.
COST
Steel - Initially the cost of steel structures is greater than the Reinforced concrete structures because the section of steel are very much expensive, moreover they are available in certain areas which also increase transport charges. Cost of steel structures can be balanced by durability of building i.e. increase in life of building.
Concrete - The cost of ready-mix concrete remains relatively stable, and even the increase in steel has had a minimal effect on reinforced concrete building projects. Concrete prices remain very steady despite the fluctuating and substantial increases in other building material prices.
MATERIAL AVAILABILITY
Steel - Availability of steel now days marked the steel industries around the world. Steel is mostly available in mega cities where easily can be placed to construction site but it can’t be easily available at different location. Availability also affects the cost of project, less availability increase the cost of project.
Concrete - Concrete and its ingredients are easily available in any native area as compare to Steel. Moreover there are many industries which make ready mix concrete and its ingredient like cement which can made available easily. Its unavailability effect very less on the project cost as compare to steel.
CONSTRUCTION SCHEDULING
Steel - Construction schedule of steel structures is more faster than RCC one because steel members are require to fasten in their position which take very less time and can easily be done with skilled labor or by fasten machines.
Concrete - Construction of RCC structures require more time than Steel structures because; while placing of concrete require to built its formwork first then placing is done. After placing it has to be cured for certain period of time which makes its construction work very slow than steel structures.
DESIGN POSSIBILITIES
Steel - has the highest strength-to- weight ratio of any construction material. And with new construction methods, steel buildings remain a popular choice for office and multifamily developers. Use of girder slab, staggered truss, and castellated beam construction enables lower floor-to-floor heights than typically expected in structural steel buildings. Steel can accomplish extremely long spans in structures [and] very open-bay footprints without intermediate columns. It’s a very flexible material in terms of different ways to address design requirements.
Concrete - concrete buildings are taking many different shapes. In addition to the unique aesthetics achieved with concrete construction, these buildings offer some very real space advantages. Public and private developers should also realize that using cast-in-place reinforced concrete to frame a high- rise office building would yield more rentable space because of lower floor- to-floor heights. With proper engineering, concrete building can also offer uninterrupted floor plates.
COMPARISON STEEL VS TIMBER
THERMAL
Steel - The coefficient of expansion of steel is very high as compare to wood so they cause failure of structures when increases. Failure may occur due to deflection structural member which are subjected to high temperature.
Timber - The coefficient of expansion of wood is very low i.e. it does not provide increase in length when temperature is raised so this may provide safety to structures and make to cool enough as compare to steel. However contraction may be danger full to timber structures because below 0 C its starts contract rapidly.
ACOUSTICS
Steel - structures may produce sound i.e. echo because they have less properties of absorption of sound waves so sound proof material may be used in these structures.
Timber - don’t reflect sound waves rather it has greater intensity to absorb sound waves so no need of providing sound proof materials in buildings which reduce cost .
COST
Steel - structures are very much costly than the timber structures because of their metallic behavior and used for high structural variations such as loads, span and strength.
Timber - The cost of timber structures is very low because these structures are design for low loads, strength and span.
DURABILITY
Steel - structures are more durable than timber because these structures got extra strength which against natural agents such as wind , rain, earthquake etc. the life time steel structures may be up to 50 years.
Timber - structures are less durable than steel structures because they have properties to against natural phenomenon. Life time may be up to 15 to 20 years.
DETERIORATION
Steel - structures are liable to be corroded this may reduce life span but if steel coated with paints then corrosion is reduced at great extent.
Timber - deteriorates more than steel because termites badly affect the timber which makes tunnel through it and make it weak enough. It can be reduced by painting.
VARIETIES
Steel - Varieties of steel is limited, they can be made variable by industrial agents i.e. by looking adding different metals, alloys etc which may get its variation.
Timber - More than 5000 kinds of wood are available in the world with different eternal structures and properties. So they can be used in engineering for various purpose.
Sheeraz RoMie, Academic Activist at Ministry of Health