Steel and Concrete Construction Frames - Term Paper Example

Running Header: Steel and concrete construction Frames Student’s Name: Instructor’s Name: Course Code: Date of Submission: Table of Contents Table of Contents 2 1.0 Introduction 3 2.0 Steel Frame Construction 3 2.1 Design and construction process 4 2.1.1 Design of main member 4 2.1.2 Design of secondary members and connections 4 2.2 Properties of Steel 5 2.3 Advantages and disadvantages of steel frame construction 6 2.4 Steel Construction details examples 7 2.5 Case studies/examples of steel frames 7 3.0 Concrete frame construction 8 3.1 Design and construction process 8 3.2 Properties of concrete 10 3.3 Advantages and disadvantages 11 3.4 Construction details 12 3.5 Case studies/examples 13 4.0 Methodology of Limit state design 14 4.1 Partial safety factors 18 5.0 Structural effects of fire on steel framed building 20 6.0 Conclusion 26 References 27 Appendices 29 1.0 Introduction Concrete and steel are the key building materials especially in high rise buildings. Frame construction of both steel and concrete are the ideal solution that provides a structure with a flexible layout of floor that is open as well as external elevations for continuous cladding and glazing. Precast floors in every level span between beams of concrete whereby they sometimes supported by billet or corbel means from a series of columns of concrete that are of full height as stated by Neville (2000). In most cases, there is formation of quick open spaces that are beneficial in terms of inherent fire, aesthetic and thermal mass characteristics. Steel on the other hand provides flexible, efficient, and open structures that can be constructed quickly and cost effectively. This is due to steel’s capability to span longer distances while at the same time retaining shallow geometry in the plan and geometry. This assessment will compare and contrast steel and concrete frame construction as well as their structural design in normal and fire conditions. This report will discuss and analyse construction materials including concrete and steel. The properties, advantages, disadvantages, and their construction design and aspects will as well be analysed. The report will also analyse the structural effects of fire on steel structures as well as analysing the limited state design methodology for structural designs. 2.0 Steel Frame Construction Steel is a common material for building that is used in the construction industry with the main aim of forming a skeleton for building or structure. Steel frame is a building technique that has a skeleton frame of horizontal I-beams and vertical steel columns as illustrated by American Institute of Steel Construction (2001). The frame is usually constructed in a rectangular grid in order to support walls, roof, and floors of a building that are attached to the frame. 2.1 Design and construction process The production of schematic, conceptual as well as design development drawings are crucial predecessor activities towards structural framework design finalization. This is mostly carried out by structural engineers in an effort to make the work of architect easy. The structural engineers design for the architects to meet their requirements by designing the entire framework of columns, beams, and bracing after which engineering calculations proceeds. The framework of structural steel design can be categorized into three main categories that include main members, secondary members, and connections as stated by Neville (2000). The structural engineering of the 3 main members comprises of girders, trusses, beams, and columns. The framework’s framework has the main members and are usually primary members carrying loads that are imparted on the structure. Secondary members comprise of bracing, decking, and stairs that are designed to carry specific loads. On the other hand, connections are usually joints or nodes of structural elements which play a key role of forces transfer between structural members or elements. 2.1.1 Design of main member Architectural drawings determine the locations of columns from where design of concrete design will be put. The columns of steel then connect to the foundation of concrete. This is through anchor bolts that have been embedded within the concrete and connected to column plates using washers and nuts. The framework of members’ configuration is determined by columns location. 2.1.2 Design of secondary members and connections Secondary members for supporting are designed after location, sizing, and determination of main members’ loads and reactions. The secondary members’ dead weight plays a key role in design of main structural members. The connection’s design involves analysing the forces and reactions such as axial forces, torsion, shear, and bending moments. All these require different designs in order to cater fully for those forces as illustrated by American Institute of Steel Construction (2001). After the design of the frame is made by engineers in conjunction with designs by architects, several elements are fabricated in the factories ready for construction process. Hot rolled elements are found in various forms of cross sections that vary from angle, square, and round sections with familiar ‘H’ and ‘I’ parts of universal beams and columns as illustrated by American Institute of Steel Construction (2001). The completed frames are taken to site where concrete foundations and other ground works are prepared. Connections are done through bolting or screwing leading to erection of frames. 2.2 Properties of Steel Steel is an alloy of carbon and iron which is most versatile and has a lot of strength as well as an attractive appearance. Steel has high tensile strength as well as compression strength. Steel has some ductile behaviour that causes its plastic deformation. It has uniform quality with proven durability with low life cycle. Unlike most of the building materials such as concrete, steel does not need time for curing process as it is always in its full strength as illustrated by American Institute of Steel Construction (2001). Steel does not twist or wrap and it does not expand or contract substantially with the weather. The amount of carbon within steel determines its strength whereby increased carbon amounts results to increased hardness and shear strength while decreasing its toughness and ductility. Steel usually absorbs shocks when subjected to sudden shock. Under tensile test, stress is directly proportional to strain to some point where plasticity is experienced. Figure 1 stress-strain curve for structural mild steel (tensile test under normal conditions) 2.3 Advantages and disadvantages of steel frame construction There are several advantages associated with steel frame construction including relatively lightweight, durability, quick erection on site mainly because of offsite fabrication, and good design in terms of flexibility. Steel frame construction has high strength to weight ratio, it does not crack split or warp, steel can be recycled indefinitely with 100percent recyclability, fire resistance, and earthquake tested. Steel allows architects to expand and improve their artistic expression through creating challenging structures that they design. It is through steel that beauty and enhancements are produced which would have been otherwise been difficult to produce using other building materials as illustrated by Buchanan (2000). However, they are some disadvantages associated with steel frame construction including its corroding properties although maintaining a cavity in external walls reduces the impact. Steel also requires large significant energy amounts to manufacture. High temperatures may soften the steel hence the need to protect it from fire. 2.4 Steel Construction details examples Steel-plated structures are usually subjected to several deformations and loads in the life time. Therefore, there is need to design a structure that can hold those demands fully in the expected time. Statistics should be used in an effort to determine the safety levels needed in the process of designing. Detailed design, fabrication, and erection are some of the major process undertaken in the steel frame construction. Pad foundations having fixing bolts are positioned for ensuring columns are attached to them as illustrated by British Steel Plc (1999). Column steel is then fixed using bolts to form around the frame. However, welded connections can as well be used to support columns. Beams and columns are usually connected using welded end plates and bolts as illustrated in figure below. The erection of flames requires constant check to ensure it is in proper level and alignment. Figure 2 connections of beams and columns using welded end plates & bolts 2.5 Case studies/examples of steel frames The New York Times building in Manhattan City is one of the world’s steel structures in the world. This is the headquarter of New York Times Company. It has exterior decorative steel wall of 840feet or 256 meters. This building has steel frame of which its 95percent has been recycled as illustrated by British Steel Plc (1999). Figure 3 New York Times building 3.0 Concrete frame construction Concrete is attained by mixing of sand, cement, and aggregates with water. Concrete can be viewed as an ‘artificial stone’ which can be moulded into any shape. In most instances, steel bars are put together with concrete to come up with reinforced concrete which is a composite material as stated by Neville (2000). The concrete structures should be designed in such a way that they have acceptable and high probability of performing satisfactorily within its set lifetime. 3.1 Design and construction process Reinforced concrete frame construction has vertical elements (columns) and horizontal elements (beams) that are joined together using rigid joints. The structures are monolithically cast in such a way that columns and beams are cast in a single operation so as to act in union. The concrete frame construction process involves three main stages that include pre-construction, construction and post construction as illustrated by Welsh (2001). The design process involves estimation of loads and forces that are likely to be applied in the frame. Other estimates done in the design and pre-construction stage include strength of material. Design of reinforced concrete structures mainly starts with assembly of various basic structural elements including columns, slabs, beams, foundations, and walls. These basic reinforced concrete elements should be designed with careful development of connections and joints. The design also includes determining the effect of combining different loads. The design is also done in such a way that it caters for other actual loads including dead loads, wind loads, earthquake loads, and live loads acting on the concrete frame structure as illustrated by Welsh (2001). There are several methods of design including work stress and limit state method. The best method of design especially for reinforced concrete members is semi-empirical limit state. Construction involves placing and compacting of concrete in well designed form work. However, required bars for reinforcement for the bending moment, axial thrust, and shear force must be put in place as well as proper preparation and accommodation of proper bending schedules as stated by Neville (2000). A properly designed frame work usually plays a great role in construction and use of concrete. This is towards reducing the loss of slurry from the concrete. Proper protection must be taken into consideration to prevent evaporation and loss of water or loss of heat during hot and cold weather respectively. The form work is reduced after the concrete attains required strength to support the structural frame (Arthur, 2004). It is important to deposit concrete near its final placing position to avoid wastage and re-handling. The compaction must start before initial setting time and should not be disturbed after the start of initial setting. There should be no displacement of reinforcement bars or movement of formwork while placing the concrete. Mechanical vibrators should be used to compact concrete especially around the corners of form work, embedded fixtures, and reinforcements to avoid honeycomb formation. Proper curing should be undertaken to reduce moisture loss from the concrete as well as maintaining satisfactory temperature. 3.2 Properties of concrete Concrete is usually obtained through the mixing of sand, cement, water and gravel in specific proportions. Mineral admixtures can also be added in an effort to improve some concrete properties as stated by Neville (2000). Therefore, the deformation and strength properties of concrete depend on the individual characteristics of water, admixtures, sand, gravel, and cement used. Curing of concrete is crucial towards the attainment of its strength. Plain concrete is known to be good in compression although it is very weak in terms of its tension as illustrated by Welsh (2001). Therefore, steel is required to be used to make the concrete sustainable in tension forces. The mix of water, gravel, sand, cement, and admixtures results to a paste after being mixed surrounding all the aggregates forming a plastic mixture. After a short period of time of less than 1hour, a hydration process occurs between cement and water where concrete changes from plastic to solid state within 2hrs as stated by Neville (2000). Concrete get more strength as it cures as illustrated in figure below. Figure 4. Typical strength-gain curve Theoretically, continued moisture conditions increases the strength of concrete although in practical terms its gains 90percent of its strength within the first 28days. Concrete has very low tensile strength hence the need for its reinforcement. On the other hand, its compressive strength depends on curing environment and proportions and quality of ingredients used. Water and cement ratio are key indicators of strength. The workability, set times, and durability properties depends on the amount of admixtures used (Arthur, 2004). In a typical composition of concrete, cement takes 7-15%, water 14-21%, and aggregate 60-80% in volume. The concrete elasticity is constant at low stress levels although it decreases at higher levels of stress as matrix cracking develops. Another property of concrete is its very low coefficient of thermal expansion. Tension and shrinkage usually makes the concrete structures to crack. 3.3 Advantages and disadvantages Concrete frame construction has some advantages including energy saving. In comparison to steel construction, concrete has less expense in terms of cooling and heating as indicated in the figure 2 below. Another advantage is that it is naturally fire resistant. Concrete qualifies as one of the fire reducers. Concrete also has high strength, low maintenance, and high durability. Concrete is also weather tight and requires less maintenance. Concrete also provides innovative solutions for architectural versatility and interest in design (Arthur, 2004). In comparison to steel, concrete has shorter floor-to-floor height by up to 2 feet per floor. Concrete has very good quality in terms of handling compression forces. It is also brittle making it good in making rigid structures. Figure 5: Thermal Reservoir Comparison: concrete vs. Steel There are various disadvantages associated with concrete including the fact that concrete cracks at some point in time. Concrete can also crack due to shrinkage in its process of drying. Concrete has a low thermal conductivity hence intense heat can damage it leading to cracks (Arthur, 2004). Furthermore, concrete requires special precautions when handling as it may lead to its failure or cracks. Concrete has poor tension handling qualities. It also requires other materials such as steel in order to reinforce it against excessive tension and shear. 3.4 Construction details Precast concrete frame Precast concrete frames involve fabrication of the structures off-site whereby they are transported to the site for incorporation. They are usually cast using some definite shapes that determine the shape and their size (Arthur, 2004). Heavier reinforcements are used to give enough strength to the precast concrete frames. Elegant connections are used between the beams and columns in an effort to transfer substantial forces with affecting the frame. 3.5 Case studies/examples CITIC plaza in Guangzhou, China is a good example of world’s tallest concrete building in the world. It’s height is 391m (1283ft) and it was completed in 1997. It has 80 stories becoming 13th tallest building in the world, 7th in Asia and 4th in China. Figure 6 CITIC plaza (source http://www.skyscrapercity.com/showthread.php?p=58998269) One of the best examples of a concrete frame constructed building is 311 South Wacker Drive in Chicago USA. This is a post-modern skyscraper that is 293m (961feet) tall with 65 storeys. It is 16th tallest building in US and at some point; it was second tallest reinforced concrete building globally. Figure 7. 311 South Wacker Drive 4.0 Methodology of Limit state design Limit states can be viewed as the acceptable limits for serviceability and safety requirements of the structure before failure takes places. The structure’s design using limit states method will make sure that limit state will not be reached and will not become unsuitable to use in their intended purposes. A limit state is a set of performance criteria including strength, twisting, vibration, stability, buckling, collapse, and deflection which requires to be met in situations where structures are subjected to the loads. Limit state comprises of two types including limit state of serviceability and limit state of collapse/strength. Limit state of collapse deals with structure’s stability and strength that are subjected to maximum design loads from possible combinations of various loads types as illustrated by Varghese (2002). Limit state of collapse makes sure that no part or entire structure collapses or even become unstable under any overloads combination situation. Limit state of serviceability deals with structure’s cracking and deflection under service loads; fire resistance, durability under working environment, and entire structure’s stability. Figure 8. Two main limit states The common design practice of concrete structure through principles of limit state comprises in taking up each of the conditions given above and providing for them independently so that the structure is secure in limit states of stability and strength. Procedure for limit states design 1. Ultimate strength condition An overload should be allowed by the structure or member’s ultimate strength. Therefore, it is important to design structures with accepted ultimate load theory in order for them to carry the overload specified. This may be torsion, tension, shear, compression, and in-flexure. 2. Durability condition The structure should be fit for the environment it is being set in. The steel, its cover, cement concrete content must satisfy the environmental conditions. 3. Deflection condition The structure’s deflection under conditions of the service loads must be within allowable limits. This can be done using 2 methods: i. Empirical method – as the crucial empirical factor controlling deflection include depth/span ratio, it is possible then to control deflection by limiting the span-depth ratios. ii. Theoretical method – this can be used to calculate deflection and controlled through appropriate structure dimensioning. iii. 4. Cracking condition Cracks of more than allowable widths should not develop in structures under condition of service load. However, this can be controlled using - i. Empirical method – through strict follow of empirical bar that detail rules ii. Theoretical method – the probable width of the crack is checked using theoretical calculations 5. Lateral stability against overall stability (accidental horizontal loads) Strict measures and conditions need to be observed when detailing and designing the peripheral, horizontal, vertical, and internal ties within structure. It is important to take into consideration all the relevant limit states within the design. This is towards making sure there is adequate degree of serviceability and safety according to Ashok, K et el (2002). The structure should be designed on the basis of most critical limit state as well as checked for other limit states. Figure 9. Structural Design The limit state of strength are related with imminent failure under action of probable and unfavourable loads combination on the structure by use of suitable partial factors of safety that are likely to endanger the life and property’s safety as illustrated by Varghese (2002). The limit of state of strength comprises of:- Fracture due to fatigue Loss of the structure’s stability or any of its part including foundations and support Loss of the structure’s equilibrium either as a whole or its section Failure due to extra deformation and rupture of the structure Brittle fracture The limit state of serviceability comprises of – Vibrations within the structure or its components leading to discomfort to people, damages to structure or that limiting the effectiveness of its functions. Special consideration is given to vibration in the floor systems prone to vibration including large open floor sections that are free of partitions towards ensuring that vibrations is accepted for the set occupancy and intended use according to Ashok, K et el (2002). Deflections and deformation that may adversely affect effective or appearance or use of structure that is likely to affect its functional effectiveness. Durability and corrosion Damage that is repairable as a result of fatigue 4.1 Partial safety factors Characteristic load is the load that has a probability of 95percent of not being exceeded during structure’s life. Some of the loads acting on the structure comprise of wind loads, live loads, dead loads, or earthquake loads among others. Characteristic load must not be more than mean/average load. According to assumptions, 95percent of characteristic loads is not exceeded in the structure’s life as illustrated in figure below according to Ashok, K et el (2002). Nevertheless, structures are as well subjected to overloading. Therefore, it is crucial to design the structures with obtained loads (factors of safety multiplied by characteristic loads) depending on the loads nature or even their combinations or limit state taken into consideration. The loads’ factors of safety are referred to as partial safety factors (γf) for loads. The design loads are calculated as:- Design load Fd=(Partial safety factor for load γf)(characteristic load FCK) Figure 10 Characteristic load Load combinations Limit state of collapse Limit state of serviceability (for short term effects only) DL IL WL DL IL WL DL+IL 1.5 1.0 1.0 1.0 1.0 DL+WL 1.5 or 0.9 - 1.5 1.0 - 1.0 DL+IL+WL 1.2 1.0 0.8 0.8 DL=dead load WL=wind load IL=imposed loads Figure 11 Figure values of partial safety factor for loads The partial safety of steel and concrete is 1.15 and 1.5 respectively. These are used when assessing the structures’ strength or structural members utilizing limit state of collapse. The method of limit state is usually based on a stochastic process. This is whereby the parameters are taken into mainly from observations done over a period of time according to Ashok, K et el (2002). The partial safety factors concept for material and load strength is usually based on probabilistic and statistical aspects. The partial safety factors for the strengths of materials are decided on the grounds of reliability of reinforcement and concrete preparation. The structure’s overloading is also taken into consideration while specifying the loads’ partial safety factors. 5.0 Structural effects of fire on steel framed building In general, structural steel when subjected to fire softens, bends, and losses strength. The engineering materials’ strength reduces with increase in temperatures including steel. Fire exposure to steel structures results to some adverse effects which can be traced in some of the major buildings globally. Such include McCormick place in Michigan, World Trade Center 5, and Alexis Nihon plaza. In some instances, building fire may reach as high as 10000C. This usually has great adverse effects on the steel and consequently the structure. At 10000C, steel exposed to fire takes approximately 25minutes to lose its stiffness and strength by over 60percent. Increased temperatures makes steel to become weaker and softer as argued by Gorenc, B et el (1996). Nevertheless, steel is incombustible and usually recovers its strength fully in most instances after fire. Steel usually absorbs some considerable thermal energy amounts when exposed to fire. However, after cooling to ambient temperature, steel gets back to its stable condition. In this cycle of cooling and heating, some individual members of steel becomes slightly damaged or even bent. However, this does not affect the entire structure’s stability. The steel’s yield stress remains the same up to a temperature of approximately 2150C. Thereafter, it progressively loses its strength. The coefficient of thermal expansion varies with temperature as illustrated by the formulae and figure 12 below that clearly illustrates it. {α(T)={12.0+T/100}×10-6 (0C)-1} Figure 12 Mechanical properties of steel at elevated temperatures The low carbon steel used in automobile and buildings usually tend to bend other than shattering. In most instances, some sections of structure subjected to extreme temperatures are likely to bend leading to that section toppling or sagging. At a temperature of 5500C, steel loses its yield strength by 50percent. At 10000C, the yield strength becomes 10percent or less. The high thermal conductivity of steel makes the temperature of unprotected steel work vary with that of the fire. Furthermore, the unprotected steelwork does not only lose its load-bearing capacity but it undergoes significant expansion when sufficiently heated. The young’s modulus does not decline with temperature rapidly as compared to its yield strength. Figure 13 graph of strength against temperature When heated, cold-worked reinforced steel lose their strength rapidly compared to hot-rolled, high-yield and mild-steel bars. On cooling, original yield stress of all bars is nearly recovered when temperatures are 500-6000C. However, cooling from 8000C, yield stress is reduced by 5percent for hot rolled bars and 30percent for cold-worked bars. Prestressing steel lose their strength at a lower stressing temperature as compared to reinforcing steel. Heat-treated and Cold-drawn steel lose some of their strength permanently when heated to temperatures more than 4000C and 3000C respectively according to Kirby & Preston (1988). The steel’s creep rate is sensitive to higher temperatures. It becomes more important for prestressing and mild steel at temperatures above 3000C and 4500C respectively. In the tests of fire resistance, the rate of temperature rise when steel is getting in to its critical temperature is very fast to mask any creep’s effect. Heated steel usually experience thermal expansion. Heated steel beams usually expand leading to high axial force on other surrounding structures. High level stress can also be developed under low temperatures although axial restraint is only partial. Cumulative Mechanical and thermal strains is usually the total strain of the structures that are redundant in fire conditions as illustrated by Mason (2000). The plastic or elastic stress is governed by mechanical strain within the structure while deformed shape is governed by total strain. In cases where there is no external loading and beams are unrestrained, thermal strain is equal to total strain that usually governs the deflection. Under fir e conditions, the induced axial force on the beam is merely as a result of thermal expansion. On buckling, the beam’s deflection rapidly increases while axial shortening is observed through the curvature of member. At post-buckling zone, there is little alteration in axial force as thermal expansion is absorbed by greater deflection. Buckling is also localised at specific sections within a structural member as stated by Rotter & Usmani (2000). There are usually three plastic hinges formed when a fully fixed beam is subjected to high temperatures. These include one at midspan, and others at each end. At the bottom flange of the span’s ends and the midspan’s top flange, the high compressive stress are induced. This causes those areas to buckle because of bending referred to as local buckling. Some steel members usually have temperature properties that are not uniform which in some instances results to high thermal gradient across the steel sections. This results to some hotter surfaces expanding much more that the sections that are cooler as illustrated by Mason (2000). This result to induced bending within the member a situation referred to as thermal bowing. Protection of steel plays a key role in determining and influencing the performance of steel. The rate of heating of protected steel is low as compared to unprotected steel. Figure 14 Rate of heating of structural steel work This report also illustrates the effects of fire on steel through some of the examples within the buildings that have occurred in some parts of the world. They include McCormick place, world trade center 5, and Alexis Nihon Plaza. McCormick Place This was a large hall of exhibition that was built in Lake Michigan shores, in Chicago. The hall was built in 1960 using reinforced structural steel and concrete. It had a large theatre, space for exhibition, supporting rooms and several other rooms. The roof of the structure was supported by large trusses of steel that spanned 210 feet column to column while cantilevering 80 feet on both sides. In January 16 at around 2 a.m., 1967, large fire began and burned 3rd floor while spreading to 2nd floor through the melted expansion joints. The structural steel of the hall had no protection against the fire as illustrated by Mason (2000). This led to excessive heating of truss members leading to roof collapse. This was after 1hour when the fire had broke out; the excessive temperatures resulted to decline of yield strength by the unprotected trusses of steel. World Trade Center 5 This was a 9 storey office building that was built around 1970. In 2001, the collapse of world trade Center towers produced a lot of debris that broke fire in World Trade Center. The subsequent fires led to shear connectors of the tree assemblies of column to fail, this initiated collapse from 8th floor through 5th floor. The 9th floor and roof which had no column trees as well as 4th floor and those others below did not collapse. All the structural members of the building were fire-protected with a mineral fiber that had been sprayed as illustrated by Mason (2000). This provided a 3-hour rating and 2 hour rating to the floors and columns respectively. However, the failure of shear tab connections was due to secondary tensile forces which developed from catenary action. Catenary action can be described as a phenomenon which takes place in composite beams due to thermal expansion in the member because of high temperatures. Generally, the fire has some adverse effects on the steel structures greatly affecting their tensile strength. Tensile strength is greatly reduced when steel is subjected to high temperatures greatly affecting their performance. However, the amount of temperature varies the rate of strength loss in the steel. Elevated temperatures lead to reduction of stiffness and strength causing failure because of excessive deformations. Insulation on the other hand plays a key role in reducing the effects of increased temperatures on the steel frames. Therefore, steel structures need to be adequately protected against the fire effects. Thermal elongation of steel is directly proportion to temperature increase as illustrated by Mason (2000). Thermal conductivity of steel decreases constantly with temperature increase, this is to a temperature of about 8000C where it remains constant. Steel has a Modulus of elasticity that decreases with increase in temperature. Steel have been discovered to have same compression and tension properties which makes it behave elastically to a specific yield point and eventually behaving in a very ductile manner afterwards. 6.0 Conclusion Concrete and steel are becoming increasingly important as building materials in the construction industry. The combination of both materials results to high strength materials for the building and structures. This becomes increasingly important when designing beams and columns of a structure. Design and construction process plays a key role in ensuring that strong buildings are constructed. In reinforced concrete structures, the concrete’s strength is enhanced by steel that carries the tensile forces. The combination of both materials provides a better insulation for fire against the steel making its impact less on the steel properties. Limit state design plays a great role in ensuring the safety of structures is maintained. However, steel has more advantages due to its high strength properties and it is not affected by fire as easily as concrete. However, it is recommendable that both be used in order to achieve possible maximum strength in the structure. References American Institute of Steel Construction 2001, Manual of Steel Construction-Load and Resistance Factor Design, 3rd ed. New York, American Institute of Steel Construction. British Steel Plc 1999, The Behaviour of Multi-storey Steel Framed Buildings in Fire, South Yorkshire, UK, Swinden Technology Centre. Buchanan, A 2000, Structural Design for Fire Safety, Christchurch, New Zealand, School of Engineering, University of Canterbury. Gorenc, B et el 1996, The Steel Designer's Handbook, Kensington, New South Wales, UNSW Press. Kirby, B. & Preston, R 1988, High Temperature Properties of Hot-Rolled Structural Steels for Use in Fire Engineering Design Studies, Fire Safety Journal, Vol 13, pp. 27-37. Mason, J 2000, Heat Transfer Programs for the Design of Structures Exposed to Fire. Fire Engineering Research Report No. 00/9. Christchurch, New Zealand, School of Engineering, University of Canterbury. Rotter, J. & Usmani, A 2000, Fundamental Principles of Structural Behaviour Under Thermal Effects. First International Workshop on Structures in Fire (Copenhagen – June) (as Report TM2). Welsh, R 2001, 2-D Analysis of Composite Steel-Concrete Beams in Fire. Fire Engineering Research Report. Christchurch, New Zealand, School of Engineering, University of Canterbury. Ashok, K et el 2002, Reinforced Concrete Limit State Design, London, Macmillan. Varghese, P 2002, Limit State Design of Reinforced Concrete, New Delhi, Prentice hall. Arthur, H 2004, Design of Concrete Structures, New Delhi. Tata McGraw-Hill Publishing Company Limited. Neville, A 2000, Properties of Concrete, New Delhi, Longman. Appendices Figure 1 stress-strain curve for structural mild steel (tensile test under normal conditions) Figure 2 connections of beams and columns using welded end plates & bolts Figure 3 New York Times building Figure 4. Typical strength-gain curve Figure 5: Thermal Reservoir Comparison: concrete vs. Steel Figure 6 CITIC plaza (source http://www.skyscrapercity.com/showthread.php?p=58998269) Figure 7. 311 South Wacker Drive Read More