Concrete Frame and Steel Frame Construction - Assignment Example

Structures, Materials and Fire Table of Contents Contents Contents 2 Part 1 2 Part 2 10 References: 14 Part 1 1. Concrete Frame and Steel Frame Construction 1.1 Typical Design and Construction Process Frame systems of concrete and steel is somewhat similar as they both have columns, primary and secondary beams, floor slabs, and joist but concrete frame systems vary widely in design and construction. For instance, frames that are normally a set of columns and beams can become bearing/shear wall, thicker slab replaced the usual joist, and moment frames replaced by smaller frames with infill shear walls to provide stronger support for lateral loads. This concrete frame construction is also known as flat-plate construction. Moreover, column spacing in concrete is between 18 to 45 ft while the depth of the beam depends of this column spacing- the wider the column spacing the deeper the beams (Ambrose & Tripeny, 2010:182). The cross section of concrete columns are normally rectangular, square, or round and it they have a minimum dimension of 10 inches and maximum dimension of 24 inches. The thickness of the frame system in concrete construction is measured from top of the slab while beam width is often similar to column width (Ambrose & Tripeny, 2010:184). Reinforced concrete frame is often moulded to a particular shape in order add compressive strength or tensile strength. In situ cast frames is widely used for structural material for multi-storey building for a number of reasons including the ability to form repetitive floor plans inside a skeleton frames of continuous columns and floors, use the same formwork and false work, support variations in loads, and others. Reinforced concrete structural frame allows partitions and framing to be built inside the frame by bolting them directly to a solid concrete backing. This type of concrete frame is usually built with one-way spanning floors cast monolithically with reinforced structural frame as shown in Figure 1 (Emmitt & Gorse, 2010:385). Box frame construction on the other hand is used when the multi-storey building has identical compartments planned on successive floors one above the other, when it is required to have permanent division, sound proofing, and fire resistance. A box frame system consist of in situ cast external and internal walls and floor can be used in multi-storey buildings without columns or beams as shown in Figure 2 (Emmitt & Gorse, 2010:385). Steel framing is commonly used for commercial and industrial buildings. Similar to concrete framing, this type of frame has columns and beams with triangular trusses as shown in Figure 3 (Brannigan & Corbett, 2010: 2002). Note that steel in this frame is generally unprotected. Figure 3- Example of Steel Frame Construction (Brannigan & Corbett, 2010) 1.2 Material Properties Concrete is made by mixing cement with sand, stone, aggregate, and water (Goody et al, 2010:178). These aggregate are held together and hardened by the chemical reaction between cement and water or hydration. Portland cement is the most common form of cement which is generally made from lime, silica, iron oxide, and alumina. Concrete strength is determined by the ratio of cement and water – less water the stronger the concrete (USDA, 1999:2). According to Brannigan & Corbett (2010:225), concrete’s compressive strength is good but it has poor tensile and shear resistance. The main properties of concrete according to Bhatt et al, (2005:13) are compressive strength, tensile strength, modulus of elasticity, creep, and shrinkage. Compressive strength of concrete is measured through a 28 days cube strength test as specified in BS EN 12390:2:2000 ( Testing Hardened Concrete: Making and Curing Specimens for Strength Test & BS EN 12390:3:2000 (Testing Hardened Concrete: Compressive Strength of Test Specimens). Tensile strength of concrete of the other hands is about “splitting” strength while modulus of elasticity is about short-term stress being experienced by concrete while with load, longitudal vibration (dynamic modulus), and creep. Creep is the gradual increase in strain while concrete is under stress and it is dependent of the concrete mix strength, type of aggregate used, curing time, temperature, and duration of sustained loading. Steel is generally made up of iron and controlled carbon content and produced through smelting (Walker, 2004:5). Carbon steel is the most common type of steel and oxidation is the most method in steel production as its strength is determined by the amount of oxygen removed from melting steel. Physical properties of steel include density, electrical conductivity, coefficient of thermal expansion, and magnetic permeability (Chandler, 1998 :41) while hardness, machinability, corrosion resistance, and tensile strength is steel’s mechanical properties (Lesko, 2008:15). In terms of strength, hardness is determined by steel resistance to penetration and commonly tested through Brinell test (Chandler, 1998:37). Machinability on the other hand is a property that determines ease of fabrication (Davis, 1995: 140) while tensile strength refers to the ability of steel to withstand stress and undergo inelastic deformation without fracture (Lindeburg, 2004:53). 1.3 Advantages and Disadvantages Reinforced concrete weighs more and has lower strength while construction involving concrete framing is slower as there are more work to do such as erection of shutter, laying reinforcing steel, pouring concrete, curing, and removing the shuttering. In contrast, steel framing is simple and fast but steel is subject to rust and its strength starts to decrease when exposed to temperatures equal or above 550 degrees – ex. fire (Reid, 1984:22). Steel framing is excellent for resisting stress from snow, wind, and seismic forces but increase risk of thermal bridging through exterior walls (Jefferis & Madsen, 2004 :506). In terms of fire, strength of steel framing with protective covering is more vulnerable compared to concrete framing but collapse and fire during construction are more prevalent in concrete (Brannigan & Corbett, 2010 :225). A building constructed with non-combustible concrete does not necessarily that it is safe from fire as according to Dornan (:149), reinforced concrete can fail if it is subjected to a fire for a long period of time and if the building contains large amount materials that can generate extreme fire loads. Similarly, unprotected steel beams can fail if exposed to fire resulting to collapse of the entire building. 1.4 Example of Construction Details a. Concrete Framing Construction In concrete construction, beams, joist, and floor slabs are integrated systems (Ambrose & Tripeny, 2010:183). They are integrated in the sense that are reinforcement connecting them and generally poured with concrete at the same time – monolithic – as shown in Figure 3. Figure 4- Slab and Beam Framing System and Cross Section of Beam and Slab (Ambrose & Tripeny, 2010) Concrete framing construction b. Steel Framing Construction Heavy steel frame is commonly used in medium to high-rise buildings as shown on the right and therefore employ the same construction method for these types of buildings. The frame includes the post, columns, beams, girders, bracing, and other elements. However, external walls in steel framing bear no weight – curtain walls. In areas where earthquake and hurricanes are of important concerns, special standard may be applied in constructing steel framed buildings (Fisher & Martin, 1991:59). Lateral stability is provided by braced bays vertically repeated on each floor as shown in Figure 5. Core construction and tube construction are the most common form of steel framed construction. Buildings constructed with core construction method (as shown by two images on the right) usually use lightweight steel or reinforced concrete frames with curtain walls consist of thin, vertical metal struts or mullions. On the other hand, steel-framed tube construction (as shown on the right) is somewhat an advanced approach where it used load-bearing exterior or perimeter walls to support the weight of the building. In this example, in order to form a torsionally rigid tube, the floors were used to stiffened the load-bearing external walls. Tube structures are resistant to lateral forces from wind and effects of earthquake (Craighead, 2009:19). Figure 5- Steel Framing Design for Stability (Reid, 1984) 1.5 High Profile Building Constructed Using Concrete and Steel Frame The General Post Office building in London is the first reinforced concrete-framed building in the United Kingdom (Emmitt & Gorse, 2010:384). The Petronas Twin Towers in Kuala Lumpur, Malaysia (right image) was built using steel framing in 1998. The Shun Hing Square tower (left image) that was built in 1996 is one of the world’s tallest all-steel buildings in China. The Walt Disney Concert Hall (left image) in Los Angeles California is also a steel-based structure while the Willis Tower (right image) formerly known as Sears Tower was built with nine square steel tubes with huge steel columns that support the whole frame. Part 2 2. Structural Effects of Fire on a Steel Framed Building Steel is a common building material, it is often used in beams, girders, columns, and bar joist to support floors and roofs, and non-combustible as it does not burn and contribute to the spread of fire. However, steel have tendencies to expand when exposed to heat particularly when temperature reached 1000 degrees Fahrenheit or 538 degrees centigrade. Moreover, steel conducts heat thus can ignite combustible materials close to it. It also loses 40 to 50 percent of its strength when exposed to extreme temperature resulting to partial or total collapse (Cote, 2004:152). Comparisons conducted by Rosalo (2001:1373) on different framing system such as heavy timber and light steel framing suggest that light steel collapse faster than timber when exposed to fire. This is because wood can take the heat and therefore takes time to self-destruct while steel takes only about 538 degrees centigrade to lose its strength and collapse. Loss of strength due to heating is the most common effect of fires in structures made of steel or reinforced concrete (Shen et al, 2005:992). However, the effects of fire is dependent on the position and function of a certain element within the building structure thus the degree of fire protection of each member of a steel-frame structure also varies (Graham et al, 1997:106). This is the reason why fire resistant design for steel structures adopted a new concept such as BS 5950 Part 8 that according to Chan & Teng (1996:291) is now bypassing the traditional 550 degrees temperature capacity of structural steel elements and allow higher temperature to be calculated for those elements with applied loads lower that the full design loads. In other words, some elements of steel framed structure only need minimal fire protection. Under fire conditions, steel behaviour creates a number of problems because it expands when it is heated. According to Norman (2012:543), a 100 ft long I-beam can expand nine and half times its length when heated to 1,000 degrees Fahrenheit. This expansion in reality can push other structure elements such as columns and walls. Heated furthermore can result to loss of strength, twisting, and sagging thus any element depending on it will fall. Moreover, extreme heat is not the only problem with steel as while sagged or twisted it has the tendency to contract back to its original length when it cooled down resulting to collapse. For instance, when an I-beam that were severely twisted by the fire cooled down as a result of fire fighting water shrink, it will be not be resting on their original support and therefore will fall. Heat can severely affect the strength of steel and particular those designed and constructed with no fire protection, their strength according to Norman (2012:545) easily deteriorate in a matter of 5 to minutes when exposed to fire. Although thicker steel can stand longer but it will ultimately sag and fail when temperature go over 1, 500 degrees Fahrenheit. For this reason, it is very important that steel structural elements are protected from heat in the form of fireproof coatings or encasing. Investigation of the 2001 World Trade Centre collapsed suggests that thermal expansion of critical interior column occurring at temperature way beyond established structural fire resistance ratings is the cause of structure collapsed. The buckling of this column led to vertical progression of floor failures due to loss of lateral support (Craighead, 2009: 247). According to Lambert (1997:18), non-combustible materials like steel are not necessarily fire resistant just because they do not provide fuel for the fire and allow flame to spread. This is because they transmit heat, expands and buckles when exposed to intense heat. However, recent studies on the behaviour of structural steel during a real fire particularly those that are conducted in the United Kingdom suggest that fire resistance of steel structures are greater when they are connected as framework. For instance, according to Tapsir (2004:2), framed steel have greater fire resistance compared to its individual elements because steel members that already reached their limiting temperature during a fire will yield and transfer the stresses to the cooler members of the frame. Moreover, contrary to common belief, steel structures can still maintain their integrity when the temperature reaches 1000 degrees centigrade if the heat is not uniformly distributed and there are cooler regions to absorb the heat. Similarly, a member that is exposed to fire but carrying lower load can withstand much higher temperature and therefore higher fire resistance. These findings are similar to the result of Wang (2002:67) analysis of the 1990 major fire incident that occurred in a 14 storey building with unprotected steel structures in London. In this fire incident, the fire temperature reached 1000 degrees centigrade affecting the unprotected floor beams, trusses, and columns but the steel frame survived fire without collapse. The heavier columns were undamaged while the lighter ones deformed and contracted by some 100 mm. The result of the investigation suggests that the steel frame survived because of “continuity” of the structure which is similar to Tapsir (2004) explanation above. Wang (2002: 68) noted that the moment plastic deformation occurred in the failed columns occur; their loads were distributed to the other cooler members of the structure and resist collapse as a whole. In response to previous high-rise fire involving steel framing such as the 1st Interstate Bank Building in Los Angeles, Broadgate Phase 8 in UK, and One Meridian Plaza in Philadelphia, the British Steel and Building Research performed a series of experiments in an eight-story Cordington building in the mid-1990s. These experiments found that the unprotected steel beam subjected to temperature up to 900 degrees centigrade did not collapse mainly because it is not isolated from the frame. These findings coincide with previous (except the World Trade Centre) fire incidents where unprotected steel framing did not collapse. For instance, the Broadgate complex was under construction when the fire incident occurs and therefore steel beams have no fire protection yet. However, similar to 1st Interstate Bank Building and One Meridian Plaza with protected steel framing and burned for about 19 hours, the Brodgate steel framing did not collapse. The structural interaction of steel members is seen as the beneficial behaviour of the complete frame during a fire which in other words suggests that the failure of one structural member does not necessarily mean structural instability of the whole building (FEMA, 2002:19). 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