Framing Construction Techniques - Report Example

Framing Construction Techniques Report 2nd October Table of Contents Introduction 3 2.Concrete Frame Construction 3 2.1.Typical Design and Construction Process 3 2.2.Material Properties 4 2.3.Advantages and Disadvantages 4 2.4.Examples of Buildings 6 3.Steel Frame Construction 6 3.1.Typical Design and Construction Process 6 3.2.Material Properties 7 3.3.Advantages and Disadvantages 8 3.4.Examples of Buildings 9 4.Timber Frame Construction 9 4.1.Typical Design and Construction Process 9 4.2.Material Properties 10 4.3.Advantages and Disadvantages 10 4.4.Examples of Buildings 11 5.Limit State Design Methodology 12 6.Structural Effects of Fire on Steel Framed Buildings 15 7.Bibliography 22 1. Introduction Framing is widely used as a construction technique and focuses on building through the use of structural members that are better known as studs. These provide the frame with stability so that interior as well as exterior wall coverings can be attached and a roof can be installed. The frames are also reinforced by the use of diagonal braces as well as with the use of rigid panels. Framing construction allows the builders to cover up large areas at minimal expense while also achieving a number of different architectural styles. The construction industry has employed a number of standards in order to ensure that materials, dimensions and construction techniques are standardised. 2. Concrete Frame Construction 2.1. Typical Design and Construction Process Concrete frames are also known better as reinforced concrete frames and are composed of vertical elements known as columns and horizontal elements known as beams. These elements are connected together using rigid joints. The structures are often cast using monolithic methods which ensure that the structure acts in unison. These frames are known to be resistant to both gravitational loading as well as lateral loading such as through bending within both beams and columns. Reinforced concrete frames are constructed in a number of different ways. The major ones are: Non-ductile reinforced concrete frames with or without infill walls; Non-ductile reinforced concrete frames with reinforced infill walls; Ductile reinforced concrete frames with or without infill walls. 2.2. Material Properties Concrete displays a high compressive strength but relatively lower tensile strength. In order to augment concrete’s tensile strength it is reinforced with other materials such as steel bars because such materials have better tensile properties. Moreover concrete displays nearly constant elasticity at low levels of stress while the elasticity tends to decrease as stress levels are increased as matrix cracking begins to form. The coefficient of thermal expansion of concrete is also low and concrete tends to shrink as it ages. The phenomenon of shrinkage and tension ensure that nearly all concrete structures develop cracks to some extent. Creep is also observable in concrete that is subjected to continuous long term loading. (Kosmatka & Panarese, 1988) 2.3. Advantages and Disadvantages Concrete frames provide an easy method of creating high rise structures that are able to respond to both gravitational and lateral loading with ease. Moreover concrete frames are rigid enough to resist earthquakes and other similar calamities with ease. The recent rise in the costs of reinforcement and steel prices has driven the costs of concrete frames up but on comparison to steel frames, the price of concrete frames and steel frames is close enough. Concrete frames cost marginally lower than steel frame construction for high rise buildings that need to bear massive loads. However the cost of timber frames is lower but timber frames cannot accommodate such massive loading. The foundation costs for concrete frames is significant but the use of post tensioned slabs helps to reduce these costs as these slabs are lighter. Moreover cladding costs represent a large area for spending so cladding area has to be minimised. The least height between floors can easily be attained using concrete frames. New regulations imposed as the new part L of the Building Regulations stipulates that a building pass pressure testing before completion. Failure to comply with these regulations means that all joints have to be revisited, inspected and sealed as required which represents significant costs. Given the fact that concrete edges are simpler to seal, many constructors have shifted over to concrete frames for this reason only. Soffit zones within concrete flat slabs allow complete areas where services can be installed and distributed without the use of any down standing beams. This allows the design to be simplified and for greater and simpler coordination to occur. Moreover the installation of various services is simplest just below a concrete flat soffit. Simplified installation allows off site fabrication to be maximised to produce better quality as well as quicker turnaround times. In addition to providing simplicity, these advantages allow the cost of concrete frames to be lowered. Moreover flat soffits also provide greater flexibility and options for adaptability. Fire protection is not mandated for concrete structures as the material is able to display sustained fire resistance for up to four hours. Given that the structure is fire proofed by its inherent material properties, the cost for separate fire protection is reduced altogether. Moreover the time required for fire proofing, the design details required for fire proofing and other such allied functions are also removed from the designers and executor’s ends. Part E of the Building Regulations code requires that acoustic isolation be achieved. For other forms of construction this often means increased costs as additional finishing have to be attached to walls and floors in order to provide acoustical isolation. Similarly concrete frames are much better at dealing with vibration than other forms of framed structures because concrete has a large mass. Again this means that no new costs have to be introduced to the construction effort to provide extra stiffening. (Sustainable Concrete, 2011) 2.4. Examples of Buildings Concrete frame construction is widely employed in creating modular structures such as apartment complexes, housing projects, office projects etc. A large number of hotels and other such applications are created out of concrete frame construction too. The buildings are first developed using the concrete structure of beams, columns and joints after which it is clad using covering walls and finished using modular construction techniques such as prefabricated bathrooms. 3. Steel Frame Construction 3.1. Typical Design and Construction Process The steel frame construction technique relies on using vertical columns of steel as well as horizontal I-beams in order to construct rectangular grids which are in turn used to support the floors, roof as well as walls of the resulting building. These features are all attached to the frame using different techniques. The advancement of steel frame constructions enabled the creation of skyscrapers as other methods were not suitable enough. The profile of steel structural members that is typically used is the letter “H” where the flanges on the column are thicker in comparison to the flanges on the beam. This ensures that these structural members can withstand greater compressive stresses within the resulting structure. The practice of using round and square tubular sections is also present especially when these are filled with concrete. In a typical steel frame structure, the beams of steel are connected to the columns using bolts as well as other threaded fasteners as well as rivets. The central web structures of the I-beams are wider within columns than in beams so as to accommodate greater bending stresses in beams. Large sheets of steel are utilised to cover the structure to provide flooring. These sheets of steel are covered up with concrete by moulding it in place. Alternatively precast concrete slabs can also be used to produce flooring. Generally a raised flooring system is used which helps to provide gaps between the walking surfaces and the actual structural floor so that service lines and cables can be installed in these gaps. 3.2. Material Properties Steel is a typical alloy of iron that is created by introducing carbon as an alloying element in quantities between 0.2% and 1.5% as per the grade. A variety of other alloying elements such as magnesium, vanadium, chromium and tungsten are also added to steel to prepare it for appropriate use. Steel is less ductile than iron but is far stronger and harder than iron. Another reason for using steel is because it displays a high corrosion resistance and displays a high degree of weld ability. The structural steel created for building and construction for example I-beams display high second moments of inertia due to their shapes. This allows them to be very stiff as far as the cross sectional area is concerned. Steel is problematic when exposed to fires especially for large durations of time. The steel member may reach its critical temperature that may cause it to fail. At high temperatures the yield stress of steel is reduced by 60%. (Zalosh, 2003) 3.3. Advantages and Disadvantages Steel has been a preferred material for construction using steel framework structures for a variety of reasons. The use of steel to create high quality and aesthetically appealing buildings is apparent through the variety of buildings created out of steel. Moreover steel structures provide low maintenance costs in the longer run as compared to other construction materials. Similarly steel is a non-combustible material so a building created out of structural steel does not need a very large amount of fire proofing such as those required for timber structures. However steel structures are more prone to fire damage than concrete frame structures. Moreover steel is an environmentally friendly material that can be recycled after use. Certain steel based components used in buildings can be reused without the need for smelting and recasting. Steel also provides a strong, stable as well as durable structure that can easily resist most environmental threats. Another advantage offered by steel that is unmatched by other materials is the speed of construction. Prefabricated steel structural members can be bolted into place in far less time than other materials used for constructing buildings. Steel structures are also not prone to problems such as termite infestations that wreak havoc on timber frame structures. Moreover steel ages rather stably when compared to concrete so it offers a larger service life in comparison to other construction materials. Overall the cost of steel structures and concrete structures is nearly equal but steel possesses clear advantages when it comes to constructing high rise buildings at a fast rate. On the down side however steel is an excellent conductor of heat in comparison to both concrete and wood. This means that steel structures tend to conduct far greater heat than other construction materials. This is highly detrimental in case of a fire in which the steel structure will conduct heat throughout because of its high conductivity. Steel structures require far greater fire proofing than concrete structures to ensure that steel structures can withstand the large amounts of heat generated by building fires. Moreover the inclusion of temperature tends to lower steel’s yield strength substantially which means that steel structures on fire will be far more prone to failure than a concrete structures. Steel is also plagued by corrosion based phenomenon that tends to reduce the life of steel structures substantially. Unless a proper corrosion resistance mechanism is set into action, steel structures are highly prone to corrosion based failures. Moreover once corrosion damage occurs to a steel structure, there is no method of reversing the damage created which means that the life of a steel structure is highly dependent on corrosion control. (Geschwindner, 2007) 3.4. Examples of Buildings Most high rise buildings today are constructed out of steel. Steel construction has been in use for well over a century now with the creation of landmarks such as the Eiffel Tower, the Empire State building, the World Trade Centres, the twin PETRONAS towers in Malaysia etc. The central structure is created out of steel while the cladding is provided as desired through the use of glass as well as other construction materials to create strong visuals. 4. Timber Frame Construction 4.1. Typical Design and Construction Process Timber framing is used to create building structures by placing heavy timbers that are joined together by using pegged mortise along with tenon joints. Lap jointing has also been used extensively to create timber structures. Diagonal braces are inserted in timber frames so that the structure does not “rack” and so that vertical beams do not misalign. The wooden members are fitted together with enough tolerance to allow the wood to season properly without placing any additional strain on the structure. The walls of timber frame structures are often in filled through the use of loam, bricks, wattle as well as rubble. The walls are subsequently plastered to create a neat finish and to improve the insulation characteristics. 4.2. Material Properties Timber is widely used for construction of buildings especially homes. When harvested timber comes in differing shapes and sizes but it is often cut into standard shapes and sizes to conform to existing standards on timber frame construction. Moreover timber is also produced artificially and is known as engineered lumber. Most of this timber serves as structural members and offers better loading characteristics than naturally harvested timber. Timber needs to be dried off before use as the harvested timber contains significant amounts of moisture. The removal of moisture tends to warp the timber a little so it is advisable to dry timber before use. Moreover the grain orientation is highly important in timber frame construction as the loading is dependent on the fibrous composition of the timber. 4.3. Advantages and Disadvantages Timber has been a preferred construction material for a variety of reasons throughout modern history. The aesthetic appeal of timber construction as well as certain structural benefits makes timber a suitable building material. Timber frame structures can be erected rapidly and allow prefabrication as well as modular construction and mass production with ease. The speed of erection can be judged from the fact that an average sized house can be erected in 48 to 72 hours using timber frame construction methods. The use of timber favours environmental friendliness as well as the option to recycle timber once the frame is torn down. Moreover the use of timber ensures greater environmental sustainability as timber can be grown and harvested over and over. The frame created using timber can be encased using standard insulated panels with ease which allows speedier placement of windows, roofs, walls and other allied service lines and mechanical systems. Timber offers a high degree of customisation as well and it can be carved relatively simply to produce aesthetically appealing structures. The seismic survivability of timber framed structures is far greater than that of other construction techniques. Even though the foundations for a number of timbers framed houses have caved in over the centuries but the houses still stand and are still in use. (Ravenswaay, 2006) On the other hand timber frames are prone to absorbing moisture under tension and compression that causes these structures to slant in any direction. The structural members may also shift positions which pose significant hazards. Timber is also susceptible to rot including dry rot as well as infection from fungus. Termites also present a major problem in timber frame structures. Constant fumigation is required to ensure that timber framed structures are not compromised. Moreover timber framed structures require a high degree of fire proofing to ensure the structure’s resistance to fire as wood has a high affinity for fire and acts as a fuel for it. (Vince, 1994) 4.4. Examples of Buildings Timber framed buildings have been in predominant use as homes throughout Europe and North America. A number of old structures have survived to date from as far back as a few hundred years. Moreover new timber framed homes are also being created especially after a revivalist movement in the 1970’s began. Timber framed buildings are easily noticeable throughout most neighbourhoods and building sizes range from small to large structures. However timbre framed buildings are developed to a few floors only as timber framed structures do not perform well in gravitational loading. (Gotz, 1989) 5. Limit State Design Methodology Limit state design (LSD) is a design methodology utilised in problems relating to structural engineering. The limit state refers to a particular condition of a concerned structure after which it fails to fulfil the specified design criteria. (Eurocode, 2001) The limit state may refer to a condition arising due to loading or other allied actions on a structure whereby the structural integrity, durability or fitness of the structure may degrade to beyond acceptable levels. The LSD approach ensures that the designed structure is ready to handle all the major design requirements that the structure would have to face throughout its lifetime. It also ensures that the structure provides a reliable state on an appropriate level which keeps the structure fit for use through its anticipated or design life. Various building codes are based on LSD and tend to specify the appropriate reliability levels in clear terms. LSD generally requires the concerned structure to satisfy two major criteria that are the ultimate limit state (ULS) and the serviceability limits state (SLS). (McCormac, 2008) The use of any design process mandates that different assumptions be utilised such as the loads that the structure would be subjected to or the sizes of the members as well as the applicable design criteria. The design criterion of all engineering approaches is the same: to ensure that the structure is both functional and safe at all times. The ultimate objective behind LSD is the same too and the use of LSD has demonstrated its abilities as an approach to engineer reliable solutions for structural design and construction. The ultimate limit state criterion designates that a structure should not collapse when it is subjected to the total peak design load that it was designed for. The ULS criteria is seen to be satisfied only and only if the total factored shear, compressive, bending and tensile stresses are seen to be lower than the factored resistance calculated for the part of the structure being considered. In these calculations the magnification factor is calculated for loads while the reduction factor is calculated for the resistance offered by the structural members in consideration. Another alternative is to set the limit state criteria in terms of the stress rather than in terms of the load. A structural element under analysis is considered to be safe when it meets the criteria that the factored magnified loads are less than the factored reduced resistance. These structural elements may include walls, columns, beams or any other load bearing member. On the other hand, the serviceability limit state criteria delineates that a concerned structure must remain functional throughout its anticipated life. Moreover the SLS criteria apply only when the structure is loaded daily with the intended loads only and the structure must also not provide uncomfortable conditions to the residents under normal day to day circumstances. The SLS criteria can be seen as satisfied only when the composing elements of the SLS criteria are all satisfied. The criteria for SLS vary by standards and codes and these criteria must not be crossed more than the limits set forth by these standards and codes. For example one such criterion could be the failure of the floors when subjected to predetermined levels of vibration for a continuous period of time. Similarly other criteria could include matters such as the allowable crack width and depth within concrete structures that need to be controlled within specific dimensional limits. Structures that fail to meet the SLS criteria cannot be deemed to fail either instantly or even in the longer run. For example a beam may offer a deflection greater than that allowed by the subject SLS criterion but the structure may not fail for years. The real purpose behind the SLS criteria is to keep the occupants satisfied fully so that the residents are kept comfortable both psychologically and physically. For example a large deflection in the floor would prompt most people to feel insecure while continual undamped vibrations would cause residents to develop physical problems such as shivering limbs. The limits imposed for these criteria are highly dependent on the materials and the finish of the materials in question. The limits delineated by most building codes are merely descriptive in nature and leave it to the engineer or architect to specify limits that would satisfy the limit state design criteria. The factors for both load and resistance are developed using statistical methods along with pre selected probabilities of failure. The variations experienced in these factors as a result of construction as well as construction materials are also accounted for. The resistance of materials being used is expressed as unity as or less than unity. The resistance of one material may vary from grade to grade as well. On the other hand the factors used for loading are independent of the material of construction but are dependent on the kind of construction methods and techniques used to create a structure. The LSD criteria have scrapped the older concepts of permissible stress levels when designing buildings and structures. The newer codes and criteria delineated by standards are more and more dependent on LSD based criteria. Generally safety factors are not provided consideration while working with LSD criteria as it promotes confusion through the design process. 6. Structural Effects of Fire on Steel Framed Buildings Steel begins to lose strength as it is heated. At a sufficient temperature, a steel member may not be able to support its load anymore. This temperature is referred to as the critical temperature. Beyond the critical temperature the steel member is unable to support the load and the steel member may collapse through buckling or any other such mechanism. Various methods of defining the critical temperature exist. Different building codes as well as standard engineering practices provide different critical temperatures for structural steel members. These variations in the critical temperature depend on the type of the structural element, the orientation, the configuration as well as the particular loading characteristics of the structural element. Studies and experimental investigation reveal that the yield stress of steel is reduced by some 60% (of its yield stress at room temperature) when it goes to critical temperature. This method has also been used to define the critical temperature for structural steel elements. A major advantage offered by steel under fire is that it is incombustible. As steel does not catch fire, it can be found in its original form after a fire. During the fire, the steel tends to absorb large amounts of thermal energy. However as steel is left to cool down this thermal energy is returned to the atmosphere and the steel structure generally returns to a stable condition. The alternate heating and cooling cycle tends to deform a few structural steel members while most structural steel is safe after a fire. The members that are distorted are checked for the total amount of deformation offered. If the deformation is low then the structural steel members are reheated and restored to their original shape. However if the deformation is too large then the structural steel members are discarded or scrapped because restoring them is too expensive an option. The structural steel members that are left unscathed or those that did not deform can be expected to function as new structural members and can be expected to fulfil their effective life without much of a problem. As a rule of thumb, if a structural steel members is found to be straight after a fire, it is considered as worthy of continued service. The ease with which steel structural elements are jointed together ensures that damaged structural steel members can be easily replaced. In contrast to structural steel, a concrete based structure will undergo irreversible degradation if subjected to a fire above 600oC for a few hours. Similarly a timber structure would burn to the ground as it is combustible. In terms of fire resistance structural steel offers unparalleled properties. Even though steel can withstand elevated temperatures and return to its original state, there is still need to find out how structural steel behaves during fires. The greatest danger for failure occurs when structural steel is subjected to fires rather than after the fire or before it. The mechanical properties of structural steel modify as steel is taken to elevated temperatures. The figure provided below shows the changes that occur in the modulus of elasticity (non-dimensional), the yield strength and the coefficient of thermal expansion as the temperature is increased in respect to the time. Figure 1 - Rate of Heating for Structural Steel The figure above clearly shows that the yield stress of steel is not changed up to temperatures of 215oC while after that the steel begins to lose its strength very gradually. Moreover both the yield stress and the coefficient of thermal expansion for steel vary with temperature through a simple relation. The change in the coefficient of thermal expansion is shown below. The figure shown below clearly indicates that the coefficient of thermal expansion varies linearly as the temperature is increased. Moreover the Young’s modulus decreases as the temperature increases although the decrease cannot be indicated as purely linear. However a linear approximation can be provided for the change in the Young’s modulus. On the other hand, the yield stress ratio is seen to stand constant to around 300oC after which it decreases gradually and continuously with increases in temperature. Based on these pieces of evidence it can be suggested that the properties of structural steel tend to degrade during its exposure to temperature though the change is gradual and nearly linearly related to changes in temperature. Figure 2 - Mechanical Properties of Steel at Elevated Temperatures The codes of practice on structural steel frames subjected to fire show strength curves. These strength curves delineate the behavior of structural steel from ambient temperatures to elevated temperatures where the change in the properties of structural steel is deemed very significant. The strain which is used to assess the strength is an important parameter for all of these curves. As an example, the BS 5950 Part 8 uses the 1.5% strain in structural steel as the strain limit versus the Euro code 3 Part 10 that uses 2% strain in structural steel as the strain limit. For most general cases, a lower strain value of 0.5% is used for columns and other components that have fire protection through the use of brittle materials. The fire resistance of structural steels can be improved by using certain grades of structural steel that are configured for fire resistance. These grades are also better known as fire resistant steels (FRS). Such grades of structural steel are subjected to thermo mechanical treatment (TMT) that aids them in performing better in fire as structural components. The properties of these structural steels tend to keep uniform for low to moderate fire temperatures. The micro structure of these steels is composed of the ferrite pearlite type of structure that ordinary steels possess. However molybdenum and chromium are also added to FRS in order to stabilize the microstructures well beyond the 600oC limit. Most FRS display at least two thirds of the yield strength at room temperature when the temperature of the structural steel element goes beyond 600oC. This enables the FRS to provide inherent fire resistance. Moreover FRS are weld able without the need for preheating and can be found commercially already shaped as channels, angles and joists. A major concern of fire engineering is to provide occupants of a structural steel building enough time for evacuation. The structural steel building should not collapse when it contacts fire for a reasonable amount of time. A simple method of improving fire resistance is to insulate structural steel members in ways that keep the temperature low. However these methods are costly and time consuming to carry out. Instead another alternative approach is also utilized to enhance the fire resistance of structural steel elements through their design. When a structural steel member is exposed to fire, a major factor that determines the rate of heating is the section factor. The section factor is the ratio between the perimeter of the section being heated by fire against the cross sectional area of the member being heated. The lower the section factor, the lower is the rate of heating of a structural member and vice versa. Structural steel members with low section factors also require lower insulation and this tends to minimize costs too. Often the section factor is the major factor that determines if a structural steel member would require additional fire protection such as through insulation or coating. In addition to the above, if the properties of structural steel at elevated temperatures are considered then another approach to fire resistance can be developed. If the relationships delineating the strength at elevated temperatures is considered then no insulation may be required because of the state of loading and the behavior of the structural steel under fire. The design temperature is considered which accounts for the maximum temperature that the structural steel member would be subject to during fire resistance. Then the limiting temperature is considered which accounts for the temperature at which a structural steel member tends to fail. Using these temperatures the equivalent acceptable loads are tabulated and a ratio between load at the design temperature and the load at the limiting temperature is calculated. If the ratio is under unity then there is no need to provide fire protection. (Kumar & Kumar, 2011) Conclusively, structural steel members tend to degrade their properties when subjected to heat. The degradation of properties is gradual and most noticeable above 215oC. The greatest threat to structural steel emerges once temperatures soar above 600oC and the internal micro structure of the steel begins to change. The structural steel members may fail under loading so design and limiting temperatures are tabulated along with equivalent loads to determine if a structural steel member requires insulation. 7. Bibliography Eurocode, 2001. Eurocode - Basis of Structural Design: EN 1990:2002 E. Standard. CEN. Geschwindner, L.F., 2007. Unified Design of Steel Structures. London: Addison Wesley. Gotz, K.-H., 1989. Timber Design & Construction Sourcebook. New York: McGraw-Hall. Kosmatka, S.H. & Panarese, W.C., 1988. Design and Control of Concrete Mixtures. Skokie: Portland Cement Association. Kumar, S.R.S. & Kumar, A.R.S., 2011. Steel structures subjected to fire. [Online] Available at: http://nptel.iitm.ac.in/courses/IIT-MADRAS/Design_Steel_Structures_I/1_introduction/6_fire_resistance.pdf [Accessed 2 October 2011]. McCormac, J.C., 2008. Structural Steel Design. 4th ed. New Jersey: Pearson Prentice Hall. Ravenswaay, C.V., 2006. The arts and architecture of German settlements in Missouri: a survey of a vanishing culture. Missouri: University of Missouri Press. Sustainable Concrete, 2011. Concrete Frame Construction. [Online] Available at: http://www.sustainableconcrete.org.uk/sustainable_design_constructio/methods_of_construction/concrete_frame_construction.aspx [Accessed 2 October 2011]. Vince, J., 1994. The Timbered House. Sorbus. Zalosh, R.G., 2003. Industrial fire protection engineering. New York: McGraw Hill. Read More