Design of Concrete Frame for Earthquake Resistance - Literature review Example

? Design of Concrete Frame based on Iranian (2800) and Euro (EC 8) for Resistance to Earthquake Design of Concrete Frame based on Iranian Code (2800) and Eurocode (EC 8) for Resistance to Earthquake Seismic hazard and history of Iran Iran stands as a common seismic country in the world. In 1976, an earthquake occurred in western Iran that claimed the lives of 15,000 people. Another incident claimed 1000 people in 1981 in Kerman while 40,000 faced the same in Gilan, a province in Northern Iran. The Ritcher scale reading struck 7.2. An earthquake that occurred in Bam in December 2003 is numbered among the 130 major earthquakes in Iranian history with estimated casualty cases of 30-50 thousand (Iranian Studies Group, 2004, p.2). The foundation of assessment of seismic hazard in Iran is on the frequency of seismicity in relation to time and space. In the assessment of seismic hazards, seismotectonic sources are considered. The success of this study relies on regional and local knowledge on geology, seismicity and tectonics. The core sources of seismotectonic forces are; Fault sources and Area sources (Takavoli, B., and Ghafory, M., 1999, P.1013). Research conducted by different firms like Wilson (1930), Niazi (1968) and many others reveal that seismicity in Iran is linked to local surface geology and tectonics (1014). Iranian plateau is characterized by active faults, active volcanoes with an Alpine earthquake belt that is elongated and of high surface. Active earthquake activities are found along Zagros fold thrust belt thus making most parts of Iran vulnerable to earthquakes (Takavoli, B., and Ghafory, M., 1999, P.1014). Design codes and Iran To effectively reduce the risks encountered in earthquakes, proper seismic design codes should be implemented at design and construction stages of a structure (Grant et al, 2006, p.54). Implementation of structural seismic design codes depend on authorities concerned with public safety. The purpose of seismic design of structures is to ensure minimum destruction of a structure in case of specified seismic waves as per the design, protection of human lives and maintenance of the structure after the earthquake (Elghazouli, 2009, p.60). Inelastic behaviour of a building is derived from capacity design rules. This allows the required dissipation energy. Capacity design protocols, parameters and fashion designs assist greatly in obtaining of design standards followed in design procedure. Seismic design codes vary from country to country due to various factors which include: The desire for seismic design codes of high quality The impact created on previous earthquakes relying on provision s indicated on the codes Research conducted at academic institutions on seismic designs and The period taken in the implementation of policies and legislative laws. Iran being an earthquake prone area has developed its own codes to be implemented in construction of structures. Reinforced Concrete Frame Buildings Reinforced concrete is composed of horizontal elements called beams and vertical elements called columns. These two components are joined together with rigid joints. The beams and columns are cast together to form a monolithic structure. Reinforced concrete frames offer resistance to gravitational and lateral loads by bending that occurs in beams and columns. Some of the subtypes of reinforced concrete frame construction are: Nonductile reinforced concrete frames with or without infill walls, Nonductile reinforced concrete frames with reinforced infill walls, Ductile reinforced concrete frames with or without infill walls. Tremendous changes have been made to design and details of reinforced concrete frame structures in seismic zones. Earlier building codes focused on requirements needed strength of a structure. The structural members were to provide resistance to lateral seismic loads. From various researches conducted about earthquakes, building codes shifted focus to sectioning and detailing of beams, columns and joints. The aim behind this step is to achieve the required value of ductility in addition to the strength required. Ductility is one of the important features needed in order to achieve the required seismic behaviour of structures. Material is referred to as ductile when it is in a position of getting deformed before it fails. Seismic detailing entails the act of determining the equity of a specific design with the amount and distribution reinforcement installed. These specifications are detailed in the building codes of different countries. The common seismic deficiencies are: Poor column detailing especially in column lap splices for flexural reinforcement and in adequate ties within the columns. Inadequate approach in designing a strong/weak beam. This means that during the design of flexural reinforcement of the beam, the design never considered the capacity of the loads. The impacts associated with post yield behaviour were not considered. As a result, shear failure in the beams or columns is experienced. This type of failure should be avoided in structures built in seismic zones. Poorly anchored beam reinforcement. Large spacing of ties for beams and columns. The beam ties are sized according to gravity shear loads. They are therefore closely spaced near the column face and widely spaced in the mid-span of the beam. Lack of enough joint ties of the beams and columns. This creates a weak area leading to failure in the joint. Figure 1: Features of nonductile RC frame construction (Ahmet, Y., 2008, p. 2). Reinforced concrete elements failures under seismic action Reinforced concrete is commonly used for construction of structures in today’s world. Concrete which can be referred to as “artificial stone” comprises of cement, sand and course aggregates mixed with water. When concrete is still fresh, it can be moulded into a preferred form to give the required finish. This makes it advantageous over other building materials. Although it was invented in the early 19th century, its wide use was limited due to weakness in tensile resistance. This weakness was overcome by embedding steel bars in concrete the so called reinforced concrete. Reinforced concrete is commonly used in most engineering applications like construction of buildings, bridges, dams and super structures. To achieve quality structures, technology, use of experts and good workmanship are needed especially in design and fieldwork. Typically, reinforced concrete is preferred in high seismic regions (Ahmet, Y., 2008, p. 1). Under experiments on seismic action on reinforced concrete by quasi-static cyclic lateral loading in both longitudinal and transverse directions reveal brittle flexural failure. Axial tension is created in shear walls because of raptured longitudinal reinforcement. Shear resistance results from various actions that include; beam action, arch and truss action. Interaction between flexural shear mechanisms is reflected in arch effects (Martinelli, L., 2007, p.1081). Other desirable failure mechanisms are concentration of plastic hinges in columns in a single storey found in a multistorey structure. Shear failure of structural elements, beam-column joints failure, succumbing of foundations and any element required to remain in an elastic condition also occur to reinforced concrete structures under seismic loading. Material requirements of the design codes for ductility classes According to Eurocode 8, material requirements for DCM (medium ductility) are: Primary seismic elements should not be made of concrete class below C 16/20. Ribbed bars should be used as reinforced steel but not closed stirrups and close ties. This also applies to critical areas of primary seismic elements. Under the same condition as stated above, the reinforced steel should be of class B or C. The use of welded wire mesh is allowed so long as it meets the required specification (European Committee for Standardization, 2004, p.88). Material requirements for DCH (high ductility) are: Concrete class below C 20/25 should not be used in primary seismic elements. Ribbed bars but not closed stirrups and cross-ties should be used as reinforced steel in critical areas of primary seismic elements Reinforced steel of class C with an upper characteristic of 95% fractile value of the real yield strength not exceeding a nominal value of more than 25% should be used (European Committee for Standardization, 2004, p. 106) Ductility classes and usage The ability of a structure to dissipate energy without any alterations in its capacity to resist horizontal and vertical loads requires a proper design of reinforced concrete. All elements necessary for adequate structural resistance must be considered. Non-linear deformation requirements in critical regions need to comply with the assumed values derived from overall ductility. When ductility demand entails the spread of the structure’s volume to various elements and parts of the storey, overall ductile behaviour is achieved. Concrete structures are classified into two ductility classes which are: DCM or medium ductility and DCH or high ductility. These classes rely on hysteretic dissipation capacity and apply to structures designed, detailed and dimension in accordance to particular provisions for earthquake resistance. Each class has its own specific amount of ductility hence the use of different values for behaviour factor q are used (European Committee for Standardization, 2004, p.80). Total base shear calculation (Iranian code and Eurocode 8 ) and loads According to Iranian code 2800, each structure is analysed for loads resulting from earthquakes and wind and designed according for the most probable critical action. Structure designed for earthquake loads takes two options, that is, horizontal and vertical components. The design for each direction is done independently apart from the following scenarios: An irregular plan for a structure and For a structure whose column holds two or more lateral forces resisted that intersect. The earthquake load for these two cases is affected at a direction realising the greatest effect. The impacts of horizontal and vertical directions are met by 100% design to seismic forces with an additional 30% prescribed seismic forces.in design, the vital area in relation to internal seismic forces is considered apart from the following cases: Column axial loads less than 20% resulting from seismic forces. Seismic loads applied to direction of 30% load. The seismic forces in the horizontal and vertical positions act reciprocally and opposite to each other. Properties related to stiffness of reinforced concrete should consider the effects of cracking. Horizontal seismic loads are determined by equivalent static or dynamic procedures. Equivalent Static Procedure is applied in the opposite direction of the structure. It includes; Base Shear, V, Base level, Design Base Acceleration Ratio, A and Building Response Factor, B. Base Shear, V, is the total of horizontal loads exerted in each direction on the structure. It is determined from: V=CW Where: V= Shear force at base level, W= Total seismic weight of the building equal to dead load plus a percentage of live and snow loads and C= Refers to seismic coefficient determined by: C= ABI/R Where: A is the design base acceleration ratio, B is the building response factor derived from design response spectrum. I is the importance factor and R is the structure behaviour factor. In general, the design base shear should not be less than: V=0.1AIW The base level which is the lower part of the structure is fixed not to allow any movement during earthquakes. It is surrounded by peripheral reinforced concrete with retaining walls and cast monolithically with the structure. The Design Base Acceleration Ratio, A is determined from seismic levels that vary depending on the regions within Iran. Table 1 Zone Description Design base acceleration(/g) 1 Very high level of relative seismic hazard 0.35 2 High level of relative seismic hazard 0.30 3 Intermediate level of relative seismic hazard 0.25 4 Low level of relative seismic hazard 0.20 From the table above, the different regions have different levels of seismic loads. This affects the type of design to be adopted (Building and Housing Research Centre, 1988, p.14) Building Response Factor, B focuses on the reaction of the structure to ground motion. It is determined by various formulae as illustrated below. For 0?T? T0the appropriate formulae is; B-I+S (T/ T0). For T0?T?Ts, the appropriate formulae is; B= S + I. For T? Ts, the appropriate formulae is B=(S+1)( Ts/T)? Where: T is the period taken by the structure in vibration in the considered direction in seconds. T0, Ts, and S are parameters derived from the type of soil profile and seismic level (Building and Housing Research Centre, 1988, pp. 13-20). Overturning effects should be resisted by the structure. At the base of the structure, overturning moment is equal to the force exerted by lateral storey force at each level multiplied by the height from the structure. Attention taken by considering the height of the structure to be from the bottom of the foundation. Overturning safety factor is derived as a ratio between the resisting moment and overturning moment which should exceed 1.75. Resisting moment is derived from vertical loads used when determining lateral seismic loads. After vertical load is derived, weight of soil on the building and that of the foundation are added. This moment is calculated from the foundation bottom level in respect to the structure’s outer edge. Vertical seismic load is considered under the following provisions: Beams of a span greater than 15 metres. The adjacent columns and walls also need to be analysed. Beams whose concentrated loads should be considered in relation to other applied loads. For this case, the adjacent columns and walls should also be analysed. The considerable concentrated loads refer to loads of at least half of the sum of the applied loads in magnitude. For these provisions, the vertical seismic load is derived from F= 0.7AIWp Where: A is derived from Design Base Acceleration Ratio, A I is derived from the Building Response Factor, B and Wp is the total weight of the element and that of live load. Load combination is used to determine vertical and horizontal seismic loads. The two combinations used are; I. 100% horizontal seismic load regardless of the direction with an additional 30% of horizontal load in perpendicular direction and a 30% of vertical seismic load. II. 100% of vertical seismic load with 30% horizontal seismic load from one of the perpendicular directions (Building and Housing Research Centre, 1988, pp.26-27). According Eurocode 8, the highest limit value for behaviour factor q, for horizontal seismic actions responsible for energy dissipation capacity is derived from: q= qokw ? 1.5 Where: qo refers to the basic value of behaviour factor and depends on the type of structural system and regularity in elevation and kw for the factor that reflects prevailing failure mode in structural systems composed of walls. For structures of regular elevation, the core values of qo for different types of structures are derived from the table below: Table 2 STRUCTURAL TYPE DCM DCH Frame system, dual system, coupled wall system 3.0 ?u/ ?1 4.5 ?u/ ?1 Uncoupled wall system 3.0 4.0 ?u/ ?1 Torsionally flexible system 2.0 3.0 Inverted pendulum system 1.5 2.0 According to the Euro Code 8, behaviour factor of structures vary depending on the seismic loads that the structures are exposed to. This code has got two categories with the required values for each type of structure (European Committee for Standardization, 2004, p. 82). For structures whose elevation is irregular, the value of qo is reduced by 20%. ?1 refers to the value multiplied with horizontal seismic action to achieve flexural resistance found in the members of the structure as other design actions are constant and ?u describes the value multiplied with horizontal seismic design action so that plastic hinges are formed in certain sections adequate to develop the instability of the whole structure with the design actions kept constant. This factor is derived from global analysis pushover (European Committee for Standardization, 2004, pp. 81-83). Beam design with Eurocode 8 and Iranian code (2800) According to the Eurocode (EC 8), the values designated to design of the bending moments and axial forces are derived from structure analysis in accordance to the situation of seismic design. Capacity design rules are used as parameters to determine shear forces for the primary seismic beams. The force should be at equilibrium with: Action of transverse load on it according to the situation in seismic design and The end moments denoted as Mi,d where i=1.2 denotes the beam’s end sections and corresponds to the formed plastic hinge positively and negatively in the direction of seismic loads. The beam designs also vary according to the class of ductility. For DCM, cyclic moments must be transferred from the primary seismic beam. This achieved by limiting relativity of beam axis to the column. To attain this, the distance between centroidal axes of the members involved must be limited below bc/4 where bc is the crossectional area dimension of column to longitudinal axis of the beam (European Committee for Standardization, 2004, p. 88). Considerations made for beams depend on the following provisions according to Iranian code (Building and Housing Research Centre, 1988, p. 26): For beams with spans greater than 15 metres and Considerable concentrated loads on beams. Euro Code 8 Iranian code 2800 The design for shear forces is determined in relation to capacity design rule. Vertical seismic loads are considered for beams of spans that exceed 15 metres. Capacity design rule is determined on the beam in relation to transverse load exerted in seismic design situation and end moments and must correspond to plastic hinge formation both in negative and positive seismic loads. Vertical seismic loads are considered for beams with considerable concentrated loads. Plastic hinges form at beam ends or vertical elements where the beams meet with the frames. Effective stiffness of cracked sections is considered when calculating fundamental period. Efficient transfer of cyclic moments from primary seismic beam to the column is achieved by limited eccentricity of beam axis to the column where it frames. Moment of inertia used in calculating fundamental period is 0.51g Design of column with Eurocode 8 and Iranian code (2800) According to the Eurocode 8, shear forces are determined according to capacity design rule. This capacity design rule relies on equilibrium of column facing end moments denoted as Mi,d where I has a value of 1.2 representing the end sections of the column. This value corresponds to plastic hinges formed in both the positive and negative directions according to seismic loads (European Committee for Standardization, 2004 p.91). For DCM in Eurocode 8, crossectional dimensions of primary seismic columns should never be below 1/10 of the greater distance between contraflexure point and bending ends unless ? ? 0.1. This is applied for bends within planes parallel to column dimensions (European Committee for Standardization, 2004, p. 89). According to Iranian code (Building and Housing Research Centre, 1988, p.36), the design load of columns is increased when the lateral-load-resisting element like shear wall or braced frame does not reach the foundation level. The column that supports them are therefore designed in accordance to load combinations. Earthquake load=dead load +live load Its strength should not exceed the maximum load exerted to them by the connecting elements. Allowable stress method can also be implemented for design hence the column design strength is 1.7 times allowable stress. Euro Code Iranian Code 2800 Cross-sectional dimension of columns in primary seismic zones should be greater than 1/10 the distance between contraflexure point and column ends. Vertical seismic loads are considered for adjacent columns with considerable concentrated loads. Bends within the plane parallel to the column dimensions must be considered. Vertical seismic loads are considered for adjacent columns of spans that exceed 15 metres. Design values for shear forces are determined by capacity design rule. The design rule depends on equilibrium of column under end moments and corresponds to plastic hinge formation. Fundamental period, T, and required stiffness of cracked sections are considered. Plastic hinges are formed at beam ends connected to joints where the column ends to frames. Moment of inertia for beam used in calculating fundamental period is Ig Flexural and shear resistance is derived from axial force value after seismic design analysis. In design, critical cases in relation to internal seismic signs are considered. The Euro code 8 is more detailed than Iranian code 2800 in beam and column design. It puts a distinction on the type of materials to be used in design for DCM (medium ductility) and DCH (high ductility). It uses the upper limit value for behaviour factor to account for energy dissipation capacity. Its behaviour factor can be used to derive seismic actions irrespective of the structural system and regular elevation. Frames Iranian code 2800 puts emphasis on frame design between the base and foundation of the structure. From the design, the strength and stiffness of the frames of a structure should not be less than that of the superstructure above the base. In case the plan and geometry of the base has no great difference with the superstructure, the properties of beams, columns, shear walls and braces should be treated as equal. Frames with “Khorjini” connections are categorized under simple building frame system as long as they conform to special technical requirements. For structures that exceed 50 meters, specific moment resisting frames are used. Columns can be used as moment resisting frames only for structures less than 10 metres (Building and Housing Research Centre, 1988, p. 34). According to the Euro Code 8, the design for moment resisting frames should allow formation of plastic hinges in the beams or where the beams are joined to the columns. The design for concentric braced frames should in a way that yielding of diagonals in tension should take place first before connections fail and before buckling of beams and columns (European Committee for Standardization, 2004, p.150). Seismic Performance Research conducted in previous years reveal the patterns of damages and failures that arise in Reinforced concrete construction. These are: Failure in concrete due to crushing and shear failure in concrete columns. This failure makes then columns not to be able to bear gravity loads hence a total collapse of the structure. Incomplete design and detailing in ductile design. This causes stiffness of a structure hence its failure. Deficient design concepts like incomplete load path, deficient architectural plan that could be irregular vertically or horizontally. Insufficient strengths in the beams and columns. It leads to failure in connections and individual members in case of the development of weak column-strong beam mechanism. Insufficient detailing of reinforcement. Seismic performance of reinforced concrete frame structures is related to effective mechanism for the building codes. Bibliography Ahmet, Y., 2008. Reinforced Concrete Frame Construction, Middle East. Technical University Turkey). Building and Housing Research Centre, 1988. Iranian Building Codes and Standards, Iranian Code of Practice for Seismic Resistant Design of Buildings. Standard No. 2800 3rd Edition. Iran. Elghazouli, A., 2009. Seismic design of buildings to Eurocode 8. Spon Press. European Committee for Standardization. 2004. Design of structures for earthquake resistance- Part 1: General rules, seismic actions and rules for buildings. Management Centre, Brussels. Grant, D. N., Blandon, C. A., and Priestley, M. J. N., 2005. Modelling inelastic response in direct displacement-based design. ROSE School, IUSS Press, Pavia, Italy. Iranian Studies Group, 2004. Earthquake Management in Iran- A compilation of literature on earthquake management. MIT. Martinelli, L., 2000. The Behaviour of Reinforced Concrete Piers Under Strong Seismic Actions. 12WCEE. Takavoli, B. and Ghafory, M., 1999. Seismic hazard assessment of Iran. ANNALI DI GEOFISICA, VOL. 42, N. 6. International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, I.R. Iran. Yuksel, B. and Kalkan, E. 2007. Behaviour of Tunnel form buildings under quasi-static cyclic lateral loading, 2006. Structural Engineering and Mechanics, Vol. 27, No. 1. Read More