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Literature Review1. Structure SpecificationsCommon building drivers for multi-story commercial buildings include long spans and clean interior spaces to provide highly flexible layouts and maximize floor slab efficiency. The most common design solution to meet these requirements is concrete frame structures, which can provide long service life with low maintenance cost. Since concrete is a brittle material that is strong under compression and weak under tension, indicating only about ten percent of its compressive strength and zero strength after cracks develop, it has to be reinforced with steel members, in order to withstand stresses generated from vibrations, wind loads and etc. In reinforced concrete beams stresses above the neutral axis are compressive and nonlinearly distributed, and in the tension zone below the neutral axis, the tensile stress is taken by the reinforcing steel. Depending on the construction specifications reinforced concrete frames can be divided into precast and cast-in-situ and are used for a variety of applications. While the precast frames are usually used for single-story or low-rise buildings in non-seismic areas, their overall performance is considered poor than that of cast-in-place frames, which perform well in seismic regions. The height and aspect ratio of the building should also be considered in the design. Because the lateral stiffness of the frame structure is small, and the lateral shift under the horizontal load is large, the overturning effect can be originated if the height and width of the structure are relatively substantial. In order to control the drift of stories of the building, the frames that are considered as moment frames must have a sufficient stiffness within the limits specified by the building code ACI 318. Typically, economical beam spans for special moment frames range between 6 to 9 m. In general, this range will result in beam depths that will support gravity loads and the requisite seismic forces without overloading the adjacent beam-column joints and columns. The beams clear span must be at least 4 times its effective depth. The depth of the beam must not exceed twice the depth of the column in the framing direction, which limits the beam-column joint aspect ratio to improve force transfer. Beams are allowed to be wider than the supporting columns within limits; however, for reasons of constructability, beam width normally does not exceed the width of the column. Beams transfer loads to columns and receive them from the slabs. Systems incorporating slabs and supporting beams, as well as joists or columns typically have multiple bays. The horizontal components can act as one-way or two-way systems, meaning they transfer loads either in a one-way or in a two-way mechanism.The flexure resisting elements are considered to be continuous and have positive and negative bending moments. These moment and shear values can be found according to beam tables, or from approximate design factors specified in the code. Mathematically their difference is found from the ratio of the longer span to the shorter span being lesser or greater than 3. While one-way slabs are mostly suitable for short-span structures, two-way slabs are widely used in long-span multi-story and commercial buildings. There are several types of two-way slabs that vary depending on their functionality. ACI 318 permits the use of Direct Design Method (DDM) and Equivalent Frame Method (EFM) for the gravity load analysis of orthogonal frames and is applicable to flat plates, flat slabs, and slabs with beams.DDM integrates the determination of positive and negative slab moments using coefficients. EFM includes the determination of the slab moments using the elastic frame analysis. In the case of a two-way slab there is always a negative moment at the supports, that is why half of the reinforcing steel bars usually require bending in order to take it into the supports.2. Design methodologies of Reinforced Concrete Frames2.1 Basic design philosophiesFrom the early 1900s to current there have been two main calculation methods. The first method is called Working Stress Design (WSD) and the second one is Ultimate Strength Design (USD). The Ultimate Strength Design method has been permitted and officially recognized in ACI 318-56 specification and since then has gradually replaced the WSD. Hence, the main design method of the ACI Code is the USD method. The Ultimate Strength Design method, also known as Strength Design Method (SDM), is based on the ultimate strength at the failure of the design member. In this method, the structure is analyzed under the stress condition at the state of impending collapse, and the non-linear stress-strain curves of concrete and steel. This method utilizes a large reserve of strength in the plastic region. If the resulting section is rather slender, excessive deformation and cracking might be formed. Load factors, however, provide the exact margin of safety against the collapse and this method allows implementing different load factors for different types of loads, such as dead and live loads. The USD method provides safety not by allowable stresses as for the ASD method but by factored loads, nominal strength, and strength reduction factors θ, both defined by the ACI code. The designed structures must satisfy the criteria of the required ultimate strength in the development of flexural, shear, compression, tension, and torsion effects under given loading conditions and their combinations. The structure should satisfy the criterion for serviceability, which limits the deflections and keeps the width of the cracks with acceptable limits. The structure should also have adequate durability, impermeability, resistance to external influences such as acids, corrosion, frost, fire, etc.2.2 The deflection and internal force calculationThe displacements in frame structural systems are calculated according to the Unit Force Method that is applicable for both linear and non-linear material behaviors. The process is done by implementing the principle of superposition for the estimation of the forces that are required to achieve compatibility with the original structure and integration of flexibility coefficients to the equations to solve the unknowns. The frame structures generally comprise a three-dimensional force system of structural elements, which is simplified into sections according to the lateral and gravity load directions during the analysis. The flexural resistance or moment capacity of a structural member is a fundamental part of the overall analysis required when designing or evaluating an assembly of structural concrete sections. The approximate hand calculation methods of cast-in-situ plane frame structures according to elastic theory include the moment distribution method, inflection point method, or D-value method. The moment distribution method is used for internal force analysis under vertical loads. It works on the assumptions that the structure joints are locked initially and released afterward and the use of different factors depending on the type of supports. The inflection point method is suitable for the calculation of the internal force of the frame structure with the beam-column stiffness ratio greater than the value of 3 under the action of horizontal force. The assumption of the method is that the inflection point occurs in the middle of beams and columns. This lets the magnitude of the axial stress in a column of a story in the building be proportional to the horizontal distance from the center of gravity of all the columns in that story. The D-value method is suitable for the beam-column stiffness less than 3 and is settled under the action of horizontal forces. When the vertical load and horizontal load are combined, the internal force and horizontal displacement of the frame can be calculated separately and then superimposed.2.3 Design of jointsThe construction joints should be able to resist all forces that may be transferred by adjacent members, using those combinations that produce the most severe force distribution at the joint, including the effect of any member eccentricity. Forces coming from deformations due to effects dependent on time and temperature should be taken into account. Research on columns subjected to severe load reversals has shown that a uniform distribution of the column longitudinal reinforcement improves confinement of the column core. A relatively uniform distribution of the longitudinal bars in the connections must be ensured. To ensure that the beam-column joint of special moment resisting frames possesses sufficient shear strength, a rational analysis of the beam-column panel zone to determine the shear forces that are generated in the joint must be performed to provide a sufficient amount of stirrups. Only joints that have a column below the joint are designed. The material properties of the joint are assumed to be the same as those of the column below the joint. The joint analysis is performed in the major and minor directions of the column. The joint design procedure involves the determination of the effective area of the joint and the design shear force of the panel zone.2.4 Seismic influence considerations in the design of frame structuresIn the occurrence of moderate-intensity seismic activity it is suggested to use base isolation devices to protect the multi-story building. They are typically installed between the building and its foundation. Under external forces, these devices act as rigid connections and under earthquakes as vibration absorbers. A base isolator with hardening behavior such as lead rubber bearing (LRB) and high damping rubber bearing (HDRB) under increasing loading has been developed for medium-rise buildings and sites with moderate earthquake risk. The isolation systems perform well with heavy masses and effective isolation is obtained with a long period of response. During an earthquake, an LRB extends the natural period of a structure due to the flexibility of the rubber. An LRB also restrains large displacement and decreases the magnitude of seismic stresses by the plastic behavior of its lead core, thereby providing a bilinear response. An HDRB modifies the rubber with enhanced dissipating properties, usually known as high damping rubber (HDR). The characteristic highly nonlinear stress-strain response of high damping rubber is affected by the type and amount of fillers, and by the amount of cross-linking.The stair component has a certain contribution to the structural rigidity, of the frame structure, meaning that the structure will be subjected to greater seismic force under the action of an earthquake. This aspect should be taken into account in the calculation of internal force and reinforcement. The asymmetry of the staircase in the plane layout will cause the uneven distribution of the structural rigidity and increase the torsion effect. The design should consider the impact of the stairs on the torsion under repeated earthquakes, the riser pedestals are alternately stretched and bent. Another thing to consider in the design is the influence of the axial force generated by the horizontal elements. The structural characteristics of platform beams and platform slab components connected to the ladder slab should be strengthened. In the earthquake, due to the tensile force of the ladder slab, part of the platform beams might get separated from the platform slab, and shear torsion damage can occur. Thus, beam-column joints should have sufficient stress resistance. Under small earthquakes, the structure is in a state of elastic stress, and the influence of construction joints can be ignored; under moderate and large earthquakes, the structure enters nonlinearity, and the influence of construction joints begins to become prominent, mainly manifested in the increase of the top displacement of the structure and the interlayer. The displacement angle increases, the angular displacement distribution of the interlayer displacement changes, and the local reaction of key components increases. Under a major earthquake construction, joints may increase the maximum displacement angle of the structure. Taking the maximum displacement angle as the control index can comprehensively reflect the adverse effects of the joints on the seismic performance of the frame structure. Concurrently, inappropriate column-beam relative strengths can also lead to failure of individual members and connections when the weak column-strong beam mechanism develops. Shear failure and concrete crushing failure in concrete columns are the most undesirable non-ductile modes of failure. This behavior can lead to the loss of gravity load-bearing capacity in the columns and potentially a total building collapse. Systems that exhibit some (limited) yielding behavior can eventually form dangerous collapse mechanisms as a result of stiffness or strength degradation at sections without ductile detailing.2.5 Fire protectionThe thermal mass of concrete makes this material inherently fire resistant.In general inherent fire resistance allows concrete structures to not require additional fire protection. This saves time and costs and removes ongoing maintenance to applied fire protection.References[1] ACI Committee 318 Building Code Requirements for Structural Concrete (ACI 318-19)[2] Hoffman E.S. Structural Design Guide to the ACI Building Code[3] Hoffman E.S., Gustafson D.P., Gouwens A.J. (1998) One-Way Reinforced Concrete Slabs.[4] Hoffman E.S., Gustafson D.P., Gouwens A.J. (1998) Two-Way Solid Flat Slab Design.[5] Hoffman E.S., Gustafson D.P., Gouwens A.J. (1998) Two-Way (Waffle) Flat Slab Design. [6] Kong F.K., Evans R.H. (1987) Reinforced concrete slabs and yield-line analysis.[7] The Concrete Centre, Economic Concrete Frame Elements a handbook for the rapid sizing of concrete frames, 2009[8] E. X. Xiuli., Design of concrete frame structure, China Construction Industry Press, 2008.[9] Jack P. Moehle John D. Hooper, Seismic Design of Reinforced Concrete Special Moment Frames A Guide for Practicing Engineers SECOND EDITION NIST GCR 16-917-40[10] Mete A. Sozen Toshikatsu Ichinose Santiago Pujol,Principles of Reinforced Concrete Design [11] A. B. M. Saiful Islam*, M. Jameel, M. A. Uddin and Syed Ishtiaq Ahmad Simplified design guidelines for seismic base isolation in multi-storey buildings for Bangladesh National Building Code (BNBC) [12] Chung G. Y., Ha D. H., Park K. N., Kim D. H. Experimental study on characteristics of LRB with low hardness rubber. Journal of the Korean Society of Civil Engineers, Vol. 22, Issue 6A, 2002, p. 1295-1307.[13] Oh J., Jung H. Y. An Experimental study for the shear property dependency of high damping rubber bearings. Journal of the Korean Society of Civil Engineers, Vol. 30, Issue 2A, 2010, p. 121-129.[14] I. Ahmed, A. Muqtadir, S. Ahmad Design Provisions for Stair Slabs in the Bangladesh Building Code [15] Wei Li*, Linzhu Sun and Kejia Yang Displacement-based Design of Frames Consisting of Composite Beams and RC Columns [16] ACI-ASCE Committee 352 Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures[17] Petrone, F., Shan, L. Kunnath, S.K. Modeling of RC Frame Buildings for Progressive Collapse Analysis.
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