COUPLED SHEAR WALL

COUPLED SHEAR WALLCOUPLED SHEAR WALLTwo or more walls are interconnected by a system of beam or slab.Commonly used in medium- and high-rise structures in combination with RC or steel moment frames. The coupling beams provide transfer of vertical forces between adjacent walls, which creates a frame-like coupling action that resists a portion of the total overturning moment induced by the seismic action It can be used economically to resist lateral loads in building up to about 40-stories

BEHAVIOUR OF COUPLED SHEAR WALL

the moment-rotation behavior of the beam under a point load applied to represent the conditions near the beam-to-wall connections during lateral loading of the coupled wall system. The left wall region is assumed to be rigid and fixed. The right wall region can move along the horizontal and vertical directions but can not rotate.The wind moment level is the resisted by the bending moment in the two walls of the axial forceCLASSIFICATION OF COUPLED SHEAR WALL FRAME STRUCTURESpecial moment frame detailing is not appropriate for short coupling beams subjected to high shear stresses.Dowel bars crossing the coupling beam/wall interface were shown to help prevent sliding shear failures, but could not prevent stiffness degradation and severe pinching in the hysteresis response. A rhombic layout of diagonal reinforcement requires less complicated detailing than diagonally reinforced coupling beams, and exhibited less stiffness degradation of coupling beams detailed as special moment frames.

Special moment frame

Full-length/cut-off dowels Rhombic reinforcement layoutDiagonal reinforcementCLASSIFICATION OF COUPLED SHEAR WALL FRAME STRUCTURE The diagonal reinforcement appeared to provide the most stable behavior and highest energy dissipation. Diagonal reinforcement, designed to carry the entire shear demand, is required in most casesColumn-type transverse reinforcement must be provided to confine either diagonal reinforcement or entire memberLittle longitudinal reinforcement, terminated at the wall near the coupling beam end.

ANALYSIS MODEL FOR COUPLED WALLThe beam-column element formulation used in an equivalent frame analysis of a CW system should satisfy a number of constraints.For linear analyses: the elements used should account for the flexural as well as shear stiffness of both wall and coupling beam members.For non-linear analysis:the elements should accurately represent flexural and shear stiffness's and strengths as well as the deformation capacities of the members. The models must also represent the behavior of unsymmetrical wall shapes that have different stiffness, strength and deformation capacity in different directions.EQUIVALENT FRAME ANALYSIS:

ANALYSIS MODEL FOR COUPLED WALLFinite element models used to analyze a CW system should be constructed with the following points in mind. In general, plane stress membrane elements or shell elements are suitable for modeling wall components. For linear analyses: The effective Youngs modulus should be reduced to account for the expected effect of cracking. For nonlinear analysis: The analysis model should account for the nonlinear behavior of concrete under tension, compression and multiaxial conditions.FINITE ELEMENT MODEL:

COUPLING BEAM MODELThe coupling beam should be modeled using element that account for both flexural and shear properties of the beam.BEAM WALL CONNECTION MODEL:The steel or steel-concrete composite coupling beams are not effectively fixed at the face of the wall. the additional flexibility needs to wall force and lateral deflection are computed with reasonable accuracy. The effective fixed point of steel or steel-concrete composite coupling beams taken one- third of embedded length from the face of wall.

ADVANDAGES It reduces the moments that must be resisted by the individual wall piers resulting in a more efficient structural system.

It provides a means by which seismic energy is dissipated over the entire height of the wall system as the coupling beams undergo inelastic deformations.

The important advantage of a coupled wall system is that it has a lateral stiffness that is significantly greater than the sum of its component wall piers permitting a reduced footprint for the lateral load resisting system.CASE STUDY The highest reinforced concrete tower, located in high seismic zone. Having a general overview of the case, some especial aspects of the tower, and the assessment of its seismic load bearing system with considering some important factors will be discussed.

A GENERAL OVERVIEW OF THE TOWERThe tower is a 56-story tall building, located in Tehran, which is the most high seismicity zone of Iran and extensively populated nowadays .As the policy of construction in Tehran is toward the vertical accommodation, so building such a tower would be helpful to approach this goal. The tower has three transverse main walls with the angle of 120 and multiple sidewalls perpendicular to each of them. It seems that this kind of architectural configuration is due to aesthetic considerations.TOWER PROPERTIESNO.OF ELEVATION56HEIGHT173mTYPICAL FOOLR AREA3000m2EFFECTIVE RESIDENTIAL AREA126000m2STRUCTURAL SYSTEMCOUPLED SHEAR WALLVOLUME OF CONCRETE125000m3WEIGHT OF REINFORCEMENT26000 tonSTEEL WEIGHT PER AREA200 Kg/m2NUMBER OF INDIVIDUALS571FOUNDATIONMATSTRUCTURAL SYSTEMMain walls are RC shear walls with regular staggered openings. Sidewalls are also RC shear walls, connected to the main walls with coupling beams. Some of sidewalls contain continuous column of openings and the rest are solid.

Plan ViewGENERAL CONSIDERATION IN THE TOWER The tower general considerations are the followings: Overall torsionTime-dependent effects Construction sequence loadingAs the tower is located in a seismic dominant site, wind effects areneglected and the evaluation of the tower behavior is limited to seismicconsiderations only. Overall torsion:In tall buildings, which have axisymmetrical lateral load resistant elements, here are three main walls; overall torsion should be considered as an important effective behavior.Regardless of the lateral in plane sidewall stiffness, the tower is not supposed to have any torsional stiffness.Therefore, not only the sidewalls are assumed to be a main gravity load bearing system of the tower, but also they are considered as a torsional resisting system.

GENERAL CONSIDERATION IN THE TOWERTime dependent effects :In the design of high-rise concrete structures, a cumulative vertical non-uniform displacement in vertical elements is another subject that must be considered.Due to the elastic nature of concrete and its basic characteristics of initial shrinkage during curing process and creep, the high-rise structure will shorten during construction and for some period thereafter. Also, differential vertical displacements due to probable different loading patterns may cause a redistribution of forces in structural components.

GENERAL CONSIDERATION IN THE TOWERConstruction sequence loading:The long been aware of the inaccurate analytical demands in the upper floors of buildings due to the assumption of the instantaneous appearance of the dead load after the structure is built. In many cases the analytical results of the final structure can be significantly affected by the construction sequence of the structure and the manner in which the structure is built and activated and the incremental dead load gets applied. The Tall buildings, which have structural elements with different longitudinal stiffness, are sensitive to these effectsSEISMIC LOAD BEARING SYSTEMThe seismic effectiveness of structural system will be explored. It should be investigated if the structure has enough level of ductility, as a seismic system, to satisfy and, effective contribution of coupled walls, which essentially depends on the behavior of coupling elements(beam interconnecting main wall and sidewalls), is of the prime importance.

EFFECT OF AXIAL LOAD ON SHEAR WALL DUCTILITYAccording to the design codes, shear walls cannot be used as both gravity and seismic bracing systems.A seismic bracing system, conceptually, should have a level of ductility,therefore the decrements of the bracing elements ductility under axial loads should be considered in conceptual design.

The main walls as a seismic bracing system and sidewalls to carry gravity loads. This tower has a considerable behavior complexity because of its especial geometric specifications such as high aspect ratio of sidewalls especial architectural plan form and some unknown facts about coupled wall system behavior. To quantify effects on gravity load distribution due to mentioned facts, numerical models of the tower assuming different number of stories over the foundation were developed.EFFECT OF AXIAL LOAD ON SHEAR WALL DUCTILITYBased on analysis results, main walls about 35% up to 60% of gravity loads varying with the story . It seems usual for a designer, to have an unreasonable judgment about gravity load distribution in the tower.for example main walls are a seismic bracing system and sidewalls are gravity load bearing system, but as it is mentioned above, not only main walls are assumed to carry seismic loads, but also they are going to a significant percentage of gravity loads.

SUMMARY the time-dependent effects, and provide for them in the design. Having concrete structural elements with different longitudinal stiffness makes the tower to be more sensitive to differential displacements due to concrete time dependency. The using shear walls for both gravity and bracing system is unacceptable neither conceptually nor economically.The concrete in shear walls is a good way to provide more level of ductility and getting more stable behavior.By considering both time dependency of concrete and construction sequence loading simultaneously in analyses, the critical demands would be found to occur in the middle height of the structure (here is somewhere between 25~35th story).

THANK YOU