Investigation on the Behavior of Conventional Reinforced Coupling Beams

— Coupled shear walls consist of two shear walls connected intermittently by beams along the height. The behavior of coupled shear walls is mainly governed by the coupling beams. The coupling beams are designed for ductile inelastic behavior in order to dissipate energy. The base of the shear walls may be designed for elastic or ductile inelastic behavior. The amount of energy dissipation depends on the yield moment capacity and plastic rotation capacity of the coupling beams. In this paper, an analytical model of coupling beam was developed to calculate the rotations and moment capacities of coupling beam with conventional reinforcement.


I. INTRODUCTION
HEN two shear walls are joined by beams at fixed intervals along its height, it is called a coupled shear wall; these beams are the primary factors which control the behavior of coupled shear walls. The coupling beams are designed for ductile inelastic behavior for the purpose of energy dissipation while the base of the shear walls can be designed for either elastic behavior or ductile inelastic behavior.
Coupling beams are quite short and deep and generally have span/depth ratios of two or even lower, since the widths of door and window openings usually range from 1.0 to 1.5 m. Being similar to deep beams, coupling beams with span/depth ratios lower than 2.0 have a predisposition to fail in shear rather than in flexure. During a major earthquake, if the coupling beams were very strong, the wall units might fail due to the large axial forces and bending moments induced in them without prior yielding of the coupling beams.
As the walls are taking vertical loads and are the major lateral loads resisting elements, any damage to the walls could endanger the safety of the building and render the repair after earthquake very difficult. On the other hand, if the coupling beams were not too strong, they would yield and dissipate the excessive vibration energy before the wall units yield thereby reducing the axial forces induced in the walls and protecting the walls from being damaged. Hence, the coupling beams should be designed to yield before the walls yield, but then the coupling beams would be subjected to a certain ductility demand. In any case, the earthquake resistance of a coupled shear wall structure is highly dependent on the nonlinear behavior, especially the strength and ductility of the coupling beams. The amount of earthquake energy dissipation is Dr. Dipendu Bhunia is with the BITS Pilani, Rajasthan, India (e-mail: dbhunia@pilani.bits-pilani.ac.in). governed by the yield moment capacity and plastic rotation capacity of the coupling beams [9]- [23].
However [1], [6]- [9] show the inconsistent modeling parameters and inconsistent evaluative parameters of coupling beams described as follow: 1. As per [7], [8], the rotational capacities of beam depends on size of wall ( w w L t , ) which is illogical. , the behavior of all types of RCC coupling beams is controlled by shear [21]. For this reason, as aspect ratio of diagonally reinforced beam is less than 1.5, it means that the behavior of diagonally reinforced beam is controlled by shear. Whereas, [1] and [7] show that diagonally reinforced coupling beam behavior is controlled by flexure which is not acceptable. 3. Conventional longitudinal reinforcement with nonconforming transverse reinforcement as per [1], [7], [8] is not accepted for new construction. 4. If the behavior of coupling beam is controlled by flexure is greater than 4], the length of the coupling beam is quite larger. It has been observed [18] that weakly coupled shear walls can be obtained for larger span of the coupling beam and the design results of this type of coupled shear walls were inconsistent with regard to the ductility and energy dissipation during earthquake motion. Hence, it can be said that rotational capacity of coupling beams controlled by flexure as per [1], [7], [8] cannot be accepted. [1], [7], [8], regarding the conditions of

As per
≥ are confusing.
6. Similarly, for aspect ratio of b b d L = 1.5 Galano and Vignoli [9] shows different results regarding the ultimate rotation of various RCC coupling beams in comparison with the results made by Englekirk [6]. Hence, in this paper an analytical model of coupling beam was developed to calculate the rotations and moment capacities of coupling beam with conventional reinforcement.
Since the above discussions show the contradictory behavior of coupling beam, more study is required to investigate into the limitations on behavior of coupling beams. ATENA2D (2006) [2] was considered to carry out this study.
Four parts are created in the PART module i) Shear wall 1 ii) Shear wall 2 iii) Reinforcement iv) Coupling beam

A. Shear Wall
Dimensions of the shear wall were 300mm thick with 4m length and 3m height. Minimum reinforcement in the shear wall was taken as 0.25% of its gross area @ 450 c/c.

B. Reinforcement Layouts
There were six RCC coupling beams with conventional reinforcement layout considered in the analytical program using ATENA2D [2]. For the layout, the cross section of the coupling beam was considered as 600mm (depth, d b ) × 300mm (width, b b ) and the beam span-depth ratio (

C. Material Properties in ATENA 2D
where, V is shear force in the beam  The rotation of coupling beam in each storey is determined as follows: Rotation of coupling beam at i th storey for symmetrical walls [6] as per Fig. 2 is given by where, wi θ is rotation of wall at i th storey and w L = depth of wall, b L = length of coupling beam.

D. Reinforcements
Reinforcement property was modeled according to Table I for each type of beam and results were analyzed by using Solution parameters with Newton-Raphson Method.

III. RESULTS AND DISCUSSIONS
Following tables show the results including discussions for the conventional reinforced coupling beams with conforming transverse reinforcement which were modeled in ATENA-2D [2] as per Fig. 1.     It was observed from the Tables II and III that the rotational limit at collapse prevention level (CP) were in the similar trends in FEMA 273, FEMA 356, ATC 40 and the results of ATENA 2D. It was also seen that the crack widths were more but rotations were less for the conditions of more shear, which is quite well understood. In addition, Tables II-IV were showing incremental rotational capacities with lesser shear and moment capacities with increases of L b or L b /d b ratio. As per the literatures [18], greater L b /d b ratio gives weakly coupled shear walls which are not accepted. Table IV  ≥ .
Finally, rotational limit at collapse prevention level (CP) for conventional reinforced coupling beam should be in the range of 0.01-0.02 radian.