2.2. CWS and SIC

During the 1970s and 1980s, many residential mid-rise RC buildings in Israel were constructed with open ground floor and slender columns. These buildings usually suffer from seismic vulnerability, do not comply with the modern seismic-design codes, and therefore require suitable engineering intervention.

The model, analyzed in this work, represents such a building. Its ground floor is open and, besides stairway walls, comprises only square-sectioned, 30/30 cm columns, made of C20 concrete. Each floor contains 60 columns. Their concrete Young’s Modulus is 23.8 GPa [11]. The floors’ heights are 2.6 m and the spans are 6 m in both directions. The floor’s area is 1548 m². The overall mass of a typical story is 1904 ton and of the roof, 1333 ton. It is assumed that internal infill walls do not contribute to the structural system’s rigidity as a long time had passed since the structure’s construction, as well, low-quality construction and supervision place a question on their ability to operate as part of the lateral load bearing system. These assumptions lead to a structure whose mechanical properties can be described by a symmetric model. A schematic of this model is depicted in the left side of Figure 1a–c where the dashed-dotted lines represent the planes of symmetry. The right half of the non-retrofitted model is the left’s reflection. A planar dynamic model was chosen by virtue of the above symmetry. In accordance with common structural practice [12], the ceilings are assumed to be rigid and the mass is concentrated at the ceilings’ levels. All the above boils down to a dynamic shear model whose details are provided below, in Section 2.4.

Floor plans of the analyzed typical building: on the right, concrete wall strengthening (Cases 1 and 2); on the left, the non-retrofitted structure (control case). (a) Ground floor (20 cm thick walls), (b) first floor (10 cm thick walls), and (c) floors two to four (10 cm thick walls).

The CWS method, which is rather classical and common, is based on strengthening the structure through improved strength elements, mainly concrete diaphragms. This is the retrofitting used in Cases 1 and 2. A vertical section of this configuration is depicted in Figure 2a. While the same live load, 490 kg_f/m², is applied to both cases, the dead load is different, due to the different compound of materials used in each one. In Case 1, the dead load is 2340 kg_f/m², whereas in Case 2, it is 1945 kg_f/m². However, the distribution of the strengthening RC diaphragms and the other retrofitting elements remains identical. It is illustrated by the right side of Figure 1. The left half of the CWS-retrofitted model is the right’s reflection. The retrofitting retains the structure’s symmetricity and accounts for two main practical issues, commonly evoked in such retrofitting. First, many structures slated for retrofitting are already occupied, thereby reducing the number of structural elements that are accessible for retrofitting operations. That is why in many retrofitting solutions, braces are implemented at the perimeter of the structure and in its internal staircase, as they are usually more accessible than other places. Another problem is that in many buildings, the ground floor is used for car parking. Hence, even though the elements in such a floor are more accessible, the need for parking places conflicts with the need to add stiffening walls, as it would block parking spots. In the suggested retrofitting, RC diaphragms are placed only at the outer walls, and no retrofitting elements are placed in the internal staircase. Such retrofitting is found sufficient for this case from a performance point of view. The number of RC diaphragms on the ground floor is reduced by half, compared to the first floor, for minimizing their impact on the parking places. However, in order to compensate for the reduced stiffness and strength, the thickness of the added diaphragms in this floor is doubled, and the perimeter beams in its ceiling level are modified accordingly. Shotcreting [13] of C30 concrete on existing walls is assumed as a method for forming these RC diaphragms. In total, 12 diaphragms of 20 cm thickness are embedded at the ground floor, 24 of 10 cm at the first floor, and 16 of 10 cm at each of the other floors. Additionally, in order to bond the reinforcing elements into a structural system, 32 beams are added at the perimeter of each ceiling. The beams’ cross-section is square, 40/40 cm in dimension, and are made of C30 concrete. Typical reinforcing elements are shown in Figure 3.

Vertical building section (A-A) of (a) concrete wall strengthening with deep green roof (Cases 1 and 2) and (b) SIC retrofitted structure with a deep green roof (Cases 3 and 4).

Concrete wall strengthening (Cases 1 and 2); vertical section of typical shear wall and peripheral beams (a); and horizontal section of typical shear wall-existing column interface (b).

Seismic isolation by seismic isolation columns (SIC) [14] is used for increasing the seismic capacity in Cases 3 and 4. Their schematic is depicted in Figure 2b. These isolators were especially developed for retrofitting buildings with open ground floor, as in the control model. They are founded on the well-known concept of friction-pendulum [15]. Essentially, the idea is to transform each existing column into a seismic-isolation device by replacing its middle portion by two steel V-shaped frames, connected by a high-load bearing chain. The remaining parts of the column are jacketed by concrete to increase their load capacity. As in Cases 1 and 2, the retrofitting of Cases 3 and 4 is identical, except for a difference in the roofs’ dead loads. The roof dead load for Case 3 is 2340 kg_f/m² and for Case 4, it is 1945 kg_f/m².

A total of 288.2 m³ of concrete and 22.5 tons of steel are required by the CWS retrofitting, and 19.2 m³ of concrete and 22.3 tons of steel by the SIC retrofitting. The mix proportions for conventional concrete with a density of 2421 kg/m³ and compressive strength of 43.2 MPa as well as for recycled concrete with density of 2334 kg/m³ and compressive strength of 35.5 MPa were adopted from Turk [16]. Thus, CWS and SIC-related mix proportions for 288.2 m³ concrete and 19.2 m³ concrete, respectively, were evaluated and are presented in Table 2.

Materials required for retrofitting with CWS and SIC-based conventional and waste-included concretes.