SEISMIC BEHAVIOUR OF BUILDINGS WITH LARGE LIGHTLY REINFORCED WALLS ALONG THE PERIMETER
Creators: name: Bibbò, Fabio Antonio
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2013

description: The use of large lightly reinforced walls in buildings founded a relevant diffusion in 50-70’s because of their good performances under seismic action with few damage in comparison with Reinforced Concrete (RC) framed buildings. Nevertheless, experimental information about this type of buildings is lacking as well as specific design indications in the technical codes. Moreover, it is important to study the behaviour of such a structural system under seismic actions because some innovative technologies for thermal insulation of external walls of buildings are leading to design structures made with large lightly reinforced walls along their perimeter.
At first, a bibliographic research to analyze the behavior of buildings realized with RC walls was carried out; the study of the technical literature pointed out that there is a lot of information on the behavior of buildings with ductile walls but a few information on the behavior of buildings with large lightly reinforced walls. 
Furthermore the bibliographic analysis pointed out that an important role is played by the in-plane behavior of the floor (rigid or flexible) when the resistant structure is realized by RC walls.
A second step in the bibliographic analysis was effected about the non-linear and cyclic behavior of RC walls in general and RC lightly reinforced walls in particular. A lot of works are available about experimental tests on RC ductile walls but a few papers were found about RC lightly reinforced walls characterized by a few percentage of reinforcement uniformly distributed and without local details. 
Then, a research in the Italian and European Code was made to analyze how the Codes treat the walls observing that all codes define the buildings with large lightly reinforced wall, and require to design them in low ductility class using the behavior factor as for buildings with ductile walls.
The bibliographic analysis was completed in the field of the non-linear models of RC plane elements. A brief summary of the materials behavior (concrete, steel and bond) was reported but the problem of RC modeling was more widely dealt with. 
After the state of art previously described, numerical finite elements models of RC panels were implemented by software SAP2000 and DIANA9.4.
The nonlinear model of a RC wall has been implemented through two software programs: SAP2000 and DIANA 9.4. 
A parametric analysis was implemented to analyze the sensitivity of the FE models respect to the tension stiffening effect and the strength of the materials. The numerical models developed by SAP2000 have shown a good agreement with the experimental tests of the walls; 
Finally, diagonal tests on concrete panels reinforced by two levels of bars grids were designed and carried out to analyze their shear deformability due to cracking. The goal of the experimental tests was to acquire experimental data for the calibration of the shear retention factor to be used in the DIANA model previously introduced. 
A final problem was approached for a lightly reinforced walls that is the design of the check of the reinforcement. 
In fact, calculation of the flexural strength of the section under axial loading is particularly onerous for RC walls due to the high number of steel bars that have to be computed with their lever arm with respect to the section centroid. To simplify the procedure, an approximation approach was developed for lightly reinforced walls. This approach can evaluate the yielding and ultimate moments, both under simple bending and under an axial load.
Through this approach the ultimate moment can be evaluated or the reinforcement designed at the serviceability limit state, but also the elastic behavior of the section can be analyzed. This model was validated about the comparison with the detailed procedure that considers each one the bar and its position.
Before the developing of the building model a study of slab flexibility was conducted in order to consider this effect in building model.
Different numerical models of the slab, considering traditional and innovative construction technologies with various levels of complexity (3D and 2D models) were implemented in SAP2000 software to establish the in-plane stiffness of the floor varying the technology of implementation (with reduction in EPS or brick) and the typology of building (with walls or frames). 
Finally the floor thickness that gives the equivalent stiffness of the 3D model was calculated to evaluate the in-plane stiffness behavior of the slab of traditional RC floor and innovative RC floor with EPS formworks. It was shown that the equivalent thickness of the innovative floors with EPS is about the one of the concrete slab.
By implementing the non-linear model of the walls within an entire building, a model of a building with large lightly reinforced walls arranged on the perimeter and with inner frames was developed. 
Currently, this type of building is realized by innovative technologies consisting of formworks composed of insulating materials that improve the overall thermal resistance of the building and serve to the construction of the walls. This solution also allows a faster erection of the structure but requires that the walls be extended along the entire perimeter covering the building to ensure thermal insulation. Furthermore, the organization of the arranged formworks does not permit the reinforcement details typical of ductile walls, and thus, it is necessary to have the reinforcement uniformly distributed both longitudinally and transversally.
On this building, a linear dynamic analysis was carried out by analyzing the results in terms of periods of vibration. A comparison of these results with simplified models of the building were made showing a good agreement with the numerical one. A comparison with provisions of national and international codes about the period of vibration was made. 
The building designed was compared with a similar building designed with internal and external frames analyzing the different behavior of two buildings in terms of periods of vibration and masses.
Firstly a parametric analysis was performed to observe the dynamic behavior of the building in terms of period of vibration, relative displacement and distribution of shear forces on vertical resisting element varying the in-plane stiffness of the slab.
In the analysis of the building, the behavior of columns and beams was considered to verify their capacity in the plastic range; particularly a parametric analysis was conducted varying the dimensions of columns and beams in order to increase the percentage of seismic shear on themselves. However the stiffness of the columns was incremented of 50 times respect to the initial hypothesis, and a low difference in the shear distribution was observed thanks to the enormous stiffness of the walls along the perimeter. Therefore, for the frame elements (beams and columns) it is possible to assume the design rules of non-seismic structures, but this result can’t be extended to all buildings with large lightly reinforced walls, because it depends on ratio between the walls and frames and the irregularity in plane and in height of the structure.
As last step, a non-linear analysis by SAP2000 was implemented using for the RC walls the model previously calibrated on a single wall, to verify the resources of building in terms of strength and ductility. 
The walls didn’t have any reinforcement details in the boundary elements. It is important because it is possible to save steel, work and time for the realization of this type of building.
The non-linear analysis was conducted according the procedure of the Italian Code and considering various intensity of earthquake and has shown a good performance. In fact, the building presents a good ductility and a good over-strength showing that the behavior factor q provided by the Italian and European Code to design this building is underestimated especially in terms of over-strength. 
The non-linear analysis has shown that this type of structure, i.e. a building with large lightly reinforced walls on the perimeter and internal frame, presents a good behavior in the post-elastic field.