## 6 Sustainable Strengthening and Seismic Improvement Techniques
### Proposed Reconstruction Measures  
The type of measures and methods that will be applied in order to increase the resistance of the specific object depends on the building configuration, the type and quality of the material from which it was built, the degree of damage, as well as the level of resistance that should be achieved. The final decision on the method of rehabilitation, i.e., strengthening, should always be based on the appropriate calculation, engineering decision, data related to the assessed condition of the object, including its damage, type, and quality of the material from which it was made.  
Strengthening and rehabilitation of the existing supporting system includes various interventions [40]:  
* strengthening by walling up individual walls in the construction system;
* introducing new walls into the basic construction system;
* connecting the construction of the walls at the level of the inter-floor constructions.  
Considering the degree of damage, the literature recommends the application of three levels of reconstruction [55]. Each of the proposed levels has its own specific characteristics that are reflected in the details. By applying these levels, the structural assembly, purpose, or area of the space does not change. Each of the proposed levels can be applied separately, with a time distance from the previous level.  
The reconstruction and rehabilitation process itself should be the least invasive and the most cost-effective, with the use of light materials and simple techniques. By applying one level or combining several at the same time, the overall earthquake resistance of the building increases.  
Level 1 is the simplest and includes restoring the building to its original state, before the earthquake, while strengthening all parts of the building that showed particular vulnerability to the earthquake. Construction calculations are not necessary for this level of rehabilitation. The measures include the following procedures:  
* Repair of observed cracks and wall damage (filling with mortar and FRP protection)
* Rehabilitation and stabilization of the roof structure and roof covering
* Connecting the floor structure and the surrounding walls
* Strengthening walls (grouting and filling cracks)
* Connecting the walls (the estimated value of the work performed at this level is 90-95 Euros/m\({}^{2}\)[26]).  
Level 2 is more complex than level 1 and its main objective is to strengthen the building in order to increase its resistance to earthquakes by a minimum of 50% of the resistance required for new buildings. This level involves the analysis of the entire building by an authorized construction expert, with the goal to make a static calculation of all damaged places on the building. Reinforcements are most often made with FRP products. Depending on the scope of the planned works, the estimated value is 70-165 Euros/m\({}^{2}\) (the price varies if the works are performed on this level or in combination with level 1) [26].  
Level 3 is a superstructure of level 2 because it requires even greater resistance and, therefore, more extensive works with the preparation of project documentation of the construction project and seismic classification of the building, with the requirement to achieve 75-100% resistance to the effects of earthquakes of the new building. The estimated value of the works on this level is 230 Euros/m\({}^{2}\), and if the works are combined with the works performed on level 1 and level 2, the estimated value of the works goes up to 400 Euros/m\({}^{2}\).  
In case of implementation of all three proposed levels, the total saving is about 15% compared to the case where each level is done separately [26].