Assessment and retrofitting of nursing faculty building of Andalas University, Padang, Indonesia

The Faculty of Nursing building is one of the administrative buildings at Andalas University. This building was designed by a planning consultant and began construction in 2013. However, during the construction period, it was found that the concrete quality was very low, that is fc'=14.32 MPa, so that the construction could not be continued because it was not in accordance with the planning quality, fc' =22.39 MPa. Therefore, it is necessary to evaluate the feasibility of the building structure before the construction is continued. In the structural analysis, the loads apllied are dead loads, live loads, and earthquake loads. The analysis was carried out using the application (software) ETABS. Based on the analysis results, it was found that the capacity of columns and beams of the building are not strong enough to withstand the loads acting on the structure. The inter story drift also does not meet the permit limit requirements according to the New Indonesian Earthquake Code, SNI 1726:2019. Therefore, it is necessary to retrofit (strengthen) the structure of the Nursing Faculty Building by re-designing the Detail Engineering Design (DED) on the beams and columns of the plan and jacketing the existing columns. Re-analysis results show that the retrofitted building structure has a strong enough capacity to carry loads acting on the structure and the inter story drift has met the permit limit requirements according to the SNI 1726:2019.


Introduction
The Faculty of Nursing building is one of the administrative buildings at Andalas University. This building was designed by a planning consultant and began construction in 2013. However, during the construction period, it was found that the concrete quality was very low, that is fc'=14.32 Mpa, so that the construction could not be continued because it was not in accordance with the planning quality, fc' =22.39 MPa [1]. The demolition of the building certainly requires a large amount of money. Therefore, it is necessary to have an action to retrofit (strengthen) the structure so that the building can carry the working load.
Therefore, it is necessary to assess of the structure and design retrofitting the structure of the Nursing Faculty Building, Andalas University.

Condition of Existing Structure
The existing structure of the Nursing Faculty Building, Andalas University consists of the foundation, slab and first floor columns, as shown in Fig. 1. * Corresponding author: fauzan@eng.unand.ac.id From a field survey on the existing structure, it is found that the steel reinforcement had corroded (Fig. 2) and the concrete has porous and overgrown with moss ( Figs. 3 and 4). From the concrete quality test results using a hammer test, the average concrete quality is fc'= 11.24 MPa, where the quality of this concrete is not in accordance with the design concrete quality.

Structural System
The structural type of the Nursing Faculty Building, Andalas University, is a reinforced concrete structure. The building risk category is type IV because the building is classified as an educational facility building with seismic design category D, so the structural system used in the analysis is a special moment resisting reinforced concrete frame system.

Building Structure Data
Data on the structure of the Nursing Faculty Building can be seen in Table 1.

Modeling of Structure
Analysis of the Nursing Faculty Building structure, Andalas University, was carried out using 3D structural modeling in the ETABS program. Columns and beams of the building structure are modeled as frame elements, while the slabs are modeled as slab elements.
The results of the modeling of the building structure can be seen in Fig. 5.

Brick Wall Load
In addition to the dead load acting on the floor of the building, another load acting on the building is the brick wall. According to SNI 1727:2020, the load on the brick walls working on the building is 250 kg/m² while the wall height is 4 m, so the uniform load acting on the building with brick walls is 4m x 250 kg/m² = 1000 kg/m [2].

Partition Loads
Based on SNI 1727:2019 Article 4.3.2, it can be seen that the partition load is at least 72 kg/m² [2]. So that for the Nursing Faculty Building, it is assumed that the partition weight is 80 kg/m² and for the height of the building between floors is 4 meters, the partition load acting on the floor beams of the Nursing Faculty Building is 4 m x 80 kg/m² = 320 kg/m² (3.14 kN/m).

Earthquake Loads
Earthquake load analysis is conducted based on SNI 1726:2019 [3]. The type of earthquake load used in the design of the structure is dynamic earthquake load. The dynamic earthquake load used in building structures is the response spectrum. The earthquake spectrum response data itself is obtained from the RSA Puskim PUPR application. The Nursing Faculty Building is located in Padang city. The response spectrum data is shown in Tables 2 and 3.  These tables are dynamic earthquake data obtained from the Puskim PU application. From these data, a graph of Earthquake Response Spectrum Design of Padang City was obtained, as shown in Fig. 6.   Table 4. Load combination Table 4 shows the load combinations in the structural analysis of the Nursing Faculty Building.     Tables 6 and 7, it can be seen that the inter story drift that occurs in the building does not meet the permit limit requirements in SNI 1726:2019.

Column Capacity
The capacity of the column is determined through the interaction diagram of the axial moment and compression of the column and the shear capacity of the column [4].

a. Momen and Axial of Column
The calculation results of the interaction diagram of the first floor and second floor columns of the building are shown in Figs. 7 and 8.   b.

Shear Capacity of Columns
The shear capacity of the building columns is shown in Table 8. From the table, it is clearly seen that the column is able to withstand the shear forces acting on the structure.  Table 9 shows the flexural capacity of the building beams.  Table 9, it can be seen that the beams on the first floor and the beams on the first and second floors are unable to withstand the loads acting on the structure.

b. Shear Capacity of Beams
The shear capacity of the building beams can be seen in Table 10. From the table, it is found that the beam is able to withstand the shear forces acting on the structure. Based on the examination of the mass participation factor, scale factor, P-delta, and structural irregularities, all of them have met the requirements of SNI 1726:2019 [3].
From the evaluation results of the structural building performance, it was found that the structure of the columns and beams could not resist the loads acting on the structure. In addition, the inter story drift that occurred did not meet the permit limit requirements according to Indonesian building standards. Therefore, the building should be retrofitted before continuing the construction.

Retrofitting of the Structure
Analysis of the retrofitting/strengthening of the structure in this building is by re-design the Detail Engineering Design (DED) of the structure where the existing columns are jacketed [5][6][7]. The re-design is carried out on the beams, the second-floor column, while the first-floor column is retrofitted using the jacketing method.

Beam
Re-design of beams was carried out on the structure of the main and secondary beams of the building. The re-design was carried out because the beams from the initial design (DED) could not withstand the loads acting on the structure [4,8].   Figs. 9 and 10 show the comparison of the details of the main beam between the initial design (DED) and the Re-design.

a. Flexural Capacity
The results of the beam flexural capacity analysis are shown in Table 11. Table 11. Re-designed beam flexural capacity From Table 11, it can be seen that all the beam structures after the re-design have been able to withstand the loads acting on the structure. b. Shear Capacity Table 12 shows the results of the beam shear capacity analysis.  Table 12, it is found that the re-designed beam is able to withstand shear forces due to external loads on the structure. The re-design of columns was carried out on the second floor columns of the building because the capacity of the planned columns could not withstand the working loads, so the dimensions and reinforcement of the column had to be changed [4,8].

Second Floor Column
A detailed comparison between the initial design (DED) and re-designed second floor columns can be seen in Fig. 11.  As seen in Fig. 12 Retrofit of the first floor columns is carried out using the Concrete Jacketing method on all existing columns because the concrete and steel quality in the column has decreased [9,10]. The Concrete Jacketing Column Position is shown in Fig. 13.

Structural Retrofitting Modeling using Concrete Jacketing Method
Retrofitting of the column using jacketing method is carried out by increasing the dimensions and adding steel reinforcement to the column with the following assumptions  The planned jacketing column is 600 x 600 (mm).
 The column quality to be achieved is fc'=25 MPa.  In order to reach a concrete quality of fc'=25 MPa, the quality of the jacketing concrete used as an addition to the column dimensions is a minimum of fc'= 36 MPa.
Calculation of the quality of the concrete jacketing is as follows: -Column quality to be achieved (A)= 25 MPa -Existing Concrete Quality (B) = 11.24 MPa -Planned column area (C) = 600 x 600 = 360,000 mm² -Existing column area (D) = 400 x 400 = 160,000 mm² -Jacketing area (E) = 200,000 mm² Quality of concrete jacketing: (D x B) + (E x X) = C x A (160,000 x 11.24) + 200,000 x (X) = 360,000 x 25 X=36 MPa  The quality and amount of added steel reinforcement are the same as the existing one, that is fy= 350 MPa with 12 D16.
From Figs. 14 and 15, it can be seen the assumption of the definition of the jacketing column in the ETABS software. Fig. 16 shows the comparison of the column cross-section between the existing column and the jacketing column.   Capacity   Fig. 17 shows the cross-sectional capacity of the column reinforced by the jacketing method. From the figure, it can be seen that all the P-M points on the graph have been in the interaction diagram, so that the column is able to withstand the working load.

Inter Story Drift
Retrofitting on the existing first floor column with the jacketing method also affects the building displacement, so it is necessary to check the displacement of the building structure. The calculation results of the inter story drift in the X and Y directions on the retrofitted building can be seen in Tables 13 and 14.
From Tables 13 and 14, it is clearly seen that the inter story drift that occurs have met the permit limit requirements.

Conclusion
Based on the structural evaluation analysis conducted at the Nursing Faculty Building, the following conclusions can be drawn: 1. The quality of the existing concrete obtained from the hammer test results is fc'=11.24 MPa, where this value is far below the concrete quality standard for reinforced concrete building structures (minimum fc' = 17 MPa).
2. The structure of the existing first floor column, second floor plan column, and the initial design of the main and secondary beams are not able to withstand the working load. 3. The inter story drift in the existing building does not meet the permit limit requirements according to the Indonesian building standard. 4. Retrofitting of the building structure is designed by redesigning the Detail Engineering Design (DED) structure, where the existing first floor column is retrofitted by using jacketing method. 5. The retrofitted building structure has a strong enough capacity to carry the working load, and the inter story drift has met the permit limit requirements.