Analysis of type X bracing on irregular building structure

An irregular structure refers to buildings or other structures that deviate from standard design principles and geometric shapes. Based on previous studies, one of the factors that causes failure of an irregular structure is the asymmetrical shape, which causes uneven load distribution in the structure. Therefore, this study aims to evaluate an irregular-structure building and bracing type X on this structure. The evaluated parameters were the period, modal participating mass ratio, base shear, story drift, and DCR. The modelling process was performed using ETABS, an element-based structural modelling software. The results indicate that failure occurred in the structure under the existing conditions in terms of the modal participating mass ratio, story drift, and DCR. The model with bracing type X indicates a reduction in the period and rotation Z (RZ) of the participating mass. The story drift on the model with bracing type X also indicates a significant reduction; however, in model 1,3, and 5, the story drift value did not meet the story drift requirement. The DCR values of the column and beam in the model with bracing type X also indicate a reduction; however, there are some elements in the structure that do not meet the requirements of model 1,3, and 5. Bracing placement at the corner of the structure has a more significant effect on the structure.


Introduction
The implementation of innovative and efficient structural solutions like X-bracing in irregular building designs contributes to the development of sustainable infrastructure.These solutions enable the construction of safer and more efficient buildings while minimizing material and energy usage.In structural engineering, irregular structures refer to buildings or other structures that deviate from standard design principles and geometric shapes [1].The seismic performance of a building depends on many factors, such as lateral strength, stiffness, ductility, regularity, and the degree of simplicity in its configuration [2] [3].When an earthquake occurs, buildings with a uniform geometric shape and a uniform distribution of mass and stiffness have a relatively better performance compared to buildings with an irregularly shaped configuration [4].
Earthquake-resistant buildings are essential for protecting human life.One way to strengthen a building is through retrofitting.Retrofitting is the process of strengthening existing structures and reducing damage caused by an earthquake [5].The reinforcement of structures, especially irregular structures, can be complex and challenging owing to their unique building characteristics and geometries.However, retrofitting can also provide significant benefits such as increased safety, increased energy efficiency, and longer building life [6].One of the main challenges in retrofitting irregular structures is ensuring structural stability.Irregular structures often have asymmetrical layouts and unusual load distributions, which make them more susceptible to structural failure [7].Therefore, IOP Publishing doi:10.1088/1755-1315/1324/1/012003 2 the retrofitting process of these structures requires a detailed understanding of their structural behavior and the use of advanced engineering techniques to ensure stability.
Irregularities in the structure cause stresses in the beams and columns during an earthquake, which can affect the behavior of the structure.The addition of steel bracing can improve the behavior of the structure during an earthquake.The process of retrofitting with steel bracing involves adding steel bracing to the existing structure, usually in the form of diagonal or X-type bracing [8].This reinforcement system is designed to provide additional strength and rigidity to the structure, helping to redistribute the load and reduce the possibility of structural failure during an earthquake [9].

Research Methodology
This research started with literature on many bracing configurations in irregular studies to obtain the results of previous studies.The structural data were then collected, such as the building structure layout and material used for modelling purposes.The structural modelling process was performed using ETABS 2020.The structural model in the existing condition was then verified for its performance in forming the bracing placement created for this study.The modelling process was performed for all configurations for further analysis.Result and Discussion This study aims to determine the performance of an irregular structure under existing conditions, and the performance of the placement of type "X" bracing on the existing structure.The parameters calculated in this study were the period, modal participating mass ratio, base shear, story drift, and crosssectional capacity of the structural element.The data collected from the shop drawing were then modeled using ETABS software for the analysis process.The design code applied was adapted from Standard Nasional Indonesia (SNI) for earthquake analysis and loading applied in the model [10].Hence, a comparison of existing and retrofitted models can be created in this study as the final output for the structure reinforcement suggestion.

Structural Modelling
Modeling and analysis of the building structures were carried out using the ETABS 20 software.The structural elements in this model include floors, beams, and columns.Structural reinforcement elements or bracing were also added to the structural modeling.Figure 1 shows the building model using ETABS software.

Load Applied
The load applied in the model included the dead, live, and seismic loads.The standard adapted for this study are SNI 1727:2020 about Minimum Load Applied for The Building and Other Structure Design.Moreover, for the applied seismic load was adapted from SNI 1726:2019 about the Earthquake Resistance Planning Procedures for Building and Non-Building Structures.The explanation for each applied load is as follows:

Dead Load
The following table lists the dead load applied in the model as a superimposed dead load and the selfweight of the structure based on its material.Self-weight or dead loads are fixed or permanent at one location and usually have types of materials, such as concrete and steel, that work vertically.The superimposed dead load is the additional load applied to the structure, such as MEP Works, finishing, ceiling, and wall bricks.= 0.492 g; • Site Class = SD.Hence, the seismic parameter data can be determined from the aforementioned basic parameters, which will eventually produce the design spectral response parameter value (Sa) for a certain period.The values (Sa) are shown in the following table.The following figure 2 show the response spectrum of the building structure.

Building Existing Condition Analysis
The building under the existing conditions was then analyzed based on SNI 1726:2019.The period, modal participating mass ratio, base shear, story drift, and cross-sectional capacity were analyzed.The modal participating mass ratio shows that the structure has run into rotation-z for approximately 10.72% in the first modal, which is not allowed for a structure to experience rotation-z in the first modal.The following figure 3 shows the story drift of the structure: As shown in Figure 4, the story drift value exceeded the drift limit based on the SNI 1726:2019.This indicates that the story stiffness does not meet the allowed standard applied.Adding reinforcement may be a solution to this problem.Moreover, the cross-sectional capacity was analyzed.The results of the analysis are presented in the following tables.From Tables 4 and 5, the value of the DCR indicates the ratio of the capacity of the beam and column to the force received by the structure.It can be seen that the column failed owing to the axial and moment forces, whereas the beam failed owing to the moment force.

Structural Modelling with Bracing Type "X"
In the modelling of the structure with bracing type "X," five configurations of bracing placement were performed.The bracing profile used in this experiment was a 250.250.9.14 IWF steel beam.The configuration of the bracing is as follows:

Analysis Result Comparison
The results of each model were analyzed for each parameter: period, modal participating mass ratio, base shear, story drift, and cross-sectional capacity.A comparison of each analysis is as follows.From Figures 10 and 11, it can be observed that the placement of bracing in the existing structure could decrease the story drift in both the x-and y-directions.Variations 2 and 4 had the most significant effect on the structure.6 shows the value of the period for each variation in the structure.It can be observed that the structure with bracing has a lower period value than the existing condition.Variations 2 and 4 show the most significant decreases in the period in both the x-and y-directions.Based on Figures 12 and 13, it can be seen that variations 2 and 4 show that translation with mass participation in the X-and Y-directions is higher than the existing condition with a value of 96.41% -96.52% for the X-direction in mode 1 and 89.34%-89.37% for the Y-direction in mode 2. As for the rotation in the Z direction, in the existing conditions, the structure experiences a rotation of 10.86% in the Z direction in mode 1.It can be seen in figure 13 that variations 2 and 4 show a significant decrease in Mode 1 to 0.46-0.48%.For other variations, the Z-direction rotation also showed a decrease of 1-5% for mode 1.The value of the base shear of the structure increased with the addition of bracing.The addition of bracing to the structure increased its stiffness value of the structure.The largest base shear force values occurred in modeling variations 2 and 4.This shows that Variations 2 and 4 had the highest stiffness values.In Table 8, it can be seen that there has been a decrease in the DCR value, but in modeling variation 1,3,5 there are still columns that have a DCR value of more than 1, while in modeling variations 2 and 4, the column DCR value is lower than 1, which indicates that the column capacity is sufficient for the load acting on the structure.The placement of the bracing on the right side of the building indicated a more significant reduction in the force in the column, as shown in models 2 and 4.
On the beam, it can be seen in Tables 9, 10, and 11 that the DCRm value in the existing conditions exceeds 1, which indicates a failure due to the moment force on the beam.Bracing structural models experienced a decrease in DCR values, but in model variations 1, 3, and 5, there were still DCR values that exceeded 1, so there were still several columns that failed.Meanwhile, in model variations 2 and 4, the DCRm value for all the beams under review is less than 1, which indicates that the cross-sectional capacity of the beam is sufficient for the applied force.

Conclusion
The following conclusions can be drawn from this study: a.Under the existing condition, in the 1st mode shape, rotation Z appeared at approximately 10%.
The base shear also did not meet this requirement; hence, it needed to be scaled.Moreover, the story drift did not meet the requirements of SNI 1726:2019.The column has failed due to axial forces and moments, while the beam has failed due to moment forces b.Structural models with type X bracing reinforcements experienced a decrease in the period value.
The smallest period values occurred in structural model variations 2 and 4. Regarding the mass participation in the structure with type X bracing reinforcement in variations 2 and 4, the translational mass participation patterns for the X and Y directions are similar to the existing conditions.With respect to mass participation in the Z-direction rotation, the model with reinforcement decreased from the existing condition.Variation 2 and 4 models experienced the most significant decrease in Z direction rotation compared to the other variations c.The value of the base shear force on the structure with type X bracing reinforcement increased compared with the existing condition.In all variations of the model with type X bracing, the value of the base shear force did not meet the value of the static base shear force; therefore, scaling of the force was necessary.The drift value between floors in the model with type X bracing reinforcement shows a significant decrease, but in models 1, 3, and 5, there is still a drift between floors that exceeds the allowable deviation value.The models with variations in 2 and 4 showed the most significant decrease.The drift value between floors in variations 2 and 4 does not exceed the permit deviation value specified in SNI 1726:2019 d.The cross-sectional capacity of the columns and beams based on their DCR values showed a decrease in the model with type X bracing reinforcement.The most significant decreases occurred in Models 2 and 4. In Models 2 and 4, all columns and beams under review met a value lower than one.Whereas in variations 1, 3, and 5 there are still columns and beams that have a DCR value of more than 1 e.The placement point of the type X bracing reinforcement, which has the most significant effect on the structure, is at the corner of the structure, as shown in Figs. 2 and 4.

Figure 3 .
Figure 3. Story drift under existing condition

Table 1 .
Dead load value applied on structure model 2.2.2.Live LoadThe applied live load was based on the function of the building itself.A live load can be applied to several parts of the structure, such as the floor and roof.The table lists the values of the live load applied to the model.

Table 2 .
Live load applied in the structure model 2.2.3.Seismic LoadBased on SNI 1726:2019, the seismic parameter can be determined based on its location in southern Tangerang.The seismic parameters are as follows:

Table 3 .
Design spectrum response parameter (Sa)

Table 4 .
Existing condition column capacity

Table 5 .
Existing condition beam capacity

Table 6 .
Structure period comparison

Table 8 .
Column capacity against moment force

Table 9 .
Beam capacity against moment force

Table 10 .
Beam capacity against shear force

Table 11 .
Beam capacity against torsion force