Structural evaluation of 3-story dormitory reinforced concrete building considering soil liquefaction potential

. The liquefaction phenomenon is the increase in water pressure in the soil, which will reduce the soil strength in supporting the load and loss of binding power between its grains. Soil liquefaction usually occurs when there is a seismic movement in the soil layer due to seismic (earthquakes) loads. Therefore, the building constructed in the soil liquefaction prone area should be designed according to the standard code. However, many design consultants do not pay attention to this condition and the building still was designed as usual even the building is located on soil liquefaction prone area. In 2018, a 3-story dormitory building structure of Hamka’s boarding school was constructed on soil liquefaction prone area in Padang city. After reviewing the design document, it was found that the consultant did not consider the soil liquefaction in its structural analysis. In this study, an evaluation of the building structure was carried out to investigate the capacity of the building in resisting the loads. From the soil evaluation using the soil Cone Penetration Test (CPT) result, it was found that the location of the dormitory building has a liquefaction potential at a depth of 1.2 - 8 meters. Considering the soil liquefaction potential in the building, the structural analysis results show that the capacity of the dormitory building, especially column, beam and foundation were not strong enough to resist the combination loads acting on the structures. Therefore, the building structure should be strengthened to face the further big earthquake that will cause the soil liquefaction.


Introduction
West Sumatra, especially Padang city, has great potential for earthquakes that can cause liquefaction.Liquefaction is a phase of solid change into a liquid phase caused by an increase in water pressure in the soil cavity [1].The impact of increasing pore water pressure causes the reduction of soil shear strength significantly due to a decrease in pore water effective stress [1].
From previous liquefaction studies, it is known that co-seismic liquefaction and the distribution of damage caused by liquefaction generally only occur in areas formed by layers of granular sediment saturated with low density, and the possibility of surface co-seismic movements exceeding the value certain threshold [2,3].Liquefactions in the soil layer are affected by the nature of soil engineering, geological environmental conditions and earthquake characteristics.Several factors that must be considered include grain size, groundwater level and maximum ground vibration acceleration [2].
Liquidation itself is a threat to construction damage in the city of Padang, which can be caused by the speed and acceleration of the earthquake and the displacement of the land surface.The potential of liquefaction is mainly in the layer of sand that is saturated with water in the presence of dynamic cyclic forces [4].It has been known that many big earthquakes occurred in Padang City, both tectonic and volcanic earthquakes.If this intensity continues to increase, it can be confirmed that land subsidence due to liquefaction in Padang city will get worse.As a result, most of the building construction in Padang city will get serious damage due to the soil liquefaction.
In order to prevent the building damages due to the earthquake on soil liquefaction potential area, a structural evaluation should be carried out using the national standard code.In this study, the structural evaluation of a 3-story dormitory building of Hamka's boarding school that was constructed on soil liquefaction area in Padang City, Indonesia, was carried out using the current standard code.

Evaluation of existing building 2.1 Structural modeling
The dormitory building of Hamka's boarding school consists of three floors with a total building height of 12.5 m.The building length and width are 37.5 m and 13.25 m, respectively.This building was designed using reinforced concrete structures.The concrete compressive strength, f c ' and steel yield strength, f y were 22.85 MPa and 400 MPa, respectively.Structural modeling and analysis were carried out using ETABS 9.7.1 software.

Loads
The dead and live loads on the building structure based on the Indonesian Building Regulation (PPIUG, 1983) are shown in Tables 1 and 2.
Table 1.Dead loads on the building structure [5].

Load Load Value
Concrete density 2400 kg/m  The earthquake loads was calculated using SNI 1726-2012 (Procedures for Planning Earthquake Resilience for Building Structure and Non-Building) [6].

Dynamic earthquake load (spectrum response)
The spectrum response is used as an analysis of dynamic earthquake loads.The spectrum response earthquake load is calculated based on SNI 1726:2012 using the 2017 Indonesian Earthquake Hazard Map.Fig. 2 shows the spectrum response for Padang City with medium and liquefied soil conditions.The spectrum response data are inputted into the structural modeling, then scale factor calculations are performed on ETABS using equation (1):

Evaluation of soil liquefaction potential
The behaviour of liquefaction on soil is affected by two main parameters, namely corrected resistance (q c1 ) and cyclic stress ratio (CSR) [7].The steps to estimate the depth of the soil which has liquefaction potential are: • Determine the number of layers and the layer numbering The number and the layer numbering are determined based on a certain depth range, which aims to simplify the analysis and calculation.The study, calculations were carried out for each layer with a data range of 20 cm of depth.
• Estimating the weight of soil volume Weight estimate of soil volume is carried out using soil behaviour graphs based on the static cone penetration data, as shown in Fig. 3, then the results of the graph are correlated to Table 3 to obtain the estimated weight of the soil volume based on the zone obtained.Table 3.The estimation of unit weight [8].• Determine the overburden ground stress Vertical stress on soil was calculated using the following formula:

The estimation of unit weight based soil description
where: σ o is vertical stress on soil (kg/m 2 ) h is depth (m) γ is weigh of soil volume (kg/m 3 ) • Determine the effective stress for the soil The vertical effective soil stress was calculated using the following formula: where: σ o ′ is the effective soil stress (kg/m 2 ) σ o is the total soil stress (kg/m 2 ) u is pore water pressure (kg/m 2 ) h is depth (m) γ is weigh of soil volume (kg/m 3 ) h w is groundwater depth (m) γ w is weight of water volume (kg/m 3 ) • Determine corrected conus resistance (q c1 ) The averaged corrected conus resistance according to type per soil depth, corrected conus resistance is calculated with the following equation: where: q c1 is corrected conus resistance q c is conus resistance CN is correction factor (Fig. 4)  • Determine the maximum magnitude and the ground acceleration (a max ) The earthquake magnitude and the maximum ground acceleration are used in the calculation of cyclic stress ratio.This parameter was obtained from Padang Pariaman earthquake data on September 30, 2009, that had magnitude 7.6 SR with a max of 0.28 g.
• Determine the stress reduction factor (r d ) The stress reduction factor is calculated based on the Liao-Whitman equation (1986) r d = 1,0 -0,00765z (for z < 9, 15 m) r d = 1,174 -0,0267z (for 9,15 m < z < 23 m) • Calculating the value of cyclic stress ratio (CSR) The calculation of CSR is averaged according to the type per depth of soil.The amount of the cyclic stress ratio is determined by: • Analyze potential liquefaction by plot CSR values (Fig. 5) From the above calculations, it is obtained the CSR value which is then plotted into the CSR chart to determine the potential for liquefaction.Table 4 shows the results of calculations and plotting results of CSR charts.From Table 4, it can be concluded that the soil of Hamka's Dormitory building has liquefaction potential at a depth of 1.2 -8 meters.

Foundation capacity
Liquidation could reduce the soil strength in supporting loads because it can make a loss of soil side resistance to axial loads.The Cone Penetration Test (CPT) data were used to analyse the foundation capacity, with a loss of side resistance at a depth that has the potential for liquefaction [9,10].

Axial bearing capacity of the foundation using the CPT data
The analysis of axial bearing capacity was carried out based on pile group calculations with CPT data along with potential liquefaction.Q u pile group = Q p pile group + Q s pile group Q p pile group = E qp x Q p Q s pile group = E qs x Q s • Determine the efficiency of the pile group E qp = (B q x L q ) / (m x n x A p ) = (0.65 x 0.65) / (2 x 2 x 0.625) = 1.69 E qs = (2 x (B q + L q )) / (m x n x Ø) = (2 x (0.65 + 0.65)) / (2 x 2 x 0.25) = 0.845 • Determine the capacity of the pile group a.The end bearing capacity of pile foundation Q p = A p x q c = 0.625 x 14709.98 = 919.3734kN b.The side resistance of pile foundation Due to the potential for liquefaction at a depth of 1.2 -8 meters, the side resistance at that depth is considered to be 0 (Table 5).Q s = Ø x JHL  From the above results, the allowable bearing capacity (Qa) is greater than the ultimate bearing capacity (Qu), it indicates that the foundation has enough capacity to resist the working load.

Analysis of lateral capacity on the pile group foundation
Other than axial load and uplift, a pile also experiences lateral loads.Potential sources of lateral loads such as wind loads, lateral soil pressure, water wave loads, ship and vehicle collisions, earthquakes, etc. Foundation deformation due to lateral loads must be within the performance criteria specified for the structure.The calculation of the lateral capacity was conducted using the Broms method with the following procedures: • Calculation parameters: n 1 : 0.4 q u : 13.72 MPa • Determine ultimate lateral load for a single pile (Q u ) (Fig. 6) With a maximum deflection requirement is 10 mm (Fig. 7), a Q a value obtained is 2.804 KN From Table 6, the reduction factor is 0.36 Q m = 0.36 x 39.83 = 14.338 kN Considering the liquefaction potential of the soil, the foundation is not strong enough to resist the lateral loads.

2.5
The column capacity

Capacity of the columns
The capacity of column was analyzed using the P-M interaction diagram method.The P-M diagram is a graph of the boundary region that shows the variety of combinations of axial loads and moments that describe the capacity of the columns.Table 7 shows the result of the bending moment capacity of the column.The checks for the column bending capacity are summarized in Table 8 and Fig. 8. Since the bending moment demand is larger than the capacity, the columns at 1 st and 2 nd floors were found to be deficient in bending under gravity and seismic loads.

Shear capacities of the columns
According to SNI-2847-2013, the shear strength of concrete structures is a combination of concrete (V c ) and steel (V s ) contributions [11].The calculation results of the column shear capacity are shown in Table 9.The table shows that the preliminary evaluation results (strength-related checks) indicate adequate in the shear stress carrying capacity of the columns.

Bending moment capacities of the beams
The detailed evaluation of the bending capacity of beam elements is summarized in Table 10.From the table, it can be seen that all the joists, almost all ring beams, and the sprandel beams with a span of 4.8 m were stressed beyond the ultimate limits.While the capacity of beams with a span of 2.1 m; 3.0 m; 4.5 m; and 5.6 m on each floor were found structurally adequate to support the working loads from floor and roof framing.

Shear capacities of the beams
Table 11 shows the calculation results of the beam shear capacities.As seen in the table, the shear capacity is less than the shear demand on some beams, it is indicating the deficiency of beam in shear under vertical and seismic loads.
The evaluation was done initially in order to determine the state and see if it is possible to strengthen the existing structures.Thus, the above evaluation suggests that the frame needs to be strengthened and retrofitted.

Fig. 2 .
Fig. 2. The comparison of the spectrum response with medium soil and soil liquefaction prone area in Padang City.
group = E qp x Q p (m x n) = 1.69 x 919.3734 x 4 = 6214.96kN Q s pile group = Σ E qs x Q s (m x n) = 0.1188 kN Q u pile group = 919.3734+ 0.196 = 6215.083kN • Determine the allowable bearing capacity (Qa) of pile group with a safety factor of 5. Q a = Q u pile group / SF = 6215.16/ 5 = 1243.017kNThe ultimate bearing capacity of the foundation based on the analysis of loading is 1000.52 kN.

• 3 • 3 •
Determine the horizontal subgrade reaction coefficient (K h ) at critical depth for cohesive soil and non-cohesive soil.K h = n 1 x n 2 x 80 q u / b = 0.4 x 1.15 x 13.72 = 2019.584kN/m Adjust the K h with loading and soil conditions (static load on cohesive soils):K h = (1/3 -1/6) x K h =1/6 x 2019.564= 336.59kN/m Determination of pile parameters a. Modulus of elasticity (E) = 29510.98MPa b.Moment of inertia (I) = 3.25 x 10 -4 m 4 c.Cross section modulus (S) = 0.002604 m 3 d.Compressive strength (f c ') = 39.425MPa e. Embedded pile length (D) = 15 m f.Diameter or width of the pile (b) = 0.25 m g.Eccentricity of applied load (e c ) = 0 h.The resistance moment of the pile (M y ) = S x f c ' = 102.6627kNm • Determine β h for cohesive soil β h = ((K h x b) / (E x I)) ¼ = ((336.59x 0.25) / (29510 x 3.25 x 10 -4 )) ¼ = 0.3059 • Determine the dimensionless length factors β h D = β h x D = 0.3059 x 15 = 4.588 • Determine whether the pile is a long pile or a short pile β h D > 2.25  then, it is a long pile • Determine other soil parameters that are located along the embedded pile γ = 13.72 kN/m 3 c u = 9.1 kN/m 2 sliding angle = 1.04

Table 4 .
The calculation results of soil liquefaction potential for Hamka's dormitory building.

Table 5 .
The side resistance of of pile foundation.

Table 7 .
The calculation of the column bending moment.

Table 8 .
The checking on column capacity.
Fig. 8. P vs M interaction diagram of column.

Table 9 .
The calculation of the column shear force.

Table 10 .
The calculation of bending moment in the beams.

Table 11 .
The calculation of shear force in the beams. .The Hamka's Dormitory Building has a liquefaction potential at a depth of 1.2 -8 meters.2. Considering the potential liquidation that occurs in the soil of the dormitory building, the pile foundation is unable to resist the required load.3. Structural elements of the building such as the columns and beams cannot resist the load if the soil liquefaction potential was considered.4. The building structure should be strengthened to prevent the damage if the big earthquake occurs that cause the soil liquefaction. 1