Seismic Fragility Assessment of Low Rise RCC and Steel Frame Buildings Founded on Soft Soil of Langol, Manipur

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Seismic Fragility Assessment of Low Rise RCC and Steel Frame Buildings Founded on Soft Soil of Langol, Manipur

https://doi.org/10.21203/rs.3.rs-7267362/v1

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This paper presents comparison and quantification of seismic responses in cases of typical low-rise steel and reinforced concrete buildings standing on soft soil of Langol (high seismic zone), considering both rigid-base and realistic soil-base conditions. The foundation design incorporate local geotechnical conditions with settlement corrections. In-elastic time history analysis is performed on the building models considering tested data set of Langol multi layered soil profile parameters. Results show that the influence of soil-structure interaction significantly increases inter-storey drifts, particularly in the steel buildings. While, the reinforced concrete buildings show better seismic resistance. Fragility analysis are conducted to derive the probability of damage to structures under varying seismic intensity measure. Comparing to the conventional fixed based condition, the probability of damage thresholds for the reinforced concrete and steel building increase to ranges of 25% − 44% and 35–47% respectively. The results underscore the necessity to consider realistic soil-base in earthquake resistant design of structures founded on soft soils and highlight the benefits of reinforced cement concrete frames building over steel frame buildings founded on similar geotechnical conditions.

Low rise buildings

Case study

Soil structure interaction

Langol soil

Seismic performance

Fragility analysis

In highly seismic regions with soft soil conditions, the role of the subsoil becomes particularly critical. Soil can significantly amplify ground motions, alter the dynamic characteristic of the structure, and increase displacement and drift demands. These effects are especially pronounced in high earthquake prone regions like Langol in Manipur, India, which also features soft to medium stiff silty clay deposits with low to moderate bearing capacity (WRD, 2024; Bhattacharjee & Sengupta, 2011). Site investigations indicate that Langol soil exhibits low Standard Penetration Test (SPT) values, high compressibility, shallow groundwater levels, and the presence of occasional sandy layers, raising concerns over potential liquefaction and differential settlement Idriss & Boulanger (2008). These characteristics lead to a soil environment that demands careful seismic design, particularly for low and mid rise structures. The dynamic properties of such soft soil profiles often shift structural natural periods and exacerbate lateral displacements, making structures more vulnerable under seismic loading (Kramer, 1996; Seed & Idriss, 1970).

Gazetas et al, (2006) and Wolf (1985) emphasized the need for considering dynamic Soil-Structure Interaction (SSI), particularly for buildings standing on soft or layered soils. Nonlinear SSI significantly modifies base shear and inter-storey drift under seismic excitations (Khosravikia & Rahgozar, 2020). Ozcelik & Aydemir (2020) observed substantial differences in performance between fixed-base and SSI-inclusive models for mid-rise buildings. Experimental and numerical investigations have indicated that Reinforced Cement Concrete (RCC) buildings may respond more favourably on soft soils, with reduced amplification effects compared to steel frames Sharma & Dey (2018). Kayhan & Arslan (2019) and Cheng & Jeremic (2017) quantified the fragility of structures under SSI conditions and showed that ignoring soil effects can lead to non-conservative design.

The present study investigates the seismic behaviour of typical low-rise RCC and steel-framed buildings on Langol soil (seismically and geotechnical high-risk regions), and provides an approximate quantification of the change in their structural responses and damage probabilities for varying intensity measure. This study intends to give more realistic information for earthquake resistant design in such cases.

The RCC and steel buildings are analysed under two conditions; when the base is assumed fixed, and when the realistic soil-base conditions is considered. For the fixed-base model, the bottom of every column is idealised as completely rigid, representing a condition where the foundation is stiff. This is an active support condition with no soil flexible effects. In the SSI model, a realistic ground behaviour is represented by directly modelling the soil using 3D layered solid elements. Soil profile in three separate layers with typical properties of Langol soils is incorporated. Direct embedding of structure base into soil zone ensures complete continuous and real stresses transfer from ground to the structure. Through such modelling, the predictions of seismic responses are more reliable and capable by compensating for deformability and dissipation of the energy in ground soils (Wolf, 1985; Gazetas et al, 2006). Boundary conditions for the soil model follows established SSI recommendations. Lateral boundaries were set at a distance twice the width of the structure to reduce wave reflection, and the soil domain depth was taken as twice the foundation width, consistent with accepted geotechnical practice (Kramer, 1996; Veletsos & Meek, 1974).

The building models have the same plan dimensions of 15 m × 10 m and a storey height of 3 m as shown in Figs. 1a and 1b. Loads are assigned based on standard design practices and guidelines prescribed in (IS 1893 Part-1: 2016, 2021; IS 875 Part-1: 1987, 2010). The material and section properties for RCC and steel structures are summarized in the Table 1.

Table 1
Key Structural Details of RCC and Steel Buildings
Element	RCC Building	Steel Building
Column (GL/FL)	0.35 m × 0.35 m / 0.30 m × 0.30 m	SHS 132 × 132 × 4.8 mm / SHS 125 × 125 × 4.5 mm
Beam Section	0.25 m × 0.30 m	ISHB 150
Wall Thickness	150 mm	8 mm
Wall Material	Cement mortar brickwork	Fibre cement board
Slab Thickness	150 mm	150 mm
Slab System	RCC slab	Hollow clay blocks with embedded reinforcement and mortar
Concrete Grade	M20	-
Reinforcement	Fe415	-

Geotechnical information is obtained from bore hole soil investigation WRD (2024). The groundwater table is found at just 0.5 meter depth below the ground level. Although the borehole log extended beyond 25 meters, only the top 25 meters of soil were considered for modelling (Fig. 2), as this depth is sufficient to capture the soil characteristics influencing the foundation behaviour. These variations in the ground conditions are considered extensively in the model, to simulate how the soil and structure will interact, especially during seismic events. Table 2 give the soil parameters for depth up to 25m.

Table 2
Soil layer properties used for SSI modelling
Soil layer	Depth(m)	E (KN/m²)	Ф (degree)	c (kg/cm²)	γ (gm/cm³)	C_c/1 + e_o
Layer 1	0.00–8.00	18000	2	0.26	1.75	0.05
Layer 2	8.00–18.00	35000	1	0.71	1.81	0.07
Layer 3	18.00–25.00	45000	1	0.71	1.82	0.07
Note: E is the modulus of elasticity, ϕ is the angle of internal friction, c denotes cohesion, C_c is the compression index, and e_₀ is the initial void ratio of the soil.

As per IS 1893 Part-1: 2016 (2021), the building location is taken as in Seismic Zone V and the soil type is taken as III. The design lateral loads are computed as per IS 1893 Part-1: 2016 (2021) at Design Basis Earthquake (DBE) level. Vertical base reactions are extracted for both the RCC and steel structures under RSA for fixed base condition. Only the contribution of vertical forces is considered in the foundation design, as vertical forces mainly affects the foundation design outcome. Smaller horizontal forces and moments are not considered. The derived values at every base node are indicative of the axial loads being passed from the structure to the foundation under seismic excitation. These values are later factored appropriately according to design load combinations to account for safety margins as per relevant code; IS 456: 2000 (2007) for RCC and IS 800: 2007 (2007) for Steel structure. Table 3 gives the maximum vertical base reactions in fixed support conditions for both RCC and steel structures. Figure 3 shows the corresponding node numbers at base supports.

Table 3
Highest Base Reactions in fixed support condition
RCC building			Steel building
Node number	Location (x,y,z) in meters	Base reaction in kiloNewton	Node number	Location (x,y,z) in meters	Base reaction in kiloNewton
25	(15,10,0)	226.967	26	(0,0,0)	61.205
26	(15,5,0)	332.028	27	(5,0,0)	119.544
27	(15,0,0)	226.967	28	(10,0,0)	119.544
37	(10,10,0)	343.258	29	(15,0,0)	61.205
38	(10,5,0)	507.747	30	(0,5,0)	118.146
39	(10,0,0)	343.258	31	(5,5,0)	247.108
40	(5,10,0)	343.258	32	(10,5,0)	247.108
41	(5,5,0)	508.747	33	(15,5,0)	118.146
42	(5,0,0)	343.258	34	(0,10,0)	61.205
43	(0,10,0)	226.967	35	(5,10,0)	119.544
44	(0,5,0)	332.028	36	(10,10,0)	119.544
45	(0,0,0)	226.967	37	(15,10,0)	61.205
Note: 'x' and 'y' - the two orthogonal horizontal plane axes. 'z' - the vertical plane axis

The seismic load-induced vertical base reactions demonstrate a clear pattern depending on the location of supports (corner, edge, interior) as well as on the structural system. The steel structure has same the pattern. However, the magnitudes are significantly lower. Comparing with the RCC structure, the steel structure loads are distributed evenly, resulting from lighter construction materials like fiber cement board walls, hollow block slabs and steel sections. The RCC structure has larger and significantly varying base reactions, requiring stronger foundations. While, the lighter and modular system of the steel building produce less concentrated vertical loads, enabling easier and more efficient foundation designs.

The soil-layers, foundation and structure model are illustrated in Fig. 4. Raft foundation is adopted for the RCC building, shown as section and layout in Figs. 4a and 4c respectively. This allows efficient load distribution and limits the chances of differential settlement. While for the steel building isolated footings are adopted, shown as section and layout in Figs. 4b and 4d respectively. In the central columns where the columns are not sufficiently spaced apart, a combined footing is used.

The soil bearing capacity is calculated using the guidelines from IS 6403: 1981 (2004). This method considers key soil parameters; cohesion and density, depth and shape of the foundation, and groundwater conditions. Shape, depth, and inclination correction factors are also applied to enhance the accuracy of the design in accordance with the IS code recommendations. To evaluate the settlement behaviour, immediate (elastic) settlement is calculated using elastic theory, considering the stiffness of the soil layers. Corrections for foundation rigidity and embedment depth are applied based on IS 8009 (Part 1):1976 (1999) and Fox (1948), which provide standard procedures for shallow foundation settlement evaluation. Given the clayey nature of parts of the Langol soil, consolidation settlement is also estimated using established methods Mesri et al (1996). Pore water pressure corrections are considered using conventional correction charts IS 1904:1986 (2006) to represent realistic field behaviour. The applied correction factors, calculated as per standard guidelines, are shown in Table 4. The foundation design parameters including footing dimensions, soil properties, correction factors, and settlements are summarized in Table 5.

Table 4
Correction Factors for settlement calculation
Foundation Type	Rigidity Correction (C_r)	Depth Correction (C_d)	Pore Water Correction (C_p)
RCC Raft Foundation	0.8	0.98	0.8
Steel Isolated − 1	-	0.78	0.8
Steel Isolated − 2	-	0.85	0.8
Steel Combined Foundation	0.8	0.94	0.8

Table 5
Foundation Design Parameters
Parameter	RCC building	Steel building
Foundation type	Raft	Isolated − 1	Isolated − 2	Combined
Size (B × L) (m × m)	12×17	1.4×1.4	2×2	2.6×7.6
Depth of Foundation (D_f) (m)	1	1	1	1
Soil Type	III	III	III	III
Cohesion (c’) (kN/m²)	27.33	17.33	17.33	17.33
Angle of friction (\(\:\varvec{\varnothing\:}\varvec{{\prime\:}}\:)\) (degree)	1.33	1.33	1.33	1.33
Unit Weight (γ) (kN/m³)	17.5	17.5	17.5	17.5
Net Ultimate Bearing Capacity (q_nf) (kN/m²)	175.93	142.41	137.11	110.54
Applied Factored Load (q_f) (kN/m²)	32.05	51.52	49.31	41.26
Factor of Safety (FoS)	5.48	2.76	2.78	2.678
Corrected Immediate settlement (S_i) (mm)	9.08	2.62	3.91	7.32
Corrected Consolidation Settlement (S_c) (mm)	63.17	20.47	25.81	29.68
Total Settlement (S) (mm)	72.25	23.36	29.72	37
Settlement Limit (allowed) (mm)	100	50	50	100
Result	Pass	Pass	Pass	Pass
Note: kN/m³ represents kiloNewton per cubic meter, kN/m² represents kiloNewton per square meter, mm represents millimeter and m represents meter.

The calculated total settlements for the steel building are 23.36 mm for Type 1 (isolated footings at corner columns), 29.72 mm for Type 2 (isolated footings at intermediate columns), and 37 mm for the combined footing at centrally located columns. Based on the relative displacement between adjacent footing types over a 5 m span, the angular distortions evaluated, ranges from 1/833 to 1/625. These are well within the acceptable threshold of 1/500, as recommended by IS 1904:1986 (2006), indicating that differential settlement is unlikely to result in any significant structural distress.

Table 6
Details of Spectrum compatible earthquake motion (PEER)
Symbol	Earthquake Name	Record no.	Magnitude (Mw)	Duration (s)
GM 1	Duzce (1999)	Duzce, 270 (ERD)	7.2	25.9
GM 2	Bhuj (2001)	N-S Component	6.9	31.9
GM 3	Gazli (1976)	Karakyr, 090	7.1	16.3

NLTHA is conducted at the design scale (DBE level, section 5) using three [15] spectrum compatible acceleration ground motion, given in Table 6. Figures 5(a)-(b) show the maximum Inter Storey Drift Ratio (ISDR) responses of the RCC and the steel buildings under NLTHA in both X and Y directions for fixed support and SSI building models. with fixed base, steel buildings again display significantly higher ISDRs. In the X-direction (Fig. 5a), ISDRs of the steel building exceeds that of the RCC building by 84.91% at the ground floor and 49.79% at the first floor respectively. In the Y-direction (Fig. 5b), the increase is 76.72% at the ground floor and 43.76% at the first floor. These values indicates the flexible nature of steel in dynamic conditions. When considering NLTHA with SSI, the trends intensify. In the X-direction (Fig. 5a), the steel building shows a 187.30% higher ISDR at the ground floor and 55.72% lower at the first floor than the RCC building, possibly due to base rocking. In the Y-direction (Fig. 5b), steel ISDR exceeds RCC by 116.01% at the ground floor and 60.15% at the first floor.

For fragility analysis, in this study, the intensity of the earthquake and the amount of building deformation are represented in terms of PGA. Five damage levels are considered, ranging from minor to severe: OP at 0.5%, IO at 1.0%, DC at 1.5%, LS at 2.0%, and CP at 2.5% drift.

Case 1

Under fixed base conditions, the RCC building demonstrates strong resistance to seismic loading, with gradual fragility curve slopes. As shown in Figs. 6(a) and 6(b), the RCC frame reaches the IO level at a 50% probability of exceedance around 0.2g-0.25g PGA, DC at 0.3g-0.35g, LS at approximately 0.4g-0.45g, and CP only beyond 0.55g PGA. In contrast, the steel building, as seen in Figs. 6(c) and 6(d), shows earlier performance limit exceedance with steeper curve gradients. The IO level is surpassed at 0.12g-0.15g PGA, DC at 0.2g-0.3g, LS near 0.3g-0.35g, and CP around 0.35g-0.4g PGA. Compared to RCC, these thresholds occur at 20%-40% lower PGA values.

Case 2

When soil-structure interaction is incorporated, as shown in Figs. 7(a)-(d), all fragility curves exhibit a noticeable leftward shift, indicating increased vulnerability due to the flexible nature of Langol soil. This shift is more pronounced in steel structures, demonstrating the crucial influence of foundation-soil flexibility on seismic performance. For the RCC building, Figs. 7(a) and 7(b) reveal that the IO threshold is reached at 0.12g-0.15g PGA, DC at 0.18g-0.22g, LS at 0.25g-0.3g, and CP around 0.3g-0.35g PGA, all at 50% probability. While this represents a leftward shift from fixed base conditions (roughly 10%-15% reduction in PGA at each performance level), the overall curve remains relatively smooth and gradual. This suggests that despite the influence of soil flexibility, the RCC building retains adequate seismic performance due to its inherent stiffness and monolithic construction.

On the other hand, Figs. 7(c) and 7(d) demonstrate that the steel structure is more affected by SSI. The IO level is reached as early as 0.08g–0.1g PGA, DC at 0.15g-0.2g, LS at 0.18g-0.2g, and CP at just 0.2g-0.25g PGA, all at 50% probability. These represent 30%-40% lower PGA values than in the RCC structure, and a leftward shift of up to 0.1g from the fixed base case. The early threshold exceedance in the curves reflect the structure’s high susceptibility to increased flexibility and base deformability introduced by the soil.

Table 7
Increase in response (NLTHA) from fixed based to SSI condition
Building type	Directions	Increase in maximum IDSR (%)	Increase in probability of damage thresholds range OP - CP (%)
RCC	X	100	25.00–43.86
	Y	120	31.35–40.00
Steel	X	200	37.87–46.67
	Y	165	35.14–46.67

Table 7 presents a comparative summary of the increase in structural responses from fixed based condition to SSI condition, based on the NLTHA results, in terms of ISDR and probability of damage thresholds ranging from OP to CP. The results underscore the critical influence of SSI on overall responses, especially for steel structures. The increases in the responses based on analysis types, structural materials, and directional responses emphasize the need to consider realistic ground behaviour in seismic design to ensure accurate prediction of structural response.

This study presents a comparative evaluation of RCC and steel low-rise buildings built on Langol soil in terms of their seismic behaviour under seismic loads with and without SSI. Foundation systems were established on the basis of actual Langol soil conditions and structural response. A raft foundation was adopted for the RCC building, while Isolated footings (with combined footings for closely spaced columns) are used for the steel building. The results demonstrated remarkable differences in the inter-storey drift and fragility performances of the two foundation conditions. The RCC building structures are more efficient for construction in Langol soil areas, because of both their intrinsic stiffness and improved performance under SSI effects. This study reaffirms the significance of site-specific soil modelling, precise reaction-based foundation design, and prudent structural system selection for safe and effective seismic performance. Over the fixed base condition, with the inclusion of SSI, the probability of damage thresholds (OP to CP) for the RCC and steel building increase to ranges of 25–44% and 35 to 47% respectively. The need for integrating reasonable SSI modelling into seismic design is emphasized, particularly for soft soils like in Langol.

Acknowledgements

The authors acknowledge the supports provided by the fellow faculties and students of our Department during the course of this study.

Funding

It is declared that no funds, grants, or support in any other forms related to the manuscript were received.

Competing Interests

There are no related financial or non-financial interests to mention.

Author contributions

All authors have similar contributions along the way starting from the study till the end of manuscript preparation. All analysis were performed by Nisha Maibam. The initial draft of the along with the tables and figures was prepared by Nisha Maibam. The Manuscript including the figures and tables was organized systematically and refined by Sunil Singh Mayengbam. All authors commented on subsequent versions of the manuscript. The final manuscript was read and approved by all authors.

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It is declared that the authors have no competing interests from other parties, in terms of finance, profession, or personal.

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Seismic Fragility Assessment of Low Rise RCC and Steel Frame Buildings Founded on Soft Soil of Langol, Manipur

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