Experimental study of geotextile as plinth beam in a pile group-supported modeled building frame
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This paper presents the experimental results of static vertical load tests on a model building frame with geotextile as plinth beam supported by pile groups embedded in cohesionless soil (sand). The experimental results have been compared with those obtained from the nonlinear FEA and conventional method of analysis. The results revealed that the conventional method of analysis gives a shear force of about 53%, bending moment at the top of the column about 17% and at the base of the column about 50–98% higher than that by the nonlinear FEA for the frame with geotextile as plinth beam.
KeywordsSoil structure interaction Plinth beam Geotextile Pile group Cohesionless soil Building frame
Soil settlement is a function of the flexural rigidity of the superstructure. The influence caused by the settlement of the supporting ground on the response of framed structures was often ignored in structural design. The structural stiffness can have a significant influence on the distribution of the column loads and moments transmitted to the foundation of the structure. Previous studies have, however, indicated that the effect of interaction between soil and structure can be quite significant. Interaction analyses have been reported in numerous previous studies such as Meyerhof (1947, 1953), Chamecki (1956), Morris (1966), Lee and Harrison (1970), Lee and Brown (1972), Subbarao et al. (and even a few studies in the recent past such as Deshmukh and Karmarkar (1991), Noorzaei et al. (Noorzaei et al. 1995), Srinivasa Rao et al. (1995), Dasgupta et al. (1998), Mandal et al. (1999), Basack (2008) and Basack and Purkayastha (2007). The common practice of obtaining foundation loads from the structural analysis without allowance for foundation settlement may, therefore, result in extra cost that might have been avoided had the effect of soil–structure interaction been taken into account in determining the settlements. This requires that the engineers not only understand the properties of the ground but they also need to know how the building responds to deformation and what the consequences of such deformation will be to the function of the building. In this regard, many analytical works have been reported on the building frames founded on pile groups by Buragohain et al. (1977), Ingle and Chore (2007) and Chore and Ingle (2008), Chore et al. and the experimental work by Reddy and Rao (2011). But no significant light was thrown on the direction of the effect of soil interaction on building frames with geotextile as plinth beam founded on pile groups.
The aim of this paper is to present an experimental investigation as well as numerical analysis through the nonlinear finite element analysis (FEA) of a model plane frame with geotextile as plinth beam supported by pile groups embedded in cohesionless soil (sand) under the static loads (central-concentrated load, uniformly distributed load (UDL) and eccentric-concentrated load). The need for consideration of soil interaction in the analysis of building frame with geotextile as plinth beam is emphasized by the experimental investigation by comparing the behavior of the frame obtained from the experimental and numerical analysis with that by the conventional method of analysis. An attempt is made to quantify the soil interaction effect on the response of the building frame in terms of displacements, rotations, shears and bending moments through the experimental investigation.
Analysis programme using ANSYS
Frame with fixed bases to evaluate the shear force and bending moment in the column, which is the usual practice done known as the conventional method.
Nonlinear analyses to evaluate the lateral displacements, vertical displacements and rotations, shear forces and bending moments on the frame.
Frame with bases released by imposing the lateral displacements, vertical displacements and rotations measured from the experiments for the corresponding loading on the frame to get the back-figured shear forces and bending moments generated in the columns.
Nonlinear finite element analysis
The following soil properties are used for sand to represent its resistance in both the lateral and axial directions: angle of internal friction ϕ (evaluated from the laboratory experiments), Poisson’s ratio ν (a typical value of 0.3 is used), ultimate skin friction τ f (evaluated from Tomlinson’s equation), ultimate tip resistance Q f, and shear modulus G i (Kulhawy and Mayne 1990). For the analysis reported herein, the following properties were employed for the loose sand: angle of internal friction ϕ of 30°, shear modulus G i of 9.615 MN/m2, unit weight of soil of 17 kN/m3 and relative density of 35%.
The frame is loaded with a central-concentrated load, UDL and eccentric-concentrated load at a nominal eccentricity of 10% of length of the beam (with eccentricity measured from the center of the beam) in increments as applied in the experimental program and the response in terms of deformations, rotations, shear forces and bending moments is obtained for each load increment.
Frame and pile groups
Scaling factors used in the study
Experimental setup and instrumentation
Static vertical loads were applied on the model frame with geotextile as plinth beam by placing weights on the hangers. The loads were applied in increments and were maintained for a minimum period to allow the deflections to stabilize, which means that the short-term deflections are considered for the analysis and long-term deflections of soil are neglected. During the application of static loads, the lateral, vertical displacements at the base of the column and the rotation of the pile cap were measured using the instrumentation setup as described earlier.
Central-concentrated load is applied in increments (1, 2, 3 kg, etc.) at the center of the beam.
The beam is loaded at third point with equal loads in increments (3, 6, 9 kg, etc.) to simulate the uniformly distributed load (UDL) condition.
Eccentric-concentrated load is applied in increments (1, 2, 3 kg, etc.) at a nominal eccentricity of 10% of the span of the beam.
Results and discussion
Lateral displacement, settlement and rotation at the base of the column from the experimental results and nonlinear FEA
Shear force in the frame by conventional method, experiments and nonlinear FEA
Bending moment at top of the column by conventional method, experiments and nonlinear FEA
Bending moment at the base of the column by the conventional method, experiments and nonlinear FEA
The bending moments given by the experiments agree well with those by the nonlinear FEA with a variation of 5–15%. Moreover, the bending moment at the base of the column changes its sign, when the load reaches some value. The sign change of the bending moment is observed to occur at an earlier stage of loading at near end than at the far end for eccentric-concentrated load. This is due to the fact that for relatively smaller loads on the frame, the column is rigidly connected to the pile cap and the soil is in its linear range, hence it behaves like a frame with fixed base. As the load on frame increases, the connection between base of the column and pile cap becomes partially rigid, the behavior of the soil will be in the nonlinear range and increase in the rotation of the pile cap will be so high, hence the nature of bending of column at the base will change its sign.
As the load on the frame increases, the behavior of the frame in terms of displacement and rotation at the base of the column predicted by nonlinear FEA and experiment appears to be linear for relatively smaller loads and for higher load range they show a nonlinear variation.
When the geotextile is used as plinth beam considerable reduction in lateral displacements and rotations is observed but not much effect is seen in settlements. In case of eccentric-concentrated load at far end the increase in lateral displacements and rotations is decreased after certain level of loading.
The displacements and rotations from the experimental results and the nonlinear FEA show a maximum difference of not more than 15%, indicating that the nonlinear curves used to characterize the soil behavior are generally good for representing the load–displacement response of the soil.
The conventional method of analysis gives a shear force of about 40–50% higher than that by the nonlinear FEA for the frame with geotextile as plinth beam. As the load acting on the frame increases, the percentage of variation of shear force predicted by the conventional method with respect to that of the experimental result also increases.
The conventional method gives a bending moment at the top of the column that is about 10–17% higher than that by the nonlinear FEA for the frame with geotextile as plinth beam but such a difference is still significant as the bending moment values are in multiples of thousands. The bending moment at the near end of the frame is higher than that of the far end for the eccentric-concentrated load case.
The conventional method gives a bending moment at the base of the column that is about 85% higher than that by the nonlinear FEA for the frame with geotextile as plinth beam. For a nominal eccentricity given for the concentrated load (10% length of the beam), the conventional method and nonlinear FEA for the frame with geotextile as plinth beam gives a higher value of bending moment at the column base of the far end from the load than the one of the near end. The reason behind this behavior is the displacements and rotations at far end were reduced when geotextile is used as plinth beam.
The response of the frame in terms of the design parameters (i.e., shear and bending moment) from the conventional method of analysis is always on the higher side irrespective of the level of loading, which reveals the need for consideration of the interaction between the building frame with geotextile as plinth beam, pile foundation, and soil.
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