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Experimental Testing and Analysis of a Beam Column Connection on Liquefiable Ground

BackgroundEarthquake-induced soil liquefaction occurs in saturated soils where the pore spaces between grains are completely filled with water. When an earthquake occurs, the relative strength and stiffness of the soil decreases due to large cyclic motions causing the grains to move readily about each other. When this happens, the ability of the soil to support the foundations of structures decrease.

Historical cases such as the Niigata earthquakes in 1964 and 2004 represent these performance failures of buildings due to combined ground shaking and permanent deformations.

Research Objectives

Beam-Column System Design SAP2000 Models of System Expected Results

Structural Fuse

Future Research

Column-Frame Connection

Strain gauges are placed by the fuse to measure bending moments, and vertical linear variable differential transformers (LVDT’s) are placed at the tip of the beam to measure its vertical displacement.

Once our system is fabricated and tested, we expect to get a similar moment- rotation plot shown below. Each “loop” on the plot represents one cycle of motion that the piston applies. We expect there to be space between each loop on our plot because this implies that the material is flexible and is absorbing energy. On the contrary, if the loops were tightly packed, the material is stiff and not absorbing as much energy.

Beam-Column Connection and cross sectional are (left). Structural fuse and cross sectional area (right).

Instron Universal Testing Machine (left). Complete system design with frame (right).

Jalila Elfejji, Dr. Shideh Dashti, Dr. Abbie B. Liel, Juan Carlos OlarteUniversity of Wisconsin-Madison, University of Colorado at Boulder

The goal of our research is to analyze how earthquake accelerations on liquefiable soils affect the behavior and damage potential of buildings. In the future, we will construct both a physical and numerical model of a 3-story, nonlinear, moment resisting structure on soil to evaluate its behavior on liquefied ground. However, our focus this summer was the design and fabrication of a prototype beam-column connection of this soon to be built structure. This is to ensure that it will hold similar structural characteristics to that of real buildings in terms of stiffness, strength, ductility and degradation.

We are testing for the moment-rotation relationship at the beam-column connection as well as the vertical displacement at the tip of the beam. We do this by applying a vertical cyclic load at the tip of the beam using an Instron Universal Testing Machine. A frame was also designed in order to keep the system securely fastened on the machine. This is represented by the red lines on the drawing.

The fuse is an important structural component of our design. It localizes plastic behavior ensuring that damage will first occur at the beams before the columns.

Parallel Axis Theorem

Moment of Inertia

SAPBeam-Column

SAPBeam-Column w/Frame

OpenSeesBeam-Column

Tip Displacement(mm)

9.66 9.66 9.6

Tip Rotation(rad)

0.046 0.046 0.045

Connection Rotation(rad)

0.013 0.013 0.012

Instrumentation

Along with a physical model, a numerical model of the beam column system was developed. In order to model the fuse, we used the cross sectional area of the real fuse and found the moment of inertia using the parallel axis theorem. We set that value equal to the moment of inertia for a rectangular section and solved for the base. This value is what was used as the base and height of the the solid square cross section of the fuse on SAP2000.

Numerical Model of Beam-Column

Parallel Axis10mm

1.6mmd

I= 1907.55 mm^4 b=12.29 mm

Strain Gauge

LVDT

We expect there to be yielding at the ends of the fuse, but the fuse itself will remain intact.

Source: Soil Dynamics and Earthquake Engineering (2012)

References:Dashti, Shideh, and Abbie B. Liel. Performance of Buildings on Liquefiable Soils: Evaluation and Mitigation : n. pag. Print.

Trombetta, N.W. , H.B Mason, Z Chen, T.C Hutchinson, J.D Bray, and B.L Kutter. "Nonlinear dynamic foundation and frame structure response observed in geotechnical centrifuge experiment." Soil Dynamics and Earthquake Engineering 32: n. pag. Web.

We will eventually build a 3-story structure that will be tested on liquefiable soil, shake table, and a centrifuge. From there we hope to analyze the sliding, tilting, and lateral drift of the structure, acceleration and frequency within the ground, pore water pressure of the soil, and techniques to help mitigate hazards due to liquefaction.