Stephen Pessiki

Axial Load Behavior of LargeScale Spirally Reinforced HighStrength Concrete Columns

The axial load behavior of 8 large.scale spirally-reinforced concrete columns was investigated. The 559 mm (22 in.) diameter columns were designed according to ACI 318 Code requirements and tested in concentric axial compression. The columns were made with concrete compressive strengths ranging from about 34.5 MPa to 69 MPa (5 ksi to 10 ksi). The influence of concrete strength, longitudinal reinforcement, and spiral reinforcement size/pitch on the strength and ductility of the columns was evaluated. The higher strength concrete columns displayed less ductility than the lower strength concrete columns. Columns with a higher longitudinal reinforcement ratio were able to maintain peak resistance for a large displacement, but exhibited less ductility as compared to columns with a relatively lower longitudinal reinforcement ratio. For the columns tested in this study, an increase in the spiral size and pitch, while maintaining a constant volume ofspiral reinforcement, lead to an increase in the column ductility. For the high-strength concrete columns, first cracking of the cover concrete was observed at a lower load relative to the peak load as compared to the low-strength concrete columns. Two failure modes were observed in the 8 specimens tested. The lower-strength concrete columns exhibited a bulging type failure mode, and the higher strength concrete columns exhibited an inclinedfailure plane.

Application of three-dimensional digital image correlation to the core-drilling method

We present a non-destructive technique for the determination ofin situ stresses in concrete structures, reterred to as the core-drilling method. The method is similar to the American Society for Testing and Materials (ASTM) hole-drilling strain gage method, except that the core-drilling method is formulated in the current work are performed with traditional photogrammetry, and the more novel (and more accurate) three-dimensional digital image correlation. In this paper we review the background elasticity theory and we discuss the results of verification experiments on steel plates. Calculated normal stresses are within 17% of applied values for photogrammetry, and 7% for three-dimensional digital image correlation.

Implications of Experiments on the Seismic Behavior of Gravity Load Designed RC Beam‐to‐Column Connections

This paper summarizes recent experimental research at Cornell University conducted on the behavior of gravity load designed reinforced concrete building frame components subjected to reversing cyclic loads (simulated seismic effects). Reinforced concrete framing systems, designed primarily for gravity loads, with little or no attention given to lateral load effects, are typically characterized by non-ductile reinforcing details in the joint regions and in the members. The seismic response of connection regions for gravity load design (GLD) frames has received relatively little attention in earlier studies, thus making it difficult to reliably evaluate GLD frames and to properly plan repair or retrofit strategies. Thirty-four full scale bare interior and exterior beam-to-column joints have been tested under reversed cyclic bending to identify the different damage mechanisms and to study the effect of critical details on strength and deformations. The discussion of test results focuses on the definition of joint shear strength factors for GLD frames to complement those provided by ACI-ASCE Committee 352 for frames designed with better details.

FLEXURAL BEHAVIOR OF CONCRETE COLUMNS RETROFITTED WITH CARBON FIBER-REINFORCED POLYMER JACKETS

A major deficiency in many existing nonductile reinforced concrete frames is the inability of the columns to undergo significant deformations while maintaining their load-carrying capacity. As a result, relatively brittle modes of column failure, accompanied by soft story structural failure mechanisms, are possible. Providing additional confinement to the columns allows them to behave in a more ductile manner. This paper investigates the use of carbon fiber-reinforced polymer composite jackets as a method of retrofitting existing nonductile reinforced concrete structural columns. A method of designing fiber-reinforced polymer composite jackets for columns to achieve the desired flexural behavior is presented and verified through a full-scale experimental study.

Resonant capacitive MEMS acoustic emission transducers

We describe resonant capacitive MEMS transducers developed for use as acoustic emission (AE) detectors, fabricated in the commercial three-layer polysilicon surface micromachining process (MUMPs). The 1 cm square device contains six independent transducers in the frequency range between 100 and 500 kHz, and a seventh transducer at 1 MHz. Each transducer is a parallel plate capacitor with one plate free to vibrate, thereby causing a capacitance change which creates an output signal in the form of a current under a dc bias voltage. With the geometric proportions we employed, each transducer responds with two distinct resonant frequencies. In our design the etch hole spacing was chosen to limit squeeze film damping and thereby produce an underdamped vibration when operated at atmospheric pressure. Characterization experiments obtained by capacitance and admittance measurements are presented, and transducer responses to physically simulated AE source are discussed. Finally, we report our use of the device to detect acoustic emissions associated with crack initiation and growth in weld metal.

Postearthquake Fire Performance of Sprayed Fire-Resistive Material on Steel Moment Frames

This paper describes damage patterns in sprayed fire resistive material (SFRM) on steel moment frame beam-column assemblages owing to a strong seismic event, and the thermal consequences of this damage when exposed to postearthquake fire. Large-scale experimental tests were performed to examine the bond of SFRM to steel in the three-dimensional configuration of a moment frame beam-column connection region. Two beam-column assemblages treated with SFRM were subject to quasi-static cyclic loading, resulting in large deformations and plastic hinges in the beam and local damage to the SFRM on the beam. Heat transfer finite-element analyses were performed to compare beam-column connections with damaged SFRM and fully insulated connections (with no damage) under the action of both standard and natural fires. Results of the heat transfer analyses show that SFRM damage on the beams adjacent to the column causes an increase in heat transferred into the column and elevated temperatures in the column. The elevated t...

Effect of earthquake-induced damage to spray-applied fire-resistive insulation on the response of steel moment-frame beam-column connections during fire exposure

Spray-applied fire-resistive material (SFRM) is prone to debonding, cracking, and spalling in steel moment-frame beam hinge regions during inelastic seismic response. This article presents an analytical case study designed to evaluate the effect of experimentally observed SFRM spall patterns on the thermomechanical response of a steel moment-frame beam-column assembly during post-earthquake compartment fire exposure. Earthquake-induced SFRM spalling is shown to significantly increase thermal degradation in the beam hinge region during fire exposure, leading to considerable temperature-induced softening of moment-rotation response for the beam-column assembly. This reduction in rotational stiffness increases the flexibility of the structural system for sideway motion, and exacerbates drift demands under the action of residual (post-earthquake) destabilizing forces. Language: en

Bond performance of SFRM on steel plates subjected to tensile yielding

Sprayed fire resistive material (SFRM) is intended to thermally protect structural steel elements during a fire. The integrity of the SFRM may become compromised during an earthquake due to yielding and damage in the underlying steel structure to which the SFRM is bonded. This in turn can influence the post-earthquake fire performance of the SFRM. Tests were performed to evaluate the bond performance of SFRM on steel plates subjected to tensile yielding. The test results show rapid degradation of the bond strength of SFRM to steel plates with increasing tensile yielding. Test results also show marked differences in bond behavior between wet mix and dry mix SFRM and between coatings on steel with a mill finish as opposed to a sand blasted finish.

Forensic Examination of a Noncomposite Adjacent Precast Prestressed Concrete Box Beam Bridge

On the evening of December 27, 2005 the fascia beam supporting the east side parapet wall of the third span of the Lake View Drive Bridge failed under the action of dead load. To gain insight into the potential causes of the failure a series of forensic analyses were conducted on the beams decommissioned from the bridge. The study correlates external observations of surface condition with internal chloride profile, depth of carbonation, and existing corrosion. The forensic investigation indicated that strand cover was reduced due to the construction methods of the time. The chloride level in the concrete at the lower layer of strands was high enough that corrosion would be expected. Chloride attack was identified to have come from the leakage of water between beams from the bridge deck surface above. Based on the research findings recommendations are made for visual inspection, and guidelines are provided for condition rating of noncomposite prestressed concrete box beam bridges.

Robust capacitive MEMS ultrasonics transducers for liquid immersion

Capacitive diaphragm MEMS ultrasonic transducers are of great interest because they offer wide bandwidth and ready integration into arrays. However, fragility of these transducers is a significant barrier to their application. In this talk, we report on robust transducers which have been fabricated using the MUMPS process. The transducer design has been optimized to minimize stray capacitance between the output node and the substrate. We report the use of a protective silicone layer which protects the transducers from liquid exposure and, to a degree, from mechanical damage. The silicone layer has been applied with high transducer yield without the need for prior closure of the etch holes, and coated transducers survive extended immersion in water. The thickness of the silicone layer must be carefully controlled, however, in order to prevent pulse distortion.