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The construction of low damage, self-centering, unbonded post-tensioned seismic systems requires special techniques and equipment such as hydraulic jacks to stress tendons that may not be readily a...

This paper proposes a new scaling procedure to consider the inelastic response of structure along with the effect of higher modes in scaling the selected ground motions for seismic response analysis. This is done by obtaining the corresponding inelastic Single-Degree-of-Freedom (SDOF) system of the structure through performing a single-run modal pushover analysis with a load pattern consistent with the combined-modal-story-shear profile obtained from response spectrum analysis. Therefore, the effect of the higher modes and interaction between them in the nonlinear phase are reflected in the inelastic SDOF. The scaling process is performed such that the peak displacement of the inelastic SDOF system under the scaled record matches the inelastic spectral displacement (target displacement). The proposed procedure is evaluated through three regular and one irregular tall building. The results demonstrate the superiority of the proposed procedure in estimating the engineering demand parameters of structures with/without important higher modes effects.

This article investigates the use of photon attenuating inclusions (PAI) for enhancing radiographic imaging of cementitious materials in composite structures. By targeting characteristic high attenuation regions in the photon energy spectrum, PAI can improve void detection in the parent material by increasing contrast in radiographic images. Radiographic imaging of cementitious grout specimens with 5% and 15% weight fractions of candidate PAI materials (Fe, BaCO3, BaSO4) was performed to characterize X-ray attenuation. Virtual radiography simulations were then conducted to evaluate the application of PAI to grout void detection in post-tensioned concrete construction. The simulations demonstrate that grout void detection is most effective when the high energy region of the emission spectrum is near the K-edge of the PAI material.

Traditional design of a seismic resistant system for a building structure has often relied on structural damage as the intended response of the structure to limit the increase in lateral force and to dissipate energy. The goal of this traditional design approach was life-safety, i.e. to prevent building collapse. Following this approach, a major seismic event can cause significant damage to the structure. This in turn requires extensive repair, or if the damage is severe enough, for the structure to be demolished. More recently, an alternative design approach has emerged that is intended to provide structures that remain damage free and self-center (i.e. exhibit no residual drift) after the earthquake. This paper describes this alternative approach, and discusses opportunities for improved sustainability through damage-resistant seismic design and renewable materials.

Spray-applied fire-resistive material (SFRM) is prone to debonding, cracking, and spalling in steel moment-frame plastic hinge regions during inelastic seismic response. To evaluate the effect of experimentally observed earthquake-induced SFRM spall patterns on building sidesway response during an ensuing fire, an analytical case study is developed for a steel special moment-frame building with a seismic hazard representative of coastal California. Response data from numerical earthquake simulations indicate that damage to SFRM insulation in beam hinge regions should be anticipated following ground shaking representative of the maximum considered seismic hazard. Thermomechanical post-earthquake fire simulations demonstrate that earthquake-induced SFRM spalling significantly increases thermal degradation in the affected beam hinge regions during fire exposure, leading to pronounced softening of moment-rotation response for the beam-column assemblies. This temperature-induced moment-frame connection softening increases the flexibility of the structural system for sidesway motion and exacerbates drift demands under the action of residual destabilizing forces.

This paper describes the static pushover response of a proposed lateral force resisting system that uses spring anchored post tensioning (PT) tendon to create a rocking system. The spring at the PT tendon anchorage is aimed at facilitating manual post tensioning of the PT tendon which helps to implement rocking systems in developing communities subjected to high seismic hazards without the need for trained workers or specialized equipment. The reduction in the lateral stiffness of this system at high intensity loads causes rocking behavior that is initiated by a gap opening at the base of the system. The PT tendon is unbonded over the length of the system to prevent yielding and the loss of prestressing force. Because the prestress force in the unbonded PT tendon is maintained, the lateral force resisting system exhibits self centering behavior. This paper derives closed form expressions that describe the lateral force deformation behavior of the proposed system. A simple finite element analytical model is presented to verify the closed form expressions and to study the effects of the system parameters on the static pushover response. Results of this study show that the closed form expressions display an excellent agreement with the analytical model including the predictions of the gap opening, clamping of the spring element, yielding and fracture of the PT tendon.

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 analy...

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...

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.

The incremental core-drilling method (ICDM) is a technique to assess in-situ stresses in concrete structures. These stresses may be either constant or vary through the thickness of the member under investigation. In this method, a core is drilled into a concrete structure incrementally. The displacements which occur locally around the perimeter of the core at each increment are related to the in-situ stresses by an elastic calculation process known as the influence function method. This paper presents the analytical and numerical techniques necessary for practical use of the ICDM. In particular, influence function coefficients for the specific geometry of a core hole are calculated using finite element techniques.

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.

Metal wythe connectors are used in a typical precast concrete sandwich panel to tie concrete wythes together and to keep the panel intact during handling and in service. Connectors interrupt the continuous insulation layer, reducing the effectiveness of the insulation. In current practice, thermal resistance (R-value) of such a panel is calculated from the zone method. However, the zone width parameter W used in the zone method was originally developed for metal-frame structures and an accurate R-value cannot be estimated for precast concrete sandwich panels containing metal wythe connectors. This paper proposes a new zone-width equation for use in the current zone method to compute the R-value of precast concrete sandwich panels containing the metal wythe connectors. The proposed zone width W n was derived from the results of a series of finite element heat-transfer analyses intended to quantify the influence of several key parameters on W n . It was found that the zone method with the proposed zone-width equation can accurately estimate R-value of a precast concrete sandwich panel containing metal wythe connectors. Also the proposed zone-width equation can effectively consider the effects of metal wythe connector sizes and spacing, material conductivities, and panel thicknesses in the zone method of R-value computation.

This paper describes an experimental program designed to verify the behavior of 3-wythe panels under later load and at the time of prestress transfer. The general flexural behavior of the 3-wythe panels was investigated using lateral load tests. Load-deflection behavior, degree of composite action, flexural strength, and horizontal shear of the panels under the action of a uniform pressure over the panel span were evaluated in this study. Fabrication of the panels for the lateral load tests revealed that cracking at prestress transfer was a concern. Thus, additional tests were performed to evaluate stresses at prestress transfer. These tests examine the actual behavior of full-scale, 3-wythe panels under the action of prestress force. Experimental results presented in this paper are compared with FEM analysis results. Descriptions of FEM analyses and models were given in a prior paper, and complete details of the work presented in this paper were provided in 2 other earlier papers.

The core-drilling method (CDM) is a stress-relief technique used to determine in situ stresses in concrete. A small hole is drilled into the concrete of a structure and the resulting displacements are measured and then converted into in situ stresses using elasticity theory. Three major factors that influence stresses determined with the CDM have been identified: (1) swelling of concrete around the core hole caused by exposure to water used during the drilling process; (2) changes in the measured deformations caused by relief of differential shrinkage stresses; and (3) steel reinforcement in close proximity to a core hole. Recent research has addressed each of these factors through analytical and numerical techniques that adjust CDM-calculated stresses. This paper describes experiments performed to verify these approaches and to show that the CDM can determine accurate in situ stresses in concrete structures. Concrete plates loaded in compression were subjected to hole drilling, and the resulting ...

The use of unbonded posttensioned (UPT) precast concrete walls with horizontal joints as the primary lateral load resisting system in seismic zones has been considered in several previous studies. This paper introduces a design-oriented analytical model that uses simple formulae to estimate the nonlinear lateral load behavior of UPT walls, and compares this simple model with available experimental results. A previously developed UPT wall model based on fiber elements is also compared with experimental results. Each model is formulated to consider several critical limit states in the lateral load behavior of UPT walls. Comparisons showed generally good agreement between analytical and experimental results for three different test walls under monotonic and cyclic loading. The simple model is found to be sufficiently accurate for seismic design of UPT walls, and the fiber model is found to be sufficiently accurate for estimating UPT wall response under earthquake loading.

Precast concrete, three-wythe sandwich wall panels were developed with potentially improved thermal and structural performance compared with that of traditional two-wythe panels. A three-wythe panel has three concrete wythes and two insulation layers. All three concrete wythes are connected by solid concrete regions, and the connections between successive concrete wythes are staggered in location so that no concrete path extends directly through the entire thickness of the panel. This paper describes the design and analysis of the three-wythe panels. A series of two- and three-wythe panels were designed, and examination of the resulting panels' behavior provides insight into the anticipated capabilities of the three-wythe panels. Finite element analyses were performed to anticipate the behavior of the three-wythe panel at both the transfer of prestressing force and under service loads. It was found that three-wythe panels can be designed using current design codes with special consideration of stresses that develop at the panel ends. A three-wythe panel can be treated as a composite panel and is suitable for longer spans compared with spans typical for a two-wythe panel. Transverse bending occurs in a three-wythe panel at the panel ends, and several approaches to reduce the transverse bending were introduced and evaluated, such as using partially debonded strands and shear connectors.

The core-drilling method is a technique that is used to determine stresses in concrete. In the method, a small hole is drilled into the concrete, and the resulting displacements are related to stresses present in the structure via elasticity theory. One source of complication when applying the method is the presence of differential shrinkage stresses. The exterior of a concrete specimen dries and shrinks faster than the interior, thus causing tension stresses on the exposed faces, and compressive stresses on the interior. These stresses tend to dissipate with time as the moisture profile becomes more uniform. However, for relatively young concrete specimens these stresses can be significant. In the current work, the moisture distribution and corresponding differential shrinkage stresses are analytically predicted in concrete plate specimens by solving a non-linear diffusion equation with appropriate boundary conditions. These differential shrinkage stresses are then evaluated numerically in the core-drilling method. It is shown that these differential shrinkage stresses can result in large discrepancies in applied versus measured stresses, especially for concrete specimens that are particularly young (less than 3 years of age), particularly thick (greater than 150 mm thick), or cast/stored in a very dry environment. Application of the procedure detailed herein to the results of a previous hole drilling study in concrete plates improved measured accuracy from 47% relative error to 34% error if it is assumed that the previous plates were stored in a 50% relative humidity environment.

Earthquake-induced damage to structural components and spray-applied fire-resistive material (SFRM) in steel gravity frame beam-column connection regions can impact connection performance during an ensuing fire. Bolted connections are particularly susceptible to failure at elevated temperatures due to a rapid loss in bolt capacity with increasing temperature, which occurs at an accelerated rate compared to the thermal degradation of structural steel. In order to characterize structural and SFRM damage over a range of seismic drift demands, two large-scale SFRM insulated beam-column connection assemblies utilizing single-plate and unstiffened seated connection designs, with composite floor slabs, were tested under combined gravity and lateral loading. SFRM cracking, debonding, and spalling were observed during both tests, exposing critical connection elements. In addition, the single-plate connection assembly, which was developed in accordance with U.S. national guidelines but contained design feat...

This paper presents a numerical modeling approach, developed within the CIVA RT virtual radiography software, for simulating radiographic imaging of Portland cement-based materials. The modeling approach is validated experimentally for 20 and 30 cm thick Portland cement concrete specimens, which were imaged over a range of source intensities using a 450 keV COMET MXR-451 X-ray tube. Both a discrete coarse aggregate (DCA) model and a computationally efficient homogenized concrete (HC) model were investigated. The study found that for concrete sections thicker than 10 cm, the HC model provides radiation transmission predictions comparable to the DCA model, which was attributed to the increase in photon scattering with specimen thickness. The experimental validation study utilized film-based radiography and considered target optical density measurements of 1.0, 2.0, and 3.5. In general, numerical predictions from the virtual radiography model were in good agreement with the experimental data. For the 20 cm thick specimens, relative error in the numerical predictions ranged from 18 to 20 %. Relative error in numerical predictions for the 30 cm thick specimens ranged from 4 to 8 %. The improvement in model prediction accuracy for the thicker specimens was attributed to increased filtration of lower energy radiation, which reduced the influence of approximation error in the lower energy region of the photon emission spectrum.

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.

A precast, prestressed concrete girder with large web openings allows building service systems (mechanical, electrical, communications, and plumbing) to cross the girder line within the member's depth, reducing a building's floor-to-floor height and the overall height of the structure. These height reductions have the potential to improve the competitiveness of total precast concrete structures versus other types of building systems. The experimental program reported in this paper tested three full-scale inverted-tee (IT) girders with large web openings (ITO girders) to failure to evaluate the openings'effect on girder behavior. The load-deflection response of the ITO girders up to peak load was similar to that of a control girder without openings. The ITO girders reached their design loads but failed at lower loads than predicted and at values approximately 20% less than the failure load for the control girder. The ITO girders failed in a brittle manner due to diagonal cracking above the opening closest to the support. The test girders were designed using available recommendations in the existing literature. Testing in this experimental program demonstrated that available recommendations in the literature inadequately predict failure loads for ITO girders.

Precast concrete sandwich wall panels are commonly constructed of two wythes of concrete separated by a layer of thermal insulation. In these two-wythe panels, solid concrete regions are often provided for embedded hardware for lifting, handling, and connections, or to provide composite action. These solid concrete regions can have a significant adverse impact on the thermal performance of the panels. This research was directed towards the development of precast concrete three-wythe sandwich wall panels with potential improved thermal and structural performance. A three-wythe panel has three concrete wythes and two insulation layers, and all three concrete wythes are connected by solid concrete regions. However, the connections between successive concrete wythes are staggered in location so that the total thermal path length through the concrete is extended. Practical panel configurations of the three-wythe panels were developed to reduce thermal bridge effects caused by regions of solid concrete. The thermal performance of the three-wythe panel was evaluated by estimating its thermal resistance (R-value) using the finite element method. It was found that, in general, the thermal performance of three-wythe panels is better than that of two-wythe panels due to the increased thermal path length through the panel.

Precast concrete sandwich wall panels are commonly constructed of two wythes of concrete separated by a layer of thermal insulation. In these two-wythe panels, solid concrete regions which extend directly through the entire thickness of the panel are often provided with embedded hardware for lifting, handling, and connections, or to provide composite action. These solid concrete regions have a significant adverse impact on the thermal performance of the panels. This research was directed towards the development of precast concrete three-wythe sandwich wall panels with improved thermal and structural performance. A three-wythe panel has three concrete wythes and two insulation layers, and all three concrete wythes are connected by solid concrete regions that are staggered in location so that no concrete path extends directly through the entire thickness of the panel. Practical panel configurations of three-wythe panels were developed, and their thermal performance was evaluated by estimating thermal resistance (R-value) using finite element method.

The paper describes a fiber based analytical model of unbonded post-tensioned precast concrete walls with vertical joint connectors. The model is used in an analytical study of the influence of selected wall design parameters on the lateral load behavior of a prototype wall. The paper verifies the accuracy of a trilinear idealized lateral load behavior based on the derived closed form expressions based on the results of the analytical parameter study. Recommendations based on the trends observed in the parameter study are provided to help the designers satisfy the criteria of the proposed design approach.

An investigation of seismic design of precast concrete wall panels that have been connected along vertical joints with ductile connectors is presented. The panels are also connected to the foundations along horizontal joints by using unbonded post-tensioning steel. The objectives of this study are (1) to derive closed-form expressions for the base shear roof displacement capacities of unbonded post tensioned precast concrete walls and (2) to propose a design approach that relates wall capacities to code-specified design demands. Study results will also apply to walls with different numbers of panels and different panel sizes.

Lateral load tests were performed on four full-scale precast concrete sandwich wall panels. The first panel was a typical precast, prestressed concrete sandwich panel that had shear transfer provided by regions of solid concrete in the insulation wythe, metal wythe connectors (M-ties), and bond between the concrete wythes and the insulation wythe. The degree of composite action developed by each shear transfer mechanism (regions of solid concrete, wythe connectors, and bond) was then evaluated by testing three additional panels that included only one mechanism each. The panels were tested in a horizontal position with simple supports under the action of a uniform lateral pressure. It was found that, for the panel geometries and materials treated in this study, the solid concrete regions provide most of the strength and stiffness that contribute to composite behavior. Steel M-tie connectors and bond between the insulation and concrete contribute relatively little to composite behavior. For design purposes, it is recommended that solid concrete regions be proportioned to provide all of the required composite action in a precast sandwich wall panel.

This paper presents the results of an experimental investigation of the axial behavior of small-scale circular and square plain concrete specimens and large-scale circular and square reinforced concrete columns confined with fiber reinforced polymer (FRP) composite jackets, subject to monotonic, concentric axial loads. Improvements in the axial load-carrying and deformation capacities of FRP jacketed concrete members over unjacketed members are reported. Factors influencing the axial stress-strain behavior of FRP confined concrete, such as transverse dilation and effectively confined regions and their relationship to jacket properties, are identified and discussed. Factors necessary to calibrate in situ jacket behavior and reported or measured FRP material properties are proposed and their interrelationships discussed.

Axial load tests were performed to evaluate the confinement effectiveness of high strength spiral reinforcement in concrete compression members. The tests included 24 and 14 in. diameter circular cross section compression members with spiral nominal yield strength that ranged from 78 to 140 ksi. It was found that while spiral steel stresses in excess of 60 ksi are achieved, the nominal strength of the spiral may not be achieved.

Unbonded post-tensioned precast concrete walls are constructed by post-tensioning precast wall panels across horizontal joints using post-tensioning steel which is not bonded to the concrete. This paper describes an analytical investigation of the seismic behavior and design of these walls. Unbonded post-tensioned precast walls with strength and initial stiffness similar to monolithic cast-in-place concrete walls can be designed to soften and undergo large nonlinear lateral drift with little damage. The nonlinear behavior is primarily due to the opening of gaps along the horizontal joints. A performance-based seismic design approach is proposed in which the walls are required to resist design level ground motions with little damage and severe survival level ground motions with damage but without failure. Shear slip along the horizontal joints is prevented by design. Nonlinear dynamic analyses show that, compared to cast-in-place walls, unbonded post-tensioned precast walls undergo larger drift, but accumulate significantly smaller residual drift during an earthquake.

This paper presents a study of the role of diaphragm deformations in the seismic performance of precast parking structures. A prototype parking structure typical of structures in the Los Angeles area prior to the 1994 Northridge earthquake was studied. Nonlinear static analyses of the diaphragms and shear walls were conducted. The response of the diaphragms and shear walls in these analyses provided stiffness and strength properties for a dynamic analysis model of the structure. Significant issues related to the role of diaphragm deformations in the seismic performance of precast parking structures were identified. These include large drift demands on the gravity load system due to excessive diaphragm deformations; shear wall locations that cause the diaphragms to twist in plan, amplifying these deformations; and cross sections in critical locations in the diaphragms that have insufficient strength.

To increase the efficiency of precast concrete buildings throughout their expected life, a set of criteria for structural floor framing systems was developed to explicitly accommodate nonstructural requirements. The nonstructural requirements include the service systems (plumbing, HVAC, and electrical) and the enclosure and finish systems, as well as spatial and functional versatility. The criteria focus on the efficiency and performance of the service, enclosure, and finish systems and the spatial and functional characteristics as influenced by the structural floor system. Associated with each criterion are specific measures that allow the direct and systematic comparison of different floor framing systems. The research applied the set of criteria to assess existing precast concrete floor systems, and used the results from the assessment to develop new concepts for precast floor systems to explicitly accommodate the nonstructural requirements. The research indicates that significant opportunities exist t...

This paper summarizes progress on research at the Center for Advanced Technology for Large Structural Systems (ATLSS) on gravity load floor systems for precast/prestressed concrete buildings. The main objective of the research is to develop precast/prestressed concrete floor systems for gravity loads for application in occupied office buildings with a regular spacing of columns. Five proposed concepts for precast/prestressed structural floor framing systems that are suitable for office building construction are presented. The concepts were developed based on previous research that included a review and assessment of existing precast/prestressed concrete floor framing systems and an identification of opportunities for the development of new precast structural systems. The proposed systems differ in relative amounts of prefabrication vs. field work, and in the manner in which the service and architectural systems are accommodated.

Over the past four decades, a large number of composite prestressed concrete beam/slab bridges have been built in North America. To evaluate the load rating of these bridges, an assumption must be made concerning the existing effective prestress force. This assumption is difficult to make because the effective prestress force is influenced by several time-dependent phenomena such as shrinkage and creep of the concrete and relaxation of the prestressing strands. This paper presents the results of an experimental study to determine the effective prestress force in two fullscale prestressed concrete bridge beams that were removed from a bridge structure after a period of 28 years in service. An average prestress loss of 18 percent was determined for the two specimens. This loss value is approximately 60 percent of the loss predicted according to design specifications.

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.

This paper and a companion paper provide a review and assessment of existing precast concrete gravity load floor framing systems suitable for office buildings. The companion paper reviews 19 precast structural systems. The assessment presented in this paper treats interactions between the structural, service, and architectural systems that comprise the building system. The assessment indicates the current state-of-the-art in precast concrete structural systems for office buildings and leads to conclusions regarding opportunities for the development of new systems and the improvement of existing systems. Work in progress is focusing on the most promising concepts evolving from this study with a view towards developing a modern, economical, and efficient precast concrete floor system.

This paper reviews 19 precast structural floor systems from North America and overseas that are suitable for office building construction. This review includes a discussion of the impact of the structural systems. The review was conducted as part of a research project directed towards the development of new precast concrete floor systems for gravity loads for application in occupied office buildings having a regular spacing of columns. A companion paper presents the assessment of these floor systems. The research addresses the need for innovation in the design and construction of precast concrete buildings.

The incremental core-drilling method (ICDM) is a nondestructive technique to assess in-situ stresses in concrete. In contrast to other available methods of in-situ stress measurement in concrete, the ICDM can quantify stresses that vary through the thickness of the concrete member under investigation, such as those due to bending or eccentric prestressing. In this method, a core is drilled into a concrete structure in discrete increments. The displacements which occur locally around the perimeter of the core at each increment are measured and related to the in-situ stresses by an elastic calculation process known as the influence function method. This paper presents the analytical and numerical techniques necessary for practical use of the ICDM, as well as results from experimental tests in which simple concrete beams were subjected to controlled loads and insitu stresses measured via the ICDM were compared to known stress distributions. The ability of the technique to accurately measure a variety of different stress distributions is demonstrated, and practical considerations for an ICDM investigation are discussed.

The incremental core-drilling method (ICDM) is a nondestructive technique to assess in-situ stresses in concrete. These stresses may be constant or vary through the thickness of the concrete member under investigation. In this method, a core is drilled into a concrete structure in discrete increments. The displacements that occur locally around the perimeter of the core at each increment are measured and related to the in-situ stresses. This paper presents results from experimental tests in which simple concrete beams were subjected to controlled loads and in-situ stresses measured via ICDM were compared to known stress distributions.

This paper discusses the nondestructive technique of core drilling to evaluate stresses in concrete. Core drilling measures the displacements found near a hole drilled in concrete and relates this to stresses present in the structure by using elasticity theory. During core drilling, the introduction of water causes concrete to swell. These swelling displacements directly lead to errors in estimating the stresses. In this paper, the author provides a way to correct these water-induced swelling displacement errors. Using the values presented in the research literature as a basis, the author estimates the depth of water penetration and the swelling strain due to water exposure. The apparent stresses are estimated using finite element modeling. Results, which are applied to an earlier non-related hole-drilling investigation, show that proper accounting for water-induced swelling displacements significantly improves accuracy in prediction.

Precast concrete sandwich wall panels are often used for building exterior cladding and may also serve as bearing or shear walls. These precast concrete sandwich wall panels are commonly constructed of two wythes of concrete separated by a layer of thermal insulation. In these two-wythe panels, solid concrete regions which extend directly through the entire thickness of the panel are often provided for embedded hardware for lifting, handling, and connections, or to provide composite action. These solid concrete regions have a significant adverse impact on the thermal performance of the panels. This research was directed towards the development of precast concrete three-wythe sandwich wall panels with improved thermal and structural performance. A three-wythe panel has three concrete wythes and two insulation layers. All three concrete wythes are connected by solid concrete regions, and the connections between successive concrete wythes are staggered in location so that no concrete path extends directly through the entire thickness of the panel. This paper describes the thermal performance of precast concrete three-wythe sandwich wall panels. Possible panel configurations of the three-wythe panels are proposed, and their thermal performance is studied by estimating R-values using the finite element method (FEM) analysis. It was found that, in general, the thermal performance of three-wythe panels is better than that of two-wythe panels due to the increased thermal path length through the panel.

Acoustic emissions are ultrasonic pulses produced in solids when irreversible damage occurs under mechanical loading. We report on the design and testing of a MEMS device, adapted from cMUT technology, to detect acoustic emissions. The device is fabricated in a surface-machined MEMS process and consists of diaphragms supported by springs above a plate. When a DC bias voltage is applied, vibration of the spring-supported diaphragm causes a time-varying current. The MEMS device contains seven independent transducers with resonant frequencies in the range of 100 kHz to 1 MHz. The availability of multiple signals allows for redundant sensing and may facilitate distinction of true acoustic emissions from other events. We report the detection of actual acoustic emission events in structural testing, comparing the performance of the MEMS device with a conventional PZT acoustic emission transducer. In the test, a MEMS device and a conventional transducer were attached to a steel beam specimen which was loaded to failure. Strong acoustic emission events were simultaneously detected by both types of transducers

In a collaborative project at Lehigh and Carnegie Mellon, a MEMS acoustic emission sensor was designed and fabricated as a suite of six resonant-type capacitive transducers in the frequency range between 100 and 500 kHz. Characterization studies showed good comparisons between predicted and experimental electro-mechanical behavior. Acoustic emission events, simulated experimentally in steel ball impact and in pencil lead break tests, were detected and source localization was demonstrated. In this paper we describe the application of the MEMS device in structural testing, both in laboratory and in field applications. We discuss our findings regarding housing and mounting (acoustic coupling) of the MEMS device with its supporting electronics, and we then report the results of structural testing. In all tests, the MEMS transducers were used in parallel with commercial acoustic emission sensors, which thereby serve as a benchmark and permit a direct observation of MEMS device functionality. All tests involved steel structures, with particular interest in propagation of existing cracks or flaws. A series of four laboratory tests were performed on beam specimens fabricated from two segments (Grade 50 steel) with a full penetration weld (E70T-4 electrode material) at midspan. That weld region was notched, an initial fatigue crack was induced, and the specimens were then instrumented with one commercial transducer and with one MEMS device; data was recorded from five individual transducers on the MEMS device. Under a four-point bending test, the beam displayed both inelastic behavior and crack propagation, including load drops associated with crack instability. The MEMS transducers detected all instability events as well as many or most of the acoustic emissions occurring during plasticity and stable crack growth. The MEMS transducers were less sensitive than the commercial transducer, and did not detect as many events, but the normalized cumulative burst count obtained from the MEMS transducers paralleled the count obtained from the commercial transducer. Waveform analysis of signals from the MEMS transducers provided additional information concerning arrivals of P-waves and S-waves. Similarly, the analysis provided additional confirmation that the acoustic emissions emanated from the damage zone near the crack tip, and were not spurious signals or artifacts. Subsequent tests were conducted in a field application where the MEMS transducers were redundant to a group of commercial transducers. The application example is a connection plate in truss bridge construction under passage of heavy traffic loads. The MEMS transducers were found to be functional, but were less sensitive in their present form than existing commercial transducers. We conclude that the transducers are usable in their current configuration and we outline applications for which they are presently suited, and then we discuss alternate MEMS structures that would provide greater sensitivity.