Browsing by Subject "Precast Concrete"
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Item Behavior of Precast Concrete and Masonry Wall Systems with Jointed Connections Subjected to Lateral Loads(2018-08) Kalliontzis, DimitriosThe significant damage and permanent deformations sustained by structures during earthquakes have motivated researchers to investigate the use of precast concrete and masonry walls with jointed connections at the wall-to-foundation interface. These walls resist the seismic lateral loads by rocking motion, which minimizes damage and re-centers the full structure effectively, providing seismic resilience. Nevertheless, there is lack of understanding of their seismic behavior. This includes energy dissipation by the impacts during rocking motion, which is a key source of energy dissipation in the walls. Impact events were inhibited in most experimental research studies, because they employed quasi-static tests. Even when dynamic tests were used, little focus was given to characterize the individual energy dissipation components in the walls, including hysteretic, impact, and other continuous energy dissipating mechanisms. Apart from the lack in experimental insight, the use of simplified models that emulate those used for monolithic walls have been able to capture only the global seismic behavior of walls with jointed connections. This dissertation combines experimental and analytical investigations of precast concrete and masonry walls with jointed connections to improve understanding of their seismic responses. Their quasi-static behavior is investigated first to characterize their hysteretic and force-displacement responses. At this stage, the dissertation focuses on masonry walls because of their more intricate behavior than that of precast concrete walls, which involves three deformation mechanisms, confinement effects due to lateral friction at the wall-to-foundation interface, and some hysteretic action. An iterative procedure is developed to estimate the envelope responses of masonry walls, using monotonic sectional analysis at the wall base. This procedure captures the deformations at the critical wall regions and accounts for the confinement in masonry due to lateral friction at the wall-to-foundation interface. To enable a methodology that can be implemented in design, a simplified procedure is also developed. Next, these procedures are extended to capture the hysteretic behavior in the walls using fiber-element sectional analysis at the wall base. The dynamics of walls with jointed connections is also investigated to capture their impact energy dissipation. Free vibration tests of carefully monitored precast concrete units are employed for this purpose. It is found in these tests that rocking takes place over a contact length and that the rotation center of rocking members migrates from one bottom toe to the other as a function of their base rotation. These observations are used to develop an expression for energy losses during impacts. The accuracy of this expression is tested further using shake-table tests of a large-scale precast concrete wall system, which was part of past research. To conclude this part of the investigation, a generalized dynamic model for walls with jointed connections is developed. The model integrates impact, hysteretic, and inherent energy dissipation, rocking and flexural deformations in the walls. Its accuracy is verified using experimental data that captures a broad range of material and geometric characteristics. Finally, recognizing that inadequate damping is available in walls with jointed connections when used in seismic regions, an investigation is used to improve their damping performance and minimize their damage during seismic motions. Elastomeric pads are strategically employed at the wall-to-foundation interface to a) increase damping in the walls; and b) minimize the strain demands on concrete and masonry by shifting most of the hysteretic action into the pads. Combining analytical and experimental means, it is shown that appropriate design of the wall-to-foundation interface allows the elastomeric pads to effectively dissipate the energy imparted to the walls through lateral seismic loads.Item Full-depth precast concrete bridge deck system: phase II(2012-11) Halverson, MaxThe Minnesota Department of Transportation (MnDOT) developed a design for a precast composite slab span system (PCSSS) to be used in accelerated bridge construction. The system consists of shallow inverted-tee precast beams placed between supports with cast-in-place (CIP) concrete placed on top, forming a composite slab span system. Suitable for spans between 20 and 60 ft., the MnDOT PCSSS is useful for replacing a large number of aging conventional slab-span bridges throughout the United States highway system. Originally developed in 2005, the PCSSS had three distinct design generations in the 12 bridges that were constructed by MnDOT between 2005 and 2011. The objective of this investigation was to evaluate the field performance of a sample of the existing bridges through detailed crack mapping and core analysis and through continued monitoring of data obtained from one of the original PCSSS bridges (Bridge No. 13004) instrumented during construction in 2005. A parametric design study was also conducted to investigate the effects of continuity design on the economy of the PCSSS. Five of the 12 PCSSS bridges, constructed between 2005 and 2011, were selected as the sample set to conduct detailed surveys of surface cracking and examinations of extracted core specimens to evaluate effects of the design changes. Surface cracking was recorded over three different inspections between the fall of 2009 and the summer of 2011. Each inspection was done using a systematic procedure of documenting crack locations and measuring crack widths. The result was a series of crack maps for each bridge, showing the surface cracking compared to major design features. Different line types were used to distinguish relative crack widths. Core specimens were taken from each of the five inspected bridges based on anticipated reflective crack locations. The cores were partial depth through the CIP concrete, taken over either the longitudinal joint between precast panels or over the precast web corner. Each core was examined under a digital microscope for cracking with particular attention paid to the regions above the longitudinal joints and web corners. The results of the core investigation were compared to the corresponding crack maps. Overall, the field inspections indicated that the changes made between each design generation improved the performance of the PCSSS. Bridge No. 13004 in Center City, MN from the first design generation showed many short, longitudinal cracks on the deck surface with very little transverse and map cracking. The longitudinal cracks were located primarily over the precast beam web, corresponding to what appeared to be insufficient consolidation of the CIP concrete around the stirrups projecting vertically from the section to facilitate composite action, which had little clearance above the precast webs. In the second generation, more clearance was provided under the stirrups projecting from the surface. Bridge Nos. 33005 and 33008 near Mora, MN from the second generation did not show the short cracks over the webs from the first generation, but more transverse cracks and longer longitudinal cracks were observed. Bridge No. 33008 showed significantly more longitudinal cracking than any of the other bridges. Significant longitudinal cracks were noted along several joints between the precast beams. Core specimens showed that these cracks were full-depth reflective cracks. The only other bridge to show reflective cracking from the core specimens was Bridge No. 13004, but these were not full-depth cracks. It was unclear from the design details of Bridge No. 33008 why it was in worse condition than the other bridges. This bridge also had noticeably different cambers between adjacent beams observed from the underside of the bridge, although it was unclear how this might be associated with the observed longitudinal cracking. For the third design generation, the thickness of the precast beam flanges was decreased and the trough reinforcement spacing (consisting of trough hooks projecting horizontally from the beams across the joint, as well as a drop-in cage) was decreased from a maximum 10 in. center-to-center to 6 in. center-to-center to better control reflective cracking. The decreased spacing was accomplished by staggering the trough hooks from adjacent precast beams. Bridge Nos. 49007 and 49036 near Little Falls, MN from the third generation did not exhibit longitudinal cracking over the precast beam joints, indicating that the design changes may have had a positive impact, though not conclusively. The most significant issue observed with the third generation was shrinkage cracking, indicated by longitudinal cracks located over the precast beam webs and more extensive transverse and map cracking. Generally, bridges with a larger length to width aspect ratio (i.e., L/W) had more transverse cracking, which could be related to more longitudinal shrinkage restraint. In addition to the field inspections, strain data from the instrumentation of Bridge No. 13004 was analyzed to evaluate performance. The bridge was instrumented in 2005 to monitor reflective cracking and continuous system behavior. Six years of strain and temperature data showed a progression of reflective cracking in several locations and significant cracking due to thermally induced restraint moments. The reflective cracking from the strain data was confirmed by observed cracks in the core specimens near the locations of the strain gauges. While the width of the reflective cracks appeared to increase over time from the strain measurements, the measurements began to plateau by the end of the six-year monitoring period. Restraint moment cracking was indicated by strain gauges attached to continuity connection reinforcement. The measured restraint moment strains were large enough for fatigue to be of potential concern, although the strains were associated with environmental effects which have a low number of cycles at once per day. Measured strains associated with both reflective cracking and restraint moments were primarily driven by seasonal and daily temperature variations, highlighting the important role of thermal effects in design. Besides the detailed field investigations, a parametric study of PCSSS designs was conducted to determine whether there was an economic benefit of continuous system design. In particular, design implications of time-dependent and thermal gradient restraint moments and their effects on continuity were studied. Because the PCSSS is a simple-span system made continuous with a CIP deck, the effects of restraint moments must be considered in the design of continuous systems. The restraint moments are those that arise from continuity, or end restraint, over the piers due to beam creep, differential shrinkage between the CIP deck and beam, and thermal gradient. Restraint moments would not develop if PCSSS were built as a series of simple spans with no continuity provided between the spans. Eight bridges covering the feasible range of span configurations were designed as both simple and continuous systems. Flexural design was performed for each case, resulting in optimized precast sections within practical design constraints. Primary design parameters were strand number, section depth, and precast concrete strength. These design parameters were compared between the continuous and simple-span designs for each configuration to evaluate economic benefit. Generally, continuous PCSSS designs were equally or less economical than simple-span designs. Spans less than 30 feet had a slight economic benefit with continuous design because large restraint moments did not develop. However, spans greater than 30 feet developed large restraint moments in continuous design, particularly due to thermal gradient effects. In addition, the restraint moments greatly reduced continuity, effectively negating the benefit to live-load capacity. It was recommended that the PCSSS be designed as a simply-supported system for live load. Furthermore, because most continuous system designs were less economical than the corresponding simply-supported designs, it was concluded that designing the PCSSS as simply-supported while also including a continuity connection would be unconservative without accounting for restraint moments. A simple method was developed to account for restraint moments for this case without time-intensive calculations. Further recommendations related to the analysis of negative moments over PCSSS bridge piers were also provided. A review of current design methods and details concluded that the current PCSSS design was generally sufficient, and recommendations for future PCSSS designs were provided. Items reviewed were related to shrinkage restraint, reflective crack control, composite action, and defining tolerances for the PCSSS. In order to try to better control top surface deck cracking, recommendations included increasing the transverse reinforcement in the CIP deck to provide a gross reinforcement ratio, ρg, of 0.0063 with a spacing no greater than 9 in., based on the work of Frosch (2006). This would translate to increasing the current transverse deck reinforcement from No. 4 bars at 6 in. (ρg =0.0056) to No. 5 bars at 6 in. (ρg =0.0086) or No. 5 bars at an increased spacing of 8 in. (ρg =0.0065) to provide the needed volumetric ratio while maintaining the maximum spacing for surface crack control. The recommendations of NCHRP 10-71 for reinforcement in the trough are adopted in order to control reflective cracking In addition, it was recommended that composite action stirrups need not be used if the required shear stress transferred between the CIP concrete deck and precast beam is less than 135 psi.