Marc O. Eberhard

Effective Stiffness of Reinforced Concrete Columns

This study uses the experimental response of 329 concrete columns to assess existing and proposed models of the effective stiffness of reinforced concrete columns subjected to lateral loads. Existing models appropriate for design applications are shown to overestimate the measured effective stiffness and are unacceptably inaccurate, because they generally neglect the influence of anchorage slip on the effective stiffness of the column. Based on simplifications of a three-component model, a new procedure is proposed that explicitly accounts for deformations due to flexure, shear, and anchorage-slip. The new procedure is shown to provide a more accurate estimate of the measured effective stiffness for the database columns. For this model, the ratio of the measured stiffness to the calculated stiffness had a mean and coefficient of variation of 1.02 and 22% for circular columns and 0.95 and 25% for rectangular columns. The proposed procedure is more accurate and rational than existing models, and could easily be incorporated into design provisions for concrete structures.

Overview of the 2010 Haiti Earthquake

The 12 January 2010 Mw 7.0 earthquake in the Republic of Haiti caused an estimated 300,000 deaths, displaced more than a million people, and damaged nearly half of all structures in the epicentral area. We provide an overview of the historical, seismological, geotechnical, structural, lifeline-related, and socioeconomic factors that contributed to the catastrophe. We also describe some of the many challenges that must be overcome to enable Haiti to recover from this event. Detailed analyses of these issues are presented in other papers in this volume.

Application of Accelerated Bridge Construction Connections in Moderate-to-High Seismic Regions

This report identifies promising details to be used for connections of bridge members in accelerated bridge construction in medium to high seismic regions and gives recommendations for further research. Existing connection details were gathered from state departments of transportation, industry, and academia and were evaluated for their performance in terms of readiness for use, construction risk, durability, and seismic performance. The material in this report will be of immediate interest to bridge engineers.

Anchorage of Large-Diameter Reinforcing Bars in Ducts

One method of connecting a column to a cap beam in bridge bents is to precast the column with projecting bars, which are then grouted into ducts in the cap beam. Bar-duct systems can be assembled rapidly if a few large bars and ducts are used to connect the column and cap beam. However, the required anchorage lengths for the large bars can exceed the length available. This study evaluates the anchorage requirements for this situation by performing 14 pullout tests on bars with sizes up to No. 18. The tests and a nonlinear finite element model showed that, under conditions similar to those tested, large bars can develop their yield strength in as few as six bar diameters and fracture strengths in as few as 10 bar diameters. In all the tests, failure occurred either by formation of a cone near the grout surface combined with shearing along the confined bar-grout interface or by bar fracture. The specimens did not fail by duct pullout or concrete splitting. Polypropylene fiber reinforcement typically lowered the peak pullout strength of the bar and had mixed impact on the post-peak response.

Seismic Resistance of Socket Connection between Footing and Precast Column

A new concept has been developed for connecting spread footings and precast columns in bridges. The socket connection is constructed by precasting the column, erecting it, and casting the reinforced concrete footing around it. This system saves construction time on site because, in little more than the time needed to construct the footing, both the column and footing can be constructed. Site erection is facilitated by the fact that the field tolerances are essentially unlimited. The longitudinal column bars are straight and are terminated with mechanical anchors. This arrangement improves constructability, because no bars cross the interface between the column and footing, and it provides better transfer of forces in the connection region than is possible with conventional bent-out longitudinal bars. The surface of the column is roughened to improve adhesion to the surrounding cast-in-place concrete. Axial-load tests demonstrated that the connection can resist column axial loads far above those expected in practice. Cyclic, lateral-load tests demonstrated that the seismic performance of the connection is at least as good as, if not better than, that of a comparable cast-in-place system. The recent deployment of the new system in a highway overpass provided both field experience and initial estimates of the potential time savings.

Post-earthquake Prioritization of Bridge Inspections

Bridge damage reports from the 2001 Nisqually earthquake were correlated with estimates of ground-motion intensity at each bridge site (obtained from ShakeMaps) and with bridge properties listed in the Washington State Bridge Inventory. Of the ground-motion parameters considered, the percentage of bridges damaged correlated best with the spectral acceleration at a period of 0.3 s. Bridges constructed before the 1940s, movable bridges, and older trusses were particularly vulnerable. These bridge types were underestimated by the HAZUS procedure, which categorizes movable bridges and older trusses as “other” bridges. An inspection prioritization strategy was developed that combines ShakeMaps, the bridge inventory and newly developed fragility curves. For the Nisqually earthquake, this prioritization strategy would have made it possible to identify 80% of the moderately damaged bridges by inspecting only 481 (14%) of the 3,407 bridges within the boundaries of the ShakeMap. To identify these bridges using a prioritization strategy based solely on epicentral distance, it would have been necessary to inspect 1,447 (42%) bridges. To help the Washington State Department of Transportation (WSDOT) rapidly identify damaged bridges, the prioritization procedure has been incorporated within the Pacific Northwest Seismic Network (PNSN) ground-motion processing and notification software.

Measures of the Seismic Vulnerability of Reinforced Concrete Buildings in Haiti

Following the 12 January 2010 Haiti earthquake, teams of students and faculty members from the United States and Haiti surveyed 170 reinforced concrete (RC) buildings in Port-au-Prince and Léogâne. This paper summarizes the survey results and compares them with results from a similar survey done after the 1999 earthquakes near Düzce, Turkey. The survey results demonstrate that the frequency of damage in RC buildings was higher in Haiti than in Turkey. This increased level of damage is consistent with practical screening criteria based on cross-sectional areas of building columns and walls. Based on these criteria, 90% of the structures surveyed in Haiti would have been classified as seismically vulnerable before the earthquake. Damage was more frequent in structures with captive columns. A two-tiered screening process is suggested to rapidly assess the vulnerability of scores of poorly built low-rise RC buildings in future earthquakes.

A weighted-stretched-wire system for monitoring deflections

Measurements of short- and long-term deflections are often helpful in understanding and evaluating the behavior of civil engineering structures. This paper describes the mechanics and design of a Weighted Stretched Wire System (WSWS), a simple but effective device that was developed to monitor structural deflections. The primary advantages of this new system are that it is accurate, robust and inexpensive, and it is suitable for continuous monitoring. The paper considers static and dynamic response, provides design recommendations, illustrates the design of a WSWS with an example and provides field verification of performance.

Shaking Table Performance of a New Bridge System with Pretensioned Rocking Columns

AbstractA new bridge bent system has been developed to reduce on-site construction time, minimize residual displacements even after a large earthquake, and reduce seismic damage in comparison with conventional cast-in-place construction. Accelerated construction is achieved through the use of precast columns and cap beams that can be assembled quickly. Postearthquake residual displacements are reduced by pretensioning the columns with partially unbonded tendons. Damage in the columns is nearly eliminated by concentrating flexural deformations to specially detailed regions at the top and bottom of the columns. In this study, the seismic performance of the new system was evaluated with a multi-shaking table test of a quarter scale, two-span bridge at the Network for Earthquake Engineering Simulation (NEES) Earthquake Engineering Laboratory at the University of Nevada, Reno. The maximum displacements of the bents were similar to those expected for a conventional bridge through the 100% design-level event [pe...

NONDESTRUCTIVE EVALUATION USING NUMERICAL SIMULATION OF IMPACT RESPONSE

A new algorithm has been developed to estimate the material properties and geometry of a concrete slab or pavement by comparing measured and simulated dynamic responses to a surface impact. The reconstruction algorithm consists of the following steps: recording an impact and the acceleration response histories at multiple locations, comparing these histories with the histories computed with a finite-difference model, and varying the properties of the model until the difference between measured and computed responses is minimized. The minimization identifies an optimal solution for the material and geometric properties of the slab or pavement. A parametric study determined that Youngs modulus, slab thickness, crack location, and crack depth would be the easiest properties to identify because they have the most significant influence on the acceleration response of a concrete pavement. Using experimental measurements of dynamic response, the algorithm simultaneously identified the slab thickness within approximately 11% of the actual depth and the Youngs modulus of the slab within 6% of comparable values obtained from laboratory tests. An important advantage of the numerical simulation approach is that, as computing power increases, the method can be applied to more complex systems.