Jack P. Moehle

Seismic evaluation of existing reinforced concrete building columns

Past earthquakes have emphasized the vulnerability of reinforced concrete columns having details typical of those built before the mid-1970s. These columns are susceptible to axial-flexural, shear, and bond failures, which subsequently may lead to severe damage or collapse of the building. Research was undertaken to investigate the lateral and vertical load-resisting behavior of reinforced concrete columns typical of pre-1970s construction. Eight full-scale specimens were constructed and were loaded with constant axial load and increasing cyclic lateral displacement increments until failure. Test data are presented and compared with behavior estimated by using various evaluation methods.

Update to ASCE/SEI 41 Concrete Provisions

A proposed supplement to ASCE/SEI 41 Seismic Rehabilitation of Existing Buildings has been developed for the purpose of updating provisions related to existing reinforced concrete buildings. Based on experimental evidence and empirical models, the proposed supplement includes revisions to modeling parameters and acceptance criteria for reinforced concrete beams, columns, structural walls, beam-column joints, and slab-column frames. The revisions are expected to result in substantially more accurate and, in most cases, more liberal assessments of the structural capacity of concrete components in seismic retrofit projects.

Axial Capacity Model for Shear-Damaged Columns

Most tests of reinforced concrete (RC) columns under seismic load conditions have been terminated shortly after loss of lateral load capacity. However, if a column can reliably carry gravity load after its lateral strength degradation begins, it may be possible to achieve considerable savings by considering the column as a secondary component. This article introduces a model to estimate the axial capacity of a column that has previously experienced shear failure. The axial load on a shear-damaged column is assumed to be supported by a combination of compression of the longitudinal reinforcement and force transfer through shear-friction on an idealized shear-failure plane. The authors relate the effective coefficient of friction from the classical shear-friction equation to the drift ratio at axial failure using the results from 12 full-scale pseudostatic column tests. Approximately half of the columns used to develop the model experienced axial failure at drifts less than those predicted by the model. The authors conclude that the model provides relations among axial load, transverse reinforcement, and the interstory drift at axial load collapse.

Lateral Displacement Ductility of Reinforced Concrete Flat Plates

It is presently unclear whether the reinforced concrete flat-slab connection possesses sufficient lateral displacement capacity to survive the lateral deformations that can be expected during a strong earthquake. The objective of this paper is to examine the available data from present and past research and from there develop an understanding of the major parameters that influence the lateral displacement capacity and ductility of reinforced concrete flat plates. The significant effects of gravity load and biaxial lateral loading are presented together with the implications of the findings with regard to seismic design and performance. An expression of the gravity level shear stress acting on the slab critical section to which it must be limited to insure adequate displacement ductility under extreme earthquake loading is given.

Drift Capacity of Reinforced Concrete Columns with Light Transverse Reinforcement

Existing reinforced concrete columns with light transverse reinforcement are vulnerable to shear failure during seismic response. Shear strength models, modeling the degradation of shear strength with increasing displacement ductility demand, have been widely used to evaluate the interstory drift capacity of such columns. The application of a shear strength model to determine the drift capacities for a database of 50 shear-critical columns demonstrates significant inaccuracies with such a method. An empirical drift capacity model based on the shear-critical column database provides a better estimate of the interstory drift at shear failure. The new drift capacity model identifies the most critical parameters affecting the drift capacity of shear-critical columns, namely, transverse reinforcement ratio, shear stress demand, and axial load ratio.

Seismic Tests of Concrete Columns with Light Transverse Reinforcement

Earthquakes and laboratory experience show that columns with inadequate transverse reinforcement are vulnerable to damage including shear and axial load failure. To study this behavior, four full-scale columns with light transverse reinforcement were tested quasistatically under unidirectional lateral load with either constant or varying axial loads. Test results show that responses of columns with nominally identical properties vary considerably with magnitude and history of axial and lateral loads. Observed behavior is compared with expected behavior based on available analytical models. The FEMA 356 assessment model predicted the column strengths well, but underestimated the displacements.

REPAIR OF EARTHQUAKE-DAMAGED BRIDGE COLUMNS

Columns supporting bridge structures are likely to respond inelastically during strong earthquakes. Restoration to serviceable conditions may require repair or replacement of damaged regions, with considerable cost and operational delay. This paper describes an experimental study undertaken to identify the performance of reinforced concrete columns repaired by varying techniques. Four spirally reinforced columns designed to meet current seismic detailing practice were damaged, repaired, and retested. The damage levels were classified as either moderate or severe. Four different repair techniques were applied. The performance of each repair technique was determined by comparing the response of the repaired column with the response of the original column as well as with the design intent.

Damage and Implications for Seismic Design of RC Structural Wall Buildings

In 1996, Chile adopted NCh433.Of96, which includes seismic design approaches similar to those used in ASCE 7-10 (2010) and a concrete code based on ACI 318-95 (1995). Since reinforced concrete buildings are the predominant form of construction in Chile for buildings over four stories, the 27 February 2010 earthquake provides an excellent opportunity to assess the performance of reinforced concrete buildings designed using modern codes similar to those used in the United States. A description of observed damage is provided and correlated with a number of factors, including relatively high levels of wall axial load, the lack of well-detailed wall boundaries, and the common usage of flanged walls. Based on a detailed assessment of these issues, potential updates to U.S. codes and recommendations are suggested related to design and detailing of special reinforced concrete shear walls.

Concrete-Steel Bond Model for Use in Finite Element Modeling of Reinforced Concrete Structures

Reinforced concrete requires bond between plain concrete and the reinforcing steel. Accurate numerical modeling of structures that exhibit severe bond-stress demand requires exact representation of bond-zone response. A bond element is presented for use in high-resolution finite element modeling of reinforced concrete structures subjected to general loading. The model is defined by a normalized bond stress versus slip relationship and a relationship between maximum bond strength and the concrete and steel stress-strain state. A finite element implementation of the model is proposed that enables a 1- or 2-D representation of bond-zone action. Nonlocal modeling is used to incorporate the dependence of bond strength on the concrete and steel material state. Comparisons of simulated and observed response for systems with uniform and variable bond-zone conditions are presented.

Performance assessment for a reinforced concrete frame building

This paper evaluates the performance of a seven-story reinforced concrete frame building that was severely damaged during the 1994 Northridge, California, earthquake. The building was designed and constructed in the 1960s and contains details that are typical of that construction era in the western United States. The building sustained severe damage that included column shear failures. The building was analyzed independently by three research teams using analysis methodólogies that were similar in concept but different in details. An objective of each analysis was to correlate observed and calculated performance. The different analyses were successful to varying degrees. The results provide a test case of the effectiveness of various seismic performance assessment methodologies.