Rajesh P. Dhakal

Path-dependent cyclic stress-strain relationship of reinforcing bar including buckling

Abstract In this paper, the formulation and verification of a cyclic stress–strain relationship of reinforcing bars are presented. The tension envelope comprises an elastic range, a yield plateau and a hardening zone. The compression envelope also includes a linear elastic range followed by a non-linear buckling model. The cyclic loops follow Giuffre–Menegotto–Pinto equations with some modifications to account for the effect of buckling. A complete path-dependent cyclic constitutive model is then obtained by combining the equations representing the two monotonic envelopes and the cyclic loops. Comparison with bare bar test results shows that the proposed model could reasonably predict the cyclic behaviour of reinforcing bars including the post-buckling loops.

Damage Avoidance Design Steel Beam-Column Moment Connection Using High-Force-to-Volume Dissipators

Existing welded steel moment frames are designed to tolerate substantial yielding and plastic rotation under earthquake loads. This sacrificial design approach can lead to permanent, and often irreparable damage when interstory drifts exceed 2%. The experimental seismic performance of a 50% full-scale damage avoidance designed structural steel beam-column connection is presented. The beam-column joint region consists of a top flange-hung beam connected to the column by an angle bracket. High-force-to-volume (HF2V) devices are attached from the column to the beam to provide joint rigidity and energy dissipation as the joint opens and closes. The HF2V devices are connected either below the beam flange or concealed above the beams lower flange. Reversed cyclic lateral load tests are conducted with drift amplitudes up to 4%. No damage is observed in the principal beam and column structural elements. The need for stiff device connections to achieve optimal device performance is demonstrated, and potential design solutions presented. Stable hysteresis and repeatable energy dissipation for a large number of cycles up to the 4% drift level is observed. It is concluded that superior and repeatable energy dissipation without damage can be achieved for every dynamic motion cycle, in contrast to conventional sacrificially designed welded moment frame connections.

Response Characteristics of Structures Subjected to Blasting-Induced Ground Motion

This paper presents a conceptual discussion on structural response to ground shocks. Numerical parametric analyses are performed on a simplified linear structural model to investigate the special features of structural response brought by short duration, large amplitude and high frequency excitations, which are the basic characteristics of ground shocks induced by blasting. Nonlinear finite element analyses on a two-storey RC frame subjected to ground shocks are carried out to qualitatively understand building response to blasting. This study shows that maximum structural response to blasting depends primarily on the amount of impulse, and it generally occurs after the major ground shock has ceased. To capture the maximum response, it is hence necessary to consider additional time duration beyond the major ground shock period in blasting analysis. It is found that the response in the forced-vibration phase includes high frequency vibration modes with small displacement but large acceleration, thus inducing high inertial shear force. However, the free-vibration response is dominated by lower frequency modes with larger displacement but smaller acceleration. Hence, buildings subjected to strong ground shocks might experience a sudden shear failure of its components. Nevertheless, if a buildings strength is enough to avoid the sudden shear failure during the major shock, it may be damaged after the ground shock during the free vibration, and the extent of damage depends on the ground shock magnitude.

High-Force-to-Volume Seismic Dissipators Embedded in a Jointed Precast Concrete Frame

An experimental and computational study of an 80-percent scale precast concrete 3D beam-column joint subassembly designed with high force-to-volume (HF2V) dampers and damage-protected rocking connections is presented. A prestress system is implemented using high-alloy high-strength unbonded thread-bars through the beams and columns. The thread-bars are posttensioned and supplemental energy dissipation is provided by internally mounted lead-extrusion dampers. A multilevel seismic performance assessment (MSPA) is conducted considering three performance objectives related to occupant protection and collapse prevention. First, bidirectional quasi-static cyclic tests characterise the specimen’s performance. Results are used in a 3D nonlinear incremental dynamic analysis (IDA), to select critical earthquakes for further bidirectional experimental tests. Thus, quasi-earthquake displacement tests are performed by using the computationally predicted seismic demands corresponding to these ground motions. Resulting ...

Seismic Performance of Damage-Protected Beam-Column Joints

This paper presents an experimental and computational study on the bidirectional behavior of a precast concrete three-dimensional (3D) beam-column joint subassembly designed with damage-protected rocking connections. A prestress system is implemented in which high-alloy, high-strength, unbonded threaded bars running through the beams are coupled to rods within the columns. The threaded bars are posttensioned and supplemental energy dissipation devices are also installed. Both wet and dry joint solutions are considered. The authors conducted a multilevel seismic performance assessment considering three performance objectives related to immediate occupancy and collapse prevention. First, bidirectional quasistatic cyclic tests were conducted and the specimen’s performance was characterized. This data was then used in a 3D nonlinear incremental dynamic analysis (IDA). Results from the IDA were used to select three critical earthquakes for further experimental bidirectional testing. Thus, quasi-earthquake displacement tests were performed. Results indicate the tested specimen met all 3 seismic performance objectives. In precast rocking frames designed to damage-avoidance principles, a cast-in-place closure pour at one beam end helps to alleviate construction tolerance issues and ensures the face of the beam is aligned properly with the column. The performance of such joints was found to be satisfactory.

Detailing of Plastic Hinges in Seismic Design of Concrete Structures

Recent changes of New Zealand structural codes for seismic design revised the way in which the level of detailing is determined for potential plastic hinges. Previously, the level of detailing was based principally on the structural ductility factor, which is broadly similar to the reduction factor used in U.S. practice. The level of detailing in the revision is based on the predicted magnitude of curvature that a plastic hinge is required to sustain in the ultimate limit state. This paper explains why the structural ductility factor does not give a reliable guide to the deformation sustained in an individual plastic hinge. It is proposed that nominal curvatures in potential plastic hinges, calculated by simplified rules, be used as an index to define the level of detailing required in the plastic hinges. Design curvature limits are proposed for different categories of plastic hinge based on test results of beams, columns, and walls. The proposed material strain limits provide a high margin of safety against failure in the ultimate limit state earthquake and an adequate margin of safety against collapse for the maximum credible earthquake.

Design of Timber-Concrete Composite Floors for Fire Resistance

This research investigated the fire performance and failure behaviour of timber-concrete composite floor systems currently under development in New Zealand, resulting in a design method for evaluating the fire resistance of these floors with different types of connections. Furnace tests were performed on two full-size floor specimens at the Building Research Association of New Zealand (BRANZ). Both floor specimens were 4 m long and 3 m wide, consisting of 65 mm concrete topping on plywood formwork, connected to double LVL (laminated veneer lumber) floor joists. They were tested over a 4 m span, subjected to a nominal design live load of 2.5 kPa. Both floors were subjected to the ISO 834 test fire for over 60 minutes. Two separate connection types were tested; concrete notches cut into the timber beams with an incorporated shear key, and metal toothed plates pressed between the double beams.It was found that the reduction in section of the timber beams due to the fire governed the failure mode of the floor...

Seismic Performance of High-Strength Self-Compacting Concrete in Reinforced Concrete Beam-Column Joints

AbstractBecause of its potentially beneficial properties, there has been an increased interest in recent years on performance of self-compacting concrete (SCC) in structural members. The capability of SCC in flowing through and filling in even the most congested areas makes it ideal for use in congested reinforced concrete (RC) structural members such as beam-column joints (BCJ). However, members of tall multistory structures impose high capacity requirements where implementing normal-strength self-compacting concrete (NSSCC) is not preferable. In the present study, six beam-column joint specimens were designed following the guidelines of the New Zealand concrete standards; namely, three high-strength self-compacting concrete (HSSCC), one conventionally vibrated high-strength concrete (CVHSC), one conventionally vibrated concrete (CVC), and one CVC with HSSCC in its joint region. Factors such as the concrete type (HSSCC, CVHSC, and CVC), amount of joint shear stirrups, axial load ratio (1% and 10% of sect...

Characteristics of High-Speed Cyclic Test of Beam-Column Joints

The detailed procedures for high-speed cyclic loading test and the authors’ first-hand experience during the tests of reinforced concrete beam-column connections subjected to cyclic displacements at frequencies as high as 20 Hz is documented in this paper. It is found necessary to give special attention to the quality of the instruments to successfully perform high-speed data acquisition. Problems faced in different phases of the high-speed tests that are not common in pseudo-dynamic tests are highlighted. The applicability of some basic criteria established for quasi-static and seismic performance in gauging the high-speed dynamic performance is also discussed. Unlike in pseudo-dynamic tests, the inertia force contributed significantly to the readings of load cells integrated with the actuators that apply the high-speed displacement cycles. The shear capacity of the tested joints was higher than that computed with the empirical equation in seismic design codes.

Elemental damping formulation: an alternative modelling of inherent damping in nonlinear dynamic analysis

To date, nonlinear dynamic analysis for seismic engineering predominantly employs the classical Rayleigh damping model and its variations. Though earlier studies have identified issues with the use of this model in nonlinear seismic analysis, it still remains the popular choice for engineers as well as for software providers. In this paper a new approach to modelling damping is initiated by formulating the damping matrix at an elemental level. To this regard, two new elemental level discrete damping models adapted from their global counterparts are proposed for its application in nonlinear dynamic analysis. Implementation schemes for these newly proposed models using Newmark incremental method and revised Newmark total equilibrium method is outlined. The performance of these proposed models, compared to existing models, is illustrated by conducting nonlinear dynamic analyses on a four story RC frame designed to Eurocodes. The incremental dynamic analysis study presented in the paper illustrates the fact that both the proposed models seem to produce more reliable results from an engineering perspective in comparison to the global models. It is also shown that the proposed elemental damping models lead to smaller and more realistic damping moments in the plastic hinges. Furthermore, these models could be easily included in existing software frameworks without adding noticeably to the computational effort. The computation time required for these models is approximately equivalent to the one required when using the tangent Rayleigh damping matrix with constant coefficients.