Lieping Ye

Lessons from the Collapse of Typical RC Frames in Xuankou School During the Great Wenchuan Earthquake

The seismic damage of typical reinforced concrete (RC) frames in Xuankou School during the Great Wenchuan Earthquake in China is introduced. A simulation method for the seismic damage sustained was developed to enable analysis of the damage mechanisms. The simulation makes use of different nonlinear finite element (FE) models, including macro-scale fibre-beam-element models and a micro-macro-scale hybrid model. The results of the nonlinear FE simulations show that the design of RC frames do not properly allow for the influence of slabs and footing rotations, which results in incorrect predictions of the internal forces and hence, the seismic damage. The collapse resistances of different buildings are compared using incremental dynamic analysis (IDA). The IDA results show that the collapse resistance of the classroom buildings is much lower than that of the office buildings because the larger axial load ratio in classroom buildings limits their lateral deformation capacity. An optimum design is proposed which would improve the collapse resistances of classroom buildings at very low cost.

Evaluation of Modal and Traditional Pushover Analyses in Frame-Shear-Wall Structures

Nonlinear static analysis (or pushover analysis) has been widely used in the last decade as a simplified and approximate method to evaluate the structural seismic performance and to estimate inelastic structural responses under severe ground motions. However most currently used pushover procedures with invariant lateral load patterns cannot fully reflect the effect of higher-order modes on structural dynamic responses. To overcome such a problem, a so-called Modal Pushover Analysis (MPA) was proposed based on the modal decoupling response spectrum method where the effect of higher modes was considered. To date, most research on MPA has been focused on frame structures. In engineering practice, however, most medium-to high-rise building structures are in the form of frame-shear-wall. Therefore it is necessary to extend the current research activity to implement the MPA to frame-shear-wall structures. In this study, two reinforced concrete frame-shear-wall structures of 10 and 18 stories are analyzed to evaluate the performance of the MPA method and the pushover procedures with invariant load patterns. The evaluation is based on the “exact” solutions of a nonlinear dynamic time-history analysis. The results show that the MPA method including higher-order modes is more accurate than the other pushover procedures. This is more evident when estimating structural responses for high-rise structures than the medium-rise counterparts.

Maximum seismic displacement of inelastic systems based on energy concept

The energy balance and energy input of Single-Degree-of-Freedom (SDF) systems under earthquake motion is studied for elastic and inelastic systems. The maximum displacement of an inelastic system is related to that of an elastic system having the same initial stiffness and mass by considering the earthquake energy input per cycle of oscillation. With an assumption that the cyclic energy input is equal for both elastic and inelastic system for intermediate- and long-period systems, a simplified relation is suggested. Newmarks equal energy rule is shown to be the upper bound of the simplified relation; the lower bound of maximum response displacement is also derived in this paper. The numerical analysis results were mostly shown to fall between the proposed upper and lower bounds. A separate approximate relation is proposed for short-period systems. The reason for divergence from the suggested relations is discussed for short-period systems. Copyright

Progressive Collapse Resistance Demand of Reinforced Concrete Frames under Catenary Mechanism

Progressive collapses are resisted by the catenary mechanism in reinforced concrete (RC) frame structures undergoing large deformations. Research to date has mainly focused on the nonlinear dynamic progressive collapse resistance demand of this type of structures under the beam mechanism (that is, for small deformations), and that the catenary mechanism is lacking. As a first attempt, this study establishes a dynamic amplification factor for evaluating the resistance demands of RC frames under the catenary mechanism. To achieve this, an energy-based, theoretical framework is proposed for calculating the aforementioned demands. Based on this framework, the analytical solution for the collapse resistance demands of regular RC frames under the catenary mechanism is readily obtained. Numerical validation indicates that the proposed equations can accurately describe the progressive collapse demand of RC frames undergoing large deformations.

Study of GFRP Steel Buckling Restraint Braces

AbstractBuckling-restraint braces (BRBs) are used extensively in seismic-resistant structures. They consist of a core energy dissipation and external restraining component. A novel glass fiber–reinforced polymer (GFRP) steel BRB is proposed. In the GFRP steel BRB, four GFRP pultruded tubes, which are tied together by GFRP wrapping layers, are used to restrict core steel component buckling instead of conventional steel tube and infilled concrete or mortar. This GFRP steel BRB is extremely lightweight. Techniques for manufacturing GFRP steel BRB were developed for convenient large-scale industrialized production. Quasi-static tests were carried out to validate the performance of the GFRP steel BRB under cyclic loading. The tests focused mainly on energy dissipation capacity and ultimate failure mode of the proposed GFRP steel BRB. The effect of applying additional reinforcements to the GFRP external restraining component on the performance of GFRP steel BRB was also investigated. The performance of the appr...

An Energy-Based Assessment on Dynamic Amplification Factor for Linear Static Analysis in Progressive Collapse Design of Ductile RC Frame Structures

Progressive collapse is a mechanical process that exhibits nonlinear and dynamic characteristics. The nonlinear dynamic effect on the progressive collapse resistance demand can be accurately evaluated by the nonlinear dynamic (ND) method. In engineering practice, however, the simplified and easy-to-use linear static (LS) method is often adopted. That is accomplished by using a dynamic amplification factor (DAF) to correct the LS resistance demand to approximate the true ND resistance demand. In this paper, the analytical expression of the DAF is established based on the energy conservation principle. The collapse-resisting substructure is firstly simplified as a single-degree-of-freedom (SDOF) equivalent. Then the energy conservation equation and the static balance equation of the SDOF equivalent are established to obtain the ND and LS demands. Finally, the DAF is obtained by dividing the ND demand by the LS demand. The DAF is validated through a series of the numerical examples including a SDOF system, a 3-storey planar frame and an 8-storey 3-D RC frame model structures.

Test and simulation of full-scale self-centering beam-to-column connection

A new type of beam-to-column connection for steel moment frames, designated as a “self-centering connection,” is studied. In this connection, bolted top-and-seat angles, and post-tensioned (PT) high-strength steel strands running along the beam are used. The PT strands tie the beam flanges on the column flange to resist moment and provide self-centering force. After an earthquake, the connections have zero deformation, and can be restored to their original status by simply replacing the angles. Four full-scale connections were tested under cyclic loading. The strength, energy-dissipation capacity, hysteresis curve, as well as angles and PT strands behavior of the connections are investigated. A general FEM analysis program called ABAQUS 6.9 is adopted to model the four test specimens. The numerical and test results match very well. Both the test and analysis results suggest that: (1) the columns and beams remain elastic while the angles sustain plastic deformations for energy dissipation when the rotation of the beam related to the column equals 0.05 rad, (2) the energy dissipation capacity is enhanced when the thickness of the angle is increased, and (3) the number of PT strands has a significant influence on the behavior of the connections, whereas the distance between the strands is not as important to the performance of the connection.

Comparison and Selection of Ground Motion Intensity Measures for Seismic Design of Super High-Rise Buildings

Ground motion intensity measures (IMs) are an important basis of structural seismic design. Extensive research has been conducted on the selection of IMs for building structures with fundamental periods shorter than 6 s. However, minimal work has been performed for super high-rise buildings whose fundamental periods are much longer than 6 s. To fill the gap in this research area, this paper aims to develop a simplified analytical model for super high-rise buildings based on the flexural-shear coupled beam model. The variation in correlation and dispersion between different IMs is evaluated and the structural seismic response demand measures (DMs) are analyzed for different structural fundamental periods. Subsequently, rational IMs for the seismic design of super high-rise buildings are suggested. In addition, the influence of different flexural and shear stiffness ratios, α0, on the selection of an IM for super high-rise buildings is also discussed. Finally, a series of incremental dynamic analyses (IDA) of the Shanghai Tower, with a total height of approximately 632 m, is performed to verify the rationality of using different IMs for super high-rise buildings. The numerical results indicate that with a minimum dispersion, the peak ground velocity (PGV) has a better correlation to the story drift ratio than any other IMs. While considering structural nonlinearity, PGV still yields the minimum coefficient of variation in the collapse analysis of the actual super high-rise building. It is therefore recommended that PGV be used as an IM for the seismic design of super high-rise buildings.

Simulation of Structural Collapse with Coupled Finite Element-Discrete Element Method

Structural progressive collapse is a great threat to life safety and therefore it is necessary to study its mechanism in detail. Numerical simulation is significant to study the whole process of progressive collapse in structural level. Since collapse is a complicated procedure from continuum into discrete fragments, numerical model should be competent in nonlinear deformation before collapse and breaking and crashing of fragments after collapse. Coupled Finite elementdiscrete element method on simulating structural progressive collapse is proposed to meet the requirements. Relatively accurate models, such as fiber model and multi-shell shell model, are introduced to construct the finite element model of structure. In the analysis, the failed finite elements will be removed and replaced with granular discrete elements according to the criteria of equivalent total mass and volume so that the impacting and heaping of fragments can be taken into account. The sample with the coupled method shows that this method not only possesses the advantages of finite element method but also simulates the behavior of fragments well.

Parameter Determination and Damage Assessment for THA-Based Regional Seismic Damage Prediction of Multi-Story Buildings

Regional seismic damage prediction based on multiple-degree-of-freedom shear model and nonlinear time-history analysis can comprehensively consider the characteristics of buildings and ground motions. Two major challenges of applying such methodology, namely (1) parameter determination and (2) damage assessment of buildings in urban scale, are addressed in this study. The reliability of the proposed methods are validated using the tests of three individual buildings and the observed seismic damages of Longtoushan Town in 2014 Ludian earthquake of China. Finally, a regional seismic damage prediction is performed for a large urban region, which demonstrates the applicability and scalability of the proposed methods.