Superstructure Mass Research Articles

New Simplified Method for Designing Seismically Isolated Highway Bridges with Massive Piers

This paper proposes two new models for the simplified seismic analysis of seismically isolated highway bridges with massive piers. Both models include two components, one of which includes the superstructure mass, the isolators, and massless pier, and the other one the pier with its distributed mass and stiffness properties. In one model, the isolator stiffness is added at the top of the pier in the second component. The total seismic response is obtained from the square root of the sum of squares (SRSS) of the two individual component results. Applicability and accuracy of the models were assessed by considering bridges with wide ranges of stiffness and mass properties of piers (Mp) and superstructures (Mss). Multimode spectral analysis (MMSA) and nonlinear time-history analysis (NLTHA) were used to define reference solutions for the superstructure displacements, vibration periods, and shears and moments at pier bases. Results from four currently available models, including the model prescribed in American and Canadian bridge codes, were also examined. The study shows that current models for the simplified analysis method yield good estimates of superstructure displacements and vibration periods; however, they generally underestimate the shear and moment demands at pier bases. The errors are more significant for more massive piers or when the stiffness of the piers is high compared to that of the isolators. In contrast, for all bridges studied, the seismic responses from the proposed models show very good agreement with those from the MMSA. Better predictions are obtained when considering the isolator stiffness in the second component of the model. The NLTHA results also show that the proposed models give overall satisfactory predictions for pier base shears and moments. The proposed models extend the range of application of the American and Canadian code simplified method to isolated bridges with massive piers while preserving its simplicity with limited extra computational effort.

Winkler model for pile seismic analysis considering end constraints effects

Beam on nonlinear Winkler foundation (BNWF) analysis of pile foundation subjected to vertically propagating earthquake ground motions is presented in this paper. The BNWF is considered a simplified approach that is capable to model nonlinear soil-pile-structure interaction. Soil nonlinearity is simulated by p-y curves according to American Petroleum Institute (API) procedure. The analyses are carried out using the open source finite element platform OpenSees. Results are verified against centrifuge experiments and numerical finite element simulations published in the literature. In this paper pile tip and head constraints effects on bending moments and displacement responses are investigated. Three types of pile tip constraints are applied: floating, end bearing, and fixed supports. Pile top constraint is modeled as either free or fixed head. Three different earthquake ground motions are applied in the model. Effect of superstructure mass is investigated for the three considered earthquake records and the different assumed pile end and head constraints. The results indicate significant influence of pile tip and head constraints on pile bending moment and displacement responses. A significant effect of superstructure mass on the analyses results is also observed. The analyses show that earthquake record characteristics remarkably influence pile response behavior. Different earthquake records scaled to the same peak ground acceleration give different pile seismic responses.

Seismic responses of asymmetric steel structures isolated with the TCFP subjected to mathematical near-fault pulse models

In this paper, the effects of mass eccentricity of superstructure as well as stiffness eccentricity of isolators on the amplification of seismic responses of base-isolated structures are investigated by using mathematical near-fault pulse models. Superstructures with 3, 6 and 9 stories and aspect ratios equal to 1, 2 and 3 are mounted on a reasonable variety of Triple Concave Friction Pendulum (TCFP) bearings considering different period and damping ratio. Three-dimensional linear superstructure mounted on nonlinear isolators are subjected to simplified pulses including fling step and forward directivity while various pulse period (Tp) and Peak Ground Velocity (PGV) amounts as two crucial parameters of these pulses are scrutinized. Maximum isolator displacement and base shear as well as peak superstructure acceleration and drift are selected as the main engineering demand parameters. The results indicate that the torsional intensification of different demand parameters caused by superstructure mass eccentricity is more significant than isolator stiffness eccentricity. The torsion due to mass eccentricity has intensified the base shear of asymmetric 6-story model 2.55 times comparing to symmetric one. In similar circumstances, the isolator displacement and roof acceleration are increased 49 and 116 percent respectively in the presence of mass eccentricity. Furthermore, it is demonstrated that torsional effects of mass eccentricity can force the drift to reach the allowable limit of ASCE 7 standard in the presence of forward directivity pulses.

Basic parameters of the static stability, loads and strength of the vital parts of a bucket wheel excavator’s slewing superstructure

Determining a bucket wheel excavator (BWE)’s slewing superstructure weight and its center of gravity (COG) is of extreme importance not only in the design phase, but also after the completion of the erection process and during the operation of the machine. This paper presents a critical comparative analysis of the basic parameters of the static stability of a BWE 1600 superstructure, with the parameters being obtained by both analytical and experimental procedures. The analysis shows that a relatively small difference in superstructure mass, obtained by calculation, leads to a relatively large unfavorable shifting of its COG, necessitating a significant increase in counterweight mass for balancing. A procedure for superstructure 3D model mass correction is presented based on results obtained by weighing after the completion of the erection process. The developed model provides enough accuracy to determine the superstructure’s COG in the entire domain of the bucket wheel boom inclination angle, and enables accurate load analysis of the superstructure’s vital parts. The importance of this analysis is reinforced by the finding that the procedure prescribed by standard DIN 22261-2 gives results which are not on the side of safety, as shown by an example of strength analysis of a bucket wheel boom stays’ end eyes.

Parametric Study and Centrifuge-Test Verification for Amplification and Bending Moment of Clay–Pile System Subject to Earthquakes

This paper examines seismic effects on fixed-head, end-bearing piles installed through soft clay. The numerical analyses were conducted using ABAQUS with a hypoelastic constitutive model for the clay. The dimensionless parameters involving the major parameters such as pile modulus, soil modulus, slenderness ratio, natural frequencies of clay layer and pile–raft, superstructure mass, density of the soil and peak ground acceleration were obtained from the parametric studies. The relationships for the amplification of ground motions and the maximum bending moment in the pile were developed based on regression of the numerical data. The computed results from the proposed relationships were compared with the results reported in the past studies.

Experimental and numerical investigation on the dynamic response of pile group in liquefying ground

The response of pile foundation in liquefiable sand reinforced by densification techniques remains a very complex problem during strong earthquakes. A shake-table experiment was carried out to investigate the behavior of a reinforced concrete low-cap pile group embedded in this type of ground. In this study, a three-dimensional (3D) finite element (FE) analysis of the experiment was conducted. The computed response of the soil-pile system was in reasonable agreement with the experimental results, highlighting some key characteristics. Then, a parametric study was performed to explore the influence of pile spacing, pile stiffness (EI), superstructure mass, sand permeability, and shaking characteristics of input motion on the behavior of the pile. The investigation demonstrated a stiffening behavior appearing in the liquefied mediumdense sand, and the pile group effect seemed negligible. Furthermore, the kinematic effect was closely connected with both EI and sand permeability. Nevertheless, the inertial effect was strongly influenced by the superstructure mass. Meanwhile, high frequency and large amplitude of the input motion could produced greater the pile’s moments. It is estimated that this case study could further enhance the current understanding of the behavior of low-cap pile foundations in liquefied dense sand.

杭位置の違いによる群杭の非線形水平地盤抵抗のモデル化

1. Introduction Pile foundation response during earthquakes is strongly affected by nonlinear soil-pile foundation interaction. The damages to pile foundations during the 1995 Hyogo-ken Nanbu Earthquake and the 2011 Tohoku-Chiho Taiheiyo-Oki Earthquake were obviously attributed to nonlinear interaction of soil-pile foundation-superstructure. The dynamic nonlinear behavior of soil around each pile affects lateral load distribution and displacement of each pile. Therefore, shaking table tests were conducted to clarify the dynamic nonlinear behavior of soil around piles, further earthquake response analyses were conducted to simulate the effect of the nonlinear soil-pile interaction system on performance of the superstructure supported by pile group. 2. Outline of shaking table tests 25-pile group foundation model was set up in Toyoura sand deposits within a laminar container. The sand deposits were prepared by air pluviation method in the laminar container. The model piles were acryl cylinder with 12mm in diameter and 400mm long. 25 piles were arranged in squares and pile spacing was 2.0 times pile diameter. The superstructure was modeled as a rigid body. The input waves were seismic waves in notification with random phase of Japan and the Hyogo-ken Nambu earthquake. 3 input acceleration levels were used to investigate the influence of input motion level. Tests with and without the mass of superstructure were conducted to investigate lateral load distribution of each pile. 3. Outline of shaking table tests simulation Two analysis methods were conducted for simulation of tests, one is nonlinear 3D-FEM, and the other is lumped mass model. Two case of Lumped mass model are conducted, one is with soil springs around pile group which were different according to pile arrangements and direction of loading, and is with soil springs without pile group effect. Soil springs were calculated by using nonlinear 3-D FEM, which were modeled appropriately according to the location of pile. 4. Conclusions The concluding remarks of these shaking tests and analytical studies are as follows. (1) In shaking tests, lateral subgrade reaction around piles depends on the location of each pile in pile group and the direction of loading. That of the corner pile is remarkably larger than that of the middle pile. (2) In shaking tests, hysteresis curve of soil spring at the corner pile is found to be asymmetric loop, while that at the middle pile be symmetric loop. (3) In shaking tests, bending moment at each pile head depends on the location of each pile in pile group. That of corner pile head is remarkably larger than that of middle pile head. As the input level of acceleration increase, bending moment at each pile head is equalized. (4) The proposed lumped mass model of the soil-pile foundation system, which consists of beam elements and nonlinear interaction springs considering the location of pile, is verified validation. It is confirmed that this proposed model well represents response of the superstructure supported by pile group.

Laboratory experiments to study ice-induced vibrations of scaled model structures during their interaction with level ice at different ice velocities

A simplified model of a typical bottom-founded structure was forced through ice sheets in a laboratory experiment to study ice-induced vibrations. The ice forces exerted on the structure are identified in conjunction with the response of the entire structure using a joint input-state estimation algorithm. Novel insights into ice-induced vibration phenomena are obtained by comparing, on different time scales, measured and estimated response quantities and forces/pressures. First, the identified forces, ice velocities and time-frequency maps of the measured responses are presented for a series of ice-induced vibration tests. It is shown that the ice forces excite more than one mode of the structure and that the transition ice velocity at which the vibrations shift from the first to the second mode increases with reduced foundation stiffness and reduced superstructure mass. Second, a detailed analysis of the interaction between the structure and the ice edge is performed on a smaller time scale by comparing the locally measured pressures at the ice-structure interface to the identified structural responses and forces. It is shown that structural vibrations at a frequency higher than the dominant vibration frequency cause cyclic loading of the ice edge during intermittent crushing. These vibrations led to an increasing loading rate prior to ice failure. During an event that shows the tendencies of frequency lock-in vibrations, the structural response was dominated by a single vibration frequency.

Effects of Liquefaction on Inelastic Demands on Extended Pile Shafts

Inelastic response of extended pile shafts subjected to liquefaction-induced lateral spreading is investigated using nonlinear dynamic analyses (NDA) covering a range of soil, pile and ground motion conditions. Each soil-structure scenario was analyzed for three cases: a baseline case with soil liquefaction and superstructure inertia; a case with liquefaction, but without superstructure inertia (i.e., superstructure mass removed); and a case without liquefaction (i.e., pore pressure generation eliminated), but with superstructure inertia. Results show that the combined effects of lateral spreading and superstructure inertia produce larger demands (often by more than 50%) than are produced by either loading case alone, such that the combined demand cannot be enveloped by analyzing the two load cases separately. The results of these parametric analyses provide a database that is used in subsequent development of an equivalent static analysis (ESA) design procedure.

Bus vibration study - finite element modelling and determination of the eigenpairs

This paper describes the finite element idealisation of a typical Indian transport bus and uses this model to predict the first few eigenpairs. The eigenvalue problem has been solved by the simultaneous iteration method and Lanczos' scheme and by using two different finite element models. The first model consists of chassis elements, the axles, the suspensions and the tyres, the mass of the superstructure being lumped at the appropriate nodes. The second is a more comprehensive model in which, in addition to the chassis, axles, suspension and tyres, the framework of the superstructure and the plates constituting the bus body and floor have been taken into account. The eigenfrequencies obtained by using both these models match well with frequencies arrived at experimentally.

Fragility curves for isolated bridges in eastern Canada using experimental results

One option to mitigate the seismic risk of highway bridges in Quebec is to replace typical elastomeric bearings used there with natural-rubber seismic-isolator devices. To support this alternative, this paper assesses the seismic vulnerability of typical bridge classes retrofitted with seismic-isolator devices through the development of fragility curves. Retrofitted-bridge fragility curves provide a powerful tool to evaluate the impact of a retrofit measure on the performance of different bridge classes. The analytical fragility approach uses nonlinear time-history analysis with 3-D detailed models for typical configurations of highway bridges. Experimental results of square bearings with different sizes and shape factors are used to account for uncertainties related to the mechanical properties of seismic-isolators. Critical load tests are conducted on slender seismic isolation bearings and a finite-element model is calibrated with the test results to define the seismic-isolator limit states. The fragility curves of different key components of the bridge system are compared and the results reveal that seismic isolation is effective significantly in reducing the probability of damage to columns and foundations. Due to insufficient clearance between the superstructure and abutment wing walls, however, the probability of damage in wing walls is increased and the fragility of this component controls the bridge-system fragility for all bridge classes evaluated. Concrete-girder bridges are found to be more vulnerable than steel-girder bridges due to the larger superstructure mass involved in the seismic response. The results from this work can be used to evaluate and select bridge retrofits, and can form the basis for cost-benefit retrofit studies.

Numerical simulations of shake-table experiment for dynamic soil-pile-structure interaction in liquefiable soils

A shake-table experiment on pile foundations in liquefiable soils composed of liquefiable sand and overlying soft clay is studied. A three-dimensional (3D) effective stress finite element (FE) analysis is employed to simulate the experiment. A recently developed multi-surface elasto-plastic constitutive model and a fully coupled dynamic inelastic FE formulation (u-p) are used to model the liquefaction behavior of the sand. The soil domains are discretized using a solid-fl uid fully coupled (u-p) 20-8 noded brick element. The pile is simulated using beam-column elements. Upon careful calibration, very good agreement is obtained between the computed and the measured dynamic behavior of the ground and the pile. A parametric analysis is also conducted on the model to investigate the effect of pile-pinning, pile diameter, pile stiffness, ground inclination angle, superstructure mass and pile head restraints on the ground improvement. It is found that the pile foundation has a noticeable pinning effect that reduces the lateral soil displacement. It is observed that a larger pile diameter and fixed pile head restraints contribute to decreasing the lateral pile deformation; however, a higher ground inclination angle tends to increase the lateral pile head displacements and pile stiffness, and superstructure mass seems to effectively influence the lateral pile displacements.

Effects of column stiffness irregularity on the seismic response of bridges in the longitudinal direction

The longitudinal seismic responses of 4-span continuous bridges designed based on the 2006 Canadian Highway Bridge Design Code were studied using elastic response spectrum and inelastic time-history analyses. Several boundary conditions including unrestrained horizontal movements at the abutments and different abutment stiffnesses were considered in the nonlinear analyses. The seismic response of more than 2600 bridges were studied to determine the effects of different design and modelling parameters including the effects of different column heights, column diameters, and superstructure mass as well as different abutment stiffnesses. The bridges were designed using two different force modification factors of 3 and 5. The effects of column stiffness ratios on the elastic and inelastic analysis results, maximum ductility demands, concentration of ductility demands, and demand to capacity ratios were investigated. The results indicate that the seismic response and maximum ductility demands in the longitudinal direction are influenced by important parameters such as the total stiffness of the substructure, the column stiffness ratio, and the aspect ratio of the columns.

The influence of design parameters on the response of bridges seismically isolated with the Friction Pendulum System (FPS)

One of the emerging tools for protecting bridges from the damaging effects of earthquakes is the use of seismic isolation systems. The Friction Pendulum System (FPS) is a sliding-based seismic isolator that can be applied to a wide range of bridge types. While the individual response of the FPS is well understood, the global effectiveness of seismic isolation as a function of varying bridge design parameters has not been adequately quantified. A detailed three-dimensional analytical model of an FPS-isolated multi-span continuous steel girder bridge was analyzed with the nonlinear time-history method for a range of six design parameters. The results illustrate that seismic isolation with the FPS is less effective for bridges that have flexible sub-structure characteristics, compared with those that are stiffer. Bridges with large superstructure flexibility tend to be affected less from the vertical components of the ground motions. Bridges with larger superstructure mass acquire smaller isolator displacements but place larger demands on the load carrying elements in the sub-structure. Unlike non-isolated bridges, the seismic response characteristics of bridges seismically isolated with the FPS were minimally affected by skew.

Closure to “Seismic Behavior of Batter Piles: Elastic Response” by A. Giannakou, N. Gerolymos, G. Gazetas, T. Tazoh, and I. Anastasopoulos

Significant advances have been made on this important research topic in the last few years because of Giannakou et al., who have faced, in urgent terms, the matter of whether inclined piles have a beneficial or detrimental role. The discusser agreeswith the authors on the fact that the horizontal drift of a superstructure supported on a group of batter piles is generally smaller than with the exclusively vertical pile group; however, it could not really be generalized that this reduction is appreciable only with tall structures. The discusser has carried out work in his Ph.D. thesis about the inclined piles, studying the behavior of a couple of piles supporting a tall superstructure (hstr 5 10m) and a short superstructure (hstr 5 2m), in different geometrical conditions and seismic excitations than those of the authors, reaching quite different results. Specifically, from results obtained performing FEM three-dimensional analysis with the computer program ADINA (ADINA 2010), and using the base of the model with two real acceleration time histories (El Centro component NS 1940 [amax 5 0:318g]; Bagnoli Irpino Earthquake 1980 [amax 5 0:358g]), the following can be concluded. The reduction of horizontal drift because of inclined piles with regard to vertical piles was more important in short superstructures than tall ones (Figs. 1 and 2). To explain these results, it is useful to keep in mind that under seismic excitation, the horizontal drift of the superstructure mass, um, is made of three parts: horizontal translation of the cap, horizontal translation because of rotation cap, and horizontal deflection of the column. The discusser has found out that the cap's horizontal translation founded on vertical piles tends to be higher than the one with the inclined piles. Instead, the contribution because of the cap rotation can increase or reduce the horizontal drift depending on the sign of the rotation. In the case of the short superstructure, Fig. 3 shows the horizontal displacement of the cap on the inclined piles, uc, the vertical displacement of the pile on the left, vcl, and the vertical displacement of the pile on the right, vcr . For better understanding of the figures, the time considered was limited to the first 3 s. Once the vertical displacements are confirmed, the rotation is obtained by dividing the difference in the vertical displacement at the head of the piles by the distance between the two piles. In Fig. 3, the horizontal oscillation frequency of the cap is quite lower than the one in the vertical direction; the vertical displacement, vcl, also has the opposite sign and basically the same amplitude of vcr. The difference in frequency

Laboratory tests and numerical simulations of barge impact on circular reinforced concrete piers

Bridge structures across navigable waterways are vulnerable to barge collisions. To protect the bridge structure, bridge piers should be specially designed to resist barge impact load. In order to quantify the impact load, barge–pier impact tests and/or reliable theoretical or numerical predictions should be conducted. However, very limited impact tests were carried out to evaluate barge impact force and pier response due to the cost and site limitations. This paper presents laboratory tests and numerical simulation results of barge impacting on bridge piers. A scaled circular reinforced concrete pier is built and tested first with a pendulum impact test system. Impact load profile and pier top displacement were recorded and analyzed. A numerical model is then developed to simulate the laboratory tests. Nonlinear materials are used to model the concrete and steel reinforcements with strain rate effects. The accuracy of the pier model is calibrated by comparing the experimental and numerical results. Using the validated numerical model, a full scale bridge pier model is developed to simulate the barge impact on bridge piers. Pile groups and soil springs are considered to model the soil–pier interaction. Parametric studies are carried out to investigate the effects of barge impact velocity, barge mass, pier diameter and height, superstructure mass and impact location, and soil–pier interaction. Impact load time history and pier displacement response are calculated. Based on the numerical results, empirical formulae are proposed to predict the peak impact force and impulse. The adequacy of the current design code is also discussed in the paper.

근단층지반운동에 대한 상부구조 질량 변화에 따른 RC 교각의 응답분석

근단층지반운동은 진앙거리가 약 10 km 이내인 지역에서 관측되는 장주기 성분의 펄스형태를 갖는 지반운동으로서, 도심지에서 발생할 경우 미국 Northridge(1994), 일본 Kobe(1995), 터키 Izmit(1999), 중국 쓰촨성(2008), 아이티지진(2010) 등과 같이 매우 큰 피해를 야기시킨다. 이에 강진지역에서는 NFGM에 대한 많은 연구가 수행되었으나 우리나라와 같은 중약진 지역에서는 매우 미흡한 실정이다. 이 연구에서는 근단층 지반운동을 받는 철근콘크리트 교각의 거동과 내진성능을 평가하기 위하여 6기의 실험체에 대하여 진동대 실험을 수행하였다. 주요 실험 변수로는 심부구속 철근비와 상부질량 크기로서 진동대 실험 결과 심부구속철근비가 적거나 상부질량이 클수록 교각의 내진성능이 저하됨을 알 수 있었다. 또한 진동대 실험 결과를 분석하여 지진동을 받는 교각의 강성변화에 대한 심부구속철근비와 질량의 영향을 분석하였으며, 변위연성도와 강성과의 관계식을 제안하였다.

Fragility analysis of track-on steel-plate-girder railway bridges in Korea

More than 40% of the bridges in conventional Korean railway lines are track-on steel-plate-girder (TOSPG) bridges. They are characterized by a superstructure consisting of railway tracks sitting directly on steel plate girders without any ballast system. Most of these bridges have been designed with little or no consideration given to seismic loading. In this paper, seismic fragility curves of TOSPG bridges in Korea are developed. Fragility curves are developed first for the components, by using the probabilistic seismic demand model. The developed component fragility curves show that the bearings are the most vulnerable components of the TOSPG bridges against seismic loading. On the other hand, the piers are much less vulnerable, although they contain no reinforcing bars. This is because the superstructure mass is very light, and therefore horizontal loading transferred from the superstructure to the piers is minimal. A generic damage measure is introduced for measuring the system-level damage of structures out of the component-level damages. The system fragility curves are then developed, using the generic damage measure. Finally, representation of seismic risk in terms of expected seismic losses is demonstrated. This demonstration shows how the fragility analysis is utilized for risk assessment and support in decision-making.

Centrifuge Modeling of Ship Impact Loads on Bridge Pile Foundations

Bridges that cross navigable waterways may be affected by accidental ship impacts. To better characterize ship impact loads on bridge pier structures, a comprehensive centrifuge model test program involving 48 ship impact tests was performed on a 2 x 3 pile group and a 3 x 3 pile group founded in saturated silty sand. These model tests simulated groups of 2-m-diameter by 31.5-m-long pipe piles. The effects of three factors related to the ship (tonnage, speed, and bow structure) and two factors related to the bridge pier structure (superstructure mass and pile-group size) were investigated through these impact tests. The characteristics of the ship impact load were identified and the mechanism of the ship-bridge collision was analyzed. The test results show that the ship impact load was highly dependent on the ship bow structure and the ship impact speed. The test results were compared with other published data and the AASHTO loads. An empirical equation was suggested to relate the ship impact load to the five influencing factors.

Probabilistic Seismic Design of Pile Foundations in Non Liquefiable Soil by Response Spectrum Approach

The behavior of pile foundations in non liquefiable soil under seismic loading is considerably influenced by the variability in the soil and seismic design parameters. Hence, probabilistic models for the assessment of seismic pile design are necessary. Deformation of pile foundation in non liquefiable soil is dominated by inertial force from superstructure. The present study considers a pseudo-static approach based on code specified design response spectra. The response of the pile is determined by equivalent cantilever approach. The soil medium is modeled as a one-dimensional random field along the depth. The variability associated with undrained shear strength, design response spectrum ordinate, and superstructure mass is taken into consideration. Monte Carlo simulation technique is adopted to determine the probability of failure and reliability indices based on pile failure modes, namely exceedance of lateral displacement limit and moment capacity. A reliability-based design approach for the free head pile under seismic force is suggested that enables a rational choice of pile design parameters.