High Seismic Risk Zones Research Articles

Behaviour of Concrete Filled Rectangular Steel Tube Column

The Concrete Filled Steel Tube (CFST) member has many advantages compared with the conventional concrete structural member made of steel reinforcement. CFSTs are frequently used for columns, caissons and for piers, deep foundations because of their large compressive stiffness. The FEA modeling using ANSYS software is adopted to investigate the load versus lateral deflection behavior of the composite sections. The effects of steel tube thickness and strength of in-filled concrete tubes are examined. The size of the column is 140 x 160 x 1500 mm and the grades of concrete infill are M20, M30 and M40. The thickness of the tube is taken as 2 mm, 3 mm, 4 mm, 5 mm and 6 mm and, the D/t ratio varies from 26.67 to 80.00. Since past few decades, composite steel-concrete construction of column and girder is being used in the construction industry. These composite sections have the rigidity and formability of reinforced concrete with the strength and speed of construction associated with structure, thereby making them economical. The steel and the concrete element in a composite member complement each other ideally. Composite members consisting of rectangular steel tubes filled with concrete are extensively used in structures involving very large applied moments, particularly in zones of high seismic risk. Concrete Filled Tubes (CFTs) have been used as girder, beam and columns in frame structures. A steel hollow section in-filled with concrete has higher strength and larger stiffness than the conventional structural steel section and reinforced concrete. Composite column are structural members, which are mainly subjected to forces and end moments. The steel tube serves as a formwork for casting the concrete, which reduces the construction cost. No other reinforcement is needed since the tube itself act as a longitudinal and lateral reinforcements for the concrete core. The continuous provided to the concrete core by the steel tube enhances the core's strength and ductility. The concrete core delays bending and buckling of the steel tube, while steel tube prevents the concrete from spalling. Also concrete filled tube (CFT) columns are suitable for tall buildings in high seismic regions since concrete delays the local buckling of steel hollow sections and increases the ductility of the section significantly. While there is a large number of studies on the behaviour of CFT columns and beam-columns; there is relatively little research reported on the flexural behaviour of concrete-filled hollow structural steel column. The structural behavior of a CFT is governed by the member strength, reflecting the fact that the load resistance is dependent not only on the material properties but also on the geometric properties of the entire member. Tests on CFTs by Bridge showed that the concrete core only provides approximately 7.5% of the capacity in members under pure bending. Lu and Kennedy studied strength on twelve beams of concrete-filled Steel Square and Rectangular Hollow Sections for examining the effects of different depth to width ratios and different values of shear span to depth. The tests showed that the ultimate flexural strengths of the composite beams are increased by about 10% - 30% over that of bare steel sections, depending on the relative proportions of steel and concrete. Although composite columns of concrete and steel were rarely used from the end of World War II until the early 1970s (Viest et al. 1997, 1.13), research had started a long time before, at the beginning of the 20th century. Combining of these materials had a number of motivations; steel columns were often encased in concrete to protect them from fire, while concrete columns were combined with structural steel as reinforcement. The model is created for different width-to-wall thickness ratios (from 26.67 to 80.00) and different concrete cube strengths (27.8, 42, 51 MPa). It is found that in general, concrete filling of the steel tube enhances strength, ductility and energy absorption especially for thinner sections. Han proposed a mechanics model that can predict the behaviour of concrete-filled hollow structural sections. In this paper, CFTs with three different wall thicknesses (Table 1) are selected (D/t = 26.67 - 80.00). Three different grades of concrete in-fills are considered. An effort is made to find the relative contributions of the steel tube and the confined concrete core to the moment capacity and deformation of CFTs under flexure.

Evidence of Quaternary active folding near Utique (Northeast Tunisia) from tectonic observations and a seismic profile

The present-day seismicity in northeastern Tunisia reported from permanent networks is of low to moderate magnitude. However, earthquakes are mentioned in the literature, specially a destructive one in the antique city of Utique. Geologic, seismic, and neotectonic investigations in this area show that the Utique fold is closely related to the recent tectonic activity in this region. Data show that the Utique fold is built on an east-west fault, and we found evidence of activity of this fault in the past 20 kyr. A seismic section and balanced cross-section show that the slip rate is of the order of 0.38 mm.yr −1. Our data show definitively the Late Pleistocene–Holocene activity of the Utique Fault; and we can predict the earthquake recurrence interval which should be of ∼10 3–10 4 yr. This high seismic risk zone deserves to be taken into account during the establishment of important regional development programs and in the application of seismic building codes.

Effect of Seismic Loading to Prefabricated Connections

Most of industrial buildings are prefabricated structures and built on high seismic risk zones. Prefabricated buildings must be earthquake resistant which can be possible if structural elements and their connections are properly built. In recent years, prefabricated buildings which have no RC walls were damaged heavily during arthquakes. In this paper, prefabricated industrial structures, damaged during Ceyhan (1998), Kocaeli (1999) and Duzce (1999) earthquakes, and reasons of structural damages are studied. Additionally design and construction precautions are given for building an earthquake resistant prefabricated structure.

Vulnerability Maps for Reinforced Concrete Structures for Mexico City's Metropolitan Area under a Design Earthquake Scenario

This paper presents vulnerability maps for reinforced concrete moment frames (RCMFs) in Mexico City's Metropolitan Area (MCMA) for a Ms=8.1 earthquake scenario assessing peak nonlinear dynamic responses using the concept of displacement ductility demand spectra (DDDS). The considered earthquake scenario is the 1985 Michoacán Earthquake. Artificial and recorded ground motions were used. Takeda model was used to simulate nonlinear behavior of RCMFs. Seismic strength for existing RCMFs structures was estimated according to the nominal strength required by versions of Mexico's Federal District Code (MFDC) that were effective before the 1985 earthquake. Overstrength sources for RCMFs were also evaluated. With all of this information, DDDS were obtained for different sites in MCMA. Later, vulnerability maps based on contours of equal displacement ductility demands and nonlinear displacements for a given structural period and a version of MFDC were defined. These vulnerability maps were compared with the geographic area of severe structural damage and collapses mapped during the 1985 earthquake. Based on the results of this study, it is believed that this procedure can be useful to forecast demands for different structural systems for a given earthquake scenario, being therefore potentially useful for urban planning of cities located in zones of high seismic risk.

Towards deformation-based capacity design of RCS beam–column connections

Hybrid RCS frame systems consisting of reinforced concrete (RC) columns and steel (S) beams have been recognized to possess several advantages in terms of structural performance and economy compared to pure RC and steel frames. However, their use has been limited to low or moderate seismic regions due to lack of appropriate design guidelines for RCS frames in high seismic risk zones. In this article, the implementation of a deformation-based capacity approach for the design of RCS connections in frames located in high seismic risk zones is described. The main purpose of the design procedure is to control joint distortions and damage, forcing most of the frame inelastic activity to occur in the beams. The design procedure is validated through the testing of two RCS beam–column–slab subassemblies under displacement reversals. Experimental results indicate that the proposed design procedure is effective in controlling joint deformations and damage, leading to the formation of plastic hinges at the beam ends. The test specimens showed excellent seismic behavior with significant energy dissipation capacity, which demonstrates the suitability of RCS frames for use in high seismic risk zones.

Modeling Shear Behavior of Hybrid RCS Beam-Column Connections

A model to predict the shear strength versus shear distortion response of hybrid connections between reinforced concrete columns and steel (RCS) beams is presented. Based on a preestablished maximum attainable joint shear deformation, design equations were developed for use in both interior and exterior RCS connections in buildings located in high seismic risk zones. The proposed model is based on the definition of the state of plane strain in the joint through the development of a ratio between the principal tensile and compression strains. The strength of the joints is given by contributions from the steel web panel and concrete diagonal struts. The joint model is capable of predicting the shear force, and stirrup and concrete strains at any level of joint shear distortion for exterior joints. In addition, the use of the proposed design equations led to excellent agreement between predicted and measured shear strength for both interior and exterior RCS joints.

Seismic Response of Exterior RC Column-to-Steel Beam Connections

The inelastic cyclic response of hybrid connections consisting of RC columns and steel beams (RCS) is studied in this paper. Experimental results from the lateral load testing of nine exterior RCS connections are presented. The primary joint details investigated are two-part U-shaped stirrups passing through holes in the web of the steel beams, steel band plates or cover plates surrounding the joint region, steel fiber concrete or engineered cementitious composite material in the connection, and bearing bars attached to the steel beam. Experimental results indicate that RCS frames are suitable for use in high seismic risk zones. All of the specimens maintained their strength at large levels of story drift without significant loss of stiffness. A joint hoop volumetric ratio of 0.9% was found to be adequate for confinement in RCS joints. However, the amount of joint stirrups can be reduced or even eliminated if steel band plates, engineered cementitious composite material, or fiber reinforced concrete are u...

Seismic Behavior of Composite RCS Frame Systems

This paper presents the results of an experimental investigation into the seismic performance of a composite moment resisting frame system that consists of reinforced concrete columns (with an embedded steel shape for erection purposes) and composite steel beam-reinforced concrete slab sections. Quasi-static reversed cyclic loading was performed on six two-thirds-scale subassemblage specimens. Various detailing options were evaluated to provide full moment connection in both orthogonal framing directions at the beam-column joints. With appropriate joint detailing, specimens exhibited a desirable beam plastic hinge mechanism with stable hysteretic response. Composite beam sections maintained near full composite behavior beyond code based drift limits with good energy dissipation characteristics and were able to undergo large plastic rotation magnitudes. This framing system may represent a viable alternative for low-to-mid-rise structures in high seismic risk zones.

Seismic Design and Constructability of RCS Special Moment Frames

This paper presents an evaluation in terms of constructability, cost, and seismic performance of an alternative structural system for high seismic risk zones made of composite reinforced concrete-steel (RCS) special moment-resisting frames. Interior beam-column joints and the effects of a reinforced concrete slab on composite metal decking are investigated. The major issues discussed are related to seismic design philosophy and constructability of a three-dimensional RCS special moment frame system for low-rise and mid-rise construction. The key conclusion from this work is that the proposed RCS frame system has advantages over steel frame systems in high seismic risk zones in terms of having a cost-effective construction process and possessing certain stable hysteretic performance during inelastic excursions.

Bridge over the Jhelum River, Pakistan

The Jhelum River in Pakistan runs through a mountainous area in a high risk seismic zone, and its flow volumes vary dramatically with the seasons. At the town of Karote, a new bridge has been built after record floods in September 1992 rose 8 m above the deck, subjecting the previous bridge to extreme loading from thousands of tons of logs, debris and boulders, leading to stability failure. In July 1993, the National Highway Authority, with a credit from World Bank, invited international bids for reconstruction on a turn-key (design-and-construct) basis. After examining comparative costs and time schedules, a bid was submitted for construction of a post-tensioned concrete segmental cantilever bridge.

Evaluation of Nonlinear Frame Finite-Element Models

In recent years nonlinear dynamic analysis of three-dimensional structural models is used more and more in the assessment of existing structures in zones of high seismic risk and in the development of appropriate retrofit strategies. In this framework beam finite-element models of various degrees of sophistication are used in the description of the hysteretic behavior of structural components under a predominantly uniaxial state of strain and stress. These models are commonly derived with the displacement method of analysis, but recent studies have highlighted the benefits of frame models that are based on force interpolation functions (flexibility approach). These benefits derive from the fact that models with force interpolation functions that reproduce the variation of internal element forces in a strict sense yield the exact solution of the governing equations in the absence of geometric nonlinearity. While the numerical implementation of force-based models at first appears cumbersome, simple examples of nonlinear analysis in this paper offer conclusive proof of the numerical and computational superiority of these models on account of the smaller number of model degrees of freedom for the same degree of accuracy in the global and local response. A numerical implementation that bypasses the iterative nature of the element state determination in recent force-based elements is also introduced, thus further expanding the benefits of flexibility-based nonlinear frame models.

Economic Considerations of Displacement-Based Seismic Design of Structural Concrete Buildings

The economic implications of using displacement-based seismic design for structural concrete building structures are presented for moderate and high seismic risk zones. Buildings from four to twenty stories tall, with the same plan layout and a series of combinations of column and girder cross sections, were designed by practicing structural engineers with a degree of detail necessary to determine their cost. The capacity of these buildings to meet different story drift requirements was investigated employing economic considerations. The use of structural walls was also evaluated for the eight story building for both seismic risk zones. Cost implications of structural walls were compared with those of frames. The results of the study are discussed from the designer’s and owner’s points of view.

Seismic analysis of complex structures: Practical application issues and case study

All large structures are vulnerable to earthquake damage. Local failures or total collapses of such structures represent not only immediate concern for loss of life and repair costs, but also major economic impacts due to loss of service. The 1989 Loma Prieta and 1994 Northridge earthquakes in California provided dramatic examples of the vulnerability of engineered structures and the extensive impact of their failure in regions classified as high seismic risk zones. Today, engineers throughout the world are faced with increasing requirements to assess the seismic vulnerability of existing structures, even in regions of relatively low seismic risk classification, and to design new, more sophisticated structures. The purpose of this paper is to discuss the particular features and benefits of ADINA with respect to the requirements of advanced seismic analysis. Major aspects of earthquake related analysis are discussed, i.e. static lateral push-over analysis, ground motion specification for dynamic analysis, and significant structural behavior phenomena associated with contact and ductile energy dissipation. The modelling of typical structural components is presented and analysis strategies are delineated via case study of a large public building. In addition to issues related to the as-built structure analysis, the philosophy of accommodating displacements via the deformation of highly ductile components or dampers introduced at existing expansion joint hinges and other appropriate structural locations is presented along with the requisite modelling requirements. With respect to post-processing, typical structural component performance is illustrated via monotonic and hysteretic energy characterizations as well as with standard displacement and force demand data. A summary of the analytical issues and their simulation within ADINA especially with respect to new capabilities, is also provided.

Short and long term radon in soil monitoring for geophysical purposes

The subduction of the Cocos Plate under the North America Plate defines a high risk seismic zone along the Pacific coast of Mexico. A seismic gap has been identified at the Guerrero State as a place where a large earthquake, up to M S=8.2, can occur within the next years. In this area an in-soil radon network, consisting of fixed SSNTD monitoring stations, has been operated during several years on a monthly exposure basis. A continuous radon probe that registers data on a short term basis is also operating since recently. The results of the two radon detection systems are compared and discussed as a function of the local seismic activity observed at the Guerrero coastal zone.

Loma Prieta earthquake

The October 17, 1989, Loma Prieta earthquake of magnitude 7.1 provided a harsh reminder of the hazards associated with life on a major plate boundary. The scientific lessons of the earthquake are still being assessed as seismic, geologic, and geodetic data are analyzed and new data collected.Probably the most striking, and deadly, aspect of the earthquake was the intense damage at sites, such as San Francisco's Marina district and the 1‐880 overpass in Oakland, 100 km from the earthquake's epicenter. Similar damage occurred in Mexico City in 1985, 350 km from the epicenter of the M 8.1 Michoacan earthquake. Seismic recordings during the Loma Prieta earthquake and its aftershocks showed that pockets of poorly consolidated sediments and Bay muds with low seismic velocities greatly amplified the ground shaking. Sites on Bay mud experienced peak ground motions many times greater than sites only a few blocks away on bedrock. Site amplification during the mainshock was less severe (at frequencies of a few Hz) than during subsequent aftershocks, demonstrating significant non‐linearity in the site effects. The damage resulted from a combination of local site amplification and pervasive failure of artificial land fills. Both effects were predicted prior to the earthquake; published earthquake hazard maps had previously designated as high seismic risk zones the sites of intense damage.