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Assessment of Response Modification Factor of Reinforced Concrete Table Top Frames Structures Subjected to Seismic Loads
Abstract
In this study, the seismic performance of table-top reinforced concrete frame structure which is commonly found in large oil refineries. Nonlinear static pushover analysis is conducted to study the inelastic behaviour of these frames. The analysis accounts for their unique detailing especially to elements that contribute to energy dissipation during major seismic events. The results show that the reserve strength of structure is greater than that prescribed by the ASCE7-10 for Special reinforced concert moment frame.
... The value of K1 can be estimated on the basis of more accurate calculations of structures of the type under consideration and, above all, on the basis of the analysis of the past earthquakes experience. The other, more serious assumption, considered in [8,[22][23][24], is that the conditional seismic load obtained by multiplying actual displacements by the rigidity of the elastic system is replaced by another conditional load obtained by calculating the elastic system for the action of actual accelerations. Unfortunately, here discrepancies can be noticeable [8], which decreases the reliability of the calculations 5. Despite possible inaccuracies in estimating seismic resistance by the RSM, especially for the ULS and the CLS, the authors consider these estimates to be more informative for the designer, than modern dynamic calculations based on earthquake accelerograms. ...
The issues of using the response-spectra method for calculating the seismic resistance of structures are considered. The physical meaning of the coefficients included in the normative calculation formulas of the response-spectra technique is analyzed, ways of refining these coefficients are indicated. In particular, the dependence of the peak ground acceleration on the design accelerogram period is noted. The issue of obtaining the displacement spectra on the base of the normative spectral curve is considered. It is noted that with the correct setting of the normative coefficients, calculation by the response-spectra method gives more reliable values of the calculated factors. Dynamic calculating of structures over time are only needed for substantially non-linear systems and, possibly, heavily damped systems with non-uniform damping. Single calculation of the structure according to an accelerogram only disorientate the designer. The refined version of the linear spectral technique presented in the paper should be considered as the main method for assessing the seismic resistance of buildings and structures.
Estimating the reduction factor for calculating massive reinforced concrete bridge piers was made. For this purpose a quasi-static “force-displacement” diagram was built up using the ANSYS software. This diagram has the form of a bilinear one, and the character of the bilinearity depends on the diameter of the reinforcing bars insignificantly. The percentage of reinforcement affects only the moment when all reinforcement bars begin to flow. The reinforcement flow takes place in the displacement interval from 3 to 5 cm. The collapse will occur when the reaction of the bearing part goes beyond the pier cross-section at pier displacements from 5 to 20 cm. Using “force-displacement” diagram, the behavior of the single-mass model with a bilinear deformation diagram and the limit displacement of 20 cm was analyzed. Then, it became possible to obtain for each accelerogram the limit elastic displacement and the limit position of the point corresponding to the maximum structure displacement during structure oscillations. It was done using real accelerograms of earthquakes with intensity 9 on the MSK scale without normalizing their amplitudes. In this case, long-period accelerograms had smaller peak accelerations, but resulted in greater plastic deformations. As a result, no evident dependence of plastic deformation on the input spectral composition was found and the value of reduction factor K1 turned out to be 0.25-0.27. However, it is shown that this reduction factor cannot be used to make transition from seismic loads obtained on the basis of time-history analysis by accelerograms to design loads.
This study attempt to evaluate over strength, ductility and response modification factors of open-frames steel platforms that are commonly found in industrial and manufacture facilities. Static pushover analyses were carried out to evaluate the seismic response modification factor for these structures considering their unique detailing requirements. The results show that the reserve strength of structure is greater than that prescribed by the ASCE7-10 for ordinary-concentric braced frame systems.
The principal objective of this investigation is to develop a pushover analysis procedure based on structural dynamics theory, which retains the conceptual simplicity and computational attractiveness of current procedures with invariant force distribution, but provides superior accuracy in estimating seismic demands on buildings.
The standard response spectrum analysis (RSA) for elastic buildings is reformulated as a Modal Pushover Analysis (MPA). The peak response of the elastic structure due to its nth vibration mode can be exactly determined by pushover analysis of the structure subjected to lateral forces distributed over the height of the building according to s*n = mφn, where m is the mass matrix and φn its nth-mode, and the structure is pushed to the roof displacement determined from the peak deformation Dnof the nth-mode elastic SDF system. Combining these peak modal responses by modal combination rule leads to the MPA procedure.
The MPA procedure is extended to estimate the seismic demands for inelastic systems: First, a pushover analysis determines the peak response rno of the inelastic MDF system to individual modal terms, peff,n(t) = −snüg (t) , in the modal expansion of the effective earthquake forces, peff,n (t) = −mιüg (t) . The base shear-roof displacement (Vbn −um ) curve is developed from a pushover analysis for force distributions*n. This pushover curve is idealized as bilinear and converted to the force-deformation relation for the nth-“mode” inelastic SDF system. The peak deformation of this SDF system is used to determine the roof displacement, at which the seismic response, rno , is determined by pushover analysis. Second, the total demand, ro , is determined by combining the rno (n= 1, 2,…) according to an appropriate modal combination rule.
Comparing the peak inelastic response of a 9-story SAC building determined by the approximate MPA procedure with rigorous nonlinear response history analysis (RHA) demonstrates that the approximate procedure provides good estimates of floor displacements and story drifts, and identifies locations of most plastic hinges; plastic hinge rotations are less accurate. The results presented for El Centro ground motion scaled by factors varying from 0.25 to 3.0, show that MPA estimates the response of buildings responding well into the inelastic range to a similar degree of accuracy when compared to standard RSA for estimating peak response of elastic systems. Thus the MPA procedure is accurate enough for practical application in building evaluation and design.
Comparing the earthquake-induced demands for the selected 9-story building determined by pushover analysis using three force distributions in FEMA-273, MPA, and nonlinear RHA, it is demonstrated that the FEMA force distributions greatly underestimate the story drift demands, and the MPA procedure is more accurate than all the FEMA force distributions methods in estimating seismic demands. However, all pushover analysis procedures considered do not seem to compute to acceptable accuracy local response quantities, such as hinge plastic rotations. Thus the present trend of comparing computed hinge plastic rotations against rotation limits established in FEMA-273 to judge structural performance does not seem prudent. Instead, structural performance evaluation should be based on story drifts known to be closely related to damage and can be estimated to a higher degree of accuracy by pushover analyses.
Vulnerability of buildings to seismic hazards is more drastic in developing countries with high seismicity, as compared to developed countries. This is primarily attributed to the lack of seismic design guidelines, which fit the type of structural systems and practices that are often applied in such parts of the world. Response reduction factors (R factors) are essential seismic design tools, which are typically used to describe the level of inelasticity expected in lateral structural systems during an earthquake. The R factors in many developing countries are often adopted from the well developed seismic design codes used in the United States or Europe. These R factors provide false representation for the structural practices applied in developing countries and thus considered unrealistic. So there is a dire need to come up with realistic R factors for various structural systems used in such countries. This study utilizes incremental dynamic analysis (IDA) and peak ground parameters to determine the R factor of reinforced concrete (RC) moment resisting frames (MRFs) in Pakistan. A suite of ground motion records from the region is used in the study. Two-dimensional building models are developed in OpenSees and subjected to nonlinear time history analysis. Fiber sections with different material constitutive behaviors are constructed for each of the building’s critical elements. The reinforcement detailing of the analyzed buildings is determined from two prototype buildings in Pakistan. A parametric study involving RC MRFs with variation in dimensional and material properties was conducted to examine the effect of these properties on R factor. Results showed that R factor is affected by both geometric configuration and material strength; however, variation in geometric parameters tends to display more significant impact on the R factor value. The results also show that the R factors recommended by the United States seismic design provisions are unconservative and over estimate the R factor values for some selection of ground motion while it is conservative for others.
Log construction is becoming increasingly popular throughout the U.S. Currently, seismic coefficients are not provided in model building codes for the design of the log shear walls as a lateral force resisting system for seismic forces. Current design practice is to use a response modification coefficient, R, of around 4.5. Several tests by other researchers on log shear walls showed strong energy dissipation and good lateral strength with stability after high displacements. This behavior of the log shear wall system is evidence that a higher R could possibly be used in design. The purpose of this study was to establish a response modification factor for single story log shear walls based on available shear wall tests using the definition of R provided in ATC-19. This research did not conduct testing according to the protocol and methodology of ATC-63. This work contains a history of the development of seismic design provisions in the U.S. and the evolution of the response modification coefficient. Common log construction practices are reviewed, with reference to ICC 400- Standard on Design and Construction of Log Structures. Using data provided by other researchers from physical testing and computer modeling of various types of log shear walls, an R of 6.0 is proposed based on the provisions of ATC-19. Finally, recommendations for further research to fully understand the behavior of the log shear wall system, including possible archetypes required by the methodology set forth in ATC-63, are provided. Master of Science Masters Department of Architectural Engineering and Construction Science Sutton F. Stephens
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