- Seismic analysis
**Seismic Analysis**is a subset ofstructural analysis and is the calculation of the response of a building (or nonbuilding) structure toearthquakes . It is part of the process ofstructural design ,earthquake engineering or structural assessment and retrofit (seestructural engineering ) in regions where earthquakes are prevalent.As seen in the figure, a building has the potential to ‘wave’ back and forth during an earthquake (or even a severe

wind storm). This is called the ‘fundamental mode’, and is the lowestfrequency of building response. Most buildings, however, have higher modes of response, which are uniquely activated during earthquakes. The figure just shows the second mode, but there are higher ‘shimmy’ modes. Nevertheless, the first and second modes tend to cause the most damage in most cases.The earliest provisions for seismic resistance were the requirement to design for a lateral force equal to a proportion of the building weight (applied at each floor level). This approach was adopted in the appendix of the 1927 Uniform Building Code (UBC), which was used on the west coast of the

USA . It later became clear that the dynamic properties of the structure affected the loads generated during an earthquake. In theLos Angeles County Building Code of 1943 a provision to vary the load based on the number of floor levels was adopted (based on research carried out atCaltech in collaboration withStanford University and theU.S. Coast and Geodetic Survey , which started in 1937). The concept of "response spectra" was developed in the 1930s, but it wasn't until 1952 that a joint committee of the San Francisco Section of theASCE and the [*http://www.seaonc.org Structural Engineers Association of Northern California*] (SEAONC) proposed using the building period (the inverse of the frequency) to determine lateral forcesref|Bozorgnia.The

University of California, Berkeley was an early base for computer-based seismic analysis of structures, led by ProfessorRay Clough (who coined the termfinite element ref|Wilson). Students includedEd Wilson , who went on to write the program SAP in 1970ref|CUREE, an early "Finite Element Analysis " program.Earthquake engineering has developed a lot since the early days, and some of the more complex designs now use special earthquake protective elements either just in the foundation (

base isolation ) or distributed throughout the structure. Analyzing these types of structures requires specialized explicit finite element computer code, which divides time into very small slices and models the actualphysics , much like common video games often have "physics engines". Very large and complex buildings can be modeled in this way (such as the [*http://www.ls-dyna.com/pages/earthquake.shtml Osaka Internation Convention Center*] ).Structural analysis methods can be divided into the following five categories.

**Equivalent Static Analysis**This approach defines a series of forces acting on a building to represent the effect of earthquake ground motion, typically defined by a seismic design

response spectrum . It assumes that the building responds in its fundamental mode. For this to be true, the building must be low-rise and must not twist significantly when the ground moves. The response is read from a designresponse spectrum , given thenatural frequency of the building (either calculated or defined by thebuilding code ). The applicability of this method is extended in manybuilding code s by applying factors to account for higher buildings with some higher modes, and for low levels of twisting. To account for effects due to "yielding" of the structure, many codes apply modification factors that reduce the design forces (e.g. force reduction factors).**Response Spectrum Analysis**This approach permits the multiple modes of response of a building to be taken into account (in the

frequency domain ). This is required in manybuilding code s for all except for very simple or very complex structures. The response of a structure can be defined as a combination of many special shapes (modes) that in a vibrating string correspond to the "harmonics ". Computer analysis can be used to determine these modes for a structure. For each mode, a response is read from the design spectrum, based on the modal frequency and the modal mass, and they are then combined to provide an estimate of the total response of the structure. Combination methods include the following:

*absolute - peak values are added together

*square root of the sum of the squares (SRSS)

*complete quadratic combination (CQC) - a method that is an improvement on SRSS for closely spaced modesIt should be noted that the result of a response spectrum analysis using the response spectrum from a ground motion is typically different from that which would be calculated directly from a linear dynamic analysis using that ground motion directly, since phase information is lost in the process of generating the response spectrum.

In cases where structures are either too irregular, too tall or of significance to a community in disaster response, the response spectrum approach is no longer appropriate, and more complex analysis is often required, such as

non-linear static or dynamic analysis.[

*http://www.isr.umd.edu/~austin/aladdin.d/matrix-eq-spectra.html A sample response spectrum analysis.*]**Linear Dynamic Analysis**Static procedures are appropriate when higher mode effects are not significant. This is generally true for short, regular buildings. Therefore, for tall buildings, buildings with torsional irregularities, or non-orthogonal systems, a dynamic procedure is required. In the linear dynamic procedure, the building is modelled as a multi-degree-of-freedom (MDOF) system with a linear elastic stiffness matrix and an equivalent viscous damping matrix.

The seismic input is modelled using either modal spectral analysis or time history analysis but in both cases, the corresponding internal forces and displacements are determined using linear elastic analysis. The advantage of these linear dynamic procedures with respect to linear static procedures is that higher modes can be considered. However, they are based on linear elastic response and hence the applicability decreases with increasing nonlinear behaviour, which is approximated by global force reduction factors.

In linear dynamic analysis, the response of the structure to ground motion is calculated in the

time domain , and all phase information is therefore maintained. Only linear properties are assumed. The analytical method can use modal decomposition as a means of reducing the degrees of freedom in the analysis.**Non-linear Static Analysis**In general, linear procedures are applicable when the structure is expected to remain nearly elastic for the level of ground motion or when the design results in nearly uniform distribution of nonlinear response throughout the structure. As the performance objective of the structure implies greater inelastic demands, the uncertainty with linear procedures increases to a point that requires a high level of conservatism in demand assumptions and acceptability criteria to avoid unintended performance. Therefore, procedures incorporating inelastic analysis can reduce the uncertainty and conservatism.

This approach is also known as "pushover" analysis. A pattern of forces is applied to a structural model that includes non-linear properties (such as steel yield), and the total force is plotted against a reference displacement to define a capacity curve. This can then be combined with a demand curve (typically in the form of an acceleration-displacement

response spectrum (ADRS)). This essentially reduces the problem to a single degree of freedom system. Nonlinear static procedures use equivalent SDOF structural models and represent seismic ground motion with response spectra. Story drifts and component actions are related subsequently to the global demand parameter by the pushover or capacity curves that are the basis of the non-linear static procedures.**Non-linear Dynamic Analysis**Nonlinear dynamic analysis utilizes the combination of ground motion records with a detailed structural model, therefore is capable of producing results with relatively low uncertainty. In nonlinear dynamic analyses, the detailed structural model subjected to a ground-motion record produces estimates of component deformations for each degree of freedom in the model and the modal responses are combined using schemes such as the square-root-sum-of-squares.

In non-linear dynamic analysis, the non-linear properties of the structure are considered as part of a

time domain analysis. This approach is the most rigorous, and is required by somebuilding code s for buildings of unusual configuration or of special importance. However, the calculated response can be very sensitive to the characteristics of the individual ground motion used as seismic input; therefore, several analyses are required using different ground motion records.**ee also***

Modal Analysis using FEM

*Seismic performance

*Seismic performance analysis

*Structural Dynamics

*OpenSees - anopen source structural analysis package developed atUC Berkeley . It is focussed on seismic analysis and is being used by theNational Science Foundation 's [*http://www.nees.org/ NEES*] program.

*National Center for Research on Earthquake Engineering (NCREE)

*Vibration control

*Earthquake Protector

*Earthquake Performance Evaluation Tool

*Earthquake simulation **References**#ASCE. (2000). Pre-standard and Commentary for the Seismic Rehabilitation of Buildings (FEMA-356) (Report No. FEMA 356). Reston, VA: American Society of Civil Engineers prepared for the Federal Emergency Management Agency.

#ATC. (1985). Earthquake Damage Evaluation Data for California (ATC-13) (Report). Redwood, CA: Applied Technology Council.

#Bozorgnia, Y, Bertero, V, "Earthquake Engineering: From Engineering Seismology to Performance-Based Engineering", CRC Press, 2004.

# [*http://www.edwilson.org/History/fe-history.pdf "Early Finite Element Research at Berkeley"*] , Wilson, E. and Clough R., presented at the Fifth U.S. National Conference on Computational Mechanics, Aug. 4-6, 1999

# [*http://www.curee.org/image_gallery/calendar/essays/1998-CUREE_excerpt.pdf "Historic Developments in the Evolution of Earthquake Engineering"*] , illustrated essays by Robert Reitherman, CUREE, 1997, p12.

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