Static Seismic Analysis Procedures (FEMA ), reaffirms FEMA's ongoing . This report (FEMA ) is the final and principal product of the. Earthquake Engineering Research Center (PEER), is in the final stages of a project The publication FEMA The Improvement of Nonlinear Static Seismic. This is an approximation that assumes all inelastic behavior is concentrated in the structure. Fixed to flexible base period shift. FEMA Eqn.
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FEMA - Ebook download as PDF File .pdf), Text File .txt) or read book online. Looking ahead, FEMA is already funding ATC to perform additional studies. In this paper, the damping defined in FEMA to include the soil-structure interaction effect is evaluated, and the accuracy of the Coefficient. Abstract: Nonlinear static procedures (NSPs) presented in the FEMA document are evaluated the ATC project is the FEMA (ATC ) report that.
For each method follow from Step 1 to Step 5 to see the computation sequence. This does not mean, however, that the method is accurate. The most complex step in the computation is the conversion from Sa to H. FEMA is a little weak on this point. Cyclic Degradation Degradation in strength and stiffness and can occur under cyclic loading. Figure 4. Hysteresis loops.
The loop in Fig. There are many other possible loops, for example as in Fig.
For the loop in Fig. It may be noted that the energy ratios are for full amplitude loops. The energy ratio is automatically 0. Example Structure Fig. Figure 5. Simple structure. Cases B1 and B2 are bilinear, with initial periods of 0. Cases C1 and C2 are curved, both with initial periods of 0.
FEMA 440- SSI Provisions)
Accuracy For the dynamic analyses a set of 6 spectrum-matched ground motions is used. Figure 6. Response spectra for the spectrum-matched ground motions. The mean spectra in Fig.
This is between the 0. The spectrum is consistent with site class B, and this class is assumed for the analyses. Results and Comparison Figure 7 shows the results. Figure 7. Results for simple structure. This is the continuous dashed horizontal line in Fig. Table 1 summarizes the results. Table 1. Summary of Results.
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The following are some key points. For the Coefficient Method this causes a 4. These improvements in modeling can lead to significant departures from fixed-base results and more accurate representation of probable structural response. Compared with the fixed-base modeling approach, the predicted period of the structure lengthens, the distribution of forces among various elements changes, the sequence of inelasticity and the modes of inelastic behavior can change, and foundation mechanisms e.
All of these effects result in more realistic evaluation of the probable structural behavior and performance. Figure c illustrates the filtering effects that soilstructure interaction can have on the character and intensity of ground motion experienced by the structural model.
Kinematic interaction results from the presence of relatively stiff foundation elements on or in soil that cause foundation motions to deviate from free-field motions. Two effects are commonly identified: The base-slab averaging effect can be visualized by recognizing that the instantaneous motion that would have occurred in the absence of the structure within and below its footprint is not the same at every point.
Placement of a structure and foundation across these spatially variable motions produces an averaging effect in which the overall motion is less than the localized maxima that would have occurred in the free field. The embedment effect is associated with the reduction of ground motion that tends to occur with depth in a soil deposit.
Both base-slab averaging and embedment affect the character of the foundation-level motion sometimes called the foundation input motion, or FIM in a manner that is independent of the superstructure i. The effects are strongly period-dependent, being maximized at small periods. The effects can be visualized as a filter.
Seismic Assessment of RC Building According to ATC 40, FEMA 356 and FEMA 440
This chapter presents simplified procedures for including the effects of interaction between a structure and the supporting soils in a structural model for nonlinear static analysis procedures. There are three primary categories of soil-structure interaction SSI effects.
These include: Current analysis procedures in FEMA and ATC partially address the flexible foundation effect through guidance on including the stiffness and strength of the geotechnical soil components of the foundation in the structural analysis model. However, these procedures do not address the reduction of the shaking demand on the structure relative to the free-field motion caused by kinematic interaction or the foundation damping effect.
Guidance on including these effects in NSPs is provided in this section. A simple example illustrates the application of these procedures. Appendix E provides detailed information on these soil-structure interaction effects.
Figure a illustrates the assumption that the structural model is mounted on a rigid base that is excited by the free-field motion. The free-field motion is the theoretical movement of a single point on the surface of the ground, assuming that there is no structure near it. The fixed-base modeling assumption is inappropriate for many structures though. Structural systems that incorporate stiff vertical elements for lateral resistance e. Relatively flexible vertical elements e. Figure b illustrates the incorporation of foundation flexibility into the structural model directly.
The impact of those effects on superstructure response will tend to be greatest for short-period buildings. A simplified procedure to apply these principles for reduction of the spectral amplitudes of the free-field motion to generate the FIM spectrum is presented in Section 8.
The foundation input motion can be applied to a fixed-base model or, as depicted in Figure c, can be combined with a flexible-base model.
Figure d illustrates foundation damping effects that are another result of inertial soil-structure interaction in addition to foundation flexibility.
Foundation damping results from the relative movements of the foundation and the supporting soil.
It is associated with radiation of energy away from the foundation and hysteretic damping within the soil. The result is an effective decrease in the spectral ordinates of ground motion experienced by the structure. Section 8. In the procedure, the foundation damping is linked to the ratio of the fundamental period of the system on the flexiblefoundation to that of a fixed-base model. Other factors affecting foundation damping are the foundation size and embedment.
The foundation damping is combined with the conventional initial structural damping to generate a revised damping ratio for the entire system, including the structure, foundation, and soil. This system damping ratio then modifies the foundation 1. The ground motions imposed at the foundation of a structure can differ from those in the free field due to averaging of variable ground motions across the foundation slab, wave scattering, and embedment effects.
These effects are referred to here as kinematic interaction effects, and they tend to be important for buildings with relatively short fundamental periods i. This section presents procedures to account for kinematic effects on building structures. A ratio of response spectra RRS factor can be used to represent kinematic interaction effects.
An RRS is simply the ratio of the response spectral ordinates imposed on the foundation i. Two phenomena should be considered in evaluating RRS: Baseslab averaging occurs to some extent in virtually all buildings. The slab-averaging effect occurs at the foundation level for mats or spread footings interconnected by either grade beams or reinforced concrete slabs.
Even if a laterally stiff foundation system is not present, averaging can occur at the first elevated level of buildings with rigid diaphragms. The only case in which base-slab averaging effects should be neglected is in buildings without a laterally connected foundation system and with flexible floor and roof diaphragms.
Foundation embedment effects should be considered for buildings with basements. Such effects should not be considered for buildings without basements, even if the footings are embedded.
Embedment effects tend to be significant when the depth of basements is greater than about 10 feet. The following simplified procedure adapted from Kim and Stewart and other sources is recommended for analysis of these two kinematic interaction effects as a function of period, T, of the structural model: An approximation to the curves in Figure is given by the following:. Ratio of response spectra for base slab averaging, RRSbsa, as a function of period, T, and effective foundation size, be.
If the structure has a basement embedded a depth e from the ground surface, evaluate an additional RRS from embedment RRSe as a function of period see Figure The curves in Figure are described by the following:.
The spectral ordinate of the foundation input motion at each period is the product of the free-field spectrum and the total RRS. Chapter 8: Ratio of response spectra for embedment RRSe, for an embedment, e, of 30 feet as a function of period, T, and shear wave velocity, vs.
Repeat steps 2 through 4 for other periods if desired to generate a complete spectrum for the foundation input motion FIM. If desired, more detailed procedures can also be used, which are described in Appendix E.
Limitations associated with application of this approach include the following, each of which is explained in Appendix E: Kinematic interaction effects should be neglected for soft clay sites such as Site Class E. Embedment effects can be neglected for foundations embedded in firm rock Site Classes A and B. The base-slab averaging model: Damping related to foundation-soil interaction can significantly supplement damping that occurs in a structure due to inelastic action of structural components.
The damping from foundation-soil interaction is associated with hysteretic behavior of soil not to be confused with hysteretic action in structural components as well as radiation of energy into the soil from the foundation i. These foundation damping effects tend to be important for stiff structural systems e. The effects of foundation damping are represented by a modified system-damping ratio. The damping attributed to foundation-soil interaction alone i.
Finally, the damping ratio of the complete structural system, accounting for foundation-soil interaction, as well as structural damping, is referred to as E0. The change in damping ratio from Ei to E0 modifies the elastic response spectrum. A number of factors influence the foundation damping factor Ef see Appendix E.
Subject to the limitations noted below, the following simplified procedure can be used to estimate Ef and the subsequent spectral ordinate change due to the modified damping ratio of the complete structural system, E0.
Evaluate the linear periods for the structural model assuming a fixed base, T, and a flexible base, T using appropriate foundation modeling assumptions. In those calculations, the strain-degraded shear modulus should be used to represent the soil stiffness.
Calculate the effective structural stiffness of the SDOF oscillator for fixed base conditions as.Hence the Applied Technology Council ATC proposed to the Federal Emergency Management Agency FEMA in that a study be conducted to determine the reasons for differing results and to develop guidance for practicing engineers on improved application of these two methods. This is illustrated in Figure Although this seems logical. The SDOF oscillators were to be analyzed as planar structures subjected to a single horizontal component of ground motion.
The NEHRP Recommended Provisions for New Buildings prescribe a scaling procedure to be used when ground motion records are used directly for timedomain dynamic analysis. Overview of Inelastic Seismic Analysis Procedures. F First. The required change in H can be calculated by iteration. Jack P.