FEMA P-750 Resource Papers - Apuntes de suelos PDF

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2009 NEHRP Recommended Seismic Provisions for New Buildings and Other Structures: PART 3, RESOURCE PAPERS (RP) ON SPECIAL TOPICS IN SEISMIC DESIGN This part of the 2009 NEHRP Recommended Seismic Provisions consists of a series of resource papers that include: •

Proposals for code and standard changes reflecting new and innovative concepts or technologies that are judged, at the time of publication of this edition of the Provisions, to require additional exposure to those who use codes and standards and to possibly require systematic trial use. Some of these potential future changes are formatted for direct adoption while others discuss only the thrust of the proposed change.

•

Discussions of topics that historically have been difficult to adequately codify. These papers provide background information intended to stimulate further discussion and research and, eventually, code change proposals.

Like Parts 1 and 2 of this volume, these resource papers have been approved for inclusion in this volume by both the 2009 Provisions Update Committee and the BSSC membership. Comments and questions about the topics treated in these Part 3 resource papers should be addressed to: Building Seismic Safety Council National Institute of Building Sciences 1090 Vermont Avenue, N.W., Suite 700 Washington, D.C. 20005 (202) 289-7800, Fax: (202) 289-1092, E-mail: [email protected]

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2009 NEHRP Recommended Seismic Provisions

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Resource Paper 1 ALTERNATE MATERIALS, DESIGN, AND METHODS OF CONSTRUCTION Early in its deliberations, the 2009 Provisions Update Committee (PUC) established Issue Team 1, Performance Criteria, to develop a proposal that would encourage the development of construction equivalent to that provided by prescriptive provisions but possibly offering economic, performance, or construction speed advantages. The PUC took this step in light of an ongoing FEMA-funded project to develop a recommended methodology for reliably quantifying building system performance and response parameters for use in seismic design, in response to growing interest in performance-based design and its use to develop alternate designs equivalent to prescriptive code provisions, and in recognition of the fact that a lack of guidance on methods of approval for such submittals might discourage the creation of needed review processes in some jurisdictions. This paper was initially prepared by BSSC 2009 Issue Team 1 as a proposal for a Provisions Part 1 modification to Section 11.1.4 of ASCE/SEI 7-05. The voting by the BSSC member organizations, however, resulted in many comments about: use of the new methodology prior to completion of the FEMA project and/or prior to complete vetting of the project recommendations, approval methods for components and products on a smaller scale than full building systems, and the lack of specificity in the suggested approval processes. Although the issue team developed complete responses to these comments, the majority of the team recommended interim placement in Part 3 of the Provisions. Due to the high interest and need for guidance on approval of submittals under the Alternate Means section, it is recommended that this or a similar change be considered for inclusion in ASCE/SEI 7-05 as soon as possible.

PROPOSED CHANGE Rearrange ASCE/SEI 7-05 Section 11.1.4.1 and add new Sec 11.1.4.2 as shown below (additions underlined). 11.1.4 Alternate Materials, Design, and Methods of Construction. 11.1.4.1 General. The provisions of this standard are not intended to prevent the use of any material, alternate design method, or alternate method of construction not specifically prescribed, provided that any such alternate has been approved and its use authorized by the authority having jurisdiction. The authority having jurisdiction may approve any such alternate, provided that the authority finds that the alternate is satisfactory and complies with the intent of the provisions of this standard, and that the alternate is, for the purpose intended, at least the equivalent of that prescribed in this standard in suitability, effectiveness, durability, and seismic resistance. 11.1.4.2 Approval of Proposals Under Sec 11.1.4. Nothing in this section shall limit the ability of the authority having jurisdiction to develop or accept general requirements for proposals under Section 11.1.4 or specific requirements for particular components or systems, such as acceptance of reports from evaluation services or other demonstration of equivalence as specified in Section 11.1.4.1. In the absence of such criteria, the approval process shall include the following elements: 11.1.4.2.1 Peer Review. Peer review of the preliminary submittal, final design, and/or construction documents. 11.1.4.2.2 Preliminary Submittal. A submittal of a detailed description and, if applicable, design criteria for the alternate material or method, for approval by the authority having jurisdiction, prior to application for a building permit. 11.1.4.2.3 Structural Design Criteria. For submittals requesting use of alternate materials, alternate design methods or alternate methods of construction for the complete seismic -force-resisting system, a structural design criteria shall be included based on the seismic performance for the Performance Category, as described in the 2006 International Code Council Performance Code, that is equivalent to the Occupancy Category of the building. The design criteria submittal shall demonstrate how the required seismic performance will be met by generally following one of the two methods described below: 1.

Nonlinear procedures described in ASCE/SEI 41-06, Seismic Rehabilitation of Existing Buildings.

2.

Probabilistic nonlinear analysis methods of Quantification of Building Seismic Performance Factor, FEMA P-695. Using these methods, it shall be demonstrated that for the required performance objectives there is an acceptably low probability of not reaching the specified performance level, given the specified ground motion.

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2009 NEHRP Recommended Seismic Provisions 11.1.4.2.4 Nonstructural Design Criteria. For seismic protection of nonstructural components not part of a designated seismic system, the design shall demonstrate that the components and systems are capable of remaining secured to the structure and will not generate life-threatening debris under the Design Earthquake Ground Motion. For designated seismic systems and components of such systems, the design shall demonstrate that the components and systems will be capable of remaining functional following design level shaking. The procedures of Section 13.2.5 and 13.2.6 may be applied as satisfactory fulfillment of these requirements.

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Resource Paper 2 NONLINEAR STATIC PROCEDURE This resource paper was prepared by Technical Subcommittee 2, Design Criteria and Analysis and Advanced Technologies, as a replacement for the Appendix to Chapter 5 of the 2003 edition of the NEHRP Recommended Provisions. It revises the information on the nonlinear static procedure (NSP) to allow its use in design of regular buildings less than 40 feet in height. The principal value of this approach as currently presented is for the design of buildings that are controlled by drift limits. Such buildings can be designed to have sufficient stiffness without using the equivalent lateral force (ELF) procedure and to have sufficient strength without conducting detailed member evaluations (R d < R/Ω0 ). In the future, the height limitation may be relaxed if, for example, the NSP is used in conjunction with a nonlinear dynamic analysis. Because requirements for the nonlinear static procedure are now specified in ASCE/SEI 41-06, it is simpler to refer to that document than to write applicable requirements into the Provisions. Modifications to the ASCE/SEI 7-05 requirements are introduced here to maintain consistency with the nonlinear static procedure information presented in the 2003 Provisions. The 40-foot height limit was selected based on the accuracy of response quantities determined for a three-story momentframe structure; no height limit was identified in the FEMA-funded Applied Technology Council project on the evaluation of inelastic seismic analysis procedures (Improvement of Nonlinear Static Seismic Analysis Procedures, FEMA 440). Although higher modes will have a similar influence on ELF quantities, the higher base shear strengths and story shears of the ELF procedure will tend to result in smaller member ductility demands. Thus, precision in the NSP estimates is especially important when system strengths are lower than those resulting from use of the ELF approach, which evaluates member deformation demands in detail. This resource paper simplifies the language used to establish whether lateral strength is nominally less than that required by the ELF procedure. This is now stated succinctly as R d > R/Ω o . Section references have been harmonized with ASCE/SEI 705 section numbers. If adopted for ASCE/SEI 7-10 or subsequent editions, the chapter number assigned to the requirements portion of this paper will have to be substituted where “X” appears below.

REQUIREMENTS X Nonlinear Static Procedure X.1 Definitions Target Displacement. An estimate of the maximum expected displacement of the control node, determined according to Section 3.3.3.3.2 of ASCE/SEI 41 Supplement1 using S a defined as a design earthquake spectral response acceleration according to the 2009 NEHRP Recommended Seismic Provisions at the effective period. X.2 Notation Q Ei Rd R max γi Ω0

Force in ith member determined according to Section 12.15.8. The system strength ratio as determined by Equation X-1. The maximum strength ratio, defined by Equation 3-16 of ASCE/SEI 41 Supplement 1. The deformations for member i. See Section 11.3.

X.3 Applicability. Regular structures less than 40 feet in height in Occupancy Categories I and II may be designed using the nonlinear static procedure following the requirements of this chapter. X.4 Seismic-force-resisting System. The seismic-force-resisting system shall conform to one of the types in Tables 12.2-1 and 15.4-1 and shall be in accordance with the seismic design category and height limitations indicated in these tables. The appropriate response modification coefficient, R, and system overstrength factor, Ω 0, identified in these tables shall be used, subject to the additional requirements of this chapter. X.5 Modeling and Analysis. Modeling and analysis shall conform to Section 3.3.3 of ASCE/SEI 41 Supplement 1 except that: (a) S a shall be defined as a design earthquake spectral response acceleration according to the NEHRP Recommended Seismic Provisions at the effective period and (b) the analysis shall be conducted for seismic actions occurring simultaneously with the effects of dead load in combination with not less than 25 percent of the required design live loads,

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2009 NEHRP Recommended Seismic Provisions reduced as permitted for the area of a single floor. P-delta effects shall be included in the analysis model, and dead and live loads acting on the entire structure shall be represented in the model. X.6 Maximum Strength Ratio. The system strength ratio, R d, is given by Equation X-1 as follows:

Rd =

Sa C Vy / W m

(X-1)

where C m , V y , and W are as defined in Section 3.3.3.3.2 of ASCE/SEI 41 Supplement 1 and S a is defined as a design earthquake spectral response acceleration according to the 2009 NEHRP Recommended Seismic Provisions at the effective period. Use of the nonlinear static procedure is not permitted when R d exceeds R max . X.7 Story Drift. The design story drift, Δ, taken as the value obtained for each story at the step at which the target displacement is reached, shall not exceed the drift limit specified in Section 12.12.1 multiplied by 0.85R/C d . X.8 Member Strength. In addition to satisfying the requirements of Section 12.15.9, member strengths also shall satisfy the requirements of Section 2.3 using E = 0, except that Section 12.4.3.2 shall apply when the effect of structural overstrength on the design seismic force must be considered. When the effect of structural overstrength is considered, the value of the individual member forces, Q Ei , obtained from the analysis at the target displacement shall be taken in place of the quantity Ω0QE. X.9 Detailed Evaluation. Detailed evaluation satisfying Sections X.9.1and X.9.2 is required if R d exceeds R/Ω 0 . X.9.1 Required Member Force and Deformation. For each nonlinear static analysis, the design response parameters, including the individual member forces, Q Ei , and member deformations, γ i, shall be taken as the values obtained from the analysis at the step at which the target displacement is reached. X.9.2 Member Capacity. The adequacy of individual members and their connections to withstand the member forces, QEi , and member deformations, γ i, shall be evaluated based on laboratory test data for similar components. The effects of gravity and other loads on member deformation capacity shall be considered in these evaluations. The deformation of a member supporting gravity loads shall not exceed: (a) two-thirds of the deformation that results in loss of ability to support gravity loads and (b) two-thirds of the deformation at which the member strength has deteriorated to less than the 70 percent of the peak strength of the component model. The deformation of a member not required for gravity load support shall not exceed two-thirds of the value at which member strength has deteriorated to less than 70 percent of the peak strength of the component model. Alternatively, it shall be permissible to deem member deformation to be acceptable if the deformation does not exceed the value provided in ASCE/SEI 41 Supplement 1 for the Life Safety performance level. Member forces shall be deemed acceptable if not in excess of expected capacities. X.10 Design Review. A review of the design of the seismic-force-resisting system and the supporting structural analyses shall be performed by an independent team having experience in seismic analysis methods and the theory and application of nonlinear seismic analysis and structural behavior under earthquake loading. The team shall be composed of at least two members including at least one registered design professional. The design review shall include: 1. 2.

Review of any site-specific seismic criteria employed in the analysis including the development of site-specific spectra and Review of the determination of the target displacement and effective yield strength of the structure.

For those structures with R d exceeding R/Ω 0 , the design review shall further include, but need not be limited to, the following: 1.

2.

Review of acceptance criteria used to demonstrate the adequacy of structural elements and systems to withstand the calculated force and deformation demands together with the laboratory and other data used to substantiate such criteria. Review of the acceptance criteria for nonlinear procedures given in ASCE/SEI 41 Supplement 1 shall be at the discretion of the design review team. Review of the final design of the entire structural system and all supporting analyses.

The design review team shall issue a report that identifies, within the scope of the review, significant concerns and any departures from general conformance with the NEHRP Recommended Provisions.

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Part 3, Special Topics in Seismic Design

COMMENTARY This resource paper presents proposed requirements for nonlinear static analysis, a seismic analysis procedure also sometimes known as pushover analysis, for review and comment and for adoption into a subsequent edition of the NEHRP Recommended Provisions. Although nonlinear static analysis has only recently been included in design provisions for new building construction, the procedure itself is not new and has been used for many years in both research and design applications. For example, nonlinear static analysis has been used for many years as a standard methodology in the design of the offshore platform structures for hydrodynamic effects and has been adopted recently in several standard methodologies for the seismic evaluation and rehabilitation of building structures, including the Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings (FEMA 350, 2000), Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06, 2007), and Seismic Evaluation and Retrofit of Concrete Buildings (Applied Technology Council, 1996). Nonlinear static analysis also forms the basis for earthquake loss estimation procedures contained in the earthquake module of the multihazard software application HAZUS-MH MR2 (FEMA, 2006) and its Advanced Engineering Building Module (FEMA, 2002). A critical review of and improvement to nonlinear static analysis methods, Improvement of Nonlinear Static Seismic Analysis Procedures, was published as FEMA 440 in 2005. Although it does not explicitly appear in the Provisions, the nonlinear static analysis methodology also forms the basis for the equivalent lateral force procedures contained in the provisions for base-isolated structures and structures with dampers. One of the controversies surrounding the introduction of this methodology into the Provisions relates to the determination of the limit deformation (sometimes called a target displacement). Several methodologies for estimating the amount of deformation induced in a structure as a result of earthquake ground shaking have been proposed and are included in various adoptions of the procedure. The approach presented in this paper is based on statistical correlations of the displacements predicted by linear and nonlinear dynamic analyses of structures recommended in the FEMA 440 report (2005) on the evaluation of inelastic seismic analysis procedures. A second controversy relates to the limited availability of consensus-based acceptance criteria to be used to determine the adequacy of a design once the forces and deformations produced by design earthquake ground shaking are estimated. It should be noted that this limitation applies equally to the nonlinear response history approach, which already has been adopted into building codes. A third controversy relates to the effects of higher modes (or multi-degree-of-freedom effects for structures responding nonlinearly) on response quantities. FEMA 440 identifies significant disparities between response quantities determined by nonlinear static analysis and those determined by nonlinear dynamic analysis for all but low-rise structures; therefore, use of the nonlinear static procedure for the design of members proposed here is limited to structures 40 feet or less in height. This limitation has resulted in the nonlinear static procedure being located in Part 3 of the Provisions. The nonlinear static procedure may be used to ensure that structures designed according to the equivalent lateral force procedure achieve strengths comparable to code expectations. Interstory drifts are compared with tabulated allowable story drifts to maintain consistency with past practice, although it is recognized that larger interstory drifts should be anticipated due to higher mode or multidegree-of-freedom effects. Nonlinear static analysis provides a simplified method of directly evaluating nonlinear response of structures to strong earthquake ground shaking that can be an attractive alternative to the more complex procedures of nonlinear response history analysis. It may be useful for characterizing system strength and stiffness and for establishing that the structure develops a desirable inelastic mechanism.

REFERENCES American Society of Civil Engineers/Structural Engineering Institute. 2006. Seismic Rehabilitation of Existing Buildings, ASCE/SEI 41-06, with Supplement 1. American Society of Civil Engineers, Reston, Virginia. Applied Technology Council. 1996. Seismic Evaluation and Retrofit of Concrete Buildings, SSC Report 96-01. S...