ACI-374.1-2005.pdf

374.1-1 ACI Committee 374 adopted ACI T1.1/T1.1R-01 as ACI 374.1-05 on October 12, 2005. ACI T1.1/T1.1R-01 superseded ACI ITG/T1.1-99 and became effective March 9, 2001. Copyright © 2005, American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. ACI Committee Reports, Guides, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom. Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer. Acceptance Criteria for Moment Frames Based on Structural Testing and Commentary An ACI Standard ACI 374.1-05 Reported by ACI Committee 374 This document defines the minimum experimental evidence that can be deemed adequate to attempt to validate the use, in regions of high seismic risk or in structures assigned to satisfy high seismic performance or design categories, of weak beam/strong column moment frames not satisfying fully the prescriptive requirements of Chapter 21 of ACI 318-99. This document consists of both a Standard and a Commentary that is not part of the Standard. Originally prepared by Innovation Task Group 1 and Collaborators Norman L. Scott Chair Neil M. Hawkins Secretary Michael E. KregerLeslie D. Martin James R. LibbyRobert F. Mast Collaborators Geraldine S. CheokSuzanne NakakiJohn F. Stanton S. K. GhoshM. J. Nigel PriestleyDean E. Stephan H. S. Lew David C. Seagren* William C. Stone *Deceased Ronald Klemencic Chair John R. Hayes, Jr. Secretary Sergio M. AlcocerMary Beth D. HuesteAndres LepageArturo E. Schultz Mark A. AschheimMohammad IqbalAli M. MemariShamim A. Sheikh John F. BonacciBrian E. KehoeVilas S. MujumdarBozidar Stojadinovic Sergio F. BrenaDominic J. KellyJaveed A. MunshiAndrew W. Taylor Paul J. BrienenRichard E. KlingnerStavroula J. PantazopoulouRaj Valluvan JoAnn P. BrowningMervyn J. KowalskyChris P. PantelidesJohn W. Wallace Duane E. CastanedaMichael E. KregerLong T. PhanEric B. Williamson Jeffrey J. DragovichSashi K. KunnathJose A. PincheiraSharon L. Wood Juan Carlos EsquivelJames M. LaFaveMario E. RodriguezFernando V. Yanez Luis E. GarciaDawn E. LehmanMurat Saatcioglu 374.1-2ACI STANDARD The document has been written in such a form that its various parts can be adopted directly into Sections 21.0, 21.1, and 21.2.1 of ACI 318-99 and the corresponding sections of ACI 318R-99. Among the subjects covered are requirements for: procedures that shall be used to design test modules; configurations for those modules; test methods; test reports; and determi- nation of satisfactory performance. The Commentary describes some of the considerations of the Innovation Task Group in developing the Standard. The section numbering for the Commentary is the same as that for the Standard, with numbers preceded by an “R” and the text in italics to distinguish them from the corresponding section numbers of the Standard. The Commentary references documentary evidence, additional to the references of Chapter 21 of ACI 318R-99, that supports the Standard. Consistent with the approach of ACI 318-99 and ACI 318R-99, no comparison is made, either in the body of the Standard or its Commentary, of research results for test modules satisfying ACI 318-99 with those for modules that, although not satisfying ACI 318-99, do satisfy the Standard. Such comparisons, both experimental and analytical, are available in the references of the Commentary. Keywords: acceptance criteria; drift ratio; energy dissipation; lateral resis- tance; moment frame; post-tensioning; precast concrete; prestressed concrete; seismic design; test module; toughness. CONTENTS Introduction, p. 374.1-2 1.0Notation, p. 374.1-2 2.0Definitions, p. 374.1-3 3.0Scope, p. 374.1-4 4.0Design procedure, p. 374.1-5 5.0Test modules, p. 374.1-5 6.0Testing agency, p. 374.1-6 7.0Test method, p. 374.1-6 8.0Test report, p. 374.1-7 9.0Acceptance criteria, p. 374.1-8 10.0Standard references, p. 374.1-9 INTRODUCTION For seismic design, ACI 318-99 specifies in Section 21.2.1.5 that “a reinforced concrete structural system not satis- fying the requirements of this chapter (Chapter 21) shall be permitted if it is demonstrated by experimental evidence and analysis that the proposed system has strength and toughness equal to or exceeding those provided by a comparable mono- lithic reinforced concrete structure that satisfies the require- ments of this chapter.” This Standard defines the minimum experimental evidence that shall be provided in order to vali- date the use, in regions of high seismic risk or for structures assigned to satisfy high seismic performance or design categories, of a weak beam/strong column moment frame not satisfying the requirements of Chapter 21 of ACI 318-99. Consistent with the ACI 318-99 requirement for analysis, this Standard specifies that, before the testing mandated by the Standard, a design procedure shall have been developed for prototype frames having the generic form for which accep- tance is sought and that design procedure shall be used to proportion the test modules. Further, the Standard assumes that the prototype frames have forms that are essentially regular, having no significant physical discontinuities in plan or in vertical configuration or in their lateral-force-resisting systems, and that the frames satisfy some, but not all, of the requirements of Chapter 21. Such frames might, for example, involve use of precast elements, precast prestressed elements, post-tensioned reinforcement, or combinations of those elements and reinforcement. Prescrip- tive requirements for moment frames constructed with such elements are not included in ACI 318-99. Such frames might also, for example, use alternate methods, other than those specified in Chapter 21, for force transfer through beam- column joints. The provisions of this Standard are intended to supplement the provisions of Chapter 21 of ACI 318-99 and not to supplant them. 1.0Notation Only symbols additional to those in ACI 318-99 are defined. Emax=maximum lateral resistance of test module deter- mined from test results (forces or moments) En=nominal lateral resistance of test module determined using specified geometric properties of test mem- bers, specified yield strength of reinforcement, spec- ified compressive strength of concrete, a strain compatibility analysis for flexural moment strength, and a strength reduction factor of 1.0 Epr=probable lateral resistance of test module determined using actual geometric and material properties of test members, an analysis for probable flexural moment strength of beams based on strain compatibility and including strain-hardening effects in the reinforce- ment, and a strength reduction factor of 1.0 =column overstrength factor used for test module =drift ratio =relative energy dissipation ratio R1.0Notation Only symbols used in this Commentary that are additional to those in Appendix E of ACI 318-99 and Standard T1.1-01 are defined in the following: Ah= area of hysteresis loop E1, E2= peak lateral resistance for positive, negative, loading for third cycle of loading sequence f1 = factor on live load defined in R2.6 h= height of column of test module, in. or mm K,K = initial stiffness for positive, negative, loading for first cycle 1,2= drift ratios at peak lateral resistance for positive, negative, loading for third cycle of loading se- quence 1 ,2 = drift ratios for zero lateral load for unloading at stiffnesses K,K from peak positive, negative, ACCEPTANCE CRITERIA FOR MOMENT FRAMES BASED ON STRUCTURAL TESTING374.1-3 lateral resistance for third cycle of loading sequence (Fig. R2.4) = lateral displacement, in. or mm. See Fig. R2.1 a= allowable story drift, in. or mm. See Table 1617.3 of IBC 2000 2.0Definitions 2.1 Drift ratioAngular rotation under load of the column chord of the test module with respect to the beam chord, where the chords are the straight lines connecting the cent- roidal axes of the points of contraflexure in the beam and the column, respectively, or the centroidal axis at the point of contraflexure to the centroid of the beam-column joint in the case where a member extends on one side of the joint only. R2.1 Where a column exists on both sides of the joint, its chord is defined by the line joining the loading (or support) points. The same is true for a beam that exists on both sides of the joint. If a column or beam exist on one side of the joint only, then the chord is defined by the line joining the end loading (or support) point and the joint centerline. The drift ratio concept is illustrated in Fig. R2.1 for an exterior column-beam module. The position of the module at the start of testing, with its self-weight only acting, is indi- cated by broken lines. The module is pin supported at A and roller supported at D. The self weight is taken by vertical reactions VAD and VDD. That weight, however, also causes a twisting about the centroid B of the joint so that opposing horizontal reactions, HAD and HCD, develop. Under self weight alone, the pin at C must be constrained to lie on the centroidal axis of the column that passes from C through B to A. That chord is the vertical reference line for drift measurements. The setup also constrains the chord joining the centroid of the joint B and the centroid of the section at D to be horizontal. For acceptance testing, a lateral force HCE is applied to the column through the pin at C and results in the specimen taking up the deformed shape indicated by solid lines. The lateral force causes reactions HAL at A and VDL at D. The column at C displaces laterally by an amount . The chord defining the reference axis for the beam, however, remains horizontal. The drift ratio is the angular rotation of the column chord with respect to the beam chord and for the setup shown equals /h where h is the column height and equal to the distance between the pin at A and that at C. 2.2 Moment frameSpace frame in which members and joints resist forces through flexure, shear and axial force. 2.3 Overstrength factorRatio of the sum of the nominal flexural strengths of the columns at their interfaces with the joint to the sum of the nominal flexural moment strengths of the beams at their interfaces with the same joint. R2.3 The column overstrength factor should be selected so that En is greater than the probable lateral resistance Epr. It is to be expected that the maximum lateral resistance of the test module Emax should be similar to Epr. In 21.4.2.2 of ACI 318-99 the ratio of the sum of the moments at the faces of the joint, corresponding to the nominal flexural strengths of the columns framing into that joint, to the sum of the moments at the faces of the joint, corresponding to the nominal flexural strengths of the beams framing into that same joint, must exceed 1.2. Further, in T-beam construction, where the slab is under tension under moments at the face of the joint, slab reinforcement within an effective width defined in 8.10 of ACI 318-99 must be assumed to contribute to the flexural strength if the slab reinforcement is developed at the critical section for flexure. Hence the specified here is a comparable quantity to, but is not the same quantity as, the 1.2 value specified in ACI 318-99. For application of this Standard, the column overstrength factor is to be specified in the design procedure and, when the contribution of the rein- forcement in the slab is considered, the requirement of 21.4.2.2 must be met. There is, however, no requirement to provide a slab on the test module. Moment strengths should take into account simultaneous application of axial force and direction of loading. The axial forces on the beam and the column should be those causing the largest and the smallest moment strength possible, respectively. Most beams, however, will have zero axial force and for most columns the smallest strength will also be for zero axial force. Directional effects require that the sign of any column axial force and beam bending moments be consistent. For example, for an end joint, column tension effects need only be considered in combination with beam positive moment strength. Where prestressing steel is used in frame members the stress fps in the reinforcement at nominal and probable lateral resistance shall be calculated in accordance with 18.7 of ACI 318-99. 2.4 Relative energy dissipation ratioRatio of actual to ideal energy dissipated by test module during reversed cyclic response between given drift ratio limits, expressed as the ratio of the area of the hysteresis loop for that cycle to the area of the circumscribing parallelograms defined by the initial stiffness during the first cycle and the peak resistance during the cycle for which the relative energy dissipation ratio is calculated. See 9.1.3. R2.4 The relative energy dissipation ratio concept is illus- trated in Fig. R2.4 for the third cycle to the drift ratio of 0.035. For Fig. R2.4, it is assumed that the test module has Fig. R2.1Deformations of exterior column-beam test module. 374.1-4ACI STANDARD exhibited different initial stiffnesses, K and K, for positive and negative lateral forces and that the peak lateral resis- tances for the third cycle for the positive and negative loading directions, E1 and E2, also differ. The area of the hysteresis loop for the third cycle, Ah, is hatched. The circumscribing figure consists of two parallelograms, ABCD and DFGA. The slopes of the lines AB and DC are the same as the initial stiffness K for positive loading, and the slopes of the lines DF and GA are the same as the initial stiffness K for negative loading. The relative energy dissi- pation ratio concept is similar to the equivalent viscous damping concept used in 13.3.3.1 and 13.9.5.2 of the 1997 NEHRP Provisions and Commentary1 for design and evalu- ation of seismically isolated structures. For a given cycle, the relative energy dissipation ratio is the area Ah inside the lateral force-drift ratio loop for the module divided by the area of the effective circumscribing parallelograms A