ACI 374.1

Description / Abstract: While only Committee 318 can determine the requirements necessary for frames to meet the provisions of 21.1.1.8 of ACI 318-11, Section 1.4 of ACI 318-11 already permits the building official to accept framing systems other than those explicitly covered by Chapter 21, provided specific tests, load factors, deflection limits, and other pertinent requirements have been established for acceptance of those systems consistent with the intent of the Code. The intent of this document is to provide a framework that establishes the specific tests, etc., appropriate for acceptance, for regions of high seismic risk or for structures assigned to satisfy high seismic performance or design categories of weak beamstrong column moment frames not satisfying all the requirements of Chapter 21. For regions of moderate seismic risk or for structures assigned to satisfy intermediate seismic performance or design categories, less stringent provisions than those specified here are appropriate.

This document assumes that the structural frame to be tested has details differing from those of 21.1 and 21.5 through 21.7 of ACI 318-11 for conventional monolithic reinforced concrete construction. Such frames might, for example, involve the use of precast elements, precast prestressed elements, post-tensioned reinforcement, or combinations of those elements and reinforcement. Alternate methods for force transfer within beam-column joints might also be approved for monolithic or precast moment frame systems based on experimental evidence and analysis using the procedures described in this document.

The fundamental requirement of ACI Code 318-11 for the weak beam/strong column action for moment frames in regions of high seismic risk is retained. The reason is because tests on subassemblages, as envisioned in this document, cannot be extrapolated with confidence to the performance of multistory frames if column sway mechanisms develop in the subassemblage test.

This document defines minimum acceptance criteria for new reinforced concrete moment frames designed for regions of high seismic risk or for structures assigned to satisfy high seismic performance or design categories, where acceptance is based on experimental evidence and mathematical analysis.

This document is not intended for use with existing construction or for use with frames that are designed to conform with all requirements of Chapter 21 of ACI 318-11. These criteria are more stringent than those for frames designed to ACI 318-11, and some frames designed to ACI 318-11 [ACI 318-99] do not meet the 0.035 drift ratio limit (Cheok et al. 1998).

Reinforced concrete moment frames, designed on the basis of a weak beam/strong column concept, shall be deemed to have a response that is, as a minimum, at least equivalent to the response of monolithic frames designed in accordance with 21.1 and 21.5 through 21.7 of ACI 318-11, when both of the following conditions are satisfied:

Tests on frame modules, in accordance with this document, establish the dependable and predictable strength, drift-ratio capacity, relative energy dissipation, and stiffnesses required by the acceptance criteria of 9.0.

For acceptance, the results of the tests on each module to be used in the frame must satisfy the criteria of 9.0. In particular, the relative energy dissipation ratio calculated from the measured results for the third cycle between limiting drift ratios of 0.035 must equal or exceed 1/8. Typical relative energy dissipation ratios at 0.030 drift ratios have been reported to be 30, 17, and 10% for reinforced concrete (Cheok et al. 1996), hybrid reinforced/prestressed concrete (Cheok et al. 1996), and prestressed concrete modules (Stanton and Mole 1994; Priestley and Tao 1993), respectively. In a building frame, as compared to a test module, damping is generally also provided by column hinging at the base of the frame. Further, that hinging is likely to be in a region of monolithic construction or one for which the relative energy dissipation characteristics differ from those of the test module. Hence, the relative energy dissipation ratios for frames with hybrid or prestressed concrete beam sections will probably be greater than the values established from module tests.

The frame as a whole, based on the results of the tests of 3.2.1 and analysis, shall be demonstrated as able to retain its structural integrity and support its specified gravity loads through peak displacements equal to or exceeding storydrift ratios of 0.035.

The criteria of 9.0 are for the test module. In contrast, the toughness criterion of 3.2.2 is for the frame as a whole and can be satisfied only by the philosophy used for the design and analysis of the frame as a whole. The criterion adopted here is similar to that described in R21.1.1 of ACI 318-11 and the intent is that test results and analyses demonstrate that the structure is still capable of supporting the specified gravity load after cycling through drift ratios of +0.035 to –0.035.