www.sefindia.org

[Dr. N. Subramanian]

To increase ease of use ACI 318-14, is being reorganized !This is the first major
reorganization of ACI 318 since 1971. See:
http://www.concrete.org/COMMITTEES/Documents/ACI_Flyer_5-March-18-2013.pdf
http://www.concrete.org/COMMITTEES/318public.htm
Reorganized Chapters of ACI 318-14:

CHAPTER 1—GENERAL
CHAPTER 2—NOTATION AND TERMINOLOGY
CHAPTER 3—REFERENCED STANDARDS
CHAPTER 4—PERFORMANCE REQUIREMENTS FOR STRUCTURAL MEMBERS AND STRUCTURAL SYSTEMS
CHAPTER 5—CONCRETE: DESIGN AND DURABILITY REQUIREMENTS
CHAPTER 6—STEEL REINFORCEMENT MATERIAL AND DURABILITY
CHAPTER 7—LOADS
CHAPTER 8—STRUCTURAL ANALYSIS
CHAPTER 9—SECTIONAL STRENGTH
CHAPTER 10—SERVICEABILITY
CHAPTER 11—ONE-WAY SLABS(draft chapter enclosed)
CHAPTER 12—TWO-WAY SLABS
CHAPTER 13—BEAMS
CHAPTER 14—COLUMNS
CHAPTER 15—WALLS
CHAPTER 16—FOUNDATIONS
CHAPTER 17—JOINTS AND CONNECTIONS
CHAPTER 18—DISCONTINUITY REGIONS
CHAPTER 19—ANCHORING TO CONCRETE
CHAPTER 20—EARTHQUAKE-RESISTANT STRUCTURES
CHAPTER 21—REINFORCEMENT DETAILS
CHAPTER 22—REQUIREMENTS FOR CONCRETE AND GROUT
CHAPTER 23—FORMs, SHORES, AND RESHORES
CHAPTER 24—STRENGTH EVALUATION OF EXISTING STRUCTURES
CHAPTER 25—PLAIN CONCRETE
318.1—CODE REQUIREMENTS FOR THIN SHELLS AND COMMENTARY

Also see the enclosed documents. Best wishes,

Subramanian


Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Maryland, USA

Posted via Email

[Dr. N. Subramanian]

New Confinement Requirements for Concrete Special Moment Frame Columns in ACI 318-14


The dependence of the amount of required confinement on the magnitude of axial load imposed on a column has long been recognized by the Canadian and New Zealand codes, but was not reflected in ACI 318 through its 2011 edition. Also, longitudinal reinforcement that is well distributed and laterally supported around the perimeter of a column core provides more effective confinement than a cage with larger, widely-spaced longitudinal bars. Confinement effectiveness is a key parameter determining the behavior of confined concrete. ACI 318, through its 2011 edition, did not explicitly account for confinement effectiveness in determining the required amount of confinement.


In view of the above, confinement requirements for columns of concrete special moment frames (Section 18.7.5) with high axial load (P_u > 0.3A_gf’_c) or high concrete compressive strength (f’_c > 10,000 psi) are significantly different in ACI 318-14 (Figure 1).

Figure 1. Changes in ACI 318-14 for Confinement of Columns in Special Moment Frames

One important new requirement is found in Section 18.7.5.2 Item (f). In columns with P_u > 0.3A_gf’_c or f’_c > 10,000 psi, and with rectilinear hoops, every longitudinal bar or bundle of bars around the perimeter of the column core is required to have lateral support provided by the corner of a hoop or by a seismic hook, and the value of h_x cannot not exceed 8 in. (Figure 1), where P_u is the largest value in compression consistent with factored load combinations including E.


In contrast, ACI 318-11 required only every other longitudinal bar to be laterally supported and a maximum permissible h_x to be 14 in. for all columns of special moment frames. Also, note a subtle difference in the way h_x is defined in the two editions of ACI 318. In the 2011 edition, h_x was the maximum center-to-center spacing of consecutive ties providing lateral support to the longitudinal bars. In the 2014 edition, however, it is the maximum center-to-center spacing of the laterally supported bars.

The other new requirement is in Section 18.7.5.4:
The amount of transverse reinforcement needs to in accordance with Table 18.7.5.4 (reproduced here as Table 1-Also see the attached file).

The concrete strength factor, k_f, and confinement effectiveness factor, k_n, are calculated by ACI 318-14 Equations 18.7.5.4a and b, respectively.
k_f = f’_c / 25,000 + 0.6 ≥ 1.0          (18.7.5.4a)
k_n = n_l / (n_l - 2)                             (18.7.5.4b)
where n_l is the number of longitudinal bars or bar bundles around the perimeter of a column core with rectilinear hoops that are laterally supported by the corner of hoops or by seismic hooks.

Equation (c) in ACI 318-14 Table 18.7.5.4 is new, so is the distinction between columns with P_u ≤ 0.3A_gf’_c andf’_c ≤ 10,000 psi and those with P_u > 0.3A_gf’_c or f’_c > 10,000 psi.

Table 1 (ACI 318-14 Table 18.7.5.4). Confinement of high-strength or highly-axially-loaded rectangular column of special moment frame

The changes discussed here will come into play for  concrete special moment frames featuring columns having high axial load (P_u > 0.3A_gf’_c) or high concrete compressive strength (f’_c > 10,000 psi).

Source:
http://skghoshassociates.com/SKGAblog/viewpost.php?id=67