Chapter 4 ‐ CONCRETE
PART 2:
CHAPTER 4, STRUCTURAL CONCRETE
Section 409 — Beams
The use of open web reinforcement for torsion and shear in slender spandrel beams by the precast concrete industry as an alternative to the closed stirrups traditionally mandated by this Code. Eliminating closed stirrups is desirable because they
cause reinforcement congestion; production costs also increase significantly because pre‐tensioning strand must be threaded through the closed stirrups.
Chapter 4 ‐ CONCRETE
Section 409 — Beams
An extensive PCI‐sponsored experimental and analytical research program was conducted at North Carolina State University (NCSU). The objective was to develop a rational
design procedure for slender precast concrete spandrel beams.
Specifically, the research was aimed at simplifying the detailing requirements for the end regions of such beams. The end
regions are often congested with heavy reinforcement cages when designed using current procedures.
In addition to the experimental program, finite element models were developed (Fig. 2) and calibrated to experimental data.
These models were used in conjunction with conventional
analysis to corroborate the experimental results and to further investigate the behavior of slender precast concrete spandrel beams.
Chapter 4 ‐ CONCRETE
Section 409 — Beams
Figure 2. Finite element model of a precast concrete spandrel beam. Image from Lucier et al., “Development of a Rational Design Methodology for Precast Concrete Slender
Spandrel Beams: Part 2, Analysis and Design Guidelines” (2011).
Chapter 4 ‐ CONCRETE
Section 409 — Beams
A new relevant Sub‐section 409.5.4.7 for solid precast sections is added to the NSCP 2015.
Section 412 — Diaphragms
NSCP 2015 Sub‐section 418.12 contained design and detailing requirements, for diaphragms in structures assigned in areas of high seismicity (Zone 4). For the first time, a new Section 412, added design provisions for diaphragms in buildings assigned in areas of low seismicity (Zone 2) The new Section applies “to the design of non‐prestressed and prestressed diaphragms,
including (a) through (d):
(a). Diaphragms that are cast‐in‐place slabs
(b). Diaphragms that comprise a cast‐in‐place topping slab on precast elements.
Chapter 4 – CONCRETE
Section 412 — Diaphragms
Figure 3
(c). Diaphragms that comprise precast elements with end strips formed by either a cast‐in‐place concrete topping slab or edge beams
(d). Diaphragms of interconnected precast elements without cast‐in‐place concrete topping. (Fig. 3)
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
There are a number of significant and substantive changes to this Section.
Column confinement ‐ The ability of the concrete core of a concrete reinforced column to sustain compressive strains tends to increase with confinement pressure. Compressive strains caused by axial load. It follows that confinement
reinforcement should be increased with axial load to ensure
consistent lateral deformation capacity. The dependence of the amount of required confinement on the magnitude of axial load imposed on a column has been recognized by some codes from other countries (such as CSA A23.3‐1419 and NZS 3101‐
0620,21) but was not reflected in ACI 318 through its 2011 edition.
The ability of confining steel to maintain core concrete integrity and increase deformation capacity is also related to the layout
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
of the transverse and longitudinal reinforcement. 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 (Mander, et al) and has been incorporated into the CSA A23.3‐14 equation for column
confinement. ACI 318, through its 2011 edition, did not
explicitly account for confinement effectiveness in determining the required amount of confinement. It instead assumed
constant confinement effectiveness independent of how the reinforcement is distributed.
In view of this, confinement requirements for columns of
special moment frames (Sub‐section 418.7.5) Fig. 4) with high
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
axial load (P u > 0.3 A f c ', where P u is the factored axial force, Ag is the gross area of the concrete section; and f c ‘ is the
special compressive strength of concrete) or high concrete compressive strength ( f c ' > 10,000 psi [6895 MPa]) are
significantly different in ACI 318M‐14.
One important new requirement for special moment frame columns is as follows:
418.7.5.2 — Transverse reinforcement shall be in accordance with (a) through (f):
(f) Where P u > 0.3 A f c ‘ or f c ' > 6895 MPa in columns with rectilinear hoops, every longitudinal bar or bundle of bars around the perimeter of the column core shall have lateral
support provided by the corner of a hoop or by a seismic hook, and the value of h x shall not exceed 200 mm. (Fig. 5). P u shall
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
Figure 4. Confinement of rectangular column of special moment frame. Note:
h1 = plan dimension of column in one of two orthogonal directions; h2 = plan dimension of column in other orthogonal direction; ℓo = length, measured from joint face along axis of member, over which special transverse reinforcement must be provided; s = center‐to‐
center spacing of items, such as longitudinal reinforcement, transverse reinforcement, tendons, or anchors. 1 in. = 25.4 mm
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
Figure 5. Confinement of high‐strength or highly axially loaded rectangular
column of special moment frame. Note: d b = nominal diameter of bar, wire, or prestressing strand; h x = maximum value of x i on all column faces greater than 200 mm.; x i = dimension from centerline to centerline of laterally supported longitudinal bars. 1 in. = 25.4 mm.
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
be the largest value in compression consistent with factored load combinations including E .
where:
h x = maximum center‐to‐center spacing of longitudinal bars laterally supported by corners of crossties or hoop legs around the perimeter of the column.
The change from prior practice is that instead of every other
longitudinal bar having to be supported by a corner of a tie or a crosstie, every longitudinal bar will have to be supported when either the axial load on a column is high or the compressive
strength of the column concrete is high.
The other new requirement for special moment frame columns is in the following section:
418.7.5.4 — Amount of transverse reinforcement shall be in accordance with Table 418.7.5.4 .
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures Table 418.7.5.4
Transverse Reinforcement
Conditions Applicable Expressions
for spiral or circular hoop
0.30g or Greatest of
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
Table 418.7.5.4 Note:
Ach = cross‐sectional area of a member measured to the outside edges of transverse reinforcement; Ag = gross area of concrete
section; for a hollow section, Ag is the area of the concrete only and does not include the area of the void(s); Asb = area of longitudinal reinforcement in shear wall boundary element; bc = cross‐sectional dimension of member core measured to the outside edges of the transverse reinforcement composing area Ash ; f c ' = specified
compressive strength of concrete; f yt = specified yield strength of transverse reinforcement; k f = concrete strength factor;
k n = confinement effectiveness factor; P u = factored axial force, to be taken as positive for compression and negative for tension;
s = center‐to‐center spacing of items, such as longitudinal
reinforcement, transverse reinforcement, tendons, or anchors.
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
Confinement requirements for columns of special moment
frames, and for columns not designated as part of the seismic‐
force‐resisting system in structures assigned to seismic zone 4 (similar to ASCE 7‐10 Seismic Design Categories D, E, and F), with high axial load or high concrete compressive strength are significantly different.
Transverse reinforcement ‐ One important new requirement for special moment frame columns are included in Sub‐sections
418.7.5.2 and 418.7.5.4. There are new restrictions on the use of headed reinforcement to make up hoops.
Special moment frame beam‐column joints – For beam‐column joints of special moment frames, clarification of the
development length of the beam longitudinal reinforcement
that is hooked, requirements for joints with headed longitudinal
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
reinforcement, and restrictions on joint aspect ratio are new.
For beam‐column joints of special moment frames, clarification of development length of beam longitudinal reinforcement that is hooked, requirements for joints with headed longitudinal
reinforcement, and restrictions on joint aspect ratio are new.
Special shear walls – Subsection 418.10 (equivalent to ACI 318‐
14M‐14 Section 18.10, previously ACI 318M‐11 Section 21.9), has been extensively revised in view of the performance of
buildings in the Chile earthquake of 2010 and the Christchurch, New Zealand, earthquakes of 2011, as wells as full‐scale
reinforced concrete building tests. In these earthquakes and laboratory tests, concrete spalling and vertical reinforcement buckling were at times observed at wall boundaries.
For ASTM A615 Grade 420 bars used as longitudinal
Chapter 4 ‐ CONCRETE
Section 418 — Earthquake‐Resistant Structures
reinforcement in special moment frames and special shear walls, the NSCP 7th Edition now requires the same minimum elongation as ASTM A706 reinforcement.
Section 419: Concrete: Design and Durability Requirements Quite a few changes have been made in concrete durability requirements, which are now located in this Section.
In previous editions (ACI 318‐11), section 5.1.5, says, “Splitting tensile strength tests shall not be used as a basis for field
acceptance of concrete,” and commentary section R5.1.5 have been deleted because in the latest edition (ACI 318M‐14 section 19.2.1.2) clearly says, “The specified compressive strength shall be used for mixture proportioning in 26.4.3 (NSCP 426.4.3) and for testing and acceptance of concrete in 26.12.3 (NSCP
426.12.3).”
Chapter 4 ‐ CONCRETE
Section 420: Steel Reinforcement Properties, Durability and Embedments
The definition of yield strength of high‐strength reinforcement for Grade 420 (Grade 60) in this Section is now, for the first
time, the same as that in ASTM specifications, except for bars with less than 420 MPa, the yield strength shall be taken as the stress corresponding to a strain of 0.35 percent.
Deformed and plain stainless steel wire and welded wire conforming to ASTM A1022 is now permitted to be used as concrete reinforcement.
Sub‐section 420.2.2.5 requires “Deformed non‐prestressed
longitudinal reinforcement resisting earthquake moment, axial force, or both, in special moment frames, special structural
walls, and all the components of special structural walls including coupling beams and wall piers” to be ASTM A706
Chapter 4 ‐ CONCRETE
Section 420: Steel Reinforcement Properties, Durability and Embedments
Grade 420 (Grade 60), ASTM 615 Grade 275 (Grade 40) or Grade 420 (Grade 60) reinforcement is permitted if two
supplementary requirements are met, which are already part of the ASTM A706 specification. A third supplementary
requirement is now added for ASTM A615 (Grade 60)
reinforcement to be permitted for use in special moment
frames, special structural walls. The minimum elongation in 200 mm (8”) must now be the same as that ASTM A615 (Grade 60) reinforcement.
One aspect of the Code compliance that the Association of
Structural Engineers of the Philippines is cautioning Designers and Constructors alike, is the introduction of ASTM 615 Grade 520 (Grade 75) in the Philippine market. Since this was not
Chapter 4 ‐ CONCRETE
Section 420: Steel Reinforcement Properties, Durability and Embedments
covered by previous editions of the NSCP Vol. 1, it creates an impression of an unregulated use of a new high‐strength
reinforcement grade.
To put it clearly, Sub‐section 420.2.2.5, corresponding to ACI 318M‐14 Section 20.2.2.5, specifies the use of deformed non‐
prestressed longitudinal reinforcement resisting earthquake‐
induced moment, axial force, or both, in special moment
frames, special structural walls, and all components of special structural walls, including coupling beams, and wall piers which shall be in accordance with (a) or (b):
(a). ASTM A706M, Grade 420 (b). ASTM A615M, Grade 280
There was no mention that ASTM A615M, Grade 520, was
Chapter 4 ‐ CONCRETE
Section 420: Steel Reinforcement Properties, Durability and Embedments
allowed, although the use of micro‐alloyed high‐strength reinforcement may be allowed in the future through the
issuance of a new ASTM or updated standard, and with proper validation by the Department of Trade and Industry’s Bureau of Standards. It will be premature to allow its use for special
moment frames, special structural, and all components of special structural walls, including coupling beams, and wall piers for Buildings located in areas of high seismicity (zone 4).
The same restrictions indicated in Sub‐section 420.7.6, on the use of quenched‐tempered thermo‐mechanically treated
(QT/TMT) reinforcing bars in structures located in seismic zone 4 for Grade 420 reinforcement, shall also be applied to Grade 520, unless proven in subsequent studies and tests.
Chapter 4 ‐ CONCRETE
Section 422: Sectional Strength
The following are the changes in Section 422:
For prestressed members, a new equation for the nominal axial strength at zero eccentricity has been introduced in Sub‐section 422.4.2.3.
New Sub‐section 422.4.3.1, which requires that the nominal axial tensile strength of a non‐prestressed, composite, or
prestressed member, not to be taken greater than the maximum nominal axial tensile strength of member.
Chapter 4 ‐ CONCRETE
Section 425: Reinforcement Details
Two changes shown in Table 7 (part of Table 425. 3.2) are made to eliminate the differences between the required tail extension of a 90‐degree or 135‐ degree standard hook, subject to a
minimum of 75 mm (3”).
Mechanical or welded splices with strengths below 125% of the yield strength of the spliced reinforcing bars are no longer
permitted. The associated stagger requirements have been deleted. Thus there is no longer a need to specify “full”
mechanical or “full” welded splices.
Chapter 4 ‐ CONCRETE
Section 426: Construction Documents and Inspection
In this section, the user will probably require some time to get used to, it starts with the following:
426.1.1 This Sub‐section addresses (a) through (c):
(a) Design information that the licensed design professional shall specify in the construction documents,
(b) Compliance requirements that the licensed design professional shall specify in the construction documents,
(c) Inspection requirements that the licensed design
professional shall specify in the construction documents, Thus, construction and inspection requirements have been consolidated, and they are now related to construction
documents. The construction requirements are designated either as “design information” or “compliance requirements.”
These are largely existing material that has been rearranged.
Chapter 4 ‐ CONCRETE
The inspection requirements in Sub‐section 426.13 are taken from Chapter 17 of the 2015 International Building Code (IBC) and were previously not part of ACI 318.
Provisions in ACI 318‐11 and earlier editions, which explained basic statistical considerations in mixture proportioning, are no longer found in ACI 318‐14. Instead, ACI 301‐10, Specifications for Structural Concrete, is referenced.
Chapter 4 ‐ CONCRETE
These are some other changes in the makeup of NSCP 2015 7th Edition that should be noted:
1. There are two new Sections: Section 404, Structural System Requirements and Section 412, Diaphragms.
2. Section 422, Structural Plain Concrete, now Section 414.
3. Section 423, Anchoring to Concrete, is now Section 417, with no significant changes. 4. Section 421, Earthquake‐Resistant
Structures, now Section 418.
5. Section 427, Strut‐and‐Tie Models is now Section 423, with no significant changes.
6. Section 420, Strength Evaluation of Existing Structures, is now Section 427.
7. Section 419, Shells and Folded Plates, is now Section 428. 8.
8. Section 424, Alternative Design Method, now Section 429, is adapted from earlier editions of the NSCP.
Chapter 4 ‐ CONCRETE
9. Section 425, Alternative Provisions for Reinforced and
Prestressed Concrete Flexural and Compression Members, and Section 426, Alternative Load and Strength Reduction Factors, have been discontinued.
10. On the other hand, Section 416, Precast Concrete, and
Section 418, Prestressed Concrete, no longer exist as separate entities. The provisions of these Sections are now spread over several of the new Sections.
Sub‐section 418.18, Requirements for post‐tensioning ducts and grouting have also been removed as being outdated. The Commentary now provides specification guidance.