Seismic Detailing of Special and Intermediate Moment ...

Seismic Detailing of Special and Intermediate Moment

Frames of Concrete Date: 15 Jun 2021

URP S-09 Training Module S11

Dr. S. K. GhoshPresident, S. K. Ghosh Associates LLC

Special Moment Frame Reinforcement

S. K. Ghosh Associates LLC International Code Council (ICC)

www.skghoshassociates.com 1

Seminar Outline

a. Introductionb. Material Qualificationsc. Beams

i. SMF (SDC D)ii. IMF (SDC C)

d. Columnsi. SMFii. IMF

e. Jointsi. SMFii. IMF

Detailing by BNBC-2020



Basis of Concrete Design Provisions of BNBC-2020

Updates of ACI 318-08



Basis

ACI 318-08 Chapter 21; rest applies except where modified

BNBC-2020 Part 6: STRUCTURAL DESIGN

Chapter 8: DETAILING OF REINFORCEMENT IN CONCRETE STRUCTURES

Section 8.3: EARTHQUAKE-RESISTANT DESIGN PROVISIONS

Chapter 6: STRENGTH DESIGN OF REINFORCED CONCRETE STRUCTURES applies except where modified

Section Numbers

All section numbers that are not in blue are from ACI 318-08

All section numbers in blue are from BNBC-2020 Part 6, Chapter 8 or Chapter 6, unless otherwise noted.



Exclusion

Special moment frames of precast concrete (Section 21.8) are outside the scope of this presentation. Intermediate moment frames of precast concrete are not recognized. There is no distinction between an ordinary cast-in-place and an ordinary precast concrete moment frame in ACI 318.Moment frames of precast concrete are not mentioned in BNBC-2020 Part 6, Chapter 8.

Idealized Force-Displacement

Lateral Displacement

Fe

Fu

e u

U

Elastic

Elastic Inelastic



General

ACI R21.1.1 The provisions of Chapter 21 relate detailing requirements to type of structural framing and seismic design category (SDC). SDCs are adopted directly from ASCE/SEI 7, and relate to considerations of seismic hazard level, soil type, occupancy, and use.

Scope of 8.3 Earthquake-Resistant Design Provisions of BNBC-2020

8.3.1 Scope

This section contains special requirements for design and construction of reinforced concrete members of a structure for which the design forces, related to earthquake motions, have been determined on the basis of energy dissipation in the nonlinear range of response.



EARTHQUAKE FORCE-RESISTING STRUCTURAL SYSTEMS OF CONCRETE —

ASCE 7-05 (BNBC-2020 Table 6.2.19)

BASIC SEISMIC FORCERESISTING SYSTEM

DETAILING REF.SECTION R 0 Cd

SYSTEM LIMITATIONS AND BUILDINGHEIGHT LIMITATIONS (m) BY SEISMIC

DESIGN CATEGORY

B C D E F

Moment Resisting Frame SystemsSpecial reinforced concrete momentframes

12.2.5.5 and14.2 8 3 51/2 NL NL NL NL NL

Intermediate reinforced concrete momentframes 14.2 5 3 41/2 NL NL NP NP NP

Ordinary reinforced concrete momentframes 14.2 3 3 21/2 NL NP NP NP NP

Dual Systems with Special Moment Frames

Special reinforced concrete shear walls 14.2 7 21/2 51/2 NL NL NL NL NL

Ordinary reinforced concrete shear walls 14.2 6 21/2 5 NL NL NP NP NP

Dual Systems with Intermediate Moment Frames

Special reinforced concrete shear walls 14.2 61/2 21/2 5 NL NL 50 30 30

Ordinary reinforced concrete shear walls 14.2 51/2 21/2 41/2 NL NL NP NP NP

21.1.4, 21.1.5 – Materials

Concrete 8.3.3.3• Compressive strength not less than 21 MPa• Lightweight – Not greater than 35 MPa

Reinforcement 8.3.3.4(b)• Low-alloy – A706 Grade 420

Alternatively,• Billet-steel – A615 (modified) Grades 275 and 420• BDS ISO 6935-2 Grades 300, 350, 400 and 420 or ASTM

A615 Grades 275 and 420 with supplementary requirements



21.1.5 – ReinforcementThe actual yield strength based on mill tests does not exceed

by more than 125 N/mm2 (retests shall not exceed this value by more than an additional 20 N/mm2); 8.3.3.4(b)(i)

Minimum elongation in 200 mm shall be at least 14 percent for bar dia. 10 mm to 20 mm, at least 12 percent for bar dia. 22 mm through 36 mm, and at least 10 percent for bar dia. 40 mm to 60 mm. 8.3.3.4(b)(iii) Added in ACI 318-14

21.1.5 , 8.3.3.4(b)(ii) – Reinforcement

BILLET STEEL REINFORCEMENT



21.1.5 – Reinforcement

ACI 21.1.5.3 — Prestressing steel resisting earthquake-induced flexural and axial loads in frame members . . . shall comply with ASTM A416 [low-relaxation seven-wire strand] or A722 [high-strength bars].

Not in BNBC-2020 Section 8.3

21.1.5 – Reinforcement

ACI 21.1.5.4, 8.3.3.4(c) — The value of fyt used to compute the amount of confinement reinforcement shall not exceed 700 MPa .

ACI 21.1.5.5, 8.3.3.4(d) — The value of fy or fyt used in design of shear reinforcement shall conform to 11.4.2, 6.4.3.2.

ACI 11.4.2, 6.4.3.2 — The values of fy and fyt used in design of shear reinforcement shall not exceed 420 MPa, except the value shall not exceed 550 MPa for welded deformed wire reinforcement.



Columns of Special Moment Frames –Rectangular Hoop Reinforcement (21.6.4)

(8.3.5.4)

Ash0.3sbc [(Ag/Ach)-1] f’’c/fyt

0.09sbc f’c/fyt

150 mm.450 mm.

High-Strength vs. Conventional Transverse Reinforcement in Columns

Courtesy: Cary Kopczynski & Co.



Columns of Special Moment Frames–Rectangular Hoop Reinforcement

hx = max. value of xi on all column faces

xi xi xi

xi

xi

6db 75 mm 6db extension

Alternate90-deg hooks

Provide add.trans. reinf. if thickness > 100 mm

Welding

21.1.7.2 — Welding of stirrups, ties, inserts, or other similar elements to longitudinal reinforcement that is required by design shall not be permitted.

8.3.3.5 Welding

Reinforcement required by factored load combinations which include earthquake effect shall not be welded except as specified in Sections 8.3.4.2(db) and 8.3.5.3(b) [splices of longitudinal reinforcement]. In addition, welding shall not be permitted on stirrups, ties, inserts, or other similar elements to longitudinal reinforcement required by design.



FLEXURAL MEMBERS

Beams of Special Moment Frames (21.5.1) (8.3.4.1)

Factored axial compressive force Agf c/10

Clear span 4 × effective depth

Width to depth ratio 0.3

Width 250 mm

width of supporting member, c2 + distances on each side of supporting member not exceeding the smaller of: (a) width of supporting member, c2, and (b) 0.75 the overall dimension of supporting member,c1



ACI 318-11 Figure R21.5.1, 8.3.4.1(d)

BNBC-2020 8.3.2(b)

… The provisions for special moment frames shall not permit the use of slab without beam as part of seismic force-resisting system.



Beams of Special Moment Frames – Longitudinal Reinforcement (21.5.2.1, 21.5.2.2) (8.3.4.2)

=0.25 / , 140/

21.5.3.2 (8.3.4.3)

h

HoopsStirrups withseismic hooks Hoops

Trans. reinf. based on Mprand factored tributary gravity load

s d/22h

s

d/48 smallest long. bar dia.24 hoop bar dia.300 mm

sd/4100 mm

mm

Beams of Special Moment Frames – Transverse Reinforcement (21.5.3.2), (8.3.4.3) ACI 318-08



Beams of Special Moment Frames – Transverse Reinforcement (21.5.3.2), (8.3.4.3) ACI 318-11

h

mm Hoops

Stirrups withseismic hooks Hoops

Trans. reinf. based on Mprand factored tributary gravity load

s d/22h

sd/46 smallest long. bar dia.150 mm

s d/4100 mm

21.5.3.2 (8.3.4.3)

Beams of Special Moment Frames –Transverse Reinforcement (21.5.3.2), (8.3.4.3)

21.5.3.2 (8.3.4.3)



318 08

318 11

Beams of Special Moment Frames –Transverse Reinforcement (21.5.3.2), (8.3.4.3)21.5.3.2 (8.3.4.3)

Beams of Special Moment Frames – Transverse Reinforcement (21.5.3.2), (8.3.4.3)

21.5.3.3 (8.3.4.3(c)) — Where hoops are required, primary flexural reinforcing bars closest to the tension and compression faces shall have lateral support conforming to 7.10.5.3 or 7.10.5.4 (8.1.9.4(c)). The spacing of transversely supported flexural reinforcing bars shall not exceed 14 in. Skin reinforcement required by 10.6.7 (9.7.2.3) need not be laterally supported.

Only the first sentence is explicit in BNBC-2020 8.3.4.3



Shear Design of Special MomentFrame Beams (21.5.4.1) (8.3.8.1(a))

Flexural Strength in ACI 318, BNBC-2020

Three different flexural strengths in ACI 318:Nominal flexural strength Mn (fs = fy)Design (underestimated)

flexural strength Mn

for flexural designProbable (reasonably overestimated)

flexural strength Mpr (fs = 1.25fy)for shear design



Hoop Reinforcement (21.5.3.6) (8.3.4.3)

( 75 mm.)

Beams of Intermediate Moment Frames –Longitudinal Reinforcement (21.3.4.1) (8.3.10.4)

(9.7.7)

140=0.25 140/ , 140/



Beams of Intermediate Moment Frames –Longitudinal Reinforcement (12.11.1, 12.11.2,

21.2.2) (8.2.8.1, 8.2.8.2, 8.1.12.2 (b), (c)

12.11.1, 12.11.2, 21.2.2, 8.2.8.1, 8.2.8.2, 8.1.2.2 (b), (c) — Beams shall have at least two continuous bars at both top and bottom faces. Continuous bottom bars shall have area not less than one-fourth the maximum area of bottom bars along the span. These bars shall be anchored to develop fy in tension at the face of support.

Beams of Intermediate Moment Frames – Transverse Reinforcement (21.3.4.2, 21.3.4.3) (8.3.4.3)

Stirrups

Trans. reinf. based on Mnand factored tributary gravity load

s d/22h

h

s

d/48 smallest long. bar dia.24 hoop bar dia.300 mm

Hoops 50 mm



Intermediate Moment Frames (21.3.3) (8.3.10.4, 8.3.10.5)

ACI 21.3.3 (8.3.10.4, beams; 8.3.10.5, columns)Vn of beams and columns resisting earthquake effect, E, shall not be less than the smaller of (a) and (b):

(a) The sum of the shear associated with development of nominal moment strengths of the member at each restrained end of the clear span and the shear calculated for factored gravity loads;

Shear Design of Intermediate MomentFrame Beams (21.3.3) (8.3.10.4, 8.3.10.5)




(b) The maximum shear obtained from design load combinations that include E, with E assumed to be a multiplier times that prescribed by the legally adopted general building code for earthquake-resistant design. The multiplier is 2 for beams and

o for columns.

Note that o = 3 for intermediate moment frames.


R21.3— Intermediate Moment FramesLoad Combination for Calculation of Vu

Beams:

U = 1.2D + 2.0E + 1.0L + 0.2S

Columns:

U = 1.2D + o E + 1.0L + 0.2S



Intermediate Moment Frames – Two-Way Slabs

ACI 21.3.6 (8.3.10.6) Two-way slabs without beamsACI 21.3.6.1 — Factored slab moment at support including earthquake effects, E, shall be determined for load combinations given in Eq. (9-5) and (9-7) (Chapter 2, Loads, Section 2.7.3). Reinforcement provided to resist Mslab shall be placed within the column strip defined in 13.2.1 (6.5.2.1).

U = 1.2D + 1.0E + 1.0L ……..ACI Eq. (9-5) /IBC Eq. (16-5)/2.7.3.1-5

U = 0.9D + 1.0E + 1.6H …............... ACI Eq. (9-7) /IBC Eq. (16-7)/2.7.3.1-7

Unbalanced momentMoment distribution: gravity load

Moment distribution: lateral load




ACI 21.3.6.2 (8.3.10.6) — Reinforcement placed within the effective width specified in 13.5.3.2 (6.5.5.3.2) shall be proportioned to resist fMslab(6.5.5.3.2). Effective slab width for exterior and corner connections shall not extend beyond the column face a distance greater than ct measured perpendicular to the slab span.

Moment Transfer by Flexure and Shear

f x Transfer Moment Transferred by Flexure

(1 - f) x Transfer Moment Transferred by Shear

f = 60% for square columns

Slightly different from 60% for rectangular columns




Fig. R21.3.6.1 (Fig. 6.8.23): Effective width for reinforcement placement in edge connection


Fig. R21.3.6.1 (Fig. 6.8.23): Effective width for reinforcement placement in corner connection



Intermediate Moment Frames –Two-way Slabs (21.3.6) (Fig. 6.8.20)

c2 c2 + 3h

½ Middle strip

½ Middle strip

Column stripAll reinforcement necessaryto resist Munbal to be placed In column strip

h = slab thickness

As

Reinforcement necessary to resist f Munbal

Reinforcement in column strip/2


Banded column strip reinforcement




Fig. R21.3.6.3 (Figs. 6.8.21 and 6.8.22): Arrangement of reinforcement in slabs

Intermediate Moment Frames –Two-Way Slabs

Slab-column frames are susceptible to punching-shear failures during earthquakes if the shear stresses due to gravity loads are high




ACI 21.3.6.8 [Cannot find in BNBC, should have been in 8.3.10.6] — At the critical sections for columns defined in 11.11.1.2 (6.4.10.1.2), two-way shear caused by factored gravity loads shall not exceed 0.4 Vc, where Vc shall be calculated as defined in 11.11.2.1 (6.4.10.2.1) for nonprestressedslabs and in 11.11.2.2 for prestressed slabs. It shall be permitted to waive this requirement if the slab design satisfies requirements of 21.13.6 (8.3.12.4).


ACI 11.11.1.2 (6.4.10.1.2) — For two-way action, each of the critical sections to be investigated shall be located so that its perimeter bo is a minimum but need not approach closer than d/2 to:

(a) Edges or corners of columns, concentrated loads, or reaction areas; and

(b) Changes in slab thickness such as edges of capitals, drop panels, or shear caps.

For two-way action, the slab or footing shall be designed in accordance with 11.11.2 through 11.11.6 (Equations 6.6.47 and 6.6.48).



Critical Sections for Two-Way Shear

Critical Shear-Transfer Sections for Flat Slabs(Source: Portland Cement Association, Notes on ACI 318-08 Building Code

Requirements for Structural Concrete, Skokie, IL, 2008)

Critical Section for Two-Way Shear

Parameters b1 and b2(Source: Portland Cement Association, Notes on ACI 318-08 Building Code

Requirements for Structural Concrete, Skokie, IL, 2008)




ACI 11.11.2.1 (6.4.10.2.1) — For nonprestressed slabs and footings, Vc shall be the smallest of (a), (b), and (c):

(a)

where is the ratio of long side to short side of the column, concentrated load or reaction area;

ACI Eq. (11-31)Eq. 6.6.72


(b)

where s is 40 for interior columns, 30 for edge columns, 20 for corner columns; and

(c)

ACI Eq. (11-32)Eq. 6.6.73

ACI Eq. (11-33)Eq. 6.6.74




ACI 21.13.6 (8.3.12.4)

For slab-column connections of two-way slabs without beams, slab shear reinforcement satisfying the requirements of 11.11.3 (6.4.10.3) and 11.11.5 (6.4.10.5) and providing Vs not less than 0.29 fc’bodshall extend at least four times the slab thickness from the face of the support, unless either (a) or (b) is satisfied:


ACI 11.11.3 (6.4.10.3)

Shear reinforcement consisting of bars or wires and single- or multiple-leg stirrups shall be permitted in slabs and footings with thickness greater than or equal to 150 mm., but not less than 16 times the shear reinforcement bar diameter. Shear reinforcement shall be in accordance with 11.11.3.1 (6.4.10.3.1)through 11.11.3.4 (6.4.10.3.4) .

ddbt

h 150 mm.

16dbt




ACI 11.11.5 (6.4.10.5)Headed shear stud reinforcement, placed perpendicular to the plane of a slab or footing, shall be permitted in slabs and footings in accordance with 11.11.5.1 through 11.11.5.4. The overall height of the shear stud assembly shall not be less than . . ..





ACI 21.13.6 (8.3.12.4)The requirements of 11.11.7 (Transfer of moment in slab-column connections) using the design shear Vug and the induced moment transferred between the slab and column under the design displacement.

The design story drift ratio does not exceed the larger of 0.005 and [0.035 – 0.05(Vug/ Vc)].


Design story drift ratio shall be taken as the larger of the design story drift ratios of the adjacent stories above and below the slab-column connection. Vc is defined in 11.11.2 [Vc shall be calculated in accordance with 6.4.10.2]. Vug is the factored shear force on the slab critical section for two-way action, calculated for the load combination 1.2D+1.0L+0.2S [due to gravity loads without moment transfer].




The load factor on the live load, L, shall be permitted to be reduced to 0.5 except for garages, areas occupied as places of public assembly, and all areas where L is greater than 5.0 kN/m2.


ACI R21.13.6 Provisions for shear reinforcement at slab-column connections were added in 2005 to reduce the likelihood of slab punching shear failure. . .. [“added in 2005” was dropped in ACI 318-14]




ACI R21.13.6 (Contd.)

Section 21.13.6(b) [no reference to this section in ACI 318-14, because Option (a) has been deleted] does not require the calculation of induced moments, and is based on research that identifies the likelihood of punching shear failure considering the story drift ratio and shear due to gravity loads. Figure R21.13.6 illustrates the requirement. The requirement can be satisfied by adding slab shear reinforcement, increasing slab thickness, changing the design to reduce the design story drift ratio, or a combination of these.


Fig. R21.13.6 (R18.14.5.1): Illustration of the criterion of 21.13.6 (b) (18.14.5.1)



MEMBERS SUBJECTED TO BENDING AND AXIAL LOADS

Columns of Special Moment Frames (21.6.1) (8.3.5)

Factored axial compressive force Agf c/10

Shortest cross-sectional dimension 300 mm.

Ratio of shortest cross-sectional dimension to perpendicular dimension 0.4



Special Moment Frame – Minimum Flexural Strength of Columns (21.6.2.2)

(8.3.5.2)

Special Moment Frame – Minimum Flexural Strength of Columns (21.6.2.2) (8.3.5.2)

Moments at the faces of the joint, corresponding to the nominal flexural strengths of the columns and the girders

For columns, flexural strength shall be calculated for the factored axial force, consistent with the direction of the lateral forces considered, resulting in the lowest flexural strength



P

M

Pu1

Mn1Mu

Pu2

Mn2

UseMn1 for 21.6.2.2

Special Moment Frame – Minimum Flexural Strength of Columns (21.6.2.2) (8.3.5.2)

21.6.2.2 (18.7.3.2) – T-beam Construction

Slab reinforcement within an effective width defined in 8.12 (6.3.2) shall be assumed to contribute to the flexural strength Mg if the slab reinforcement is developed at the critical section for flexure

bw s2s1

h

< ¼ xbeam span

< 8h, s1/2 < 8h, s2/2



o

o

Tension lap splicew/in center half ofmember length

Transverse reinf. per21.6.4.2 and 21.6.4.3(8.3.5.4)

0.01 g 0.06

Columns of Special Moment Frames –Reinforcement Requirements (21.6.3)

(8.3.5.3)

A A

Section A-A

Columns of Special Moment Frames – Spiral or Circular Hoop Reinforcement (21.6.4.4)

(8.3.5.4)

Clear space*75 mm

larger of 25 mm. or1.33(max. agg.)

s

0.12f’c/fyt

0.45[(Ag/Ac)-1] f’c/fyt

*Clear spacing for spiral reinforcement. Circular hoops to be spaced per 21.6.4.3(8.3.5.4).



Spiral Column Crushing

Low ratio of spiralreinforcement

Very high ratio oflongitudinalreinforcement

1985 Mexico City

Spiral Column Crushing

1985 Mexico City



Columns of Special Moment Frames –Rectangular Hoop Reinforcement (21.6.4)

(8.3.5.4)

Ash0.3sbc [(Ag/Ach)-1] f’c/fyt

0.09sbc f’c/fyt

450 mm. 150 mm.

0.3 1 /0.09 /

Transverse Reinforcement in a SpecialMoment Frame Column (8.3.5.4)



Columns of Special Moment Frames–Rectangular Hoop Reinforcement (8.3.5.4)

xi 350 mm.hx = max. value of x on all column faces150 mm. sx = 100 + [(350 – hx)/3] 100 mm.

xi xi xi

xi

xi

6db 45 mm. 6db extension

Alternate90-deg hooks

Provide add.trans. reinf. if thickness > 100 mm

Columns of Special Moment Frames – Shear Strength Requirements (21.6.5) (8.3.8)

Design shear force based on Mpr of the member associated with the range of factored axial loads on the memberMember shears need not exceed those determined from joint strengths based on Mpr of the transverse members framing into the jointMember shear forces must not be less than those determined from analysis



Mcol

Mcol

u

prrprl

u

colcol

prrprlcol

MMMV

MMM

22

Vcol

Vcol

Mcol

Vcol

Vcol

Vcol

prrMprlMAs

'sA

).( ys fA 251

).(' ys fA 251 ysfA '

ysfA

bafc'.850

xjointV

colysysjoint VfAfAV ).().( ' 251251

Assumed: columnsand beams areidentical in the storiesabove and below thejoint.

u

xbafc'.850

Short Column Failure

1985 Mexico City



Shear Failure

1994 Northridge

Columns Supporting Discontinued Shear Walls

1971 San Fernando



Columns of Special Moment Frames Supporting Discontinued Stiff Members

(21.6.4.6) (8.3.5.4 (f))

Development length of largest longitudinal column reinforcement in accordance with 21.7.5 (8.3.7.4)

Wall

Transverse reinf. per 21.6.4.2 – 21.6.4.4 (8.3.5.4)over full height of columns

300 mm

Footing or mat

Development length of largest longitudinal column reinforcement in accordance with 21.7.5 (8.3.7.4)

Reinforcement not shown for clarityShearwall

Columns of Intermediate Moment Frames Supporting Discontinued Stiff Members (21.3.5.6) (8.3.5.4 (f))

ACI 21.3.5.6 (8.3.5.4 (f)) — Columns supporting reactions from discontinuous stiff members, such as walls, shall be provided with transverse reinforcement at the spacing, so, as defined in 21.3.5.2 (8.3.5.4 (b)) over the full height beneath the level at which the discontinuity occurs if the portion of factored axial compressive force in these members related to earthquake effects exceeds Agfc’/10. . . . This transverse reinforcement shall extend above and below the columns as required in 21.6.4.6(b) (8.3.7.4).



Columns of Intermediate Moment Frames–Transverse Reinforcement (21.3.3, 21.3.5)

(8.3.10.5)

s to conform to 7.10 and 11.4.5.1

h2

h1

so/2Joint reinf.per 11.10(6.4.9)

Trans. reinf. based onMn and factoredtributary gravity load

so

8 smallest long. bar dia.24 hoop bar dia.0.5 min. (h1 or h2)12”

o

o

Larger of h1 or h2Clear span/618”

Hoops

450 mm.

300 mm.

BEAM-COLUMN JOINTS



Joints of Special Moment Frames (21.7) (8.3.7)

c2

Ash

Elevation Plan

Standard Hook

dh

/ . (21.7.5) (8.3.7.4)

Joints of Special Moment Frames (21.7.2.3) (8.3.7)

Elevation

20 largest long. bar dia. (normal weight concrete)26 largest long. bar dia. (lightweight concrete)c1



Effective Joint Area

Horizontal Shear in Joint



Mcol

Mcol

u

prrprl

u

colcol

prrprlcol

MMMV

MMM

22

Vcol

Vcol

Mcol

Vcol

Vcol

Vcol

prrMprlMAs

'sA

).( ys fA 251

).(' ys fA 251 ysfA '

ysfA

bafc'.850

xjointV

colysysjoint VfAfAV ).().( ' 251251

Assumed: columnsand beams areidentical in the storiesabove and below thejoint.

u

xbafc'.850

Nominal Shear Strength of Special Moment Frame Joint

ACI 21.7.4.1 (8.3.7.3)Joints confined on all 4 faces:

Vn = Joints confined on 3 faces/2 opposite faces:

Vn = For others:

Vn =



Nominal Shear Strength of Special Moment Frame Joint

ACI 21.7.4.1 (8.3.7.3)A member that frames into a face is considered to provide confinement to the joint if at least three quarters of the face of the joint is covered by the framing member. Extensions of beams at least one overall beam depth h beyond the joint face are permitted to be considered as confining members. Extensions of beams shall satisfy 21.5.1.3, 21.5.2.1, 21.5.3.2, 21.5.3.3 and 21.5.3.6 (8.3.4.1(d), 8.3.4.2, 8.3.4.3(c), 8.3.4.3(d), and 8.3.4.3(b). A joint is considered to be confined if such confining members frame into all faces of the joint.

Interior Joint Failure

Longitudinal beam barsnot confined withincolumn longitudinalbars or ties

1985 Mexico City



Column and Joint Failure

1994 Northridge

Joints of Intermediate Moment Frames

ACI 21.3.5.5 (8.3.7.2) — Joint transverse reinforcement shall conform to 11.10 (8.3.5.4).

ACI 11.10.1 (6.4.9.1) — When gravity load, wind, earthquake, or other lateral forces cause transfer of moment at connections of framing elements to columns, the shear resulting from moment transfer shall be considered in the design of lateral reinforcement in the columns.




ACI 11.10.2 (6.4.9.2, 6.4.3.5.3) — Except for connections not part of a primary seismic load-resisting system that are restrained on four sides by beams or slabs of approximately equal depth, connections shall have lateral reinforcement not less than that required by Eq. (11-13) (6.4.3.5.3) within the column for a depth not less than that of the deepest connection of framing elements to the columns. See also 7.9 (8.1.13).

Av,min = (11-13)

but shall not be less than (Eq. 6.6.55).


7.9 (8.1.13) — ConnectionsACI 7.9.1 (8.1.13.1) — At connections of principal framing elements (such as beams and columns), enclosure shall be provided for splices of continuing reinforcement and for anchorage of reinforcement terminating in such connections.ACI 7.9.2 (8.1.13.2) — Enclosure at connections shall consist of external concrete or internal closed ties, spirals, or stirrups.



Precast Parking Garage

1994 Northridge



Questions?Thank you



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