special rc frames

7/29/2019 Special Rc Frames

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Seismic Design and Detailing ofReinforced Concrete Systems

Chapter 21 - ACI 318-05

General Requirements

21.2.1 ScopeThe additional requirements for the designand construction of reinforced concrete

structures that resist earthquake forces aregiven in Chapter 21 of the ACI Building Code.

The provisions are tied to the outdated

concept of seismic zones. Application of theprovisions to seismic design categories is nottransparent.


YesNoMaterial Properties

YesNoGravity Frames

YesNoFoundations

YesNoDiaphragms

YesYesPrecast Walls

YesNoWalls and CouplingBeams

YesYesFrame Members

High Risk

(Zones 3 and 4)

Moderate Risk

(Zone 2)Component

ACI 318-08

Committee 318 has approved the changefrom seismic zones to seismic design

categories for the 2008 edition of the Code.

Additional changes are still possible, as two

levels of external review are required.


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YesYesNoNoAnchors

YesNoNoNoGravity frames

YesNoNoNoFoundations

YesNoNoNoStructural diaphragms and trusses

YesYesNoNoPrecast structural walls

YesNoNoNoStructural walls / coupling beams

YesYesYesNoFrame members

YesNoNoNoMaterials

D, E, FCBA

Seismic Design CategoryComponent


Special Structural Systems

Beams in moment-resisting frames 21.3

Columns in moment-resisting frames 21.4

Beam-column joints 21.5

Precast moment-resisting frames 21.6

Structural walls and coupling beams 21.7

Precast structural walls 21.8 Diaphragms 21.9

Foundations 21.10

Non-participating frames 21.11


Intermediate Structural Systems

Beams in Moment-Resisting Frames 21.12

Columns in Moment-Resisting Frames 21.12

Two-way slabs without beams 21.12

Precast structural walls 21.13

Special Material Properties

21.2.4 Concrete Specified concrete compressive strength must be at least

3000 psi.

Specified concrete compressive strength shall not exceed5000 psi for lightweight concrete.

21.2.5 Reinforcement ASTM A706 reinforcing steel must be used in frame

members and boundary elements of walls.

ASTM A615 may be used if the actual yield stress does notexceed the nominal yield stress by more than 18 ksi and ifthe ratio of the actual tensile strength to actual yield stressexceeds 1.25.


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Special Moment-Resisting Frames

Beams Flexural members of specialmoment frames

Columns Special moment frame memberssubjected to bending and axial load

Joints Joints of special moment frames

Detailing Provisions forBeams

Section 21.3

21.3.1 - Scope

A beam is defined as any frame member thatresists earthquake-induced forces and isproportioned primarily to resist flexure.

Beams must satisfy the following:

Factored axial compressive force must not exceed Agfc/10. Clear span must be more than 4 times the effective depth.

Width of member must not be less than the smaller of 0.3hand 10 in.

Width of member shall not exceed width of supportingmember plus 3/4h of the beam on each side.

21.3.2 Longitudinal Reinforcement

All Locations:Minimum of 2 continuous bars per face.

3 200cs w wy y

fA b d b d

f f

0 025.

1 2

4 4n n

nM M

M max ,

1nM

+ 112n

nM

M

2nM

+ 222n

nM

M


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Lap splices are not permitted within thebeam-column joints, within a distance oftwice the member depth from the face of the

joint, or at locations where flexural hinging isexpected.

Closely-spaced hoops are required over the

length of the lap. Spacing of hoops shall notexceed the smaller of d/4 and 4 in.

ClosedHoops

21.3.3 Transverse Reinforcement

ClosedHoops

2h 2h

h

First hoop shall be located not more than 2 in. from the face of thesupporting member.

Spacing of closed hoops shall not exceed the smallest of d/4, 8 times thediameter of the longitudinal bar, 24 times the diameter of the hoop bar,and 12 in.

Stirrups withseismic hooks

Spacing of stirrups shall not exceed d/2.


Hoops in beams are permitted to be made oftwo pieces of reinforcement: a stirrupshaving seismic hooks at both ends and across tie.


Where hoops are required, longitudinal barson the perimeter shall have lateral support

conforming to 7.10.5.3.

Hoops shall be arranged such at that every

corner and alternate longitudinal bar shallhave lateral support provided by the corner ofa hoop and no bar shall be farther than 6 in.

from such a laterally supported bar.


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21.3.4 - Shear Strength Requirements

eV

nL

eV

1prM 2prM

uw

Capacity Design Approach:

+= 1 22

pr pr u ne

n

M M w LV

L

21.3.4 - Shear Strength Requirements

eV

prM

= design shear force (factored shear)

= probable flexural strength, calculated using astress in the reinforcement of 1.25 fy and astrength reduction factor of 1.0.

21.3.4 Shear Strength Requirements

Transverse reinforcement in the regionswhere hoops are required shall be

proportioned to resist shear assuming that

Vc = 0 when both of the following conditions

occur: The earthquake-induced shear force represents at

least 50% of the required shear strength.

The factored axial compressive force including

earthquake effects is less than Agfc/20.

Detailing Provisions forColumns

Section 21.4


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21.4.1 - Scope

A column is defined as any frame memberthat resists earthquake-induced forces andhas a factored axial force that exceedsAgfc/10.

Columns must satisfy the following: Shorter cross-sectional dimension must be at

least 12 in.

Aspect ratio for the column must not be less than0.4.

21.4.2 - Minimum Flexural Strength of

Columns

Collapse of individual stories in reinforcedconcrete buildings often occurs when flexural

hinges occur at both ends of the columnsduring an earthquake.

ACI 318-05 includes two procedures to avoidthis problem.

21.4.2.2 addresses frames with strong columnsand weak beams.

21.4.2.3 addresses frames with strong beams and

weak columns.

21.4.2.2 Strong Columns/Weak Beams

A strong column-weak beam system mustsatisfy:

( ) nc nb M / M6 5ncM = sum of moments at the faces of the joint corresponding to thenominal flexural strength of the columns framing into that joint.

nbM = sum of moments at the faces of the joint corresponding to thenominal flexural strength of the girders framing into that joint.In T-beam construction, where the slab is in tension under the

moments at the face of the joint, slab reinforcement within theeffective slab width defined in 8.10 shall be assumed to contributeto the flexural strength if the slab reinforcement is developedat the critical section for flexure.

21.4.2.2 Strong Columns/Weak Beams

The nominal flexural capacities of the members are summed such that columnmoments oppose the beam moments. The column strengths must satisfy therelationship for beam moments acting in both directions.

ncM 1

ncM 2

nbM 1

nbM 2

ncM 1

ncM 2

nbM 1

nbM 2


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21.4.2.3 Weak Columns/Strong Beams

If the columns do not satisfy the requirementsfor strong columns, closely-spacedtransverse reinforcement is required along

the entire height of the column.

In addition, the lateral strength and stiffnessof columns that do not satisfy 21.4.2.2 must

be ignored when calculating the strength andstiffness of the structure.


The longitudinal reinforcement ratio must notbe less than 0.01 nor more than 0.06.

Lap splices are only permitted within thecenter half of the member and must be

proportioned as tension splices.


In order to ensure adequate deformationcapacity, a considerable amount of

transverse reinforcement must be provided inall columns that are part of the lateral force

resisting system. Because rectangular hoops are less effective

than circular spirals in confining the core, the

volumetric reinforcement ratio for rectangularhoops is larger.

21.4.4 Spirals and Circular Hoops

Volumetric reinforcement ratio shall not beless than the larger of:

= 0 45 1g c

s ch yt

A f.

A f

= 0 12 csyt

f.

f


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21.4.4 Rectangular Hoops

The total cross-sectional area of rectangularhoop reinforcement shall not be less than thelarger of:

= 0 3 1gc

sh cyt ch

AfA . sb

f A

= 0 09c

sh cyt

fA . sb f

21.4.4 Rectangular Transverse

Reinforcement

Single or overlapping hoops may be used asthe transverse reinforcement. Crossties of

the same bar size and spacing of the hoopsmay also be used.

Crossties or legs of overlapping hoops shall

not be spaced more than 14 in. on centerhorizontally.

1shA

2shA

1cb

2cb

xh 14 in.

21.4.4 Vertical Spacing ofTransverse Reinforcement

Within Lo, smust not exceedthe smallest of:

b ob h

d s, , ,64 4

Outside Lo, smustnot exceed thesmaller of:

bd ,6 6 in.

= + xoh

s14

43

hx is the maximum spacing of thehoop or crosstie legs on all facesof the column, in.

Lois the largest of:

nLh, , 18 in.6

oL

oL xh 14 in.

os 6 in.


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Spacing of Transverse Reinforcement

0

2

4

6

8

0 2 4 6 8 10 12 14 16

hx, Maximum horizontal spacing of hoops, in.

so,M

aximumv

erticalspacingofhoops,

in.


The design shear force in columns iscalculated based on the maximum forces thatcan be generated at the faces of the joint ateach end of the member.

The end moments for the columns need notexceed the moments generated by theprobable flexural strength of the beams

framing into the beam-column joint. The design shear must not be less than the

factored shear determined from analysis ofthe structure.


Transverse reinforcementover the length Lo, shall beproportioned to resist shearassuming that Vc = 0 whenboth of the followingconditions occur:

The earthquake-inducedshear force represents atleast 50% of the requiredshear strength.

The factored axialcompressive force includingearthquake effects is lessthan Agfc/20.

eV

nH

eV1prM

2prM

P

P

+= 1 2pr pr en

M MV H

Detailing Provisions forBeam-Column Joints

Section 21.5


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21.5.1 General Requirements

eV

eV

1C

1T

2T

2C

> 20 bh d

0 85.=for shear in joints

= + 1 2u eV T T V

21.5.1 General Requirements

Forces in longitudinal beam reinforcement atthe joint face shall be determined byassuming that the stress in the flexural

reinforcement is 1.25 fy.

Beam reinforcement that terminates in abeam-column joint must extend to the far

face of the confined core and be anchored intension per 21.5.4 or in compression perChapter 12.


Transverse hoop reinforcement, as specifiedfor the ends of columns in 21.4.4, must be

provided within the joint.

21.5.3 Shear Strength

The nominal shear strength of the joint shallnot exceed the values given below:

Joints confined on all four faces

Joints confined on three faces or

on two opposite faces Other joints

It is not possible to increase the shear

strength of the joint by adding morereinforcement.

20 c jf A15

c jf A

12 c jf A


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21.5.3 Shear Strength

jA

21.5.4.1 Development Lengths

The development length for a bar with a 90hook shall not be less than the largest of:

8 db 6 in.

The 90hook must be located within theconfined core of a column or boundaryelement.

65y b

c

f d

f

21.5.4.1 Development Lengths

The development length of a straight bar in tension

(#3 through #11) must not be less than 2.5 times the

development length for a hooked bar if the depth ofconcrete does not exceed 12 in., and 3.5 times the

development length for a hooked bar if the depth of

concrete exceeds 12 in.

Straight bars terminated in a joint must pass throughthe confined core of a column or boundary element.

Any portion of the straight embedded length that is

not within the confined core must be increased by afactor of 1.6.

Members not Designated asPart of the Lateral-Force-Resisting System

Section 21.11


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Non-Participating Frames

A large number of reinforced concretebuildings were damaged during the 1994Northridge earthquake when columns that

were designed to resist only gravity loadsfailed.

Codes distinguish between the lateral-loadresisting system and gravity-load resisting

system, and the detailing provisions aredifferent for members that are not intended tocarry lateral loads.

Non-Participating Frames

However, the building must deform as asingle unit.

Therefore, all members must have sufficientreinforcement details to behave in a ductile

manner when pushed into the inelastic regionof response during an earthquake.

21.11.1 - Overview

In previous versions of the code, the designerhad to subject each of the frame members

that was not assumed to contribute to thelateral resistance to twice the calculated

lateral displacements under the factoreddesign loads.

These members were then designed for the

corresponding moments and shears.

21.11.1 - Overview

These provisions were difficult to interpret, soACI 318-99 (and subsequent versions) gives

the designer two options:

subject the member to the design displacements

and design accordingly use more stringent detailing provisions for all

members.


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21.11.1 - Overview

The change from using twice the designdisplacement to using the designdisplacement results from the change in the

1997 UBC to strength design levels.Previous versions of the UBC were based onworking stress design levels for earthquakeloads.

21.11.2 Displacements Calculated and

Forces are Low

When the induced moments and shears

under the lateral displacements defined in21.11.1 combined with the factored moments

and shears due to gravity loads do notexceed the nominal flexural and shear

capacities of the frame members, thefollowing provisions apply:

21.11.2

Beams must satisfy the longitudinalreinforcement requirements in 21.3.2.1 and

stirrups must be spaced no more than d/2along the entire length of the member.

21.11.2

Columns must satisfy the longitudinalreinforcement requirements in 21.4.3,

requirements for hoops and crossties asdefined in 21.4.4.1(c) and 21.4.4.3, and

design shears are calculated per 21.4.5. Themaximum longitudinal spacing of ties may notexceed 6db or 6 in. along the entire length of

the column.


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21.11.2

If the factored gravity axial load exceeds 0.35Po, the area of transverse reinforcement inthe column must be at least one-half the

amount required in 21.4.4.1.

21.11.2

For the purpose of this analysis, the followingcombinations of gravity loads must be used:

1.2 D + 1.0 L + 0.2 S or 0.9 D

The load factor on L may be reduced to 0.5except for garages, areas occupied as placesof public assembly, and all areas where L isgreater than 100 psf.

21.11.3 Displacements Calculated andForces are High OR Displacements areNOT Calculated

When the induced moments and shears

under the lateral displacements defined in

21.11.1 combined with the factored momentsand shears due to gravity loads exceed the

nominal flexural and shear capacities of theframe members or if the induced forces are

not calculated, the following provisions apply:

21.11.3

The material properties must satisfyprovisions in 21.2.4 and 21.2.5.

Longitudinal reinforcement in beams mustsatisfy the provisions in 21.3.2.1, design

shears must be calculated using theprovisions in 21.3.4, and stirrups must bespaced at more than d/2 along the entire

length of the member.


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21.11.3

Longitudinal reinforcement in columns mustsatisfy 21.4.3.1, transverse reinforcementmust satisfy provisions in 21.4.4, design

shears must be calculated using theprovisions in 21.4.5, and transversereinforcement in joints must satisfy theprovisions in 21.5.2.1.

Inconsistencies in 21.11

If the engineer does not evaluate the effectsof the design displacements, the designprovisions should be more stringent than if

the effects of the design displacements arechecked and the member has sufficientstrength.

Inconsistencies in 21.11

21.11.3 permits column lap splices to belocated at the base of the column. (But these

columns are at risk of forming plastic hinges.)

21.11.2 requires that column lap splices belocated in the center half of the column. (But

these columns are unlikely to form plastichinges.)

Proposal for 318-08

21.11.2.2Columns must satisfy 21.4.3.1, 21.4.4.1(c)

and 21.4.4.3, and 21.4.5.

21.11.2.3Columns must satisfy 21.4.3, 21.4.4, 21.4.5,

and 21.5.2.1.

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