seismic design and detailing_pp151-176

8/17/2019 Seismic Design and Detailing_pp151-176

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Rev1 04-04 / CE573-151Mapua Institute of Technology (MAPUA Tech)

Adam C Abinales f.asep, pice

Seismic Design of Concrete Structures

5. Structural Design and Detailing for Earthquake ResistanceStructural Design and Detailing for Earthquake Resistance5.2.5 Seismic Shear Forces in Beams and ColumnsSeismic Shear Forces in Beams and Columns.

Shear failure in reinforced concrete members is regarded as brittle failure. Therefore, in designingearthquake-resistant structures, it is important to provide excess shear capacity over and abovethat corresponding to flexural failure. The code requirements are based on the strong column-

weak beam concept. Hence plastification of the critical regions at the ends of the beams will haveto be considered as a possible loading condition.

The shear force is then computed based on the moment resistances in the developed plastichinges, labeled as probable moment resistance M PR , developed when the longitudinal flexural steelenters into the hardening stage. Consequently, the computation of the probable momentresistance, 1.25f y , is used as the stress in the longitudinal reinforcement. In order to absorb the

energy that can cause plastic hinging, the earthquake-resistant frame has to be ductile in partthrough confinement of the longitudinal reinforcement of the columns and the beam-column jointsand in part through the provision of the excess shear capacity.

Figure 5.6 shows the deformed geometry of

and the seismic moment and shear forces fora beam subjected to gravity loading and

reversible sidesway. (a) sidesway to the left;

(b) sidesway to the right.


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5. Structural Design and Detailing for Earthquake ResistanceStructural Design and Detailing for Earthquake Resistance

The seismic shear forces are

where = span, L and R subscripts = left and right ends, and M PR = probable moment strength at

the end of the beam based on steel reinforcement tensile strength of 1.25f y and strength reductionfactor = 1.0. These instantaneous moments M PR should be computed on the basis of equilibrium

of moments at the joint where the beam moments are equal to the probable moments of resistance.

The shear forces in the columns are computed in a similar manner, so the horizontal V e at top andbottom of the column is

except that end moments for columns (M PR 1

and M PR 2

) need not be greater than the momentsgenerated by the M PR of beams framing into the beam-column joint, where h = column height, andthe subscripts 1 and 2 indicate the top and bottom column end moments, respectively, as seen inFigure 5.7.

2

7141

2

7141

LDMMV

LDMMV

PRPR

R

PRPR

L

..

..

+−+=

++

+=

−+

+−

l

l

h

MMV PRPR

e

21+

=


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Figure 5.7 Seismic moments and shears at

column ends: (a) joint moments; (b) sway toright; (c) sway to left.


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5. Structural Design and Detailing for Earthquake ResistanceStructural Design and Detailing for Earthquake Resistance5.2.6 Strong ColumnStrong Column- - Weak Beam Concept Weak Beam Concept .

As previously discussed in 5.2.2, the strong column-beam concept is ensured by the followinginequality:

For a joint subjected to reversible base shear forces, as shown in Figure 5.8, the above equationbecomes

where

= 0.90 for beams, 0.65 for

tied columns, and 0.70 for

spiral columns.

For beam-columns,

= 0.90 to 0.65.

Figure 5.8 Seismic moment summation atbeam-column joint: (a) sidesway to the left;

(b) sidesway to the right.

beamcol MM Σ

≥Σ

5

6

beamnncolnn MMMM )()( −+−+ +

≥+ φ φ φ φ

5

6


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5. Structural Design and Detailing for Earthquake ResistanceStructural Design and Detailing for Earthquake Resistance5.2.7 Design of Confining Reinforcement for BeamDesign of Confining Reinforcement for Beam- - Column ConnectionColumn Connection.

Transverse reinforcement in the form of closely spaced hoops (ties) or spirals has to be adequatelyprovided. The aim is to produce adequate rotational capacity within the elastic hinges that maydevelop as a result of the seismic forces.

For column spirals, the minimum volumetric ratio of the spiral hoops needed for the concretecore confinement cannot be less than the larger of:

whichever is greater, where

s = ratio of volume of spiral reinforcement to the core volume measured out-to-out.

Ag = gross area of the column section.

Ach = core area of section measured to the outside of the transverse reinforcement.f yh = specified yield strength of transverse reinforcement.

yh

c

ch

g

s

yh

c

s

f

f

A

A

f

f

'.

or '.

−≥

≥

1450

120

ρ

ρ


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For column rectangular hoops, the cross-sectional area within spacing s cannot be less thanthe larger of:

whichever is greater, where

Ash = total cross-sectional area of transverse reinforcement (including cross ties) within

spacing s and perpendicular to dimension hc .

hc = cross-sectional dimension of column core measured center-to-center of confiningreinforcement.

h x = maximum horizontal spacing of hoops or cross-ties on all faces of the column.

Ach = cross-sectional area of structural member, measured out-to-out transversereinforcement.

s = spacing of transverse reinforcement within length o. Whose value should not exceed

150mm and need not be taken less than 100mm.

smax = one-quarter of the smallest cross-sectional dimension of the member, 6 times

diameter of longitudinal reinforcement.

yh

c

ch

g

c sh

yh

c

c sh

f

f

A

A sh A

f

f sh A

'.

or '

.

−≥

≥

130

090


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Additionally, if the thickness of the concrete outside the confining transverse reinforcementexceeds 100mm, additional transverse reinforcement has to be provided at a spacing not toexceed 300mm. The concrete cover on the additional reinforcement should not exceed

100mm. The confining transverse reinforcement in columns should be placed on both sides of a

potential hinge over a distance o. The largest of the following three conditions governs the

length o:

(a) depth of member at joint face

(b) one-sixth of the clear span

(c) 450mm

Increase o by 50% or more in locations of high axial loads and flexural demands such as the

base of a building.

When transverse reinforcement is not provided throughout the column length, the remainderof the column length has to contain spiral or hoop reinforcement with spacing not exceedingthe smaller of 6 times the diameter of the longitudinal bars or 150mm.


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For beam confinement, the confining transverse reinforcement at beam ends should beplaced over a length equal to twice the member depth h from the face of the joint on eitherside or of any other location where plastic hinges can develop. The maximum hoop spacing

should be the smallest of the following four conditions:(a) one-fourth effective depth d

(b) eight times diameter of longitudinal bars

(c) 24 times diameter of the hoop

(d) 300mmhowever, the Code requires that confining reinforcement spacing need not exceed 100mm.Figure 5.9 summarizes typical detailing requirements for a confined column.


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Figure 5.9 Typical detailing of seismically reinforced column: (a) spirally confined; (b) confined with

rectangular hoops; (c) cross-sectional detailing of ties. X ≤ 350mm. Consecutive cross ties have 90°hooks on opposite sides.


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Reduction in confinement at joints: a 50% reduction in confinement and an increase in theminimum tie spacing to 150mm are allowed by the code if a joint is confined on all four facesby adjoining beams with each beam wide enough to cover three-quarters of the adjoining

face. The yield strength of reinforcement in seismic zones (particularly zone 4) should not exceed

410 MPa.

Horizontal Shear in BeamHorizontal Shear in Beam- - Column ConnectionColumn Connection

Test of joints and deep beams shave shown that shear strength is not as sensitive to joint (shear)reinforcement as for that along the span. On this basis, the code has assumed the joint strength asa function of only the compressive strength of the concrete and requires a minimum amount oftransverse reinforcement in the joint. The effective area A j in Figure 5.10 should in no case begreater than the column cross-sectional area.

The minimal shear strength of the joint should not be taken greater than the forces V n specifiedbelow for normal-weight concrete.


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Figure 5.10 Seismic effective area of joint.


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Reduction in confinement at joints: a 50% reduction in confinement and an increase in theminimum tie spacing to 150mm are allowed by the code if a joint is confined on all four facesby adjoining beams with each beam wide enough to cover three-quarters of the adjoining

face. The yield strength of reinforcement in seismic zones (particularly zone 4) should not exceed

410 MPa.

Horizontal Shear in BeamHorizontal Shear in Beam- - Column ConnectionColumn Connection

Test of joints and deep beams shave shown that shear strength is not as sensitive to joint (shear)reinforcement as for that along the span. On this basis, the code has assumed the joint strength asa function of only the compressive strength of the concrete and requires a minimum amount oftransverse reinforcement in the joint. The effective area A j in Figure 5.10 should in no case begreater than the column cross-sectional area.

The minimal shear strength of the joint should not be taken greater than the forces V n specifiedbelow for normal-weight concrete.

Confined on all faces by beams framing into the joint:

jcn Af V '.661≤


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Confined on three faces or on two opposite faces:

All other cases:

A framing beam is considered to provide confinement to the joint only if at least three-quarters ofthe joint is covered by the beam.

The value of allowable V n should be reduced by 25% if lightweight concrete is used. Some test dataindicate that the value of V n for all other cases is unconservative when applied to corner joints. A j =effective cross-sectional area within a joint in a plane parallel to the plane of reinforcementgenerating shear at the joint. The code assumes that the horizontal shear in the joint is determined

on the basis that the stress in the flexural tensile steel = 1.25f y .

jcn Af V '.251≤

jcn Af V '.01≤


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5. Structural Design and Detailing for Earthquake ResistanceStructural Design and Detailing for Earthquake Resistance5.2.8 Design of Confining Reinforcement for BeamDesign of Confining Reinforcement for Beam- - Column ConnectionColumn Connection.

E E xample Problem 5.3. Design the transverse confining reinforcement of joint A in a ductilemoment-resisting frame of a building as shown in the figure below. The structure is situated inseismic zone 4. The following design criteria applies to the building frame as:

All beams are 300mm x 600mm with 4- 25 longitudinal bars top and bottom and columns are400mm x 600mm. Stirrup size is 12.

27.6 MPaf’c =

460 kN-mMPR =

410 MPafy =

36 kN/mwL =

21 kN/mwD =

600 7500 600

3 6 0 0

All beams are 300mm

x 600mm with

4-Ø25 bars top and

bottom. 6 0 0

6 0 0

Column size

400mm x 600mm

FRAME ELEVATION

Joint

A

Joint

B

Ad C Abi l


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Check the web shear reinforcement along beam span outside the inelastic zone. Consider thefigure of isolated joint A below showing schematic of the lines of action of the beam-column jointforces.

600 7500 600

3 6 0 0

All beams are 300mm

x 600mm with

4-Ø25 bars top and

bottom. 6 0 0

6 0 0

Column size

FRAME ELEVATION

Joint

A

MEQVu

col

Vcol h 1

/ 2

= 1 8 0 0

h 2

/ 2

= 1 8 0 0

Shear forces at beam-column joint.

Ad C Abi l f i


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depth of reinforcement, d = 600-(40+12+25/2) = 535.5 mm

reinforcement, As = 4*491 = 1964 mm2

longitudinal steel ratio, :

therefore, 4-

25 bars at top and bottom are sufficient.

V A VB

Beam AB Equilibrium

M A

MB

Ln = 7500

wD = 21 kN/m

wL = 36 kN/m

025001220

5535300

1964

..

).(


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Determine the beam transverse confining reinforcement in the inelastic zone of plastic hinging.Using the following equations for seismic shear forces:

Computing shear strength provided by the concrete beam,

Calculate the nominal shear force at a distance d from the

face of the column support,

V A VB

Beam AB Equilibrium

M A

MBLn = 7500

wD = 21 kN/m

wL = 36 kN/m

VDA = 21(7.5/2) = 78.75 kN

VLA = 36(7.5/2) = 135 kN

kN.

.

)(.).(.

.

..

667575

750

1

2

27071515741

57

460460

1

2

7141

=

++

+=

++

+=

L

L

n

B AL

V

V

LDMMV φ l

kN.

).)()(.)(/(')/(

664140

1000

55353006276161

=

==

c

wcc

V

dbf V

kN.

)/.(

)./.(.

462493

257

53550257667575

=

−=

n

n

V

V

Adam C Abinales f asep pice


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Then, the nominal shear strength to be resisted by the reinforcement,

Using 12 hoops, Av = 2(113) = 226 mm2, the required spacing is

These confining hoops shall be placed over beam within a distance of o = 2h = 2(600) = 1200 mm

and shall be spaced not to exceed the least value of

(d /4) = 535.5/4 = 133 mm……….. Governs, say 100 mm

(8*smallest longitudinal bar d b) = 8(25) = 200 mm

(24*hoop diameter) = 24(12) = 288 mm or

(maximum spacing of ) = 300 mm

Therefore, within o = 1200mm, use 12 hoops and crossties at 100 mm c-c over this distance.

Further, use 12 closed hoops at 150 mm c-c beyond critical section, then increase spacing to d /2= 535.5/2 = 267 mm, say 250 mm approaching midspan and stop stirrups at V c /2.

kN.

..

798352

664140462493

=

−=−=

s

cn s

V

V V V

mm

)(.

).)()((

140

1000798352

5535410226

=

==

s

V

df A s

s

yv

Adam C Abinales f asep pice


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Determine the confining reinforcement in the column in beam-column joint. Calculate the jointshear strength. Column shear forces should not exceed those base on the probable end momentstrengths M PR of the beams framing into the joint.

then

and this

where A j = 400(600) = 240,000 mm2, then allowable

Hence, the confined column joint is adequate to resist the seismic shear.

kN.

/././/

778127

26032603460

2221

=

+=

+=

col

PRcol

V

hhMV

kN.

.))((

462677

7781271000

4101964

=

−=−=

n

coly sn

V

V f AV

jcn Af V '.251≤

kN.actualkN.

),(..

4626770711576

1000

000240627251

=>=

=

nn

n

V V

V



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Determine the column confinement in the inelastic zone.

column d = 600 - (40+12+25/2) = 535.5 mm

At the A j plane, the nominal shear strength provided by concrete is given also as

then, the nominal shear strength to be resisted by confinement is

Using 12 hoops, Av = 2(113) = 226 mm2, the required spacing is

kN.

).)()(.)(/(')/(

552187

1000

55354006276161

=

==

c

cc

V

bdf V

kN.

..

91489

552187462677

=

−=−=

s

cn s

V

V V V

mm

)(.).)()((

101

100091489

5535410226

=

==

s

V

df A s

s

yv



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p p



Determine the greater value of the following expressions.

where

hc = column core dimension measured c-c of confining reinforcementhc = 600 – 2(40+12) = 496 mm

try spacing s = 90 mm

Check with the maximum spacing, the least value of

(smaller column dimension/4) = 400/4 = 100 mm…….. governs

(6*longitudinal bar diameter) = 6(25) = 150 mm(maximum spacing of ) = 100 mm or

yh

c

ch

g

c sh

yh

c

c sh

f

f

A

A

sh A

f

f sh A

'

.

or '

.

−≥

≥

130

090

controls---mm.**))((.

or mm.

))((.

2

2

>=

−≥

=≥

570410

6271496264

6004004969030

270410

62749690090

sh

sh

A

A

mm144/).(

mm

mm

=−−

+=

>


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5. Structural Design and Detailing for EarthquakeStructural Design and Detailing for EarthquakeResistanceResistance

Determine the distance o over which these confinements

shall be placed in the column of both sides of potential hinge

and shall be the largest of

(depth of the member h) = 600 mm

(beam clear span over 6) = 7500/6 = 1250 mm or……… governs

(minimum of ) = 450 mm

Hence, provide 12 hoops and 12 crossties at 90 mm c-cover the distance of say o = 1250 mm.

hc

= 496

400296

6008- 25

14 spaces @90mm = 1260mm

12 @ 90mm

50

4- 25

4- 25

12 spaces @100mm =1200mm

12 @ 90mm

150 150

12 @ 100mm



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5. Structural Design and Detailing for Earthquake ResistanceStructural Design and Detailing for Earthquake Resistance5.2.9 InIn- - situ Concrete Detailingsitu Concrete Detailing – – General RequirementsGeneral Requirements. The following notes and associated detail

drawings have been compiled to enable the elements of reinforced concrete structures to bedetailed in a consistent and satisfactory manner for earthquake resistance. These details should besatisfactory in regions of medium and high seismic risk in so far as they reflect the present state-of-the-art. However considerable uncertainty exists regarding effective details for some members,

particularly columns and beam-columns connections. In low risk regions, relaxations may bemade to the following requirements, but the principles of lapping, containment and continuity mustbe retained if adequate ductility is to be obtained.

LapsLaps. Laps in earthquake resisting frames must continue to function while the members or joints undergo large deformations. As the stress transfer is accomplished through theconcrete surrounding the bars, it is essential that there be adequate space in a member to

place and compact good quality concrete.

Laps should preferably not be made in regions of high stress, such as near beam-to-columnconnections, as the concrete may become cracked under large deformations and thusdestroy the transfer of stress by bond. In regions of high stress, laps should be consideredas an anchorage problem rather than a lap problem, i.e. the transfer of stress from one bar to

another is not considered; instead the bars required to resist tension should be extendedbeyond the zone of expected large deformations in order to develop their strength byanchorage.

Laps should preferably be staggered but where this is impracticable and large numbers arelapped at one location (I.e. columns) adequate links or ties must be provided to minimize thepossibility of splitting in concrete. In columns and beams even when laps are made in

regions of low stress at least two links should be provided as shown in the details.Code provisions on laps are given in NSCP Section 412.15 to 412.20.



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AnchorageAnchorage. Satisfactory anchorage may be achieved by extending bars as straight lengths,or by using 90° and 180° bends, but anchorage efficiency will be governed largely by the stateof stress of the concrete in the anchorage length. Tensile reinforcement should not beanchored in zones of high tension. If this cannot be achieved, additional reinforcement in theform of links should be added, especially where high shear exists, to help to confine theconcrete in the anchorage length. It is especially desirable to avoid anchoring bars in the‘panel’ zone of beam-column connections. Large amounts of the reinforcement should not becurtailed at any one location. See NSCP Section 412 for development and splices ofreinforcement.

Bar bendingBar bending. The code has adopted standardization of bar shapes but due attention must bemade to the bearing stresses in bends. The bearing stress inside a bend in a bar which doesnot extend or is not assumed to be stressed beyond a point four times the bar size past theend of the bend need not be checked, as the longitudinal stresses developed in the bar at thebend will be small. See NSCP Section 407.2 through 407.407.4 for details of reinforcement.

The bearing stress inside a bend in any other bar should be calculated as

where

F t = tensile force due to ultimate loads in a bar or group of bars, N

r = internal radius of the bend, mm

= diameter of the bar or, in bundle, the diameter of a bar of equivalent area, mm

b

ct

pa

f

r

Ff

/

'. stressbearing

φ φ 21

51

+≤=



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ab = center-to-center distance perpendicular to the plane of the bend between bars orgroups of bars for a particular bar or group of bars in contact, respectively, mm

Concrete cover Concrete cover. Minimum cover to reinforcement as set forth in NSCP Section 407.8.1.

Concrete qualityConcrete quality. The minimum recommended 28-day compressive strength, f’ c for structuralconcrete is 20 N/mm2.

The use of lightweight aggregates for structural purposes in seismic zones should be verycautiously proceeded with, as many lightweight concretes prove very brittle in earthquakes.

Appropriate advice should be sought in order to obtain a suitably ductile concrete. It cannotbe over-emphasized that quality control, workmanship and supervision are of the utmostimportance in obtaining earthquake-resistant concrete.

Reinforcement qualityReinforcement quality. For adequate earthquake resistance, suitable quality of reinforcementmust be ensured by both specification and testing. As the properties of reinforcement varygreatly between manufacturers, much depends on knowing the source of the bars, and on

applying the appropriate tests.

The following points should be observed:

- Adequate minimum yield stress may be ensured by specifying steel to an appropriatestandard (PNS 49 or ASTM A615).

- Grades of steel with f’ c in excess of 410 N/mm2

may not be permitted in areas of highseismic risk, but slightly greater strengths may be used if adequate ductility is proven bytests.



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- Cold worked steel are not recommended in high seismic risk areas.

- Steel of higher strength than that specified should not be substituted on site.

- The elongation test is particularly important for ensuring adequate steel ductility. InPNS 49 or ASTM A615 appropriate requirements are set out for steels conforming tothose standards.

- Bending tests are most important for ensuring sufficient ductility of reinforcement inthe bend condition. In PNS 49 or ASTM A615 appropriate requirements are set out forsteels conforming to those standards.

- Minimum bend radius for bars as set forth in NSCP Section 407.3.

- Resistance to brittle fracture should be checked by a notch toughness test conductedat the minimum service temperature, where this is less than about 3-5°C.

- Strain-age embrittlement should be checked by re-bend tests.

- Welding of reinforcing bars may cause embrittlement and needs special consideration.- Galvanizing of reinforcing bars may cause embrittlement and needs specialconsideration.

- Welded steel fabric (mesh) is unsuitable for earthquake resistance because of itspotential brittleness. However, it may be used for the control of shrinkage in non-

structural elements such as ground slabs.

seismic design and detailing_pp151-176

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