STRC22 Lateral Forces Concrete Seismic 0716

StructuralEngineeringReviewCourse

LateralForces:ReinforcedConcrete(SeismicDesign)

Reinforced Concrete: Lateral Forces Structural Engineering Review Course

STRC ©2016 Professional Publications, Inc.

2016 ©Professional Publications, Inc.

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Reinforced Concrete: Lateral Forces

Exam Specifications NCEES Specifications Concrete(LateralForces)

5questions(12.5%)

A. ordinary orintermediate shearwalls B. special shear walls

STRMChap. 7 N/A

C. ordinary or intermediate moment-resisting frames D.specialmoment-resistingframes

STRM Chap. 7 N/A

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Lesson Overview •

general seismic considerations

•

ordinary moment frames

•

intermediate moment frames

•

special moment frames

•

special structural walls

•

diaphragms

•

members not designed as part of the seismic force-resisting system

•

anchoring to concrete

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Prerequisite Knowledge You should already be familiar with •

reinforced concrete design

•

general seismic design

•

design standards •

•

•

IBC load combinations ASCE/SEI7 loading criteria (seismic loads and wind loads)

moment frame and shear wall components

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Referenced Codes and Standards Codes •

Building Code Requirements for Structural Concrete (ACI 318, 2011)

•

International Building Code (IBC, 2012)

•

Minimum Design Loads for Buildings and Other Structures (ASCE/SEI7, 2010)

References •

NEHRP Seismic Design Technical Brief No. 1 (NEHRP, 2008)

•

PCA Notes on ACI 318-11 Building Code (PCA Notes on ACI 318, 2011)

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General Seismic Considerations Seismic and Wind Lateral Forces Design This lecture focuses on seismic detailing and design requirements. Given the disproportionate amount of detailing required for seismic design compared to wind design, this will likely be reflected on the SE exam.

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General Seismic Considerations Concrete Seismic Design •

In most concrete buildings (risk category II), economical earthquake-resistant design will exhibit the following performance based on seismic force intensity. •

•

•

minor intensity: no damage, forces in elastic range moderate intensity: negligible structural damage with some repairable nonstructural damage design level intensity: structural and nonstructural damage, yielding and inelastic effects, building remains stable but requires repair before occupancy

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General Seismic Considerations Concrete Seismic Design •

•

How can a material that typically fails inelastically in a brittle manner have a ductile inelastic response? Toughness requirements and extensive details confine the plastic hinges to ductile yield mechanisms.

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General Seismic Considerations General Design Requirements •

•

•

•

applies to all seismic design categories (SDC) positive reinforcement of flexural member in seismic-load-resisting system must be anchored to supports to develop its yield strength (ACI 318 Sec. 12.11.2) shear reinforcement required in some beam-column connections (such as at the exterior of buildings) (ACI 318 Sec. 11.10.2) general structural integrity requirements to improve redundancy and ductility (ACI 318 Sec. 7.13)

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General Seismic Considerations General Design Requirements (continued) •

•

•

ACI 318 Chap. 21 contains the vast majority of seismic force detailing provisions. •

relatively self-contained

•

many provisions lengthy, but can be summarized neatly in a few figures

All structures must satisfy the applicable provisions of Chap. 21, unless assigned to SDC A. (ACI 318 Sec. 1.1.9.2) For SDC A and for non-seismic loads to which ACI 318 Chap. 21 does not apply, use Chaps. 1–20, and Chap. 22.



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General Seismic Considerations Seismic Design Category B General Requirements For SDC B, only the requirements of ACI 318 Sec. 21.1.2 must be met. •

•

•

Consider interaction of all structural and non-structural members. Consider consequences of failure of structural and non-structural members that are not part of the seismic-force-resisting system (SFRS). (ACI 318 Sec. 21.1.2.2) Below grade structures that are required for seismic force resistance and transfer forces to the foundation should be designed per Chap. 21, consistent with the above grade seismic-force-resisting system (SFRS). (ACI 318 Sec. 21.1.2.3)



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General Seismic Considerations Seismic Design Category C General Requirements •

requirements of ACI 318 Sec. 21.1.2 and 21.1.8 (similar to SDC B) •

•

Sections 21.1.3 through 21.1.7 do not apply to SDC C.

Appendix D anchors resisting seismic forces must conform to ACI 318 Sec. D.3.3. (ACI 318 Sec. 21.1.8)



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General Seismic Considerations Seismic Design Category D, E, & F General Requirements •

same general requirements of SDC C

•

requirements of ACI 318 Sec. 21.1.2 through 21.1.8, and Sec. 21.11 through 21.13 •

ACI 318 Sec. 21.11 covers structural diaphragms and trusses.

•

ACI 318 Sec. 21.12 covers foundations.

•

•

ACI 318 Sec. 21.13 covers members not designed as part of the SFRS (covered later in the lecture).

ACI Sec. 21.1.3 to 21.1.7 do not apply to SDC C.



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General Seismic Considerations SDC D, E, & F Foundation Requirements ACI 318 Sec. 21.12 provides additional requirements for buildings supported on reinforced concrete foundations in SDC D, E, & F.

from PCA Notes on ACI 318-11



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General Seismic Considerations SDC D, E, & F Foundation Requirements(continued) •

•

ACI 318 Sec. 21.12.3 contains additional requirements for grade beams and slab on grade foundations. Grade beam provisions •

•

•

•

18 in ≥ minimum grade beam dimension ≥ column clear spacing divided by 20 (ACI 318 Sec. 21.12.3.2) closed ties the length of the grade beam, spaced a minimum of 12 in or half the smallest grade beam dimension (ACI 318 Sec. 21.12.3.2) grade beams may be thickened areas of slabs-on-grade grade beams resisting flexure from SFRS columns must be designed to similar requirements as the SFRS beams and conform to ACI 318 Sec. 21.5 (ACI 318 Sec. 21.12.3.3) STRC ©2016 Professional Publications, Inc.


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General Seismic Considerations SDC D, E, & F Foundation Requirements(continued) •

•

•

Slabs-on-ground must be designed as diaphragms when subject to seismic forces from walls or columns (ACI 318 Sec. 21.11 and 21.12.3.4). Piles, pier, and caisson requirements when supporting members of the SFRS are given in ACI 318 Sec. 21.12.4. ACI refers to slab on grade as “slab on ground.”



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General Seismic Considerations Levels of Seismic Detailing •

•

ACI 318 Chap. 21 uses the following terms (similar to steel seismic detailing). •

ordinary

•

intermediate

•

special

ACI 318 Table R21.1.1 summarizes the requirements of elements in the SFRS elements. STRC ©2016 Professional Publications, Inc.


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General Seismic Considerations Strength Reduction Factors (φ) •

•

Seismic strength reduction factors given in ACI 318 Sec. 9.3.4. φ is modified per ACI 318 Sec. 9.3.4(a) through (c) for structures that resist earthquake effects, E, using the following. •

intermediate precast structural walls in SDC D, E, or F

•


•




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General Seismic Considerations Strength Reduction Factors (φ) (continued) •

•

•

φ = 0.60 for shear if the nominal shear strength of the member is less than the shear corresponding to the development of the nominal flexural strength of the member (ACI 318 Sec. 9.3.4(a)) nominal flexural strength determined considering the most critical factored axial loads and including E typically applies only to short structural walls or wall piers between openings



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General Seismic Considerations Strength Reduction Factors (φ)(continued) •

•

•

For diaphragms, φ for shear must not exceed the minimum φ for shear used for the vertical components of the primary seismic-force-resisting system (ACI 318 Sec. 9.3.4(b)). For joints and diagonally reinforced coupling beams, for shear φ = 0.85 (ACI 318 Sec. 9.3.4(c)). Otherwise, φ = 0.75 for shear (as normal).



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General Seismic Considerations Minimum & Maximum Material Strengths •

for special moment frames and special structural walls only (ACI 318 Sec. 21.1.4 and 21.1.5): f c  3000 psi

•

minimum specified concrete compressive strength of

•

maximum specified reinforcement yield strength f y  60, 000 psi

•

flexural and axial rebar must be ASTM A706 grade 60 low-alloy steel: •

ASTM A615 billet steel bars of grade 40 or 60 may be used if tested for actual yield strength and complies with ACI 318 Sec. 21.1.5.2(a) and (b)



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General Seismic Considerations Minimum & Maximum Material Strengths (continued) •

maximum specified lightweight concrete compressive strength of

f c  5000 psi

(no maximum compressive strength for normal-weight concrete) •

prestressing steel requirements given in ACI 318 Sec. 21.1.5.3



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General Seismic Considerations Strong–Column/Weak–Beam Principle •

•

Strong–column/weak–beam relationship applies to reinforced concrete (similar to as discussed for steel seismic design) (ACI 318 Sec. 21.6.2.2) Substituting grade 60 rebar for grade 40 can be detrimental to the SFRS. •

•

•

(Providing reinforcement with higher tensile strengths than specified could increase shears beyond what is anticipated. (ACI 318 Sec. 21.1.5.2))

Rebar probable yield and fracture strengths must be known (similar to AISC seismic design requirements). Providing reinforcement with higher tensile strengths than specified could increase shears beyond what is anticipated. (ACI 318 Sec. 21.1.5.2) STRC ©2016 Professional Publications, Inc.


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General Seismic Considerations Splice Requirements for Special Frames and Walls •

•

mechanical splice requirements for special moment frames and special structural walls given in ACI 318 Sec. 21.1.6 welded splice requirements for special moment frames and special structural walls given in ACI 318 Sec. 21.1.7



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General Seismic Considerations Ordinary Moment Frame Requirements •

•

•

•

requirements given in ACI 318 Sec.21.2 ordinary moment frames permitted only in SDC A & B per ASCE7 Table 12.2-1 Beams must have at least two of the longitudinal bars continuous along both the top and bottom faces. (These bars are developed at the face of the support.) Columns having clear height, ℓ  5 , are designed for shear in accordance with ACI 318 Sec. 21.3.3.2. •

c1 = dimension of column measured in the direction of the span for which moments are being determined



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General Seismic Considerations Ordinary Moment Frame Requirements (continued)



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Intermediate Moment Frames Intermediate Moment Frame Requirements requirements given in ACI 318 Sec. 21.3

•

•

• •

•

beam requirements given in ACI 318 Sec. 21.3.4

beam shear strength requirements per ACI 318 Sec. 21.3.3.1 φVn not less than smaller of •

column requirements given in ACI 318 Section 21.3.5 •

•

intermediate moment frames not permitted in SDC D, E, and F per ASCE7 Table 12.2-1. STRC ©2016 Professional Publications, Inc.


shear associated with development of nominal moment strengths max. shear obtained from design load combinations that include E, with E taken as two times normal (2.0E, not oE)

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Intermediate Moment Frames Intermediate Moment Frame Shear Strength Requirements beam shear strength requirements per ACI 318 Sec. 21.3.3.1; φ Vn not less than the smaller of •

•

shear associated with development of nominal moment strengths, Mn, at each restrained end due to reverse curvature bending and shear from factored gravity loads max. shear obtained from design load combinations that include E, with E taken as two times normal (2.0E, not oE)



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Intermediate Moment Frames Intermediate Moment Frame Shear Strength Requirements (continued) •

For option ACI 318 Sec. 21.3.3.1(a) and 21.3.3.2(a), determine the design shear force as follows. •

•

•

Construct two free-body diagrams with end moments equal to nominal flexural strength (Mn) for the beams and columns in reverse curvature bending. (One diagram exhibits clockwise moments, the other counter-clockwise.) Consider nominal moment strengths (φ = 1.0 for Mn). The factored axial force, Pu, should be chosen for the largest flexural strength of the column, consistent with the direction of the lateral force considered. •

Use the flexural strength corresponding to the balanced point. STRC ©2016 Professional Publications, Inc.


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•

Assume that the nominal moment strengths are developed at each end of the member simultaneously. Include the beam shear due to factored gravity loads under the following load case. •

U  1.2D  fL f1 S

2

IBC Eq. 16-5



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For option ACI 318 Sec. 21.3.3.1(b) and 21.3.3.2(b), determine the design shear force as follows. •

Calculate the maximum shear force in the beam using the normal load combinations but with the earthquake effects, E, doubled. •

•

•

U  1.2 D

2.0  Ef L  fS1

U  0.9 D  2.0 E

2

IBC Eq. 16-5 (modified)

IBC Eq. 16-7 (modified)

Column shear is obtained by increasing E by o, not 2.0 (ACI 318 Sec. 21.3.3.2 (b)).



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Intermediate Moment Frames Intermediate Moment Frame Beams (ACI 318 Sec. 21.3.4.1) •

•

positive moment strength at face of the joint not less than one third of the negative moment strength at face of joint negative and positive moment strength at any section not less than one fifth of the max. moment strength at either joint



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Intermediate Moment Frames Intermediate Moment Frame Beams

•

smallest of

(continued) •

At both ends of beam, hoops (not stirrups) should be provided over lengths not less than 2h. •

•

Spacing of hoops must not exceed the

•

•

measured from the face of the supporting member toward midspan first hoop located not more than 2 in from the face of supporting member

•

(8)(diameter of the smallest long. bar enclosed)

•

(24)(diameter of hoop bar)

•

12 in

stirrups (away from hoop regions) spaced not more than d/2 throughout the length of the beam



d/4

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Intermediate Moment Frames Intermediate Moment Frame Beams (continued)



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Intermediate Moment Frames Intermediate Moment Frame Columns (ACI 318 Sec. 21.3.5.2 ) Columns should be spirally reinforced per ACI 318 Sec. 7.10.4 or conform with ACI 318 Sec. 21.3.5.2 through 21.3.5.4. ACI 318 Sec. 21.3.5.2 requires hoops at both ends of columns at spacing, so, over a length, ℓo, measured from the joint face.



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Intermediate Moment Frames Intermediate Moment Frame Columns (ACI 318 Sec. 21.3.5.2 continued) Hoop spacing, so, must not exceed the smallest of •

(8)(diameter of the smallest long. bar enclosed)(24)(diameter of hoop bar)

•

half of the smallest cross-sectional column dimension

•

12 in

Length ℓo must not be less than the largest of •

1/6 clear span of column ℓu

•

maximum cross-section dimension of the column

•

18 in



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Intermediate Moment Frames Intermediate Moment Frame Columns (continued) •

•

First hoop must be located not more than so/2 from the joint face (ACI 318 Sec. 21.3.5.3). Outside the length, ℓo, spacing of transverse reinforcement must conform to ACI 318 Secs. 7.10 and 11.4.5.1. (ACI 318 Sec. 21.3.5.4).



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Intermediate Moment Frames Intermediate Moment Frame Columns (continued) •

•

joint reinforcement must conform to ACI 318 Sec. 11.10 (ACI 318 Sec. 21.3.5.5) special transverse reinforcement requirements for columns supporting discontinuous stiff members (ACI 318 Sec. 21.3.5.6): •

•

•

structural walls supported on moment frame columns requires transverse reinforcement spacing, so, given ACI 318 Sec. 21.3.5.2 full length of column transverse reinforcement extends above and below the columns (required in ACI 318 Sec. 21.6.4.6(b))



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Intermediate Moment Frames Intermediate Moment Frames with TwoWay Slabs ACI 318 Sec. 21.3.6 gives requirements for two-way slabs without beams in intermediate moment frames.



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Intermediate Moment Frames Intermediate Precast Structural Walls Requirements for precast structural walls with intermediate detailing given in ACI 318 Sec. 21.4.



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Special Moment Frames Special Moment Frame General Requirements •

required for moment frames in seismic design category (SDC) D or above

•

beam requirements given in ACI 318 Sec. 21.5

•

column requirements given in ACI 318 Sec. 21.6 •

•

beam with large axial loads may fall under requirements of ACI 318 Sec. 21.6

joint requirements given in ACI 318 Sec. 21.7



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Special Moment Frames Special Moment Frame General Requirements (continued) •

•

•

•

•

precast moment frame requirements given in ACI 318 Sec. 21.8 special structural wall requirements and diagonally reinforced coupling beams given in ACI 318 Sec. 21.9 special precast structural wall requirements given in ACI 318 Sec. 21.10 ACI splits requirements between members with flexure only and members with axial and flexure. A “beam” with an axial load would fall under 21.6. add diagonally reinforced coupling beams



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Special Moment Frames Special Moment Frame Beams geometric constraints given in ACI 318 Secs. 21.5.1.2 through 21.5.1.4 n

d

•

clear span,

•

width to depth ratio

bw  0.3 h

•

width bw  10 in

•

width bw  c2  distance on each side of min 

 c2 0.75 c1

•

c1 = depth of supporting member parallel to direction of analysis

•

c2 = width of supporting member perpendicular to direction of analysis



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Special Moment Frames Special Moment Frame Beams (continued)



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Special Moment Frames Special Moment Frame Beams Minimum Reinforcement (ACI 318 Sec. 21.5.2.1) •

Except as provided in ACI 318 Sec. 10.5.3, top and bottom reinforcement in any section not less than •

•

•

3 f ' c  fy 

As , min  

 200bw d   bw d    fy 

reinforcement ratio ρ ≤ 0.025

at least 2 bars must be provided continuously at both top and bottom



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Special Moment Frames Special Moment Frame Beams Minimum Reinforcement (ACI 318 Sec.21.5.2.2) •

•

•

positive moment strength at face of the joint not less than half the negative moment strength at face of joint negative and positive moment strength at any section not less than half the maximum moment strength at either joint similar to the requirement for intermediate moment frames



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Special Moment Frames Special Moment Frame Beams Minimum Reinforcement (continued)

from PCA Notes on ACI 318-11



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Special Moment Frames Special Moment Frame Beams Transverse Reinforcement •

•

•

lap splices of flexural reinforcement only permitted if hoop or spiral reinforcement is provided over the lap length (ACI 318 Sec. 21.5.2.3) Spacing of hoop or spiral reinforcement must not exceed the smaller of •

•

d/4

Lap splices must not be used •

•

•

within the joints within a distance of twice the member depth from the face of the joint where analysis indicates flexural yielding is caused by inelastic lateral displacements of the frame

4 in STRC ©2016 Professional Publications, Inc.


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Example – Lap Splice Location During pre-pour inspections in the construction of a special moment frame, it is discovered that the contractor mislocated the lap splice for a 24 in deep beam at 36 in from the face of the column. Is this acceptable? If not, what remedies can be used?



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Example – Lap Splice Location Solution Lap splices are special moment frames which are not permitted within a distance of twice the members’ depth, so the lap splice is not permitted. However, options exist to salvage the construction without replacing the existing reinforcement. ACI 318 Sec. 21.5.2.3 is in regard to lap splices only. Mechanical splices or welded splices are permitted and can be used if the requirements of ACI 318 Sec. 21.1.6 (mechanical) or 21.1.7 (welded) are met. As the bars are already placed, assume a mechanical splice is the most feasible.



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Example – Lap Splice Location Solution (continued) Per ACI 318 Sec. 21.1.6, there are two types of splices. •

•

A type 1 mechanical splice has similar limitations to lap splices and is insufficient. A type 2 mechanical splice is permitted at any location in the special moment frame or special structural wall. (Use a type 2 mechanical splice in place of the srcinal lap splice.)



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Special Moment Frames Special Moment Frame Beams

•

smallest of

Transverse Reinforcement At both ends of beam and at locations where flexural yielding is likely to occur, hoops that meet ACI 318 Sec. 21.5.3.3 must be provided over lengths ≥ 2h from ends and 2h on either side of the flexural yielding location. •

•

Spacing of hoops must not exceed the

measured from the face of the supporting member toward midspan first hoop must be located ≤ 2 in from the face of supporting member

•

•

•

•

(6)(diameter of smallest long. bar enclosed) 6 in

Stirrups with seismic hooks at both ends (away from hoop required regions) must be spaced not more than d/2 throughout the length of the beam.



d/4

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Special Moment Frames Special Moment Frame Beams Transverse Reinforcement (continued)

Hoop and Stirrup Location and Spacing Requirements from NEHRP Seismic Design Technical Brief No. 1



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Special Moment Frames Special Moment Frame Beams Transverse Reinforcement (continued) •

bars nearest the tension and compression faces must have lateral support meeting ACI 318 Secs. 7.10.5.3 or 7.10.5.4 (ACI 318 Sec. 21.5.3.3) •

spacing of supported bars ≤ 14 in

•

does not apply to skin reinforcement



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Special Moment Frames Special Moment Frame Beams Transverse Reinforcement (continued) Hoops in flexural members can be made up of two pieces of reinforcement as outlined in ACI 318 Sec. 21.5.3.6. •

•

a stirrup, having seismic hooks at both ends and closed by a crosstie consecutive crossties engaging the same long. bar (must have their 90 hooks at opposite sides of the member) °

If long. rebar secured by crossties is confined by a slab on only one side, the crosstie 90 hooks must be placed on the side by the slab.



°

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Special Moment Frames Special Moment Frame Beams Transverse Reinforcement (continued)



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Special Moment Frames Special Moment Frame Shear Strength Requirements (ACI 318 Sec. 21.5.4) •

concrete shear strength ignored V ( c = 0) per ACI 318 Sec. 21.5.4.2 when: •

•

earthquake-induced shear force, Ve, per 21.5.4.1 represents half or more of the max. required shear strength Ag f c factored axial compressive force, Pu, including E, is less than 20



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Special Moment Frames Design Shear Force for Special Moment Frames Design shear force, Ve, is determined from assuming that the moment of opposite sign corresponding to the probable flexural strength, Mpr, acts at the joint faces (ACI 318 Sec. 21.5.4.1). •

•

member loaded with the factored tributary gravity load along its span similar to intermediate moment frame shear force determination per ACI 318 Sec. 21.3.3.1(a)



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Special Moment Frames Design Shear Force for Special Moment Frames (continued) For special moment frames, determine the design shear force, Ve, as follows. •

Construct two free-body diagrams with end moments equal to the probable flexural strength (Mpr) for the beams and columns in reverse curvature bending. •

•

•

One diagram for clockwise moments, the other for counter-clockwise moments Mpr based on the steel tensile stress of 1.25 fy For a rectangular section with tension reinforcement only,

 

M pr  1.25 As f y  d 

a where 2

a

1.25 As f y 0.85 f c'b



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Special Moment Frames Design Shear Force for Special Moment Frames (continued) •

•

•

End moment Mpr for columns must not be greater than moments generated by Mpr of the beams framing into the beam-column joints. The factored axial force, Pu, should be chosen for the largest flexural strength of the column, consistent with the direction of the lateral force considered. (Use the flexural strength corresponding to the balanced point.) Include the beam shear due to factored gravity loads under the following load case.

U  1.2D fL 1fS

2

IBC Eq. 16-5



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Special Moment Frames



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Special Moment Frames Special Moment Frame Columns •

Columns must be provided with sufficient strength so that they will not yield prior to the beam-column joint. •

•

strong–column/weak–beam principle

For this reason, columns are designed with 20% higher flexural strength as compared to beams meeting at the same joint.



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Special Moment Frames Special Moment Frame Columns (continued)

Fig. 29-7,“Strong–Column/Weak–Beam” Frame Requirements for Special Moment Frames, from PCA Notes on ACI 318-11 STRC ©2016 Professional Publications, Inc.


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Special Moment Frames Special Moment Frame Columns Longitudinal Reinforcement •

geometric constraints given in ACI 318 Sec. 21.6.1.1 and 21.6.1.2 •

•

shortest cross-sectional dimension cmin ≥ 12 in

•

longitudinal reinforcement requirements given in ACI 318 Sec. 21.6.3.1 and 21.6.3.2 •

cmin  0.4 cmax •

area of long. reinforcement, Ast, not less than 0.01Ag or more than 0.06Ag columns with circular hoops require at least 6 longitudinal bars



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Special Moment Frames Special Moment Frame Columns Longitudinal Reinforcement (continued) Mechanical and lap splice requirements given in ACI 318 Sec. 21.6.3.3. Lap splices •

permitted only within the center half of the column length

•

designed as tension lap splices

•

must be enclosed within transverse reinforcement conforming to ACI 318 Sec. 21.6.4.2 and 21.6.4.3



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Special Moment Frames Special Moment Frame Columns Transverse Reinforcement (continued) Transverse reinforcement requirements given in ACI 318 Sec. 21.6.4.1 through 21.6.4.7 •

•

Lo ≥ largest of the following.

transverse reinforcement required over length, Lo, from each joint face and on both sides of any section where flexural yielding is likely to occur, similar to intermediate moment frames (ACI 318 Sec. 21.6.4.1)



•

•

•

depth of the member at the joint face 1/6 the clear span of the member 18 in

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Special Moment Frames Special Moment Frame Columns Transverse Reinforcement (continued) Transverse reinforcement must be provided by single or overlapping spirals satisfying ACI 318 Sec. 7.10.4, circular hoops, or rectilinear hoops (with or without crossties) (ACI 318 Sec. 21.6.4.2). •

•

Crossties of the same or smaller bar size as the hoops are permitted. Each end of the crosstie must engage a peripheral long. bar. STRC ©2016 Professional Publications, Inc.


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Special Moment Frames Special Moment Frame Columns Transverse Reinforcement (continued) Spacing of transverse reinforcement along the length ℓo must not exceed the smallest of the following. •

1/6 of the min. column dimension

•

(6)(diameter of the smallest long. bar)

•

so  4 

14  hx 3

4in  so 6in Fig 29-8, Typical Lap Splice Details for Columns in Special Moment Frames,from PCA Notes on ACI STRC ©2016 Professional Publications, Inc.


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Special Moment Frames Special Moment Frame Columns Transverse Reinforcement (continued) The volumetric ratio of spiral or circular hoop reinforcement is given in ACI 318 Sec. 21.6.4.4(a).

Fig 29-9, Confinement Requirements at Column Ends (a) Spiral or Circular Hoop Reinforcement, from PCA Notes on ACI STRC ©2016 Professional Publications, Inc.


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Special Moment Frames Special Moment Frame Columns Transverse Reinforcement (continued) The cross-section area requirements of rectangular hoop reinforcement is given in ACI 318 Sec. 21.6.4.4(b).

Fig 29-9, Confinement Requirements at Column Ends (b) Spiral or Circular Hoop Reinforcement, from PCA Notes on ACI STRC ©2016 Professional Publications, Inc.


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Special Moment Frames ) Special Moment Frame Columns Transverse Reinforcement (continued •

•

amount of transverse reinforcement per ACI 318 Sec. 21.6.4.4 and 21.6.5 unless a larger amount is required by ACI 318 Sec. 21.6.3.3 (splices) or ACI 318 Sec. 21.6.5 (design shear strength, Ve), center-to-center spacing (s) of transverse reinforcement beyond ℓo ≤ smaller of the following •

(6)(the smallest long. column bars)

•

6 in



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Reinforced Concrete: Lateral Forces Example – Special Moment Frame Transverse Reinforcement A special moment frame column with a clear height of 10.5 ft has a compressive strength of 4000 lbf/in2 and a yield strength of 60,000 lbf/in2. The diameter of the bar is 1.25 in. The transverse reinforcement must be hoops of #4 bar. Ignoring shear strength requirements, what is the minimum required area and spacing of the transverse reinforcement in the length ℓ ?



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Reinforced Concrete: Lateral Forces Example – Special Moment Frame Transverse Reinforcement Solution •

•

The clear span is 10.5 ft (126 in). c1  c2  24 in

c

 21 in

6

 24 in  21in 18 in

(ACI 318 Sec. 21.6.4.1)

•

o

•

Transverse reinforcement must not exceed the following spacing. •

•

•

min. c1 or c2  4

6db in 4

 4s0

 14  hx  6  3 in  

ACI 318 Eq. 21-2



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Reinforced Concrete: Lateral Forces Example – Special Moment Frame Transverse Reinforcement Solution (continued) hx is the largest spacing between transverse hoops/ties in the column. hx must not be greater than 14 in. The hoop dimensions are 24 in 3 in  3in 0.5  in

17.5 in

This is too far, so cross-ties must be added. Assume that 3 cross-ties are required for minimum transverse reinforcement requirements.



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74



Reinforced Concrete: Lateral Forces Example – Special Moment Frame Transverse Reinforcement Solution (continued) hx  5.7in

The maximum spacing becomes 24 in

 6 in controls 4  6 1.25 in  7.5 in 14   5.7 in     5.43 inin 6 3  

so  4 

Use a spacing of 5 in for the transverse #4 hoops and cross-ties within a length of 24 in from the face of the column.



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Reinforced Concrete: Lateral Forces Example – Special Moment Frame Transverse Reinforcement Solution (continued) The minimum rectangular hoop area is calculated from the maximum of ACI 318 Eq. 21-4 and 21-5. The width of the confined core, bc, is 17.5 in.  lbf      5i n 17.5in 4,0 00 in 2     24 in 2       11   0.3   17.5 in2  lbf   60,000     in 2    lbf      5i n 17.5in 4,0 00 in 2   sbc fc'     0.53 in2 Ash  0.09   0.09   lbf f yt   60,000 2   in  

sb f '   Ag   Ash   0.3  c c    1 f yt   Ach  



2 .54 incontrols 



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Reinforced Concrete: Lateral Forces Example – Special Moment Frame Transverse Reinforcement Solution (continued) 2 For #4 bars 0.20 in2/bar, 1.54 in 2  7.7 legs (8 legs)

0.20 in

8 cross-ties and hoop legs may present congestion issues for the column. Revise for a smaller spacing of the hoops and cross-ties. Use a spacing of 3 in.  3 in  2  1.54in   0.925in  5 in 

Ash   0.925 in 2 0.20 in 2

2

 4.6 legs (5 legs)

Provide one hoop with three cross-ties (5 legs) of #4 bars, spaced at 3 in on center, within 24 in of the face of the joint, symmetrical in both orthogonal directions of the column. STRC ©2016 Professional Publications, Inc.


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Special Moment Frames Special Moment Frame Columns, Other Requirements •

•

•

•

ACI 318 Sec. 21.6.4.6 has special transverse reinforcement requirements for columns supporting discontinuous stiff members, such as walls. ACI 318 Sec. 21.6.4.7 has special transverse reinforcement requirements for when concrete cover outside transverse reinforcement is greater than 4 in. ACI Sec. 21.6.5 gives shear strength requirements. The shear reinforcement in the columns is based on the probable moment strengths Mpr that can be developed at the ends of the column, as discussed previously.



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Special Moment Frames Beam-Column Joints of Special Moment Frames (ACI 318 Sec. 21.7) Geometric constraints given in ACI 318 Sec. 21.7.2.1 through 21.7.2.3. •

•

•

Forces in long. beam bars at the joint face are determined by assuming that the stress in the flexural tensile rebar is 1.25 fy. Beam long. bars terminated in a column must extend to the far face of the confined core and anchored in tension per ACI 318 Sec. 21.7.5 and in compression per ACI 318 Chap. 12. For beam long. bars extending through a joint, the column dimension parallel to the bar beam reinforcement c1  20 diameter largest long.  •

For lightweight concrete, the dimension must not be less than 26 times the bar diameter.



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Special Moment Frames Beam-Column Joints of Special Moment Frames Transverse Reinforcement •

Joint transverse reinforcement must satisfy either ACI 318 Sec. 21.6.4.4(a) or (b) •

•

column transverse reinforcement requirements

Transverse reinforcement shall also satisfy •

ACI 318 Sec. 21.6.4.2 (spiral, circular hoop, and hoop crosstie requirements)

•

ACI 318 Sec. 21.6.4.3 (transverse reinforcement spacing requirements)

•

ACI 318 Sec. 21.6.4.7 (requirements for clear cover > 4 in), except as permitted in ACI 318 Sec. 21.7.3.2



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Special Moment Frames Beam-Column Joints of Special Moment Frames, Shear Strength •

joint shear strength requirements per ACI 318 Sec. 21.7.4

•

For normal weight concrete, shear strength is as follows.

•

Aj = effective cross-section area within a joint computed from joint depth times effective joint width (ACI 318 Sec. 21.7.4.1).



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Special Moment Frames Beam-Column Joints of Special Moment Frames, Shear Strength (continued)



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Special Moment Frames Additional Special Moment Frame Requirements •

ACI 318 Sec. 21.7.5 provides modified development length requirements.

•

Applies to bars in tension of special moment frames.

•

Key Requirements •

For bars with a standard 90 hook, the hook must be anchored within the confined core of a column or boundary element f y db °

 dh

•



ACI 318 Eq. 21-6

65 f c'

Straight bars terminated at a joint pass through the confined core. (Portions of ℓ not within the confined core will be increased by a factor of 1.6.) STRC ©2016 Professional Publications, Inc.


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Special Structural Walls Special Structural Walls (ACI 318 Sec. 21.9) •

ASCE/SEI7 refers to walls that are part of the SFRS as shear walls and special shear walls.

•

ACI 318 refers to them as structural walls and special structural walls.

•

special structural walls required for SFRS shear walls in SDC D, E, & F

•

•

hw  160 ft in SDC D and E

•

hw  100 ft in SDC F

•

hw  height of wall above base

above height limits can be increased per ASCE/SEI 7-10 Sec. 12.2.5.4



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Special Structural Walls Special Structural Walls (continued) Structural walls resist seismic forces though combination of flexure and shear. Boundary elements = areas at extreme ends of the structural walls. •

•

special boundary element detailing often required in special structural walls (ACI 318 Sec. 21.9.6) structural walls can have enlarged boundary elements (flanges) added to the ends STRC ©2016 Professional Publications, Inc.


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Special Structural Walls Special Structural Walls (continued) A vertical wall segment refers to a part of a wall bounded horizontally by openings or by an opening and an edge. For general structural wall design and vertical wall segments, ρt refers to the horizontal reinforcement ratio andρl refers to the vertical reinforcement ratio.



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Special Structural Walls Special Structural Walls (continued) A horizontal wall segment is a wall segment between two vertically aligned openings. •

•

This can also be a coupling beam if the segments are aligned. ρt refers to the vertical reinforcement ratio and ρl refers to the horizontal reinforcement ratio.



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Special Structural Walls Special Structural Walls (continued) Vertical wall segments may be further defined as wall piers, depending on the width to thickness ratio of the vertical wall segment (ACI 318 Sec. 21.9.8).



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Special Structural Walls Special Structural Walls (continued) •

•

Wall piers with columns

w

 2.5hw behave as

Wall piers with 2.5hw   w  6hw are designed as special moment frame columns, unless designed per ACI 318 Sec. 21.9.8.1(a) through (f).



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Special Structural Walls Special Structural Walls •

•

Properly designed structural walls have historically performed better than regular frame elements, resulting in greater levels of safety and damage control. Structural walls are relatively stiffer than frames and can be subjected to correspondingly higher lateral forces during seismic events. •

•

•

greater stiffness → less lateral displacement efficient solutions to multistory structures where a critical occupancy must be maintained or where post-seismic repairs must be kept to a minimum h



Isolated, slender structural walls with  w  2  behave as large cantilever beams and are  w  controlled by flexure at the base. STRC ©2016 Professional Publications, Inc.


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Special Structural Walls Special Structural Walls (continued) Coupled walls (multiple walls linked by short, rigid "coupling beams" at each story) typically exhibit significant plastic hinging at the coupling beams. w

 overall length of wall



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Special Structural Walls Special Structural Walls General Requirements Strain does not need to be proportioned to the distance from the neutral axis as is normally required by ACI Sec. 10.2.2 (ACI 318 Sec. 21.9.5.1). For seismic cyclic loading of special structural walls, it is often assumed that all vertical reinforcement yields in either tension or compression.



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Special Structural Walls Coupling Beams (ACI 318 Sec. 21.9.7) •

Coupling beams with  n  4 designed as a special moment frame beam. (ACI 318 h Sec. 21.9.7.1) •

•

•

h = height of coupling beam  n=

clear length of the coupling beam

Coupling beams with

n

h

 2 and Vu  4 c fcw' A must be reinforced with diagonal

reinforcement. (ACI 318 Sec. 21.9.7.2) •

Coupling beams with 2 

n

h

 4 may be designed using either section.



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Special Structural Walls Coupling Beams (continued) •

•

•

•

If diagonal bars are provided, they should be placed symmetrically at steep angles to the horizontal to be effective. Diagonal reinforcement elements must have at least 4 bars and must be confined. (Two confinement options are presented in ACI 318 Sec. 21.9.7.4(c) and (d).) Diagonal bars must be developed into the supporting wall a minimum of 1.25 the development length for fy (in tension). For diagonally reinforced coupling beams, Vn Af2vdy •

sin Af  10 c cw

ACI 318 Eq. 21-9

φv = 0.85 for diagonally reinforced coupling beams per ACI 318 Sec. 9.3.4



'

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94




Special Structural Walls Coupling Beams Confinement Options



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Special Structural Walls Coupling Beams Confinement Options



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Example – Coupling Beam Reinforcement A special structural shear wall in SDC E has a series of coupling beams between regularly spaced openings in the wall. An analysis is performed and the greatest factored shear is determined to be 480 kips for the coupling beam as shown in the figure. For the coupling beam shown the compressive strength of concrete is 4000 lbf/in2 and the yield strength is 60,000 lbf/in2. The bars are plain steel. What are the area and development requirements of the diagonal reinforcement (if such reinforcement is required)? STRC ©2016 Professional Publications, Inc.


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Example – Coupling Beam Reinforcement Solution Calculate the coupling beam reinforcement per ACI 318 Sec. 21.9.7. Check if diagonal reinforcement is required or if the coupling beam should be reinforced as a special moment frame beam.

Vu Acw



lbf 



kip 

 480kips  1000  f c' n

h

16 in 

82 in 76 in

76 in

4000



lbf

 6.24  4.0

(ACI 318 Sec. 21.9.7.2)

in 2

 1.08  2

The coupling beam requires diagonal reinforcement. A minimum of 4 bars per diagonal group is required per ACI 318 Sec. 21.9.7.4(b). STRC ©2016 Professional Publications, Inc.


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Example – Coupling Beam Reinforcement Solution (continued) Avd = area of reinforcement in each diagonal group.

Trial rebar configurations:

φ = 0.85 for shear in coupling beams (ACI 318 Sec. 9.3.4) Vn  Af2vdy

sin  Af  c10 cw

'

ACI 318 Eq. 21-9

(4) #11 bars = 6.24 in2

[insufficient]

(6) #10 bars = 7.62 in2

[insufficient]

(6) #11 bars = 9.36 in2

[acceptable]

Since Vn  Vu , rearrange and solve for Avd. Avd 

Vu  2 f y sin 



480 kips kips    0.85  2  60  2  sin3 6  in  

 8.01 in2



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Example – Coupling Beam Reinforcement Solution (continued) Using 6 #11 bars per diagonal group creates a balance between required reinforcement and reduced congestion. The larger bars will require a large development length. If insufficient wall length is provided outside the coupling beam, an increased number of smaller bars may be required.

Vn   2 vd A y f sin   561.1kips    76in lbf 16 in 10 f c' Acw  10 4, 000 2  in   lbf   1,000  kip   

   769 kips 561.1kips   


2016 © Professional Publications, Inc.

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Example – Coupling Beam Reinforcement Solution (continued) Calculate the required development length. Assume clear cover to diagonal bars outside of coupling beam is > db and that the clear spacing of the bars is > 2db. Because the bars have more than 12 in of concrete placed below them, ψt = 1.3 . The commentary in ACI 318 indicates that this factor should apply to inclined bars. Because the bars are uncoated,ψt = 1.3. Assume normal weight concrete.

d

 0.05 f y ψψ t  db   λ f' c 

 e   1.375 in  

    0.05    

 lbf    2 1.3 1.0   60,0 00 in    in 84.79 lbf  1.0  4,0 00 2  in 



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Example – Coupling Beam Reinforcement Solution (continued) Per ACI 318 Sec. 21.9.7.4(b) the diagonal bars must be anchored into the wall a minimum of 1.25 the development length of the yield strength.

1.25 d  105.99 in

(106 in)

The diagonal bars should be extended into the wall a minimum of 106 in.



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Special Structural Walls Diaphragm General Requirements •

Floor and roof slabs acting as diaphragms in the SFRS in SDC D, E, & F must be designed to ACI 318 Sec. 21.11 (also applies to collectors and trusses which are part of the SFRS).

•

General diaphragm principles regarding lateral force distribution apply.

•

Diaphragm shear strength is calculated per ACI 318 Sec. 21.11.9.

•

Cast-in-place composite and non-composite topping slabs over a precast floor or roof may be used to create a diaphragm resisting seismic forces.



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Special Structural Walls Diaphragm General Requirements (continued) •

Flexural design of diaphragms is similar to normal reinforced concrete beams and is performed per ACI 318 Sec. 10.2 and 10.3. •

•

Nonlinear strain distribution of deep beams need not be considered (ACI 318 Sec. 21.11.8).

•

Openings must be considered.

•

Areas of high axial compression may require confinement (ACI 318 Sec. 21.11.7.5) .

•

Shear strength of diaphragms per ACI 318 Sec. 21.11.9.

Add shear strength of diaphragms per ACI 318 Sec. 21.11.9.



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Non-SFRS Members Members Not Designated as Part of the SFRS •

given in ACI 318 Sec.21.13

•

applies to SDC D, E, & F only

•

Summary of Design Requirements •

•

Compute moments and shears in the members due to δu. Use the gravity load combinations •

•

•

1.2 D  f1 L 0.2 S 1.0 E 0.9 D  1.0 E

Determine the factored moment, Mu, and the factored shear,Vu. STRC ©2016 Professional Publications, Inc.


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Non-SFRS Members Members with High Axial Load •

If

u

 φM n , Vu  φVn , but Pu 

f 'c , satisfy the following (ACI 318 Sec. 21.13.3.2). 10 g

 Ast  0.06 Ag )

•

ACI 318 Sec. 21.6.3.1 ( 0.01

•

ACI 318 Sec. 21.6.4.2 (transverse reinforcement, see ACI 318 Fig. R21.6.4.2)

•

•

g

ACI 318 Sec. 21.6.5 (shear strength requirements similar to special moment frame columns) ties spaced ≤ so (given in ACI 318 Eq. 21-2) and provided full height of column •

6db of smallest long. bar   6 in  

so  minimum 



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Non-SFRS Members Members with High Axial Load (continued) •

•

Po = nominal axial strength at zero eccentricity If

u

 φM n , Vu  φVn , but Pu 

Ag f 'c 10

, satisfy the following. (ACI 318 Sec. 21.13.3.3)

•

all provisions from ACI 318 Sec. 21.13.3.2

•

ACI 318 Sec. 21.6.4.7 (additional transverse reinforcement if cover > 4 in)

•

50% of the hoops/spirals required by ACI 318 Sec. 21.6.4.4 with spacing ≤ so



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Non-SFRS Members Members with Seismic Loads Exceeding Their Elastic Design Strengths if M u  φM n , or Vu  φVn, or if induced moments are not calculated, then must satisfy the following (ACI 318 Sec. 21.13.4) •

•

ACI 318 Sec. 21.13.4.1 (material and splice requirements similar to special moment frames and special structural walls) ACI 318 Sec. 21.13.4.2 and 21.13.4.3 (see following slides)



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Non-SFRS Members Members with Seismic Loads Exceeding Their Elastic Design Strengths (continued) •

ACI 318 Sec. 21.13.4.2 (limited special moment frame beam detailing) g f 'c applies only if Pu  10 ACI 318 Sec. 21.5.2.1 (longitudinal beam reinforcement) •

•

•

•

ACI 318 Sec. 21.5.4 (shear strength requirements similar to special moment frame beams) stirrups spaced at no more than d/2 must be provided throughout the length of the member



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Non-SFRS Members Members with Seismic Loads Exceeding Their Elastic Design Strengths (continued) •

ACI 318 Sec. 21.13.4.3 (special moment frame column ductile detailing) g f 'c applies only if Pu  10 ACI 318 Sec. 21.6.3 (longitudinal reinforcement requirements) •

•

•

ACI 318 Sec. 21.6.4 (transverse reinforcement requirements)

•

ACI 318 Sec. 21.6.5 (shear strength requirements)

•

ACI 318 Sec. 21.7.3.1 (joint transverse reinforcement requirements)



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Non-SFRS Members Precast Members and Wall Piers •

•

Precast frame members must satisfy ACI 318 Sec. 21.13.5. (These are additional requirements beyond those discussed earlier.) Wall piers not part of the SFRS must satisfy ACI 318 Sec. 21.13.7. (Requires satisfying ACI 318 Sec. 21.9.8) •

Design shear forces may be determined from ACI 318 Sec. 21.6.5.1 or from 0 times the shear induced under design displacements δu.



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Non-SFRS Members Slab-Column Connections in Two-Way Slab Systems Slab-column connections in two-way slab systems without beams must meet ACI 318 Sec. 21.13.6. Slab shear reinforcement that meets ACI 318 Sec. 11.11.3 and 11.11.5 is required. •

•

Vs  3.5 fbd c'

o

Shear reinforcement should extend a minimum of 4 times the slab thickness from face of support.



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Non-SFRS Members Slab-Column Connections in Two-Way Slab Systems The shear reinforcement of ACI 318 Sec. 21.13.6 is not required if one of the following is satisfied. •

ACI 318 Sec. 21.13.6(a) The requirements of ACI 318 Sec. 11.11.7 are satisfied using the design shear Vug and the induced moment transferred between the slab and the column under the design displacement. STRC ©2016 Professional Publications, Inc.


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Non-SFRS Members Slab-Column Connections in Two-Way Slab Systems •

ACI 318 Sec. 21.13.6(b)

V  design story drift ratio < 0.005 or 0.035  0.05  ug   Vc  (whichever is larger) (calculation of design story drift ratio given by ACI 318 Sec. 21.13.6) •

Vug is the factored shear force on the slab critical section for two-way action, calculated for the load combination



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Non-SFRS Members Slab-Column Connections in Two-Way Slab Systems •

Vug is the factored shear force on the slab critical section for two-way action, calculated for the load combination



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Non-SFRS Members Anchoring to Concrete (2011 ACI 318 Appendix D) •

•

Convoluted requirements regarding seismic requirements were modified to clarify and simplify sections from the 2008 ACI 318 specification. IBC 2012 provisions were written based on ACI 318 2008 and replace many sections of the ACI 318-11 Appendix D, but due to the difference in editions these changes conflict with 2011 provisions.



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Non-SFRS Members Anchoring to Concrete (continued) •

IBC 2012 Sec. 1905.1.9 and 1905.1.10 modify ACI 318 D.3.3.4 through D.3.3.7 and D.4.2.2. •

•

•

•

IBC errata provide some clarification, but still use ACI 318-08 and directly adopt the 2008 ACI 318 for IBC 2012 Sec. 1905.1.9. Per the NCEES SE Exam Design Standards, the IBC 2012 should be used without supplements Modified App. D provisions give a large penalty for anchors in SDC C, D, E, and F and are counterintuitive.

ASCE/SEI7-10 also provides conflicting details regarding the correct application of overstrength factors for non-structural component anchorage. STRC ©2016 Professional Publications, Inc.


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Lesson Overview •

general seismic considerations

•

ordinary moment frames

•

intermediate moment frames

•


•


•

diaphragms

•

members not designed as part of the seismic force-resisting system

•

anchoring to concrete



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STRC22 Lateral Forces Concrete Seismic 0716

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