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Chapter 11 - Walls

ACI 318-14:Reorganized for Design


Walls

• Chapter sub-headings– 11.1 Scope– 11.2 General– 11.3 Design limits– 11.4 Required strength– 11.5 Design strength– 11.6 Reinforcement limits– 11.7 Reinforcement detailing – 11.8 Alternative method for out-of-plane slender

wall analysis



Scope

• Scope of walls– Ordinary structural walls– Precast and Tilt-up walls– Cantilever retaining walls

• Not covered– Plain concrete walls → Ch. 14– Special structural walls → Ch. 18– Grade beams → Ch. 13

11.1 Scope11.2 General11.3 Design limits11.4 Required strength11.5 Design strength11.6 Reinforcement limits11.7 Reinforcement detailing


Load distribution

• Effective length of wall for load distribution– Lesser of:

• Center-to-center of loads• Bearing width + 4h

– Shall not cross wall joints

Effective horizontal length of wall

heig

ht o

f wal

l

wall

conc. load

bearing plate




Minimum wall thickness, h

• Bearing • Nonbearing• Basement wall

Bearing wallHt > 8’-4” controlsNonbearing wallHt > 10’-0” controls

}}



Required strength

• Loads → Ch. 5• Analysis → Ch. 6

– Slenderness effects → Ch. 6




Simplified method

• Axial load and out-of-plane moment

• Resultant is within middle third

– 0.55 1 ℓ


h/3 h/3 h/3

Pe = M/P

h


In-plane shear

• Very similar to beam/slab shearVn = 2λ√f’chd + Avfytd/s ≤ 10√f’chd

• If wall height ≤ 2 ℓw, strut-and-tie method can be used


ℓw h

d = 0.8ℓw



Reinforcement limits

• In-plane Vu ≤ 0.5φVc– Min. reinf. → Table 11.6.1

• In-plane Vu > 0.5φVc– Min. ρℓ is greater of:

• 0.0025• ρℓ = 0.0025 + 0.5(2.5 - hw/ℓw)(ρt – 0.0025)• But need not exceed ρt

– Min. ρt = 0.0025


Longitudinal bars

Transverse bars


Spacing of longitudinal bars

• Maximum spacing s = lesser of 3h, 18 in.– If in-plane shear steel required, s ≤ ℓw/3

– If wall thickness h > 10 in.• Two layers of bars• ½ to 2/3 of reinf. placed near exterior face• Balance of reinf. placed near interior face


Longitudinal bars



Spacing of transverse bars

• Maximum spacing s = lesser of 3h, 18 in.– If in-plane shear steel required,

s ≤ ℓw/5

11.1 Scope11.2 General11.3 Design limits11.4 Required strength11.5 Design strength11.6 Reinforcement limits11.7 Reinforcement 5etailing

Transverse bars

s


Reinforcement around openings

• One No. 5 bar required for each layer of reinforcing

• Anchored to develop fy in tension• Window and door sized openings


ℓd (typ.)

1-No. 5 each layer of bars1-No. 5 each layer of bars



Alternative slender wall analysis

• Limitations– Constant cross section– Tension-controlled– φMn ≥ Mcr

– Pu ≤ 0.06f’cAg

– Δs ≤ ℓc/150• For walls within these limitations:

– Better estimate of deflections – Better estimate of slenderness effects

11.1 Scope11.2 General11.3 Design limits11.4 Required strength11.5 Design strength11.6 Reinforcement limits11.7 Reinforcement detailing11.8 Alternative slender wall analysis


Chapter 18 – Earthquake resistant structures

• Beams• Columns

• Beams• Columns• Two-way slabs• Precast structural

walls

• Beams• Columns• Beam-column joints• Moment frames

using precast concrete

• Diaphragms• Members not part

of seismic force resisting system

• Structural walls• Foundations

Ordinary Systems (Min. for SDC B)

Intermediate Systems (Min. for SDC C)

Special Systems (SDC D, E, F)



Ch. 18 – Special structural wall *

• Two curtains of steel minimum in some walls

• Several changes to requirements for walls and boundary elements.

• Clarify connection to code strain limits

• Walls and boundary elements need to meet minimum thickness

Shear wall

Pu

Mu

Vu

ℓw

hw Special boundary element

δu


Reinforcement → 18.10.2

• Vu > Acvλ√f’c, ρℓ and ρt ≥ 0.0025

• Vu > 2Acvλ√f’c or hw/ℓw ≥ 2.0 *, must have two curtains of steel

ACI 318-14, 18.10.2.2: At least two curtains of reinforcement shall be used in a wall if Vu > 2Acvλ√f’c or hw/ℓw ≥ 2.0, in which hw and ℓw refer to height and length of entire wall.



Reinforcement → 18.10.2

• Long. bars extend 0.8ℓw past where no longer needed (except at top)

• Develop 1.25fy in long. bars at yielding locations

• Mechanical and welded splices develop 1.25fy


Shear strength → 18.10.4• Vn = Acv(αcλ√f’c + ρtfy)• For segments of walls, hw/ℓw is

greater of entire wall or segment

• If hw/ℓw < 2.0, then ρℓ ≥ ρt

• For horizontal wall segments, the longitudinal bars are horizontal and the transverse bars are vertical

hw/ℓw

αc

3.0

2.0

1.5 2.0



Wall segments

• For vertical wall segments– Overall Vn ≤ 8Acv√f’c

– Segment Vn ≤ 10Acw√f’c

• For horizontal wall segments– Vn ≤ 10Acw√f’c

11 22 33

Acv wall = Acw1+Acw2+Acw3

Acw2

Vertical wall segments


Boundary elements → 18.10.6

• Two procedures• 18.10.6.2

– Walls or wall piers with hw/ℓw ≥ 2 * and continuous– Special compression zone boundary elements

required where c ≥ ℓw/[600(1.5δu)/hw] *



Special boundary element

• ACI 318-14, 18.10.6.2• c = depth of wall neutral axis to extreme

compressive fiber• Need boundary element if *≥ ℓ600 1.5 ℎ⁄• Ratio δu/hw shall not be taken less than 0.005.


Boundary element requirements

• Requirement per 18.10.6.2, 18.10.6.4, and 18.10.6.5



Hoop spacing for boundary bars *

ACI 318-14, 18.10.6.5 (a): If the boundary reinforcement ratio exceeds 400/fy, …hoop spacing < the lesser of 8 in. and 8db of the smallest vertical bars, except hoop spacing < the lesser of 6 in. and 6db within the greater of ℓw and Mu/4Vu above and below critical sections where yielding of longitudinal reinforcement is likely to occur as a result of inelastic lateral displacements.


Hoop spacing for boundary bars *

• For walls without special boundary elements

• Confinement of boundary bars



Boundary elements→ 18.10.6

• Second procedure• 18.10.6.3

– Calculate σc using linear model with gross section properties

– Special boundary elements at edges and openings when σc > 0.2f’c


Boundary element requirements

• Requirement per 18.10.6.3, 18.10.6.4, and 18.10.6.5



Boundary element dimensions

• ℓbe ≥ greater of (c-0.1ℓw) and c/2

• b ≥ hu/16 *• b ≥ 12 in. if hw/ℓw ≥ 2.0 *

and c/ℓw ≥ 3/8 • Develop horizontal bars

in special boundary element with hooks or straight bars


Ch. 18 – Wall & boundary element width *

• Width of wall compression zones (ACI 318-14, 18.10.6.4)



Transverse reinforcement

• Transverse reinforcement configuration per 18.7.5.2(a) to (e) and 18.7.5.3 for special moment frame columns

• Amount of trans. reinf. in Table 18.10.6.4(f)• Spacing

– Within ℓw or Mu/4Vu from critical section, s ≤ lesser of 6 in. or 6db *

– ρ > 400/fy, s ≤ lesser of 8 in. or 8db *


Ch. 18 – Structural truss elements *

xi

xi

hx is the largest xi

ACI 318-14, 18.12.11.1: For high axial forces in the strut of a structural truss (Pu > 0.3Agf’c) for high concrete compressive strength (f’c > 10.000 psi) every perimeter bar shall have lateral support, and hx shall not exceed 8 in.

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