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Chapters 21, 22, 24, 25 – Toolbox Chapters

ACI 318-14:Reorganized for Design

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Toolbox Chapters

• Ch. 21 – Strength Reduction Factors• Ch. 22 – Sectional Strength• Ch. 24 – Serviceability Requirements• Ch. 25 – Reinforcement Details

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Toolbox Chapters

• Referenced mainly by member chapters in part or in whole

• Requirements organized by topic• Contain requirements that would otherwise

be repeated in several chapters• Allows simpler modification in future editions

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Ch. 21 – Strength Reduction Factors

• Ch. 21 is a simple reference chapter• Contains all reduction factors in tabular

form• Section 21.2 referenced by member

chapters

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Ch. 21 – Strength Reduction Factors

• Lists strength reduction factor requirements based on:– action or structural element– axial load, moment, and combined, – ends of prestressed members

• Special requirements for φ in structures resisting earthquake effects

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Ch. 21 – φ for pretensioned members *

Clarified φ for sections near the end of pretensioned members *

ACI 318-11, 12.9.3: Where bonding of a strand does not extend to end of member, and design includes tension at service load in precompressedtensile zone as permitted by 18.3.3, ld specified in 12.9.1 shall be doubled.

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Ch. 21 – φ for pretensioned members *

Clarified φ for sections near the end of pretensioned members *

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Ch. 22 – Sectional Strength

• Referenced in member chapters by section• Used as a reference for strength calculations• Not intended to be read from beginning to

end

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Ch. 22 – Sectional Strength• Chapter subheadings

– 22.1 Scope– 22.2 Design assumptions for moment and axial str.– 22.3 Flexural strength– 22.4 Axial strength or combined flexural and axial str.– 22.5 One-way shear strength– 22.6 Two-way shear strength– 22.7 Torsional strength– 22.8 Bearing– 22.9 Shear friction

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22.2 - Design assumptions for moment and axial design

• Maximum strain at extreme concrete fiber, εc = 0.003

• Neglect concrete tensile strength

• “Plane sections remain plane”

εt

εc = 0.003

c C = 0.85f’cba

T=Asfy

neutral axisa

d

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Flexural, axial, or combined strength

• 22.3 - Flexure– Mn based on assumptions in 22.2

• 22.4 - Axial or Combined Axial & Flexural– Mn and Pn based on assumptions in 22.2– Pn,max = (Factor) (Po)

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22.4.2.3 - Axial strength for prestressed member

Calculate Po (previously undefined) *

= . − − + − − .(Eq. 22.4.2.3)

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22.4.3.1 – Maximum axial tensile strength *

Added equation 22.4.3.1 for the maximum nominal axial tensile strength of any member *

Where:• fse = effective stress in prestressing reinforcement, after allowance for all prestress losses, psi• Δfp = increase in stress in prestressingreinforcement due to factored loads, psi

, = + + ∆ (22.4.3.1)

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Shear strengths

• 22.5 - One-way shear– Vn = Vc + Vs

• 22.6 - Two-way shear• vn = vc + vs

• 22.9 - Shear friction– Vn = μ Avf fy

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22.9 - Shear-Friction Behavior *

T= Avf fy

Shear Plane(Assumed Crack)

Shear FrictionReinforcement

Vn = Cμ = Tμ = Avf fyμ

Vn

Vn Slip

Vn =Cμ

Note: C=T

Vn

Gap

C

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Ch. 22 – Shear friction *

Clarification that shear friction does not apply when reinforcement is in compression *

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Torsion and bearing strengths

• 22.7 - Torsion– Torsion threshold – can

neglect torsional effect

– Cracking torsion – can design for cracking torsion if torsion can be redistributed

• 22.8 - Bearing– Bn = 0.85 A1 f’c

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Ch. 24 – Serviceability Requirements

• Referenced in member chapters by section• Not intended to be read from beginning to

end• Used as a reference for serviceability

calculations

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Ch. 24 – Serviceability Requirements

• Chapter subheadings – 24.1 Scope– 24.2 Deflections due to service-level gravity loads– 24.3 Distribution of flexural reinforcement in one-

way slabs and beams– 24.4 Shrinkage and temperature reinforcement– 24.5 Permissible stresses in prestressed concrete

flexural members

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24.2 - Deflections due to service-level gravity loads

• Allowable deflection limits• Time dependent deflections• Effective moments of inertia

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Ch. 24 – Change in permissible deflection *

• Table 24.2.2• Removed exception for exceeding allowable

deflection after attachment of nonstructural element when camber is provided

• Applies to roof or floor construction supporting or attached to nonstructural elements not likely to be damaged by large deflections

[4]Limit shall not be greater than tolerance provided for nonstructural elements. Limit may be exceeded if camber is provided so that total deflection minus camber does not exceed limit. *

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24.3 - Distribution of flexural reinforcement– Maximum spacing for crack control– Flanges of T-beams

24.4 - Shrinkage and temperature– Minimum reinforcement– Maximum spacing is the lesser of 5h or 18 in.

24.5 – Stresses in prestressed members

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Ch. 25 – Reinforcement Details

• Referenced in member chapters by section• Not intended to be read from beginning to

end

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Ch. 25 – Reinforcement Details

• Chapter subheadings – 25.1 Scope– 25.2 Minimum spacing of reinforcement– 25.3 Standard hooks, seismic hooks, crossties, and

minimum inside bend diameters– 25.4 Development of reinforcement– 25.5 Splices– 25.6 Bundled reinforcement– 25.7 Transverse reinforcement– 25.8 Post-tensioning anchorages and couplers– 25.9 Anchorage zones for post-tensioned tendons

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25.2 - Minimum spacing of reinforcement

• Nonprestressed bars in a horizontal layer– 1 in.– 1 bar diameter– (4/3) nominal maximum aggregate size

• Longitudinal reinforcement in columns, pedestals, struts, and boundary elements– 1.5 in.– 1.5 bar diameters– (4/3) nominal maximum aggregate size

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25.3 - Standard hooks, seismic hooks, crossties, and minimum inside bend diameters

*

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25.4 – Development of reinforcement

• Tension development length – Straight bar – Hooks– Headed bars– Mechanical anchors– Welded wire reinforcement– Strand

• Compression development of deformed bars and wires

• Reduction for excess reinforcement

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Development length table *

ACI 318-14, R25.4.2.4: Should be considered for inclined reinforcement

ACI 318-14, Table 25.4.2.4: Changed from “Top Bar” to “Casting Position” *

*

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Ch. 25 – Commentary – closely spaced headed bars *

• ACI 318-14, R25.4.4.2: Where closely spaced headed bars are used, the potential for concrete breakout failure exists. …concrete breakout failure can be precluded by providing anchorage length equal to or greater than d/1.5 …or by providing reinforcement in the form of hoops and ties…

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25.5 Splices

• Lap splices– Based on development lengths– Excess reinforcement reduction for development

length not permitted– Contact and noncontact– Bundled bars– Tension and compression

• End-bearing, mechanical, and welded splices

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Yield strength of mechanical and welded splices *

• Removed “full” splice language *• Eliminated mechanical and welded splices not

meeting 125% of fy for No. 5 bars and smaller *

ACI 318-11 ACI 318-14

Removed provision

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25.6 – Bundled reinforcement

• Bundled bars– Number– Size– Stagger– Development– Splices

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25.7 – Transverse reinforcement

• Configuration and anchorage • Stirrups• Ties

– Rectilinear– Circular

• Spirals• Hoops

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25.7.2.4.1 — Anchorage of Circular Tiesa) Ends shall overlap by at least 6 in.b) Ends shall terminate with std

hooks …c) Overlaps at ends of adjacent

circular ties shall be staggered around the perimeter enclosing the longitudinal bars.

New in 2011

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Fig. R25.7.2.3b — Continuous Ties *New in 2014

Continuously wound bars or wires can be considered as ties, provided their pitch and area are at least equivalent to the area and spacing of separate ties. *

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Post-tensioning anchorage

• 25.8 - Post-tensioning anchorages and couplers

• 25.9 - Anchorage zones for post-tensioned tendons