318-14 Chapter 14 Code, Approved 2013-03-19

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CHAPTER 14 — COLUMNS 1

2

14.1 — Scope 3

4 14.1.1 — Provisions of this chapter shall apply to the design of nonprestressed ,and prestressed, 5 and composite columns. The provisions shall also apply to the design of reinforced concrete 6 pedestals: <10.1> 7 8

(a) pedestals 9 (b) composite columns constructed of concrete and structural steel 10

11 14.1.2 — The provisions of Chapter 25 shall apply for the design of pPlain concrete pedestals. 12 shall be designed according to Chapter 25. <22.2.1> 13 14 15

14.2 — General 16

17 14.2.1 — Materials 18 19 14.2.1.1 — Design properties for concrete shall conform to Chapter 5. <~> 20 21 14.2.1.2 — Design properties for steel reinforcement and structural steel used in composite 22 columns shall conform to Chapter 6. <~> 23 24 14.2.1.3 — Design properties for structural steel used in composite columns shall conform to 25 Chapter 6. <~> 26 27 14.2.2 – Composite columns 28 29 14.2.2.1 — Columns shall be designed as composite columns where If a structural steel shape, 30 pipe, or tubing is used as longitudinal reinforcement, the column shall be designed as a 31 composite column. <10.13.1> 32 33 14.2.3 — Connection to other members 34 35 14.2.3.1 — For cast-in-place columnsconstruction, beam-column and slab-column joints shall 36 satisfy the requirements of 17.2. <10.12> 37 38 39 14.2.3.2 — For precast columnsconstruction, connections shall satisfy the force transfer 40 requirements of 17.3. <~> 41 42 14.2.3.3 — Connections of columns to foundations shall satisfy the requirements of 17.4. <~> 43 44 45 46

Ref to 17.2 based on LB12-7

Ref to 17.3 based on LB12-7

Ref to 17.4 based on LB12-7

318-14 Chapter 14 Code, Approved 2013-03-19

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14.3 — Design limits 47

48

14.3.1 — Dimensional limits 49 50 14.3.1.1 — For columns of any cross-sectional shapewith a square, octagonal, or other shaped 51 cross section, it shall be permitted to base gross area considered, required 52 reinforcementminimum longitudinal reinforcement, and design strength on a circular section 53 with a diameter equal to the least lateral dimension of the actual shape. <10.8.3> 54 55 14.3.1.2 — For columns with cross sections larger than required by considerations of loading, it 56 shall be permitted to base gross area considered, minimum longitudinal reinforcementrequired 57 reinforcement, and design strength on a reduced effective area, not less than one-half the total 58 area. This provision shall not apply to columns in special moment frames designed in 59 accordance with Chapter 20. <10.8.4> 60 61 14.3.1.3 — For columns built monolithically with a concrete wall, the outer limits of the 62 effective cross section of the column shall not be taken greater than 1.5 in. outside the transverse 63 reinforcement. <10.8.2> 64 65 14.3.1.4 — For columns with two or more interlocking spirals, outer limits of the effective cross 66 section shall be taken at a distance outside the spirals equal to the minimum required concrete 67 cover. <10.8.1> 68 69 14.3.1.54 — If a reduced effective area is considered as permitted by 14.3.1.1 through 14.3.1.4, 70 14.3.1.2, or 14.3.1.3 are used, structural analysis and design of other parts of the structure that 71 interact with the column shall be based on the actual cross section.the effects of the actual cross 72 section on member stiffness shall be included in the structural analysis and considered in the 73 design of the other parts of the structure that interact with the column. <R10.8.2, R10.8.3, 74 R10.8.4> 75 76 14.3.1.5 — For columns with two or more interlocking spirals, outer limits of the effective cross 77 section shall be taken at a distance outside the spirals equal to the minimum required concrete 78 cover. <10.8.1> 79 80 14.3.1.6 — For composite columns with a concrete core encased by structural steel, the thickness 81 of the steel encasement shall be at least (a) for rectangular sections or (b) for circular sections: 82 <10.13.6.1> 83 84

(a) 3

y

s

fb

E

for each face of widthwhere b is the width of each face 85

86

(b) y

s

fh

E8 for circular sections of diameterwhere h is the section diameter 87

88

318-14 Chapter 14 Code, Approved 2013-03-19

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89

14.4 — Required strength 90

91 14.4.1 — General 92 93 14.4.1.1 — Required strength shall be calculated in accordance with the factored load 94 combinations defined in Chapter 7 and analysis procedures defined in Chapter 8. <~> 95 96 14.4.2 — Factored axial force and moment 97 98 14.4.2.1 —Pu and Mu occurring simultaneously for each applicable factored load combination 99 shall be considered. <~> 100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

14.5 — Design strength 118

119 14.5.1 — General 120 121 14.5.1.1 — Design strength at all sections along the column shall be in accordance with (a) 122

through (c)satisfy Sn ≥ U, including (a) to (c), for each applicable factored load combination. 123 Interaction between axial force and moment, as well as interactions between other load effects, 124 shall be considered. <9.1.1> <11.1.1> <11.5.3.5> 125 126 127

(a) Pn ≥ Pu 128 129

(b) Mn ≥ Mu 130 131

14.4.2 — Factored axial force and moment

14.4.2.1 — Combined Pu and Mu shall be considered where Pu is calculated considering

loads on all floors or roof and Mu is calculated considering loads on a single adjacent

span of the floor or roof under consideration. Loading condition giving the maximum

ratio of moment to axial load shall also be considered. <8.10.1> -- Provision reworded

as presented above.

14.4.2.2 — Consideration shall be given to the effect of unbalanced floor or roof loads

on both exterior and interior columns and of eccentric loading due to other causes.

<8.10.2> -- Provision deleted as it is covered in Chapter 8 (8.6.2.4).

14.4.3 — Factored shear

14.4.3.1 — When gravity load, wind, earthquake, or other lateral forces cause transfer of

moment at connections of framing elements to columns, the shear resulting from

moment transfer shall be considered. <11.10.1> -- Provision deleted as it is covered in

Chapter 17 (17.2.1.2).

318-14 Chapter 14 Code, Approved 2013-03-19

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(c) Vn ≥ Vu 132 133 14.5.2 — Axial load force and flexuremoment 134 135

14.5.2.1 —Pn and Mn shall be calculated in accordance with 9.4. <~> 136 137 14.5.2.2 — For composite columns, forces shall be transferred between the steel section and 138 concrete by direct bearing, shear connectors, or bond in accordance to the axial strength assigned 139 to each component. <~> <10.13.3> <10.13.4> 140 141 142 143 144 145 146 147 148 149 150 151 14.5.3 — Shear 152 153

14.5.3.1 — Vn shall be calculated in accordance with 9.5. <11.1.1> 154 155 14.5.4 — Torsion 156 157

14.5.4.1 — If u thT T as defined in 9.7, torsion shall be considered in accordance with Chapter 158

13. 159 160 161 162 163 164

14.6 — Reinforcement Limits 165

166

14.6.1 — Minimum and maximum longitudinal reinforcement 167 168 14.6.1.1 — For noncomposite columns columnswith average fpe < 225 psi, area of longitudinal 169 reinforcement Ast shall not be less than 0.01Ag or more than 0.08 Ag.satisfy Eq. (14.6.1.1). 170 <10.9.1> <18.11.2.1> <18.11.2.2> 171 172 0.01Ag ≤ Ast ≤ 0.08Ag Eq. (14.6.1.1) 173 174 14.6.1.2 — For prestressed columns with average fpe ≥ 225 psi, 14.6.1.1 need not be satisfied. 175 <18.11.2.1> <18.11.2.2> 176

Commentary will be added to indicate that torsion rarely needs to be considered in

column design.

The provision below has been rewritten to reflect composite design practice as presented

above.

14.5.2.2 — For composite columns with a structural steel core, axial load strength

assigned to concrete shall be transferred to the concrete by members or brackets in direct

bearing on the composite column concrete. Axial load strength not assigned to concrete

shall be transferred by direct connection to the structural steel shape, pipe, or tubing.

<10.13.3> <10.13.4>

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177 14.6.1.3 2 — For composite columns with a structural steel core, area of longitudinal reinforcing 178 bars located within the transverse reinforcement shall not be less than 0.01(Ag – Asx) or more 179 than 0.08(Ag – Asx). satisfy Eq. (14.6.1.1), where Ast is the area of longitudinal bars and Ag is the 180 net area of the concrete section. <10.13.7.3> <10.13.8.5> 181 182 14.6.1.4 — For composite columns with a concrete core encased by structural steel, minimum 183 longitudinal reinforcement shall not be required. <10.13.6> 184 185 14.6.2 — Minimum shear reinforcement 186 187 14.6.2.1 — A minimum area of shear reinforcement, Av,min, shall be provided in all regions 188

where u cV V0.5 . <11.4.6.1> 189

190 14.6.2.2 — If shear reinforcement is required, Av,min/s shall be the greater of (a) or (b). 191 <11.4.6.3> <11.4.6.4> 192 193

(a)

'0.75 wc

yt

b sf

f 194

(b)

50 w

yt

b s

f 195

in accordance with Table 14.6.2.2 <11.4.6.3> <11.4.6.4> 196 197

Table 14.6.2.2 —Required Av,min/s 198

Column type

Av,min/s

Nonprestressed

and

Prestressed with effective prestress

force < 40 percent of the tensile

strength of the flexural reinforcement

Greater

of:

'0.75 wc

yt

bf

f

(a)

50 w

yt

b

f

(b)

Prestressed with effective prestress

force ≥ 40 percent of the tensile

strength of the flexural reinforcement

Lesser

of:

Greater of:

'0.75 wc

yt

bf

f

(c)

50 w

yt

b

f

(d)

80

ps pu

yt w

A f d

f d b

(e)

199 200

14.7 — Reinforcement: detailing 201

202

318-14 Chapter 14 Code, Approved 2013-03-19

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14.7.1 — General 203 204 14.7.1.1 — Concrete cover for reinforcement shall be in accordance with 6.8.1. <~> 205 206 14.7.1.2 — Development lengths of deformed and prestressed reinforcement shall be calculated 207 in accordance with 21.4. <~> 208 209 14.7.1.3 — Bundled bars shall be detailed in accordance with 21.6. <~> 210 211 14.7.1.4 — The most restrictive requirements for reinforcement spacing and placement shall 212 apply. <11.5.3.8> 213 214 14.7.2 — Reinforcement spacing 215 216 14.7.2.1 — Minimum spacing s shall be in accordance with 21.2. <~> 217 218 219 14.7.32 — Longitudinal reinforcement 220 221 14.7.2.1 — Minimum spacing s shall be in accordance with 21.2. <~> 222 223 14.7.3.12.2 — For nonprestressed columns and prestressed columns with average fpe < 225 psi, 224 the minimum number of longitudinal bars shall satisfy be in accordance with (a), (b), or (c): 225 <10.9.2> <18.1.3> 226 227

(a) 3 within triangular ties; 228 (b) 4 within rectangular or circular ties; 229 (c) 6 enclosed by spirals or for columns of special moment frames enclosed by circular 230 hoops. 231

232 14.7.3 — Longitudinal reinforcement of composite columns with structural steel cores 233 234 14.7.3.12 — For composite columns with structural steel cores, A a longitudinal bar shall be 235 located at every corner of a rectangular cross section, with other longitudinal bars spaced not 236 farther apart than one-half the least side dimension of the composite column. <10.13.8.6> 237 238 14.7.3.2 — Ends of structural steel cores shall be accurately finished to bear at end bearing 239 splices, with positive provision for alignment of one core above the other in concentric contact. 240 Bearing shall be considered effective to transfer not greater than 50% of the total compressive 241 stress in the steel core. <7.8.2.1> <7.8.2.2> 242 243 14.7.4 – Offset bent longitudinal reinforcement 244 245 14.7.4.1 — The slope of the inclined portion of an offset bent longitudinal bar relative to the 246 longitudinal axis of the column shall not exceed 1 in 6. with axis of column. Portions of bar 247 above and below an offset shall be parallel to axis of column. <7.8.1.1> <7.8.1.2> 248

318-14 Chapter 14 Code, Approved 2013-03-19

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249 14.7.4.2 — If the column face is offset 3 in. or greater, longitudinal bars shall not be offset bent. 250 Separate dowels, lap spliced with the longitudinal bars adjacent to the offset column faces, shall 251 be provided. <7.8.1.5> 252 253 14.7.5 — Splices of longitudinal reinforcement 254 255 14.7.5.1 — General 256 257 14.7.5.1.1 — Lap splices, mechanical splices, butt-welded splices, and end-bearing splices shall 258 be permitted. <12.17.1> 259 260 14.7.5.1.2 — Splices shall satisfy requirements for all factored load combinations. <12.17.1> 261 262 14.7.5.1.3 — Splices of deformed reinforcement shall be in accordance with 21.5 in addition to 263 14.7.5.2 for lap splices or 14.7.5.3 for end-bearing splices. <~> 264 265 14.7.5.2 — Lap splices 266 267 14.7.5.2.1 — If the bar stress force due to factored loads is compressive, compression lap splices 268 shall be permitted. It shall be permitted to decrease the compression lap splice length in 269 accordance with (a) or (b), but the lap splice length shall not be less than 12 in. <12.17.2.1> 270

271 (a) For tied columns, where ties throughout the lap splice length have an effective area 272 not less than 0.0015hs in both directions, lap splice length shall be permitted to be 273 multiplied by 0.83. Tie legs perpendicular to dimension h shall be used in determining 274 effective area. <12.17.2.4> 275

276 (b) For spiral columns, where spirals throughout the lap splice length satisfy 21.8.3, lap 277 splice length shall be permitted to be multiplied by 0.75.<12.17.2.5> <R12.17.2.5> 278

279 14.7.5.2.2 — If the bar stress force due to factored loads is tensile, tension lap splices shall be 280 provided according to Table 14.7.5.2.2. <12.17.2.2> <12.17.2.3> 281 282

Table 14.7.5.2.2 – Tension lap splice class 283

Tensile Bar

Stress Splice Details Splice Type

0.5 yf

≤ 50% bars spliced at any section

and

alternate lap splices on adjacent bars

staggered by at least d

Class A

Other Class B

0.5 yf All cases Class B

284 285

318-14 Chapter 14 Code, Approved 2013-03-19

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14.7.5.3 — End-bearing splices 286 287 14.7.5.3.1 — If the bar stress force due to factored loads is compressive, end-bearing splices 288 shall be permitted provided the splices are staggered or additional bars are provided at splice 289 locations. The continuing bars in each face of the column shall have a tensile strength not less 290 than 0.25fy times the area of the vertical reinforcement in along that face. <12.17.4> 291 292 14.7.5.3.2 — For composite columns, ends of structural steel cores shall be accurately finished to 293 bear at end-bearing splices, with positive provision for alignment of one core above the other in 294 concentric contact. Bearing shall be considered effective to transfer not greater than 50 percent 295 of the total compressive force in the steel core. <7.8.2.1> <7.8.2.2> 296 297 298 14.7.6 — Transverse Reinforcement 299 300 14.7.6.1 — General 301 302 14.7.6.1.1 — Transverse reinforcement shall be provided in accordance with this section. The 303 most restrictive requirements for reinforcement spacing shall apply. <~> 304 305 14.7.6.1.2 — Details of transverse reinforcement shall be in accordance with 21.8.2 for ties, 306 21.8.3 for spirals, or 21.8.4 for hoops. <7.11.2> 307 308 14.7.6.1.3 —For prestressed columns with average fpe ≥ 225 psi, transverse ties or hoops need 309 not satisfy the 16db spacing requirement of 21.8.2.1 <18.11.2.2> 310 311 14.7.6.1.4 — For composite columns with a structural steel core, transverse ties or hoops shall 312 have a minimum db of 0.02 times the greater side dimension of the composite column but shall 313 be at least No. 3 and need not be larger than No. 5. Spacing shall satisfy 21.8.2.1 but not exceed 314 0.5 times the least dimension of the composite column. Deformed wire or welded wire 315 reinforcement of equivalent area shall be permitted. <10.13.8.3> <10.13.8.4> 316 317 14.7.6.1.3 5 —Longitudinal reinforcement shall be laterally supported using ties or hoops in 318 accordance with 14.7.6.2 or spirals in accordance with 14.7.6.3 unless tests and structural 319 analyseis demonstrate adequate strength and feasibility of construction. <7.10.1> <7.10.2> 320 <7.10.3> 321 322 14.7.6.1.64 — If anchor bolts are placed in the top of a column or pedestal, the bolts shall be 323 enclosed by transverse reinforcement that also surrounds at least four longitudinal bars within the 324 column or pedestal. The transverse reinforcement shall be distributed within 5 in. of the top of 325 the column or pedestal and shall consist of at least two No. 4 or three No. 3 bars. <7.10.5.7> 326 327 14.7.6.2 — Lateral support of longitudinal bars using ties or hoops 328 329 14.7.6.2.1 — In any story, the bottom tie or hoop shall be located not more than one-half the tie 330 or hoop spacing above the top of footing or slab. <7.10.5.5> <18.11.2.2(c)> 331

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332 14.7.6.2.2 — In any story, the top tie or hoop shall be located not more than one-half the tie or 333 hoop spacing below the lowest horizontal reinforcement in the slab, drop panel, or shear cap. If 334 beams or brackets frame into all sides of the column, the top tie or hoop shall be located not 335 more than 3 in. below the lowest horizontal reinforcement in the shallowest beam or bracket. 336 <7.10.5.5> <7.10.5.6> <18.11.2.2(c & d)> 337 338 14.7.6.2.3 —For prestressed columns with average fpe ≥ 225 psi, transverse reinforcement shall 339 be at least No. 3 in size and center-to-center spacing shall not exceed the lesser of 48db of tie bar 340 and least dimension of column. Deformed wire or welded wire reinforcement of equivalent area 341 shall be permitted. 342 343 14.7.6.2.4 — For composite columns with a structural steel core, spacing of transverse 344 reinforcement shall be in accordance with 21.8.2.1 but not to exceed 0.5 times the least 345 dimension of the composite column. Transverse reinforcement shall have a minimum db of 0.02 346 times the greater side dimension of the composite column but shall not be smaller than No. 3 and 347 is not required to be larger than No. 5. <10.13.8.3> <10.13.8.4> 348 349 14.7.6.3 — Lateral support of longitudinal bars using spirals 350 351 14.7.6.3.1 — In any story, the bottom of the spiral shall be located at the top of footing or slab. 352 <7.10.4.6> 353 354 14.7.6.3.2 — In any story, the top of the spiral shall be located as provided in Table 14.7.6.3.2. 355 356

Table 14.7.6.3.2 —Spiral extension requirements at top of column 357

Framing at column end Extension requirements

Beams or brackets frame into

all sides of the column

Extend to the level of the lowest horizontal

reinforcement in members supported above.

<7.10.4.6>

Beams or brackets do not

frame into all sides of the

column

Extend to the level of the lowest horizontal

reinforcement in members supported above.

Additional column ties shall extend above

termination of spiral to bottom of slab, drop

panel, or shear cap. <7.10.4.7>

Columns with capitals

Extend to the level at which the diameter or

width of capital is twice that of the column.

<7.10.4.8>

358 359 14.7.6.4 — Lateral support of offset bent longitudinal bars 360 361 14.7.6.4.1 — Where longitudinal bars are offset, horizontal support shall be provided by ties, 362 hoops, spirals, or parts of the floor construction and shall be designed to resist 1.5 times the 363 horizontal component of the calculated force in the inclined portion of the offset bar. <7.8.1.3> 364

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365 14.7.6.4.2 — If transverse reinforcement is provided to resist forces that result from offset bends, 366 ties, hoops, or spirals shall be placed not more than 6 in. from points of bend. <7.8.1.3> 367 368 14.7.6.5 — Shear 369 370 14.7.6.5.1 —If required, shear reinforcement shall be provided using ties, hoops, or spirals. 371 <11.5.6.2> <11.5.4.1> <7.11.3> <7.11.2><7.10.1> 372 373 14.7.6.5.2 — Maximum spacing of shear reinforcement shall be in accordance with Table 374 14.7.6.5.2. <11.4.5> <11.4.5.1> <11.4.5.3> 375 376

Table 14.7.6.5.2 — Maximum spacing of shear reinforcement 377

Vs

Maximum s, in.

Nonprestressed

Column

Prestressed

Column

'4 c wf b d Lesser of: 2d

34h

24

'4 c wf b d Lesser of: 4d

38h

12

378 379

ACI 318-14 CR145/LB13-5 2013-08-07

CHAPTER 14—Columns 1 COMMENTARY 2

3 Notes: 4 1. Written for the revision of Columns chapter available on the reorganization website as 5 the Latest Version. 6 2. Purple highlights indicate reference numbers that will need to be updated. 7 3. Revisions of the 318-11 commentary relative to the first ballot are noted in red 8 strikeout/underline. 9 10 Ballot History: 11 This commentary was balloted by Sub D on LB13D-2. Ballot results were Y-5, N-8, C-0, 12 and A-0. There were a total of 111 comments of which 34 were negatives. All comments 13 were addressed, and all negatives were resolved. This commentary was balloted for the 14 first time by the main committee on this ballot, LB13-4. The comments from this ballot 15 were subsequently addressed by Sub D at the summer meeting in San Antonio. This is the 16 second ballot by the main committee on LB13-5. 17 18 19 R14.2 — General 20 21 R14.2.2 — Composite columns 22 23 R14.2.2.1 — Composite columns include both structural steel sections encased in 24 concrete and hollow structural steel sections filled with concrete. Composite columns are 25 defined without reference to classifications of combination, composite, or concrete-filled 26 pipe column. Reference to other metals used for reinforcement has been omitted because 27 they are seldom used in concrete construction. <R10.13.1> 28 29 30 R14.3 — Design limits 31 32 R14.3.1 — Dimensional Limits 33 34 R14.3.1 — With the 1971 Code, Explicit minimum sizes for columns are not specified 35 were eliminated to permit the utilization use of reinforced concrete columns with small 36 cross sections in lightly loaded structures, such as low-rise residential and light office 37 buildings. When If small cross sections are used, there is a greater need for careful 38 workmanship, and shrinkage stresses have increased significance. <R10.8> 39 40 For column design,10.25 the Code provisions for quantity of reinforcement, both vertical 41 and spiral, are based on the gross column area and core area, and the design strength of 42 the column is based on the gross area of the column section. 43 44 R14.3.1.2 — In some cases, however, the gross area of a column is larger than necessary 45 to resist the factored load. In those cases, the minimum reinforcement percentage may be 46

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calculated on the basis of the required area rather than the provided area but cannot be 47 less than one-half percent of the actual cross sectional area. 48 49 The basis of 14.3.1.1 through 14.3.1.4 is that it is satisfactory to design a column of 50 sufficient size (called “the reduced effective area” in 14.3.1) to carry the factored load 51 and then simply add concrete around the designed section without increasing the 52 reinforcement to meet the minimum percentages required by 14.6.1.1. The additional 53 concrete should not be considered as carrying load; however, the effects of the additional 54 concrete on member stiffness should be included in the structural analysis. The effects of 55 the additional concrete also should be considered in design of the other parts of the 56 structure that interact with the oversized member as is required by 14.3.1.5 <R10.8.2, 57 R10.8.3, R10.8.4> 58 59 Note: the last part of the deleted commentary is no longer need as this has become a code 60 provision in 14.3.1.5. 61 62 R14.3.1.6 — Steel-encased concrete sections should have a steel metal wall thickness 63 large enough to attain the longitudinal yield stress before buckling outward. <R10.13.6> 64 65 66 R14.4 — Required strength 67 68 R14.4.2 — Factored axial force and moment 69 70 R14.4.2.1 — The critical load combinations may be difficult to discern by inspection. As 71 illustrated in Figure R14.4.2.1, only considering the factored load combinations with 72 maximum axial force (P1) and with maximum bending moment (P2) does not ensure a 73 safe design for all load combinations such as P3. <R8.10> 74 75 Section 8.10 has been developed with the intent of making certain that the most 76 demanding combinations of axial load and moments be identified for design. <R8.10> 77 78

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79 80 Fig. R14.4.2.1 – Critical column load combination 81 82 83 R14.5 — Design strength 84 85 R14.5.1 — General 86 87 R14.5.1.1 —The design conditions 14.5.1.1 (a) through (c) list the typical forces that 88 need to be considered for a column. However, the general condition nS Uφ ≥ indicates 89 that all forces, such as torsion, that are relevant for a given structure need to be 90 considered. <~> 91 92 93 R14.5.2 — Axial force and moment 94 95 R14.5.2.2— Force transfer between structural steel and concrete can be accomplished 96 using various resistance mechanisms. AISC 360(XX) provides guidance regarding the 97 calculation of force transfer capacity in composite columns. 98 99 While bond can be considered as a force transfer mechanism, it may not be appropriate 100 for certain cases. For example, bond is typically considered for the strength of filled 101

Mu  

Pu

Mu  maxMu3Mu1

Pu  max

Pu2

Pu3

P1

P2

P3

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composite columns. However, a concrete encasement around a structural steel shape may 102 stiffen a column, but not necessarily increase its strength. AISC 360 does not permit 103 bond to be considered for concrete encased steel columns and does not permit bond to be 104 combined with other transfer mechanisms. <R10.13.3><R10.13.4> 105 106 Add new reference: 107 108 XX. AISC 360-10, “Specification for Structural Steel Buildings”, American Institute of 109 Steel Construction, Chicago, IL, 2010. 110 111 NOTE: Relevant section to review in AISC 360-10 is I6.3. 112 113 R14.5.3 — Shear 114 115 NOTE: Request sent to Sub E to provide guidance in commentary of Section 9.5 on 116 calculation of d when all reinforcement is in compression. This will be balloted as new 117 business. 118 119 R14.5.4 —Torsion acting on columns in buildings is typically negligible; torsion is rarely 120 a governing factor in the design of columns. 121 122 123 R14.6 — Reinforcement limits 124 125 R14.6.1 — Minimum and maximum longitudinal reinforcement 126 127 R14.6.1.1 — Limits are provided for both the minimum and maximum longitudinal 128 reinforcement ratios. 129 This section prescribes the limits on the amount of longitudinal reinforcement for 130 noncomposite compression members. If the use of high reinforcement ratios would 131 involve practical difficulties in the placing of concrete, a lower percentage and hence a 132 larger column, or higher strength concrete or reinforcement (see R6.2.2) should be 133 considered. The percentage of reinforcement in columns should usually not exceed 4 134 percent if the column bars are required to be lap spliced. <R10.9.1> 135 136 Minimum reinforcement — Since the design methods for columns incorporate separate 137 terms for the load carried by concrete and by reinforcement, it is necessary to specify 138 some minimum amount of reinforcement to ensure that only reinforced concrete columns 139 are designed by these procedures. Reinforcement is necessary to provide resistance to 140 bending, which may exist whether or not computations show that bending exists 141 regardless of analytical results, and to reduce the effects of creep and shrinkage of the 142 concrete under sustained compressive stresses. Tests have shown that creep Creep and 143 shrinkage tend to transfer load from the concrete to the reinforcement, with a consequent 144 increase in stress in the reinforcement, and that this the resultant increase in 145 reinforcement stress is becomes greater as the reinforcement ratio decreases. Therefore, a 146 minimum limit is placed on the reinforcement ratio to prevent reinforcement from 147

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yielding under sustained service loads.10.26 Unless a lower limit is placed on this ratio, 148 the stress in the reinforcement may increase to the yield level under sustained service 149 loads. This phenomenon was emphasized in the report of ACI Committee 10510.26, and 150 minimum reinforcement ratios of 0.01 and 0.005 were recommended for spiral and tied 151 columns, respectively. However, in all editions of the Code since 1936, the minimum 152 ratio has been 0.01 for both types of transversely reinforced columns. <R10.9.1> 153 154 Maximum reinforcement —The amount of longitudinal reinforcement must be limited 155 to ensure that concrete can be effectively consolidated around the bars and to ensure that 156 columns designed according to the Code are similar to the test specimens by which the 157 Code was calibrated. Extensive tests of the ACI column investigation10.26 included 158 reinforcement ratios no greater than 0.06. Although other tests with as much as 17 159 percent reinforcement in the form of bars produced results similar to those obtained 160 previously, it is necessary to note that the loads in these tests were applied through 161 bearing plates on the ends of the columns and the problem of transferring a proportional 162 amount of the load to the bars was thus minimized or avoided. Maximum ratios of 0.08 163 and 0.03 were recommended by ACI Committee 10510.26 for spiral and tied columns, 164 respectively. In the 1936 Code, these limits were made 0.08 and 0.04, respectively. In the 165 1956 Code, the limit for tied columns with bending was raised to 0.08. Since the 1963 166 Code, it has been required that bending be considered in the design of all columns, and 167 the maximum ratio of 0.08 has been applied to both types of columns. This The 0.08 limit 168 can also be considered a practical maximum for longitudinal reinforcement in terms of 169 economy and requirements for placing. The percentage of Longitudinal reinforcement in 170 columns should usually not exceed 4 percent if the column bars are required to be lap 171 spliced, as the lap splice zone will have twice as much reinforcement if all lap splices 172 occur at the same location. <R10.9.1> 173 174 R14.6.1.2 — Longitudinal and transverse reinforcement is necessary to prevent spalling 175 and ensure that the concrete outside the structural steel core behaves as reinforced 176 concrete. Limitations on longitudinal reinforcement are necessary for the reasons 177 described in R14.6.1.1. Transverse reinforcement requirements are provided in 178 14.7.6.1.4. <~> 179 180 For composite columns with a concrete core encased by structural steel, no reinforcing 181 bars are required. The minimum steel wall thickness of 14.3.1.6 inherently provides 182 adequate reinforcement. <~> 183 184 R14.6.2 — Minimum shear reinforcement 185 186 R14.6.2 — The basis for the minimum shear reinforcement is the same for columns and 187 beams. See R13.6.3 for more information. 188 189 190 R14.7 — Reinforcement detailing 191 192 R14.7.3 — Longitudinal reinforcement 193

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194 R14.7.3.1 — Prestressed columns with average fpe ≥ 225 psi are exempted from this 195 requirement based on a history of successful performance of prestressed members with 196 fewer longitudinal bars than required by 14.7.3.1 <~> 197 198 For compression members, a A minimum of four longitudinal bars are is required when 199 bars are enclosed by rectangular or circular ties. For other shapes, one bar should be 200 provided at each apex or corner and proper transverse reinforcement provided. For 201 example, tied triangular columns require a minimum of three longitudinal bars, with one 202 at each apex of the triangular ties. For bars enclosed by spirals, six bars are required. 203 <R10.9.2> 204 205 When If the number of bars in a circular arrangement is less than eight, the orientation of 206 the bars will may significantly affect the moment strength of eccentrically loaded 207 columns and should be considered in design. <R10.9.2> 208 209 R14.7.5 — Splices of longitudinal reinforcement 210 211 R14.7.5.1 — General 212 213 R14.7.5.1.2 — Note that the column splice should satisfy requirements for all load 214 combinations for the column. Frequently, the basic gravity load combination will govern 215 the design of the column itself, but a load combination including wind or seismic loads 216 earthquake effects may induce greater tension in some column bars.; and the column 217 Each bar splice should be designed for this the maximum calculated bar tensile force. 218 <R12.17> 219 220 R14.7.5.2 — Lap splices 221 222 In columns subject to flexure moment and axial loadsforce, tension tensile stresses may 223 occur on one face of the column for moderate and large eccentricities as shown in Fig. 224 R14.7.5.2. When If such tensions stresses occur, 14.7.5.2.2 requires tension splices to be 225 used. or an adequate tensile resistance to be provided. Furthermore, a minimum tension 226 strength is required in each face of all columns even where analysis indicates 227 compression only. <R12.17> 228 229

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230 231 Fig. R14.7.5.2—Special lap splice requirements for columns. 232 233 Staff to change figure text: 234

• 12.17.2.1->14.7.5.2.1 235 • 12.17.2.2-> “Table 14.7.5.2.2 (Class A splices allowed with certain conditions)” 236 • 12.17.2.3->“Table 14.7.5.2.2 (Class B splices required)” 237

238 239 The 1989 Code clarifies this section The splice requirements have been formulated on the 240 basis that a compressive lap splice has a tension tensile strength of at least one-quarter 241 0.25fy , which simplifies the calculation requirements in previous Codes. Therefore, even 242 if columns bars are designed for compression according to 14.7.5.2.1, nominal tensile 243 strength is inherently provided. <R12.17> 244 245 R14.7.5.2.1 —Reduced lap lengths are allowed permitted when if the splice is enclosed 246 throughout its length by minimum sufficient ties. The tie leg areas perpendicular to each 247 direction are computed calculated separately and the requirement satisfied in each 248 direction to apply the 0.83 reduction factor. This is illustrated An example is provided in 249 Fig. R14.7.5.2.1, where four legs are effective in one direction and two legs in the other 250 direction. This calculation is critical in one direction, which normally can be determined 251 by inspection. <R12.17.2.4> 252 253 Compression lap lengths may also be reduced when if the lap splice is enclosed 254 throughout its length by spirals because of due to increased splitting resistance. Spirals 255 should meet requirements of 14.7.6.3 and 21.8.3.3. <R12.17.2.5> 256

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257 258 Figure R14.7.5.2.1 Example of application of 14.7.5.2.1(a) 259 260 R14.7.5.3 — End-bearing splices 261 262 R14.7.5.3.1 263 End-bearing splices used to splice column bars always in compression should have a 264 tension strength of 25 percent of the specified yield strength of the steel area on each face 265 of the column, either by staggering the end-bearing splices or by adding additional steel 266 through the splice location. Details for end-bearing splices are provided in 21.5.6. The 267 end-bearing splice should conform to 21.5.6. <R12.17.4> 268 269 R14.7.5.3.2 — The 50 percent limit on transfer of compressive load by end bearing on 270 ends of structural steel cores is intended to provide some level of tensile strength at such 271 splices (up to 50 percent) since because the remainder of the total compressive stress load 272 in the steel core are is to be transmitted by dowels, splice plates, welds, etc.or other 273 mechanisms. This provision should is intended to ensure that splices in composite 274 compression members columns meet essentially the same tensile strength requirements as 275 as required for conventionally reinforced concrete compression members columns. 276 <R7.8.2> 277 278 R14.7.6 — Transverse Reinforcement 279 280 R14.7.6.1 — General 281 282 R14.7.6.1.4 — Research10.46 has shown that the required amount of tie reinforcement 283 around the structural steel core is sufficient for the longitudinal steel bars to be included 284 in the flexural stiffness of the composite column as permitted by Chapter 8. <R10.13.8> 285 286 R14.7.6.1.5 — All longitudinal bars in compression should be enclosed within transverse 287 ties reinforcement. Where longitudinal bars are arranged in a circular pattern, only one 288 circular tie per specified spacing is required. This requirement can be satisfied by a 289 continuous circular tie (helix) at larger pitch than required for spirals under 10.9.3,; the 290 maximum pitch being equal to the required tie spacing (see also 7.10.4.3). <R7.10.5> 291

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292 Unusual columns such as pPrecast columns with cover less than 1-1/2 in., prestressed 293 columns without longitudinal bars, columns smaller than minimum dimensions 294 prescribed in earlier Code editions, columns of concrete with small size coarse aggregate, 295 wall-like columns, and other unusual cases may require special designs for transverse 296 reinforcement. Wire, W4, D4, or larger, may be used for ties or spirals If such unusual 297 columns are considered as spiral columns for load strength calculations in design, the 298 volumetric reinforcement ratio for of the spiral, ρs, is to should conform to 21.8.3.3. 299 <R7.10.3> 300 301 R14.7.6.1.6 — Provisions for confinement of anchor bolts that are placed in the top of 302 columns or pedestals were added in the 2002 Code. Confinement improves load transfer 303 from the anchor bolts to the column or pier for situations where the concrete cracks in the 304 vicinity of the bolts. Such cracking can occur due to unanticipated forces caused by 305 temperature, restrained shrinkage, and similar effects. <R7.10.5.7> 306 307 R14.7.6.2 — Lateral support of longitudinal bars using ties or hoops 308 309 R14.7.6.2.2 — For rectangular columns, beams or brackets framing into all four sides at 310 the same elevation are considered to provide confinement restraint over a joint depth 311 equal to that of the shallowest beam or bracket. For columns with other shapes, four 312 beams framing into the column from two orthogonal directions are considered to 313 provided equivalent restraint. <~> 314 With the 1983 Code, the wording of this section was modified to clarify that ties may be 315 terminated only when elements frame into all four sides of square and rectangular 316 columns; for round or polygonal columns, such elements frame into the column from four 317 directions. <R7.10.5.6> 318 319 R14.7.6.3 — Lateral support of longitudinal bars using spirals 320 321 R14.7.6.3.2 — See R14.7.6.2.2. 322 323

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