Bending of Reinforcement Bars and the Minimum Bending Pin Diameter





The bending of reinforcement bars Minimum bending pin diameter for bars

The minimum diameter Øm (mandrel diameter) to which a bar is bent, shall be such to avoid bending cracks to the bar and ensure the integrity of concrete inside the bent of the bar where large forces appear.  The smaller the mandrel diameter is, the larger these forces are and consequently the concrete may fail thus leading to the disintegration of the cross-section.

Minimum bending pin diameter for wired mesh

Bar anchorage with simple hook

Beam bar anchorage with simple hooks and transverse bars (metal pins) ≥Ø within the bent section

The term ‘anchorage’ represents the extension of a reinforcement bar inside the concrete, in order to achieve adequate cohesion of the two materials, steel and concrete. With this specific procedure, steel can contribute up to its maximum strength, whenever it is required. The anchorage can be either straight or hooked.

Apart from the satisfaction of the bending standards, the bent section of a bar is accounted in the overall anchorage length lbd (see  next paragraph ) when the three following conditions are met:

(1)  The anchorage of the bar does not require a length larger than  beyond the bending point,

(2)  At least one transverse bar (metal pins) with diameter ≥Ø is fitted within the bent section,

The bending level is not situated too close to the surface of the concrete.

Beam bar anchorage with simple hooks and 2 welded pins

Beam bar anchorage with simple hooks and 2 welded transverse bars (metal pins) could be ≥0.6Ø within the bent section

Drum anchorage

Whenever the above requirements cannot be met, then the anchorage is achieved
with a large diameter drum

The minimum bend diameter Øm,min depends on the steel strength fyd, the strength of the con-crete fcd, the diameter Ø of the reinforcement bar and the axial distance ab between the adjacent bars according to the following formula:

• For bar or group of bars near the surface of the structural element ab=cover+Ø/2

• Strength fcd always lower than fcd of concrete class C55/67

Column minimum dimension

In case of anchorage of beam reinforcement bars inside a column (which is the most common case), for common concrete, ab=37mm+Φ hence the minimum bend mandrel diameter becomes:

Øm,min=(PI/4)×Ø2×fyd× [1/(37+Ø) + 1/2Ø]/ fcd

For these cases, the minimum dimensions of the column must be:

lmin=cnom+ Østir+ Ø + Øm,min/2+5Ø

Minimum diameters of bend bar anchorage and minimum column dimensions lmin where these bars are anchored

For steel B500c (fyd=434.78MPa) and γc=1.50 and for cnom=35mm, Østir=10mm

Ø (mm)   12 14 16 18 20 22 25 28 32 40
fck(MPa) mm mm mm mm mm mm mm mm mm mm mm
12 Øm,min 382 463 548 636 726 820 964 1112 1316 1741
lmin 310 360 410 470 530 590 680 770 900 1160
16 Øm,min 286 347 411 477 545 615 723 834 987 1305
lmin 260 300 350 390 440 480 560 630 730 940
20 Øm,min 229 278 329 381 436 492 578 667 790 1044
lmin 230 270 310 340 380 420 480 550 630 810
25 Øm,min 183 222 263 305 349 393 463 534 632 836
lmin 210 240 270 310 340 370 430 480 550 700
30 Øm,min 153 185 219 254 291 328 386 445 527 696
lmin 190 220 250 280 310 340 390 440 500 630
35 Øm,min 131 159 188 218 249 281 330 381 451 597
lmin 180 210 230 260 290 320 360 400 460 580
40 Øm,min 114 139 164 191 218 246 289 334 395 522
lmin 170 200 220 250 270 300 340 380 430 550
45 Øm,min 102 123 146 169 194 219 257 297 351 464
lmin 170 190 210 240 260 290 320 360 410 520
50 Øm,min 92 111 131 153 174 197 231 267 316 418
lmin 160 180 210 230 250 280 310 350 390 490
55 Øm,min 83 101 119 139 158 179 210 243 287 380
lmin 160 180 200 220 240 270 300 330 380 480

Example: For bar Ø20, in case of C20, lmin=380 mm, in case of C30, lmin=310 mm and in case of C40, lmin=270 mm

 

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