Bending of Reinforcement Bars and the Minimum Bending Pin Diameter

The bending of reinforcement bars Minimum bending pin diameter for bars

The minimum diameter Ø_m (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 l_bd (see next paragraph ) when the three following conditions are met:

(1) The anchorage of the bar does not require a length larger than 5Ø 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, a_b=37mm+Φ hence the minimum bend mandrel diameter becomes:

Ø_m_,_min=(PI/4)×Ø²×f_yd× [1/(37+Ø) + 1/2Ø]/ f_cd

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

l_min=c_nom+ Ø_stir+ Ø + Ø_m,min/2+5Ø

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