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How can the length of a bolt to be used in drop-in anchors be determined?

Two factors will determine their length

If they are too short it may mean that the bolt thread does not grip enough of the female thread of the sleeve, causing the male thread of the bolt or the female thread of the sleeve to fail.

If the bolt is too long, its point may come into contact with the anchor’s expansion cone before the head of the bolt reaches the material to be fastened. In such a case, the bolt is unable to move any further, meaning that further torque may break the anchor’s sleeve due to the traction that the bolt thread exercises on the sleeve.

Example of the breakage of an M6 drop-in anchor (top photo) during installation due to excessive bolt length

In order to calculate the correct length of the bolt, follow the indications in the Index technical documentation, as well as European Technical Assessment ETA 14/0068 and 14/0135:

Minimum bolt length = tfix + l_s,min
Maximum bolt length = t_fix + l_s,max

where:

t_fix: thickness of the fixture
l_s,min, l_s,max: values depending on approval assessment

For example, in the case an M8 drop-in anchor (Code no. HEHOM08), where the thickness of the material to be attached: t_fix = 9 mm

Table C1: Installation parameters for HECHO / HECLO anchor

Installation parameters			Properties
Installation parameters			M6	M8	M10	M12	M16	M20
d_o	Nominal drilling diameter	[mm]	8	10	12	15	20	25
D	Thread diameter	[mm]	M6	M8	M10	M12	M16	M20
d_f	Fixture clearance hole diameter	[mm]	7	9	12	14	18	22
T_inst	Max. installation torque ≤	[mm]	4	11	17	38	60	100
l_s,min	Min. thread length	[mm]	6	8	10	12	16	20
l_s,max	Max. thread length	[mm]	10	13	17	21	27	34
h_min	Min. concrete thickness	[mm]	100	100	100	100	130	160
h₁	Bore hole depth	[mm]	27	33	43	54	70	86
h_nom	Embedment depth in the concrete	[mm]	25	30	40	50	65	80
h_ef	Effective embedment depth	[mm]	25	30	40	50	65	80
s_min	Minimum allowable spacing	[mm]	60	60	80	100	130	160
c_min	Minimum edge distance	[mm]	105	105	140	175	230	280

Minimum bolt length: 9 mm + 8 mm = 17 mm
Maximum bolt length: 9 mm + 13 mm = 22 mm

Therefore an M8 x 20 should be used

For other installation parameters, follow the values indicated in in the Index technical documentation, as well as in the European Technical Assessment.

For the installation process, consult applicable documentation and FAQ XXXXX

hef : Effective embedment depth
h1 : Bore hole depth
hnom : Embedment depth in the concrete
hmin : Min. concrete thickness
tfix : Fixture thickness
do : Nominal drilling diameter
df : Fixture clearance hole diameter

Latest revision: FAQ4 rev0

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