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Steel Designers' Manual - 6th Edition (2003)
830 Foundations and holding-down systems
27.3.2 Bolt anchorage
(a) (b)
Fig. 27.13 Typical pull-out from concrete block
Fig. 27.14 Conical pull-out
x
Anchorage of the holding-down bolts into the concrete foundation should be sufficient
to cater for any uplift forces and to provide for any shears applied to the bolts.
Attention is particularly directed to the last paragraph of clause 6.7 of BS 5950,
which states that rag bolts and indented foundation bolts should not be used to resist
uplift forces. The elastic elongation of indented screwed rods or bolts under tension
causes the breakdown of the grout surrounding the bolt. This is even more critical
in the case of resinous grouts.
The failure mode of bolts pulled from a concrete block is shown in Fig. 27.13(a);
a reasonable approximation is shown in Fig. 27.13(b).The surface area of the conical
pull-out (Fig. 27.14) is 4.44D2 , where D is the depth of embedment. The factored
tensile capacity of an M20 (4.6) bolt is 245 ¥ 195 ¥ 10-3 = 47.7 kN. For an M20 HD
bolt 450 mm long with an embedment of say 350 mm
the conical surface is 4.44 ¥ 3502 = 544 ¥ 103 mm2 47. 7 kN
the surface stress is
544 ¥ 10
3
= 009 . N/mm
As holding-down bolts usually act in pairs the conical pull-outs often overlap
depending on the depth of embedment.The BCSA have tabulated the surface areas
including those which overlap (Table 27.2).
2
2h