Foundations - Analysis - Steel Structures.

1 Bolt forces
The area required to transmit the compressive forces under the  baseplate is calculated at the appropriate bearing strength of the concrete.The stress block may be assumed to be rectangular with a maximum stress of 0.6 fcu where fcu is the characteristic cube strength of the concrete base or the bedding material, whichever is less. The lever arm for the design of the bolts is then from the centroid of this  stress block to the bolt position as shown in Fig. 27.12. The centroid of the stress block is often less than the edge distance from the compression edge of the plate to the holding-down bolt: it is therefore often assumed that the lever arm for  the bolts is equal to the centres of the bolts. It is also very  likely that the point of application of the compressive forces will be near to the holding-down bolts  due to the extra stiffening that is often included in the vicinity of the bolts. This is illustrated in the typical design given as the fourth worked example at the end of this part.

Distribution of forces and equations of equilibrium
Fig. 27.12 Distribution of forces and equations of equilibrium

2 Bolt anchorage
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 x 195 x 10-3
= 47.7kN. For an M20 HD bolt 450mm long with an embedment of say 350mm

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).

Typical pull-out from concrete block
Fig. 27.13 Typical pull-out from concrete block

Conical pull-out
Fig. 27.14 Conical pull-out

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