4. ANCHORAGE AND FOUNDATION BOLTS

Check setting out positions and levels for conformity with drawings and

tolerance requirements. Do not do any casting or drilling until all checks are

to the satisfaction of the designer. Cast HD bolts in sleeves/boxes allowing

enough movement of bolts for final stanchion base location. Rotate HD bolts

periodically within the sleeve/box while concrete is 'green' to prevent rigid

set. For setting out, use accurate templates.

    Adequate reinforcement must be provided on the outside of HD bolt boxes

where these occur at the edge of a base or foundation slab. If there is any

doubt, consult the designer.

    For lighter loads the alternative to cast-in anchors is to use a heavy duty

anchor of the type shown in figure 14.3. There is a wide range of such anchors

available from a number of suppliers, suitable for use in concrete and masonry.

The size range is M8 up to M24 with the maximum working pull-out loads

6.9 to 45.5kN respectively (for 30N/mm2 concrete). These fixings can also

take shear (lateral) loads and manufacturers provide interaction performance

curves for combinations of tension and shear loading.

    Anchorage is obtained by the wedging action of an expanding sleeve which

is forced radially outwards against the side of the hole as the bolt is torque tightened.

Once the bolt is torqued to the recommended value the anchorage

load capacity of the bolt is automatically achieved since the two phenomena

interact with each other.


Figure 14,3  Heavy duty anchors for concrete and masonry.


    When using these anchors the manufacturer's procedures must be

followed fully. Accurate positioning and sizing of the holes is vital, and

templates will always be necessary since there is no possibility for

repositioning after drilling. Also if the hole is drilled too large the essential

compression stresses between the sleeve and the hole cannot be developed.

The depth of the hole drilled is also important since the fixing must penetrate

into the base material to at least the minimum depth required. For this reason

there is a maximum thickness of base plate which can be used with a

particular length of anchor. The surface of the base material must be correct level, flat

and sound since grouting is not possible.

    Because of the compression stresses induced in the base material there

are limits to the minimum acceptable spacing between anchors, to the

minimum distance of an anchor from an edge, and to the minimum thickness

of the base itself.

    'Chemical' anchors do not induce compressive stresses in the base

material since they are fixed by injecting epoxy grout into the hole and hold

the anchor by the shear strength of the grout.