1. Introduction
Typical connection details for simple framing between tubular columns and open
section beams usually employ a fitting welded to the column which supports the beam and allows clearance for site bolting. Fig. 1 indicates examples of some of the
more common types of simple joints which include the seating angle, web cleats,
top and bottom angles or the fin plate (tab plate). A detailed survey of the use of
these connections has been conducted by the SCI [1]. The tab plate or fin plate is
probably the most frequently used of all because of its simplicity and the ease with
which the beam and column can be bolted together. However, all of these conventional
methods suffer from the disadvantage of requiring fittings to the column which
can prove costly to fabricate and make the section more difficult to transport without
damage. The ideal system is one which allows site bolting, leaves the column exterior
without projections, is similar to traditional beam to open column joints and uses
ordinary bolts. To achieve these objectives, the face of the tubular column must
incorporate the thread to allow the bolt connection.
For many tubular sections the use of ordinary bolts in tapped holes is not normally
possible due to the restricted length of thread available in the relatively thin tube
wall and the problems of adequate bolt bearing capacity. A relatively new thermal
drilling technique, the flowdrill process, allows a thread to be incorporated into relatively
thin steel by locally displacing the metal and increasing the thickness sufficiently
to permit tapping of a thread into the steel. Although the method was
developed as an alternative to conventional drilling as long ago as 1923 by Jan
Claude de Valliere [2], practical applications were not forthcoming until almost sixty
years later when technical developments made possible the manufacture of tungsten
carbide drill bits. The diagrams of Fig. 2 illustrate the process. A tungsten carbide
bit is rotated at high speed and gradually applied to the base material (Fig. 2a) which,
due to the heat generated by friction, begins to soften allowing the flowdrill bit to
be forced through the steel (Fig. 2b–d). As the drill bit passes through the thickness
of material a conical lobe is produced on the inside which locally thickens the steel
in the vicinity of the bolt. A small upstand is produced on the outside as the drill
is forced through, which is removed by a cutter on the drill bit to leave a clean finish
on the exterior of the tube (Fig. 2e,f). A cold-formed thread capable of accepting a
standard bolt without any special modifications can now be successfully introduced (Fig. 2g). The process is only required up to 12.5 mm thickness as greater thicknesses
of wall can be drilled and tapped in the normal way. Fig. 3 shows both the bolt and
flowdrill thread.
|
