Wire and Fabric Development Length

Development Length L_sy.t For Plain Wire

AS3600 Clause 13.1.2 (c) allows a development length of 50 d for hard drawn
wire. The D500L grade bars are cold rolled wires.

It is often not appreciated that the requirements of these Standards are closely
inter related.

AS/NZS 4671, Clause 7.2.5 gives the minimum welded connection shear
force for mesh as 0.5 _Rek.L A_s, that is, half the yield force of the largest bar
at the weld.

To develop the design stress of the wire (R_ek.L A_s) at least two welded
intersections must be embedded (2 x 0.5 (R_ek.L A_s). To transfer this stress
from one sheet to another, a minimum overlap of two welded intersections
must be specified. These are the reasons for the requirements of Clause
13.2.3 of AS3600.

Thus embedment of two cross wires by 25 mm or more, from where full
strength is required can be assumed to develop the full design strength of the
mesh wire.

Each wire of welded wire fabric develops its stress by normal adhesive bond
with concrete together with bearing, against the concrete, of cross wires
welded transversly to it. Adhesive bond to the wire is usually neglected for
cross-wire spacings up to 200 mm. No advantage is given to deformed wires.
Most overseas Standards specify the minimum overlaps of 200 mm or more
because the weld-shear strength is limited to 0.3f_sy. Thus at least three, and
often four, wires must be overlapped leading to serious bunching.

Development Length for Mesh with an Overhang

In certain circumstances, such as at a support, the bond of the overhanging wires
can be utilised for stress development. Clause 13.1.2 applies to all deformed
bars including hard drawn wire.

The following table shows the development strength of one cross wire plus an
overhang of 25 d_wire.

Embedment length of one weld plus an overhang of 25d_wire + one weld + 25mm

Note: Values are provided as a basis for an engineering judgement.