Bar Compression Lap Length and Anchorage

The lengths below rationalise the values given in AS3600. If both tension and
compression can act at different times on the same cross-section, anchorage
must be designed for the worst case situation.

If the factors given in Clause 13.1.7 for bundled are applied directly to
compression development length given in Clause 13.1.5.2, for ƒ'_c = 25 MPa
and ƒ_sy = 500 MPa (22d_b) they become 26.4d_b and 29.3d_b respectively.
Bundled bars in compression are not commonly used other than in columns of
very high buildings so that, for practial use, all development lengths have been
rounded up to 30d_b. This value will decrease as ƒ’_c increases.

The lap splice for a single bar in compression is a minimum 40d_b (Clause
13.2.4(a)) and rounded up to 54d_b for a bundled bar (Clause 13.2.5).

Lap splices are always based on full yield strength ƒ_sy. To permit other values
would create uncertainty in the mind of fixers and inspectors, and would
certainly require more work from detailers. AS3600-2009 Clause 13.2.4 does
not allow reductions in compression lap lengths.

Compression Development Length for Single Bars, L_sy.c (Clause 13.1.5)

Although it is not stated in Clause 13.1.5, the minimum cover and spacing
rules still apply. Compression causes splitting of the cover in a different way
to tensile forces. A realistic spacing is required to ensure concrete can be
consolidated properly. Encircling ties may also be advisable in zones of heavy
reinforcement.

Compression bars must not be hooked.

Compression Lap Splices for Single Bars (Clause 13.2.4)

For 500 MPa bars in a compression zone of the concrete, the lap length is 41d_b.
This is twice the compressive stress development length. This value also applies to
lap splice lengths for column bars.

Compression bars must not be hooked. This is not restricted to columns and
walls. It applies to all members.

Where the concrete at the bottom of a beam over a column carries an
excessively large compression load, extra bars lapped for compressive stress
transfer will be required there.

Development Length of Bundled Bars in Compression
(Clauses 8.10.8, 10.7, 13.1.6, 13.1.7)

When two bars are tied together over their full length, to form a two-bar bundle,
an increase in development length is not required.

Three or four bars can be tied tightly together to form a bundle. Each bar of
the “unit” therefore presents a smaller surface in contact with the surrounding
concrete. This requires an increased development length for bundled bars
(Clauses 13.1.6, 13.1.7).

In beams, the bar cut-off point of each bar in a bundle must be staggered by
40d_b (Clause 8.10.8).

Compression Lap Splices for Bundled Bars
(Clauses 13.2.4, 13.2.5)

Lap splicing of bundled bars is messy, complicated, uses excessive steel, and
causes overcrowding of the column area. These lap splices must be avoided.

Wherever possible bundled bars should be spliced by end bearing (no laps) or
by mechanical splice because these give a simpler solution.

The values in the table also apply to an extra splice bar added to a bundle which
did not have a sawn-end preparation for an end-bearing splice. The splice bar
must be at least twice the lap length given above, and located centrally about
the section where the splice is.