Civil Engineering Reference
In-Depth Information
7.5
HOOKS
When sufficient space is not available to anchor
tension
bars by running them straight for
their required development lengths, as described in the last section of this text, hooks may be
used. (
Hooks are considered useless for compression bars for development length purposes.
)
Figure 7.12 shows details of the standard 90
and 180
hooks specified in Sections
7.1 and 7.2 of the ACI Code. Either the 90
hook with an extension of 12 bar diameters
hook with an extension of 4 bar diameters (4
d
b
), but not
less than in., may be used at the free end. The radii and diameters shown are measured
on the inside of the bends.
The dimensions given for hooks were developed to protect members against splitting
of the concrete or bar breakage no matter what concrete strengths, bar sizes, or bar
stresses are used. Actually, hooks do not provide an appreciable increase in anchorage
strength because the concrete in the plane of the hook is somewhat vulnerable to splitting.
This means that adding more length (i.e., more than the specified 12
d
b
, or 4
d
b
values) onto
bars beyond the hooks doesn't really increase their anchorage strengths.
The development length needed for a hook is directly proportional to the bar diame-
ter. This is because the magnitude of compressive stresses in the concrete on the inside of
(12
d
b
) at the free end or the 180
2
2
Figure 7.12
Hooks.
