1. Anchor
Channel Systems
2. HAC
Portfolio
3. HAC
Applications
Due to the multiple responses of seismic action, the assumed
compression zone under static action may suddenly become
the tension zone. The possibility of cracks intersecting the
anchor location can therefore be assumed to be highly
probable, even if the original anchoring location was assumed to
be uncracked, as indicated.
Static loading: cracks may occur in defined tension zones
5. Base material
6. Loading
The movement of concrete components under seismic actions
results in opening and closing of cracks in combination with
load cycling on the anchor. This crack opening and closing
pattern is different to the patterns found under static conditions,
as described in Figure 5.5.2.3.
The crack opens and closes with the
changing of live load and rebar restrain,
which is less severe compared to seismic
conditions.
Concrete beside the cracks is alternately
under compression and tension, resulting in
the worst conditions for the anchor zone.
Figure 5.5.2.3 — Comparison of crack width under static and seismic
conditions
Seismic loading:cracks may occur almost everywhere in
concrete members.
10. Design
Software
11. Best
Practices
12. Instructions
for Use
13. Field Fixes
14. Design
Example
6. LOADING
Acceptance Criteria 232 (AC232) provides design guidelines for cast-in anchor channels. Anchor
channels are now easier to show compliance with the International Building Code via an Evaluation
Service Report. The lack of explicit provisions for anchor channels in the anchor provisions of
ACI 318 are addressed by the design guidelines provided in AC232, which are written as
amendments to ACI 318.
Historically, anchor channels have been designed using ASD (allowable stress design) concepts.
With the introduction of AC232, the use of LRFD (strength design) for the design of anchor channels
was made possible. Whereas ASD assigns global safety factors to cover all aspects of the design
problem (variability, consequences of failure, etc.) strength design permits explicit consideration of
the variability in resistance and loads. In many cases, strength design may result in more efficient
design solutions. The ACI 318 model code has used strength design since 1971, and its use for
anchorage problems is now accepted practice throughout the U.S.
According to section ACI 318-14 §17.2.3.4.4, the anchor
concrete design tensile strength for resisting earthquake forces
shall be reduced by an additional 0.75. The reduced anchor
nominal tensile strengths associated with concrete failure
modes is to account for increased cracking and spalling in the
concrete resulting from seismic actions.
Source: Hoehler,
M. S. (2006)
Behavior and
testing of
fastenings to
concrete for
use in seismic
applications.
9. Special Anchor
Channel Design
The width of cracks generated during earthquakes is, on
average, significantly greater than those resulting from static
loading. Under static conditions, cracks are normally restricted
to a width of 0.3 mm under service load conditions, and at the
load levels of designed resistance they may reach a width of up
to 0.5 mm. However, during seismic events, cracks can easily
reach a width of up to 0.8 mm. This has been confirmed by tests
with groups of 4 anchors carried out in 2006, as shown in Figure
5.5.2.2.
According to ACI §318-14 17.2.3, anchors in structures assigned
to Seismic Design Category (SDC) C, D, E, or F shall satisfy the
additional requirements of ACI 318 § 17.3.2 through §17.2.3.7.
Commentary §R17.2.3 states that these additional requirements
shall be satisfied regardless of whether earthquake loads are
included in the controlling load combination for the anchor
design.
8. Reinforcing
Bar Anchorage
Because seismic design generally assumes that all or portions
of the structure are loaded beyond yield, it is likely that the
concrete is cracked throughout for the purpose of determining
the anchor strength. In locations where it can be demonstrated
that the concrete does not crack, uncracked concrete may be
assumed for determining the anchor strength as governed by
concrete failure modes.
Source:
Eligehausen, R.;
Bozenhardt, A.
(1989): Crack widths
as measured
in actual structures
and conclusions
for the testing of
fastening elements
Figure 5.5.2.2 Comparison of crack width under seismic and static
conditions
142
Figure 5.5.2.1 — Comparison of potential crack positions under seismic and
static conditions (reinforcement not shown for clarity).
The nature of seismic loads is very different than the static
loads, hence there is a different testing criteria for the anchor
channel used in seismic zones replicating the seismic activity
and also the design methodology is more stringent.
7. Anchor Channel
Design Code
5.5.2 BEHAVIOR OF THE MATERIALS IN
WHICH ANCHORS ARE SET
4. Design
Introduction
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