Code Compliant Precast Connections

REID CONSTRUCTION SYSTEMS
• Compliant Bolted Connections
Connection of precast wall panels to in-situ concrete elements
(eg suspended slabs) requires (i) a fixing in the precast element
to anchor (ii) the reinforcement starter bar in the in-situ element.
For a correctly designed connection the reinforcing bar will be
fully anchored by the fixing in the panel, and the precast to in-situ
element connection will yield in a ductile manner in the event of
overload.
Connection of precast elements to temporary or
permanent steel members requires (i) a threaded
fixing in the precast element to anchor (ii) a bolted
steel fastening for typical purposes of ;
• Temporary bracing or strongback lifting supports
• Mechanical, electrical or hydraulic services
• Structural steel framing
• Clip connections for precast façade panels
• Panel to Panel restraint connections
A correctly designed bolted connection will specify a
bolt steel grade of lower strength than the fixing
anchor steel grade and concrete pullout capacity, and
the bolted connection will yield in a ductile manner in
the event of overload.
Steel to Concrete Connections
requirements for the design and construction of
concrete structures, and states that in an overload
condition the reinforcement must yield in a ductile
manner before the failure of the concrete.
AS3600 Cl 19.3.1(c) states fixings shall be designed
to yield before ultimate failure in the event of an
overload.
AS3600 Cl 13.1.4 requires the area of the anchor
head to be greater than, or equal to, 4 times the
cross-section of the bar.
AS3850.1:2015 Prefabricated concrete elements
defines ferrules as fixings for threaded bolts only
(not threaded reinforcing bars)
To comply with AS3600, reinforcing bar connections must use headed anchorages that develop the yield capacity of the starter bar being anchored in order for the connection to fail in a ductile manner
AS3600 & AS3850
Australian Standards
Lsy.hb = 0.5 Lsy.tb
Lsy.hb = 6 db
Concrete tensile capacity of headed anchors will be influenced by;
• Embedment depth (anchor length + rebate depth)
• Concrete design strength
• Anchor spacing centres
CCD Method References for Headed Anchors
ACI318: Building Code Requirements for Structural Concrete
EN1992-1-1 Eurocode 2: Design of Concrete Structures
AS3600:2018 Concrete Structures, Section 19 references AS3850.1 Appendix B
AS 3850:2015 Prefabricated Concrete Elements, Appendix B
AS5216:2018 Design of Post-Installed & Cast-In Fasteners
NZS3101:2107 Concrete Structures Amendment A3, Section 17
Concrete Capacity Design (CCD) Principles
The designer must consult technical design data and/or advice provided by the anchor manufacturer/supplier to ensure correct design and detailing of the structural connection to achieve the required tensile and shear strength, and investigate load transfer and mode of failure for ductile behaviour.
Concrete Capacity Design Principles
Calculated Concrete Tensile Capacity – Pullout Cone Mode of Failure for Headed Anchorages
Nuc = k. f’c 0.5 hef
1.5 (Isolated direct tension of a single anchor*) Where: Nuc = Characteristic Ultimate Concrete Tensile Capacity (kN) k = 12.5 for Uncracked Concrete, k = 10 for cracked concrete f’c = characteristic concrete compressive strength (MPa) hef = effective embedment depth of anchor (mm)
*Group effects apply: effect of spacing on anchors in a single row or multiple rows to be calculated as; Xnai = a / 3 x hef
Where: • a = Anchor spacing distance (mm) • hef = effective embedment depth of anchor (mm)
Code Compliant Precast ConnectionsHeaded Anchorage Design Principles & Methods
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Code Compliant Precast Connections
Elastic design requires ductile mode of failure at connections, where Concrete strength exceeds Steel strength, ie; Nuc > Nus, and design for steel capacity as limiting factor
*CCD Method recommends not using values above 60MPa
Concrete Strength Effect
Code Compliant Precast ConnectionsMetric Ferrule & Metric Bar Capacity
• Metric ferrules are short anchors typically less than
100mm length / effective embedment depth.
• The concrete tensile capacity is limited by the short
embedment depth in the range 55- 95mm.
• The failure load of the concrete surrounding the anchor
will be lower than the yield capacity of the bar.
• The failure mode is brittle, and the connection design
does not comply with AS3600 Section 19.3.1 (c).
• Therefore metric ferrules are not compliant to anchor
the tension capacity of a reinforcement bar.
hef <100mm
following conditions;
ferrules out of precast wall panels.
• Progressive brittle failures along wall to slab joints
including single row of connections acting as a hinge
joint, and double row in a rigid moment resisting joint.
• Reinforcing bar designed as shear dowels subject to
unexpected axial loads from bending moments in the
supported slab.
•
Metric Ferrule Brittle Mode of Failure (Concrete Pullout Cone)
Code Compliant Precast Connections
Published Test data indicates that cutting a metric thread in 500N grade reinforcing bar
reduces the tensile strength as equivalent to 2 bar sizes smaller, as shown in the table;
Metric thread cut bars are limited as structural members as the cut threads act as notches.
Reinforcing bars are not designed to be threaded and have insufficient diameter to achieve a full
thread cut in the same nominal metric diameter.
Therefore N20 bar is threaded as M16 to fit an M16 ferrule but achieves reduced tensile strength
equivalent to N12 bar.
Metric Threaded Reinforcing Bar - Tensile Capacity
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Metric Ferrule & Threaded Bar Compatibility
Metric ferrules and metric threaded starter bars are
often manufactured, supplied and installed by different
parties in the construction project supply chain.
Where the connection has not been proof tested with
specified components, the combined tensile properties
on the components is unverified and the connection is
not validated as a system.
No quality control can be applied to the reinforcing
connection using components of unknown
manufacturing source.
No guarantee can be provided by the ferrule or bar
supplier as the combined performance of the
components is unknown.
• Mechanical, electrical or hydraulic services
• Structural steel framing
• Clip connections for precast façade panels
• Panel to Panel restraint connections
Designing connections for these applications requires the
capacity of the ferrule to exceed the capacity of the bolt.
A typical metric ferrule will be grade 5.8 steel and suitable
for a grade 4.6 steel bolt only, ensuring the bolt will fail in
tension in a ductile manner.
Use of a grade 8.8 steel bolt or stud could result in
component failure in concrete pullout or steel fracture.
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optimised effective embedment depths.
• The concrete tensile capacity is checked for the
rebated embedment depth of the anchors, and their
spacing centres.
engineered for design capacity to full tensile yield capacity
of 500N Grade reinforcing bar with ductile mode of failure
at the full , and the connection design complies with AS3600
Section 19.3.1 (c).
RBA16 @300 RBA20 @400
Code Compliant Precast ConnectionsCompliant Reinforcing Bar Connections
N16 Reinforcing Connection: 20mm EF Metric Ferrule vs 16mm ReidBar Inserts
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Collaborative Reporting for Safer Structures Australasia (CROSS-AUS) -
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