CAPTIVE NUT BENEFITS
Document # : GEN-BE-001
Date :
16/02/2017
Dextra Manufacturing Co. Ltd. | Bar Systems department
I. CAPTIVE NUT BENEFIT
Captive nut solution as proven being efficient and cost effective. It was adopted on large marine tierod projects. This section lists the main benefits of this solution.
Allow usage for both D-Wall and Combi-wall:
Captive nuts can be used in both Diaphragm walls and Combi walls. The only available alternative is
ball cage. Hinge solutions or pad eye solution do not allow use in D-Wall since a portion of bar would
protrude.
Alignment adjustment:
Captive nut provide the tie rod with the possibility of ±5° rotation in every direction when hinge of pad
eye connection provide only vertical alignment adjustment.
Stress concentration at anchorage are therefore minimized.
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Dextra Manufacturing Co. Ltd. | Bar Systems department
Length adjustment possibility:
Captive nut can be fitted with special extended cap to allow length adjustment at the anchor point.
This unique capability cannot be achieved by ball cage, hinge or pad eye connection.
Simple construction:
This is the simplest, lightest and most compact solution. It involves the minimum quantity of part: 1
preassembled item only including starter bar.
In comparison:
- T-plate solution requires 2 independent parts: heavy T-plate assembly and a pin connection.
- Pad eye solution requires 4 independent parts: plate, nut, short bar, pad eye.
- Ball cage solution requires 5 independent parts: plate, nut, short bar, ball cage, ball nut.
Dirt ingress prevention:
Captive nuts are delivered fully pre-assembled from our factory.
Ingress of material inside the captive nut during concreting phase is prevented by a fabric sleeve or
protective cap.
Primary Fabric sleeve
Plastic cap
A secondary fabric sleeve prevent ingress of backfilling material inside captive nut. This sleeve deploy
as the insert bar is extracted
Secondary fabric sleeve
(Primary sleeve not shown for clarity)
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Dextra Manufacturing Co. Ltd. | Bar Systems department
No structural welding:
Its simple construction does not require use of full penetration structural weld.
In comparison, for a T-Plate solution (hinge) the complete tie-rod load is transferred through structural
welding to the base plate.
No overhanging anchor element protruding for anchor wall:
Load is transferred directly into the center on the wall. There are no stress (bending moment)
generated by a load being applied to an overhanging item as this is the case for pad eye or hinge
connection.
Ease of installation:
When installed in a circular pile or D-wall, captive nut is the only solution that allows combining the
following:
- Easy to seal the cut made in circular pile. T-plates typical required two cuts.
- No precaution to take to ensure nut is in contact with plate prior concreting the pile.
- Ease of placing. Captive can simply be attached to pile or D-wall reinforcement at the right
elevation.
- Allow use of precast concrete block in pile on which captive can be assembled. This allow
applying tension on tie-rod hence allowing backfilling before concrete is poor in tubular pile.
A simple coupler connection allow to connect the captive nut starter bar to the main tie-rod.
Fig: Examples of captive installed in cage prior placing the cage
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Dextra Manufacturing Co. Ltd. | Bar Systems department
Fig: Examples of captive installed inside precast block (with or within continuity rebar)
Flexibility in case of compression load occurring:
In the event tie-rod would be applied compression load (for example under ship berthing phase), the
captive nut design allows the nut to freely lose contact to plate and to return at its exact position later.
This suppresses risk of tie-rod buckling or accessories bending.
No compression stress
on tie-rod.
Displacement of wall
under load
Ex: berthing of vessel
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Dextra Manufacturing Co. Ltd. | Bar Systems department
Cost effective and short delivery lead-time:
While tube tend to generate slightly thicker plate, captive nut remains the most cost effective solution
thanks to its simple construction. Its limited number of part also allow short lead-time for advanced
delivery of captive nuts to be inserted in wall.
Refer to Dextra Technical Reference Guide for captive nut typical dimensions and details.
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