Requirements for shear reinforcement

2-essential requirements for shear reinforcement

The basic requirement for shear reinforcement is to strengthen the shear load capability that members can bear. However, another important condition that must be met is that, unlike the sudden brittle failure behavior of the member that occurs after reaching the maximum shear force, the ductile behavior must be improved by using shear reinforcement.

(1) Strengthening the shear capacity (before peak value)

Improved resistance to shear load

Improved shear resistance performance (P2) is secured compared to the shear resistance capacity (P1) when unreinforced. (See [Figure 1] on the left)

Reduction in crack width

It has the effect of reducing the width of cracks that occur. This is due to the effect that the displacement value (d2) is reduced compared to the value (d1) when not reinforced. In particular, the enlarged head type shear reinforcement ‘SRD’ has the characteristic of a significantly reduced displacement value (d2) compared to the bending anchorage type

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(2) Improved ductility (after peak value)

Strengthening ductility

Shear reinforcement must secure not only shear resistance performance but also the ductility of the member to alleviate the problem of brittle behavior when unreinforced.

In shear-reinforced members, it can be seen that the failure behavior after the peak value shows ductile, unlike when it is unreinforced. (See [Figure 1], [Photo 1])

In particular, among various types of shear reinforcements that secure ductility, the enlarged head method (SRD, etc.) shows more improved ductility than the bending anchorage method (Stirrup, etc.). (Refer to the comparison of SRD and closed-Stirrup ductility index in the example of [Photo 1] on the left)

Shear reinforcement design

Shear reinforcement design model

Strut-and-Tie model

This is a truss model composed of Struts, Ties, and Nodes that transmit the cross-sectional force of the Struts and Ties to a Node or nearby area. Reinforced concrete members can be designed according to traditional shear design methods or the Strut-and-Tie model.

Tie, Node and Anchorage

The strut responsible for compression corresponds to the concrete, and the tie refers to the surrounding concrete having the same central axis, including reinforcing bars, tendons, or these. Additionally, the surrounding concrete is also used to define nodal areas where the forces of struts and ties can be anchored. In particular, in the Strut-and-Tie model of reinforced concrete members, the ties must be firmly anchored (in the node area) by mechanical devices, post-tension anchoring devices, standard hooks, extensions of reinforcing bars, etc. in order to effectively exert tensile force.

Shear failure process

As can be expected from ‘Shear Stress Distribution of cross-section’, in stage 5, the initial shear crack occurs at the center of the cross section. (Refer to step 5) Afterwards, the shear crack gradually expands in both directions on the top and bottom of the member. (see step 6)

If the member is an isotropic single material, the shear crack can be observed to progress along the Principal Stress Curve. However, the reinforced concrete member is a heterogeneous composite material, and the crack progression on the lower surface shows a different trend (inflection point) from the PSC diagram. (see step 6)

In the later stages of shear crack occurrence, the flexural reinforcement placed at the bottom of the member also plays the role of a continuous collapse prevention bar that suppresses the brittle shear failure phenomenon of the member. (Dowel action, S-shaped crack)

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Performance Evaluation

Shear reinforcement and Pull-Out Test

Quotations : R. Beutel, J. Hegger / Germany / Cement & Composiete 24 (2002) 539-549

Shear reinforcement is placed across the shear crack location, and it plays a role in confining the concrete to prevent the shear crack widening. (See [Photo 2] on the left)

And the performance of shear reinforcement can be ultimately evaluated by comparing the anchorage characteristics with a pull-out tensile test, as shown on the left [Figure 2].

(Analytical method) Example of comparison of pull-out tensile performance according to anchorage type

Among the many graphs (A~H) shown, the conventional bending anchorage type such as ‘Stirrup’ corresponds to ‘B’, and the enlarged head type such as SRD corresponds to ‘H’. In addition, in other graphs (E to H), the phenomenon of strengthening anchorage performance by additionally reflecting welded rebar is evaluated through analysis.

Compared to the bending anchorage type (B), the enlarged head type (H) not only has the best anchorage performance (=tensile strength), but also causes relatively the least anchorage slip (=minimizes crack width). (Refer to left example [Figure 3])

Anchorage Method

굽힘정착방식 확대이미지

Bending anchorage type vs. Enlarged-head type

The general classification of shear reinforcement methods includes 1) Bending anchorage type represented by Stirrup, 2) Shear head type in which steel beams are arranged in a lattice form, and 3) Enlarged head type introduced in the revised domestic structural standards in 2012. In particular, it can be explained in two types. The main features compared to the bending anchorage method and the enlarged head method are summarized as follows.

Bending anchorage method

Bending anchorage method
Method of securing anchorage performance

· ⓐ Bending part + ⓑ Penetrating rebar + ⓒ Bonding force

· Secure anchorage performance through the 3-factors

· Rebars must be tightly surrounded.

Anchorage displacement (slip) · (relatively) large
Cracks and peeling

· The width of cracks is (relatively) large.

· Structural performance degradation when the peeling

Whether yield strength is reached · Not reaching yield strength (thin slab, etc.)

· Closed, single-stranded, multi-stranded stirrups, etc. (see figure below)

· This method also includes the use of steel wires or thin plates instead of rebars

Example Example
Enlarged head method

Enlarged head method

Enlarged head method
Method of securing anchorage performance

· ⓐ Enlarged head alone

· Securing anchorage performance with the head alone

· No need to surround rebar

Anchorage displacement (slip) · (relatively) small
Cracks and peeling

· The width of cracks is (relatively) small.

· Maintains performance even when the peeling

Whether yield strength is reached · Reaching yield strength (thin slab, etc.)

· T-type enlarged head such as SRD/ Stud-type enlarged head

· In particular, T-type enlarged heads provide the most improved anchorage performance

Shear and Durability

Improved durability of structures: Cracks and peeling

『Concrete Structure Standard Design Commentary KDS 14 20 22, 4.11.5 (1)』

According to the results of an experimental study (Joint ACI-ASCE Committee 421 1999), the studs with mechanical anchorage that are as close to the top and bottom of the slab as possible effectively resist punching shear. Compared to shear reinforcing bars with hooks at the ends, the enlarged heads of shear studs have less slip, so the shear crack width is relatively small.

The enlarged head method has the characteristic of maintaining anchorage performance with little deformation (slip) by using the enlarged head alone without the help of adhesion even in situations where cracks progress or the coating peels off.

This means that the performance of the structure can be stably maintained even in unexpected situations such as sudden shocks, and this can be said to be a very necessary feature, especially in public SOC structures where safety is important.

Main fields of application

Main fields of application

· Plate-type concrete members such as flat plate slab, foundation, walls

· SOC underground structures such as underpasses, tunnels (entrances and exits), subways, etc

Applicable fields

Applicable fields

· Applicable to beams, columns and other members

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