2
 
• There are two types of anchorage: bending and mechanical.
• Stirrup reinforcement is an example of bending anchorage



• The improved geometry of SRDs decreases compressive stress in concrete.
• SRDs encompass a larger volume of concrete than stud type.
• SRD's improved performance has been certified by authorized institutions.
Geometric Details of Anchorage ; Anchoring Shear Reinforcement can be
classified as either bending anchorage or mechanical anchorage.



In the case of bending anchorage, like a stirrup, the bent portion takes the
role of the anchor.
This part has an asymmetric shape. The process of slippage is as follows:




Tension in a stirrup leg (or a cross tie)
• Very high compressive stresses are developed and exerted on the concrete
  in contact with the inner face of the bend or hook
• These compressive stresses are sufficient to crush the concrete inside the
   bend, resulting in a measurable slip of the leg and dislocation of the hook.
• Such slip causes significant strain losses in the leg and diminishes the
  stirrup's capacity to prevent the widening of a crack.

Journal of American Concrete Institute (Vol. 77, No.1, January/February 1980, pp.28-35,by F.Seible, A. Ghali and W. H. Dilger) and in Bautechnik (Vol. 42, October 1965, by F. Leonhardt and R. Walther, in German)

Tests have shown that movement occurs at the bends of shear reinforcement, at point A, before the yield strength can be reached in the shear reinforcements, causing a loss of tension. Furthermore, the concrete within the bend in the stirrups is subjected to stresses that could exceed 0.4 times the stirrup’s yield stress, fyv, causing concrete crushing. (ACI 421.1R-99 Shear Reinforcement for Slabs, Reported by Joint ACI-ASCE Committee 421)

Hooks and bends require a bar of larger diameter, running in the perpendicular
direction to the plane of the bend to enhance the anchorage. This provides partial mechanical anchorage, which can be lost when, because of imprecise workmanship, the heavier bar is not lodged in the appropriate position in contact with the inner surface of the hook.

But With headed shear reinforcement(mechanical anchorage), the heads secure sufficient anchorage without the need of rebar for enhancement. ('Anchoring with Double-Headed Studs' by Amin Ghali and Walter H. Dilger)




The heads should be shaped to cause less compressive stress.

Shear failure begins when part of the concrete is crushed due to the focused increase of pressure at some parts inside the anchor.


Concrete crushing leads to the head slipping.
• Such slip causes significant strain losses in the stem and diminishes the
  shear reinforcement's capacity to prevent the widening of a crack.
• Therefore the head shape is crucial in avoiding concrete crushing and
  slipping.
• concrete crushing under the head (especially adjacent to the stem)


In stirrup reinforcement, the compressive stress is also maximized at the bent part adjacent to the vertical lag.

In stud reinforcement, stress is also maximized inside the head adjacent to the stem (vertical leg).

 

 

 

 

Therefore shear reinforcing is more effective if the anchor head is shaped to reduce the compressive stress (P/A) adjacent to the stem.

Additional Geometric Characteristics
• The holes in the stem have a secondary function: to reduce the
  chance of slipping.
• Strips, link with transverse welded bars, can improve anchor quality.
  ('The effect of anchorage on the effectiveness of the shear reinforcement
  in the punching zone' by Rüdiger Beutel and Josef Hegger)




The volume of concrete confined by the shear reinforcement should be maximized.

 

Effective anchorage is essential and its location must be as close as possible to the structural member’s outer surfaces.
This means that the vertical part of the shear reinforcement must be as tall as possible to avoid the possibility of cracks passing above or below it. (ACI 421.1R-99 Shear Reinforcement for Slabs, Reported by Joint ACI-ASCE Committee 421)

 

.... Thus, by placing the anchor heads as close as possible to the outer surfaces, the volume of concrete confined between the two anchor heads of a stem is maximized. (Anchoring with double-head studs, Amin Ghali, Walter H. Dilger, 1998)

 

In order to increase the volume of the cone, the head area (A) adjacent to the stem must be maximized, as well as the height of the cone (B).

 

Due to the geometric features mentioned above, SRDs perform better in shear reinforcement than other methods. The head area of an SRD is more than 10 times the cross-sectional area of the stem, making for a larger confining volume than the other methods. These characteristics have been verified by authorized institutes.

 

 

 

For several years, Pusan National University's Research Institute of Industrial Technology has been conducting experimental tests and comparative analyses on SRD, headed stud, stirrup, and non-shear reinforcement.

 

 

 

Tests have been conducted for punching shear and comparative analyses of vertical loads and lateral loads have been done. The results are that SRDs show excellent shear reinforcement and ductility compared to the others. SRDs were shown to have a very high displacement ductility index and an outstanding energy dissipation capacity.