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Shear Friction Check: A Worked Example

 

 

In our previous article Construction Joints in Slab we tackled about how, when and where we are considering a construction joint in slabs. We have learned that terminating a slab casting has a shear friction check to consider before casting another fresh concrete. Because originally slabs were designed as a continuous slab which means that the reinforcements are fully continuous, but since we are casting the slab at different times, we are terminating the continuity of the other thus the output results in stresses is different already compared when it is a continuous one. In this aspect we need to check for any reinforcement needed in between the construction joint to resist the shear transfer across the slab plane and any potential cracks that may occur.

The shear friction concept assumed that such a crack is formed when concrete will be cast at different times and that reinforcement is provided across the crack to resist relative displacement along it. The reinforcement in tension provides a clamping force, Avf x fy across the crack faces.

How to Perform a Shear Friction Check?

To recap, shear friction check can be performed using the following equation set forth by the code:

According to ACI 318-14, Shear friction check shall be performed to address the possible failure by shear sliding on a plane and design shear strength across the assumed shear plane shall satisfy:

ΦVn ≥ Vu             [Section 22.9.3.1]

where:

  • Φ=0.75 for shear
  • Vn:  Nominal Shear (shear due to applied reinforcements)
  • Vu:  Maximum Shear (maximum shear due to factored loadings)

If shear-friction reinforcement is perpendicular to the shear plane, nominal shear strength across the assumed shear plane shall be calculated by:

Vn = µ x Avf x fy               [Section 22.9.4.2]

where:

  • Avf : area of reinforcement crossing the assumed shear plane to resist shear (e.g. mesh reinforcement in case of slab)
  • µ: coefficient of friction according to table 22.9.4.2. of ACI 318-14. Refer to previous article.

Worked Example:

In an approved first floor framing plan shown in the figure below, we have a slab ready for casting, since the slab is too wide in dimensions, a series of concrete casting was proposed by the contractor and it will be cast in two different phases. The shaded slab will be cast first and the remainder will be poured after some time. It is required to check or design if any additional reinforcement is needed as shear bar according to slab pouring termination. Use steel reinforcement yield stress, fy=460Mpa.

Solution:

Based on the above drawing, the given mesh is T12@200mm c/c Top and Bottom, Both ways. We will consider the tension reinforcement and conservatively assumed that the shear force will be resisted by the bottom mesh.

Solving for the area of Mesh Reinforcement, Avf:

Avf= (113/0.2)  = 565mm2/m x 1m = 565mm2

Vn = µ x Avf x fy

      = [1.0 x 565mm2 x 460 N/mm2][1/1000]

Vn = 260 kN

ΦVn = 0.75(260kN)

ΦVn = 195 kN

The maximum shear, Vu shall be taken from the factored loadings. In this example, the slab has been designed by the SAFE program. It is more convenient for the designer to use the existing SAFE model to get the maximum shear, Vu. To be noted that Vu can also be taken at any means or through manual calculations depending on the designer’s prerogative. For the purpose of presentation, we will get the maximum shear force, Vu through the shear force diagram of the SAFE model of the shaded slab above and the figure below are the shear force diagram results.

Strip Shear Force Diagram

From Shear force diagram taken from SAFE result above; Vu=315kN, since ΦVn < Vu, then additional shear reinforcement is required!

To satisfy the equation ΦVn ≥ Vu of ACI 318-14, Section 22.9.3.1, additional rebar is needed on the areas having the maximum shear. Let us say that we are adding an additional reinforcement T10@200mm c/c, then the mesh reinforcement, Avf will be:

Avf= (113/0.2)  = 565mm2/m x 1m = 565mm2

Additional Avf= (79/0.2) =  395mm2/m x 1m = 395mm2

Total Avf= 960 mm2

Vn = µ x Avf x fy

      = [1.0 x 960mm2 x 460 N/mm2][1/1000]

Vn = 441.6 kN

ΦVn = 0.75(441.6kN)

ΦVn = 332 Kn  >  Vu=315kN, Hence OK!

Therefore an additional T10@200mm c/c should be added on the portion having the maximum shear as per the SAFE shear diagram results.

Drawing Details

As a designer, you should also be able to interpret the above calculation results in the drawing for the site engineer to be implemented prior to concrete casting. Refer to the figure below for the recommended details:

Revised First Floor Slab Showing Additional Rebar for Shear
Revised First Floor Slab Showing Additional Rebar for Shear

It is noticed on the above figure that an additional steel reinforcement of T10@200mm c/c has been added on the areas of the slab that the maximum Vu exceeds ΦVn. The rests along the pouring line don’t need additional bars since it satisfies the code requirement.

Slab Section along the Pour Line

Construction Joint Detail with Shear Friction Reinforcement
Construction Joint Detail with Shear Friction Reinforcement

Constructability

  1. Construction joint shall be cleaned and laitance removed before new concrete is placed.
  2. The surface of the first pour should be intentionally roughened to increase the bond strength and to provide an aggregate interlock.



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READ ALSO:   The Sequence of Actual Construction

2 Comments

  1. Harris

    Interesting sharing! What about check for large slab pour, for example to a 4m thick piled raft, where construction joint is horizontal in plane? I imagine the shear friction this time is resisted by concrete section only. Any example on that?

    Reply
    1. Mahmoud

      Interested to know??

      Reply

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