Hanger Reinforcement

Hanger Shear Reinforcement Bars

Hanger bars shall be provided in concrete beams which supports secondary beams. Reinforcement shall be provided in the primary beam,  and they shall be placed around the joint of the two beams. Latter figures shows the method of detailing of reinforcements.

Design Technique
Generally we provide primary beams to support the secondary beams. Reaction of the secondary beams is carried by the primary beam. We normally design the primary beams for the reaction of the secondary beams, and most of the the time many engineers miss the design at the joint. Joint should be design to carry the reaction of the secondary beam. We provide hanger reinforcement at the joint, in the primary in addition to the normal shear reinforcements.

Provision of hanger reinforcement can be avoid the if the shear force at the edge of primary beam is less than 3(√f'c)b_w2d₂  as the inclined cracking is not fully developed at this shear. Here b_w2 is the width of the secondary beam and d₂ is the effective depth of the secondary beam.

Method of design and detailing are as per ACI guide lines.

Depth of the primary and secondary beams are h₁ and h₂ respectively. h_b is the vertical distance between bottom of the primary beam to bottom of the secondary beam.

           ϕA_hf_yt ≥ (1-h_b/h₁) V_u2

Here,

 ϕ  = 0.75
A_h = area of hanger reinforcement adjacent to one face of supporting beam
f_yt = yield strength of reinforcement
V_u2 = factored shear at the end of supporting beam

Design example

• Factored shear force at end of the supporting beam = 200 kN
• Yield strength = 460 N/mm2
• Primary beam height 600 mm
• Secondary beam height 400 mm

h_b = 600 - 400

     = 200 mm

Form above equation,

0.75x460xA_h ≥ (1-200/600) (2x200x1000)

                 A_h  ≥773 mm²

Provide four 12 mm bars

Provided area of reinforcement  = 113x2x4

                                                    = 904 mm²

Four links are adequate.

There are some other methods as well for designing hanger reinforcements. The book, "Design of Concrete Structures" written by Arthur H. Nilson, David Darwin and Charles W. Dolan also have give a method to calculate requirement of reinforcements.

They a suggested to use the ration of the beam height to calculate required shear force.

V_d = (h_secondary / h_primary ) Shear force

For this shear force we can find the hanger reinforcements. If we use the same concept above, we can write area of reinforcements as follows.

ϕA_hf_yt ≥ V_d

Using this we can find the hanger reinforcements.

Click here for more information about shear design.

Beam Shear Design to BS 8110

Beam Shear Design to Eurocode 2