Table of Contents
Hanger bars in beams shall be equipped with concrete that backs secondary beams.
Reinforcement shall be equipped in the primary beam, and they shall be kept around the joint of the two beams.
1. Design Technique
Normally, we equip primary beams to reinforce the secondary beams.
The load of the secondary beams is held by the primary beam.
We usually design the primary beams for the load of the secondary beams, and most of the time multiple engineers ignore the design at the joint.
The joint should be well prepared to hold the reaction of the secondary beam.
Hanger reinforcement is provided at the joint, in the primary in addition to the normal shear reinforcements.
The requirement of hanger reinforcement can bypass if the shear force at the edge of the primary beam is less than 3(√f’c)bw2d2 as the inclined cracking is not fully formed at this shear.
bw2 is the width of the secondary beam and d2 is the effective depth of the secondary beam. The method of design and detailing are as per ACI guidelines.
The depth of the primary and secondary beams are taken as h1 and h2 respectively. Here, hb is the vertical distance between the bottom of the primary beam to the secondary beam.
ϕAhfyt ≥ (1-hb/h1) Vu2
ϕ = 0.75
Ah = area of hanger reinforcement adjacent to one face of the supporting beam
fyt= Yield Strength of Reinforcement
Vu2= Factored Shear at the End of Supporting Beam
2. Design Example of Hanger Bars in Beam
Factored shear force at end of the supporting beam Vu2 = 200 kN
Yield strength fyt = 460 N/mm2
Primary beam height= 600 mm
Secondary beam height= 400 mm
hb = 600 – 400 = 200 mm
From the above equation,
0.75x460x Ah ≥ (1-200/600) (2x200x1000)
Ah ≥773 mm2
Provide 4 Φ 12 mm bars
Here, area of reinforcement = 113x2x4 = 904 mm2
Here 4 links are sufficient.
There are a few additional methods as well for developing hanger reinforcements. The textbook, “Design of Concrete Structures” authored by H. Nilson, David Darwin, and Charles W. Dolan also provided a method to compute the requirement of reinforcements.
They suggested using the ratio of the beam height to calculate the required shear force.
Vd = (hsecondary / hprimary ) (Shear force)
For this shear force, we can calculate the hanger reinforcements. If we employ the exact idea above, we can note the area of reinforcements as follows.
|ϕAhfyt ≥ Vd|
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