Reinforcement Detailing of Beams from ETABS output

REINFORCEMENT DETAILING OF BEAMS FROM ETABS OUTPUT

Aim:

• To carryout the analysis of the given Etabs model and to design the RCC moment resisting frame.
• To provide the detailing of longitudinal and shear reinforcement for two continuous beams.
• To provide the reasons of failure of middle span along grid A.

Introduction:

In this challenge two beam detailing is done:

a) 3 span continuous beam along grid A.
b) 7 span continuous beam along grid 3.

Then inorder to calculate the maximum shear force in any of the spans of the continuous beam, IS13920-2016, clause 6.3.3 (b) is followed which states that

Shear force capacity of a beam shall be more than larger of,

a) Factored shear force as per linear structural analysis

b) Factored gravity shear force plus equilibrium shear force ehn plastic hinges are formed at both the ends of the beam given by:

1. For sway to right:

2. For sway to left:

Procedure:

• Initially open the given Etabs model.
• Then go to analysis --> Check model.

• After that again go to analysis and carry out the analysis of the model by choosing Run analysis option.
• After analysis have been completed go to concrete frame design drop down and carry out the design of the beam by choosing start design/ check option.
• After completion, reinforcement values are obtained from display design info option in Concrete frame design dropdown.

• Now, we need to carry out the detailing of grid A and grid 3 continuous beams.
• Initially detailing is done for grid A.
  • The maximum value of reinforcement at top is 2045(mm)2(mm)2 and at bottom is 1945(mm)2(mm)2.
  • Hence, provide 2 no.s of 32mm dia bars at top and bottom i.e 1608(mm)2(mm)2
  • Then, we will have to provide additional one 25mm dia bar to satisfy the remaining.
  • Cover provided is 30mm.
  • The shear reinforcement is provided as 10mm dia bars @ 50mm c/c throughout the beam.

• Now, detailing is carried out for grid 3:
  • The maximum value of reinforcement at top is 1940(mm)2(mm)2 and at bottom is 1501(mm)2(mm)2.
  • Hence, provide 2 no.s of 32mm dia bars at top and bottom i.e 1608(mm)2(mm)2
  • Then, we will have to provide additional one 25mm dia bar to satisfy the remaining(only at the top).
  • Cover provided is 30mm.
  • The shear reinforcement is provided as 10mm dia bars @ 100mm c/c throughout the beam.

Hence, beam detailing is completed.

• Possible reasons for failure of the middle span along grid A.

From the above design result it is clear that failure is due to higher shear stress. Here the shear stress developed is 3.11MPa which is greater than the allowable shear stress i.e, 3.1MPa which leads to the failure of this beam.

Possible ways by which this issue can be resolved are:

• Using higher grade of concrete which increases the permissible stresses in it.
• Shear strength of the beam can be increased by reducing the effective depth of the beam.

Now, for For grid A:

sway to right

Hogging moment of resistance = 173.974 kN m

Sagging moment of resistance = 292.99 kN m

Factored shear force due to vertical loads acting on the span ,V_u,b^D+L=114.9754kN

Clear span,

L_AB= 4.725m

So as per IS13920 sway shear is calculated as:

V_u,b= 253.32kN for sway to right

Now, for sway to left the sway shear is calculated as:

V_u,b= 252.43kN for sway to left.

Shear force demand calculated by etabs : 237.54 kN

Take the shear sway to be 253 kN.

Grid 3:

Sway to right

Hogging moment of resistance = 161.07 kN m

Sagging moment of resistance = 219.39 kN m

Factored shear force due to vertical loads acting on the span ,V_u,b^D+L= 88.23 kN

Clear span,

L_AB= 3.95 m

So as per IS13920 sway shear is calculated as:

V_u,b= 240.4 kN for sway to right

Now, for sway to left the sway shear is calculated as:

V_u,b= 243.386 kN for sway to left.

Shear force demand calculated by etabs : 209.552kN

Hence take the shear sway as 240 kN.

Result :

Hence, the entire detailing and designing of beam is carried out.