Reinforcement Detailing of Beams in ETABS output

REINFORCEMENT DETAILING OF BEAMS IN ETABS OUTPUT

TASK:

Detail design of beams in Etabs 2018.
The ETABS file for a G+4 building is provided. Run the analysis and design of the RCC Moment ResistingFrame. The following challenge deals specifically with two continuous beams on the 1st floor.

1) 3 span continuous beam along grid A
2) 7 span continuous beam along grid 3

Address the following issues thereafter.

• Provide details of longitudinal and shear reinforcement for the two continuous beams. Sketch the beam elevation details or draft it using AutoCAD software. A sample of how beam elevation details are prepared in consultancy firms is provided below. Participants can prepare it as per their preference:

• Provide reasons for the failure of the middle span along grid A. What are the possible ways this issue can be resolved?
• Calculate the value of the maximum shear force in any one of the spans in both the continuous beams, as per clause 6.3.3 (b) in IS 13920 – 2016. Also, confirm these values from shear force demand calculated by ETABS. Please note that if longitudinal reinforcement provided is more than required (as per ETABS results), the shear force demand will vary from what is provided by ETABS.

AIM:

• To design the beams in detail using ETABS.
• To Run the analysis and design the RCC Moment Resisting Frame.
• To provide details of longitudinal and shear reinforcement for the two continuous beams.
• To provide reasons for the failure of the middle span along grid A
• To calculate the value of the maximum shear force in any one of the spans in both the continuous beams.

INTRODUCTION:

• Beams are horizontal structural elements used for supporting lateral loads.
• In conventional reinforced concrete structures, beams usually receive a load from the floor slab, but may also be subjected to other loads such as wall loads, finishes, services installation, etc.
• The design and detailing of RC beams involves the selection of the proper beam size and the quantity of longitudinal and shear reinforcement that will satisfy ultimate and serviceability limit state requirements.
• it is very important that the beam reinforcements are placed and arranged properly on site, to avoid construction errors.
• In order 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 the Shear force capacity of a beam shall be more than larger of,
  • Factored shear force as per linear structural analysis
  • Factored gravity shear force plus equilibrium shear force plastic hinges are formed at both the ends of the beam given by:

For sway to right:

`V_(u,a)=V_(u,a)^(D+L)-1.4(M_u^(As)+M_u^(Bh))/L_(AB)` and

`V_(u,b)=V_(u,b)^(D+L)+1.4(M_u^(As)+M_u^(Bh))/L_(AB)`

For sway to left:

`V_(u,a)=V_(u,a)^(D+L)-1.4(M_u^(Ah)+M_u^(Bs))/L_(AB)` and

`V_(u,b)=V_(u,b)^(D+L)+1.4(M_u^(Ah)+M_u^(Bs))/L_(AB)`

PROCEDURE:

Initially open the given attached ETABS model.

Then go to analysis --> Check model.

After that again go to analysis and carry out the analysis of the model by choosing the Run analysis option.

After an analysis has been completed go to the concrete frame design drop down and carry out the design of the beam by choosing the start design/ check option.

After completion, reinforcement values are obtained from the display design info option in the 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 the top is 2045(mm/ and at the bottom is 1945(mm)2.
• Hence, provide 2 no .s of 32mm dia bars at the top and bottom i.e 1608(mm)2
• Then provide an additional 25mm dia bar to satisfy the remaining.
• The cover provided is 30mm.
• The shear reinforcement is provided as 10mm dia bars 0 50mm c/c throughout the beam.

Now, detailing is carried out for grid 3:

• The maximum value of reinforcement at the top is 1940 (mm)2 and at the bottom is 1501(mm)2.
• Hence, provide 2 no .s of 32mm dia bars at the top and bottom i.e 160 8 (mm)2
• Then provide an additional 25mm dia bar to satisfy the remaining(only at the top).
• The 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 the 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 3.11MPawhich is greater than the allowable shear stress i.e. 3.lMPa which leads to the failure of this beam.

Possible ways by which this issue can be resolved are:

• Using a higher grade of concrete increases the permissible stresses in it.
• The 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)=114.9754 kN`

Clearspan `L_(AB)=4.725m`

So as per IS 13920 sway shear is calculated as `V_(u,b)= 253.32kN` for sway to the 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)=83.23 kN`

Clearspan `L_(AB)=3.95m`

So as per IS 13920 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: 209.552 kN

Hence take the shear sway as 240 kN.

RESULT:

Hence, the entire detailing and designing of the beam are carried out. The results of each drawing drawn in ETABS and AutoCAD are attached.