Reinforcement Detailing of Beams from ETABS output

Ans 1: The details of longitudinal and shear reinforcement for two continuous beams are provided with the attached document.

Ans 2: The reason for middle span failure along grid A is shear force overstressed. It happens beacuse Earthquake forces on short span exceeded the requirements as per code. The shear force resistance required in the beam is more than the capacity of the beam.

This issue can be resolved (1) by increasing the depth of the beam or increasing the cross sectional area of the beam (while increasing cross section the ratio of width to depth must be maintained).

(2) By decreasing the load on the beam.

But if the load cannot be decreased as per requirements then the first method is more suitable for resolving this issue.

Ans 3: Maximum Shear force in third span along grid 3:

As per calculations total unfactored load on the beam is 236 kN (including self weight and live load=3kN/m2). Therefore factored load is 1.2*236 = 284 kN.

Therefore load on one simple support is 284/2 = 142 kN (shear force due to gravity).

Now for capacity shear (calculations are done on provided longitudinal reinforcement on conservative side and not on reinforcement calculated by Etabs):

For sway to right: (1)Sagging moment at point A

There are 2 bars of 25mm are provided at bottom on left side. Therefore total cross section area of bars is 981 mm2. From this the moment can be calculated as 981*fyd*z = 981*415*440 = 179 kN-m.

(2) Hogging moment at point B

There are 3 bars of each 16mm and 25mm are provided at top of right side. Therefore total cross section area of bars is 2075 mm2. From this the moment can be calculated as 2075*fyd*z = 2075*415*440 = 379 kN-m.

• Therefore shear force at point A (total gravity + seismic)=142-(1.4(179+379)/7.35) = 35 kN
• And Shear force at point B (total gravity +seismic)=142+1.4(179+379/7.35) = 248 kN

For Sway to Left: (3) Hogging moment at point A

There are 3 bars of each 16mm and 25mm are provided at top of right side. Therefore total cross section area of bars is 2075 mm2. From this the moment can be calculated as 2075*fyd*z = 2075*415*440 = 379 kN-m.

(4) Sagging moment at point B

There are 2 bars of 25mm are provided at bottom on left side. Therefore total cross section area of bars is 981 mm2. From this the moment can be calculated as 981*fyd*z = 981*415*440 = 179 kN-m.

• Therefore shear force at point A (total gravity + seismic)=142+(1.4(379+179)/7.35) = 248 kN
• And Shear force at point B (total gravity +seismic)=142-1.4(379+179/7.35) = 35 kN

Conclusion: The value of maximum shear force is 248 kN which is calculated as per longitudinal reinforcement provided on conservative side in the 3rd span of along grid 3.