Hira Malik

Absolute or relative deflection in ETABS

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While checking the allowable deflection in member in ETABS, which deflection is to be considered, absolute or relative? Which one represents the actual deflection of the member?

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It should be relative deflection. For example a beam in X supporting on girders in Y direction. You need to take the net deflection of supports and mid-span (for s.s. case) to compute deflections.

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    • By Engr Waqas
      Assalam o alaikum,
      For assuring deflection check of flexural members, we compare immediate deflection due to live load with L/180 or L/360. Also we compare total deflection (sum of immediate deflection due live load and long term deflection due to "dead +sustained loads") with L/240 or L/480.
      These deflections are calculated using corresponding effective moment of inertia "Ie" using equation 9-8 of section 9.5.2.3.
      To find out immediate deflection due to dead load we find out it using Ie calculated using cracking moment due to dead loads.
      To find out immediate deflection due to sustained loads we find out it using cracking moment due to sustained loads.
      To find out immediate deflection due to D+L  we find out it using cracking moment due to dead + live loads.
      But to find out immediate deflection due to live load, we don't find it using cracking moment due to live load. We find out it by the difference of deflection due to dead + live load with the deflection due to dead load only.
      as shown by following results of manual calculations (Results from example 10.1 of PCA notes of ACI 318)
      Del.dead = 0.098 in      (calculated using Ma due to dead load only and corresponding Ie came out to be 10648 in4 = i.e. Ig because Ma is less than cracking moment)
      Del.live = 0.0744 (calculated using Ma due to live only and corresponding  Ie came out to be 10648 in4 = i.e. Ig because Ma is less than cracking moment)
      Del.(dead+live) = 0.344 in (calculated using Ma due to dead+live combo and corresponding  Ie came out to be 5345 in4 < Ig because Ma is greater than cracking moment)
      Note that 0.344 is not the simple sum of 0.098(Del.dead) + 0.0744 (Del.live) because here for Del.live, Ie used was based on live only i.e. Ig =10648 in4; for Del.dead, Ie used was based on dead load only i.e. Ig = 10648 in4; and for Del.(dead+live) Ie used was based on (dead+live) load combo i.e. 5345 in4.
      All three "Ie" are different".
       
      My question is, how to cater this in etabs modeling because in etabs it simply super impose deflections due to dead and live for D+L combo which must not be the case. More over, does modifier 0.35 is enough to cater effect of effective moment of inertia?
      Thanks.
       
       
       
       
       
    • By Ahmad Shabaneh
      I work in a project with large spans (9.2m X 8m) and large service loads (4kn/m^2 SDL+Partitions), (7.5kn/m^2 Live load)
      In modeling the project on ETABS I set the property modifiers for slab (thin shell as 0.25) and this affected the deflection in slabs ( about 5 cm deflection from service loads)
      Hence: Slab thickness is 22 cm and interior beams 50X50 and the owner didn't allowed to increase the thickness
      Any suggestions to solve the problem.  
    • By Nasir Malhi
      Do etabs or sap has allowable deflection checks for beams if it is analysed and designed through these softwares.