10-31-2010, 02:45 PM
dear user...
for beam deflection, refer to ACI 318 chapter 9 section "CONTROL OF DEFLECTIONS"...or refer to PCA Notes on ACI 318.
calculate the desired section properties, moment of inertia, cracked moment of inertia, effective moment of inertia etc...and calculate deflection.
Now ETABS does not take cracked section properties in its analysis, that is why you will have to define FRAME PROPERTY MODIFIERS...
Also in your manual calculations do not calculate Icr (Cracket moment of inertia)...rather take Ieff (effective mom. of inertia) = some factor x Ig (Gross mom. of inertia...)....
for example if you have 12 inch by 12 inch rect. section...your Ig is bd^3/12 = 12*12^3 / 12 = 1728 in^4...now take your Ieff = some factor * Ig....
Most common factor for beams = 0.35 so your Ieff = 0.35*Ig = 0.35*1728 = 604.8
Now use this value in your hand calculations and to check it in ETABS do this:-
1) draw a simple beam and apply Uniformly dist. load on it...
2) make its end offsets value 0...by ASSIGN>FRAME LINE>END OFFSETS and select DEFINE and put 0 in x & y.
3) now again ASSIGN>FRAME LINE>FRAME PROPERTY MODIFERS and put 0.35 in mom. of inertia along 3 -axis (major axis) and press OK.
4) define the desired load comb on which you want deflection..e.g. 1.0D+ 1.0L etc
5) Run the analysis
6) Click DISPLAY>SHOW MEMBER FORCES/STRESSES> FRAME/PIER/FORCES and select desired combination and select Moment 3-3 then Ok.
7) Right click on the bending moment diagram and there its at the last...deflection...dont forget to click ABSOLUTE option (first one) at the bottom to check deflection...this will be the exact deflection you calculated manually...
if you have further doubts dont hesitate to ask....but for the actual cracked section properties without using this factor of 0.35 etc...USE SAFE...
to check the long term deflection...manually put the long term deflection combination e.g. 3SW+3DL+3LL etc...
for beam deflection, refer to ACI 318 chapter 9 section "CONTROL OF DEFLECTIONS"...or refer to PCA Notes on ACI 318.
calculate the desired section properties, moment of inertia, cracked moment of inertia, effective moment of inertia etc...and calculate deflection.
Now ETABS does not take cracked section properties in its analysis, that is why you will have to define FRAME PROPERTY MODIFIERS...
Also in your manual calculations do not calculate Icr (Cracket moment of inertia)...rather take Ieff (effective mom. of inertia) = some factor x Ig (Gross mom. of inertia...)....
for example if you have 12 inch by 12 inch rect. section...your Ig is bd^3/12 = 12*12^3 / 12 = 1728 in^4...now take your Ieff = some factor * Ig....
Most common factor for beams = 0.35 so your Ieff = 0.35*Ig = 0.35*1728 = 604.8
Now use this value in your hand calculations and to check it in ETABS do this:-
1) draw a simple beam and apply Uniformly dist. load on it...
2) make its end offsets value 0...by ASSIGN>FRAME LINE>END OFFSETS and select DEFINE and put 0 in x & y.
3) now again ASSIGN>FRAME LINE>FRAME PROPERTY MODIFERS and put 0.35 in mom. of inertia along 3 -axis (major axis) and press OK.
4) define the desired load comb on which you want deflection..e.g. 1.0D+ 1.0L etc
5) Run the analysis
6) Click DISPLAY>SHOW MEMBER FORCES/STRESSES> FRAME/PIER/FORCES and select desired combination and select Moment 3-3 then Ok.
7) Right click on the bending moment diagram and there its at the last...deflection...dont forget to click ABSOLUTE option (first one) at the bottom to check deflection...this will be the exact deflection you calculated manually...
if you have further doubts dont hesitate to ask....but for the actual cracked section properties without using this factor of 0.35 etc...USE SAFE...
to check the long term deflection...manually put the long term deflection combination e.g. 3SW+3DL+3LL etc...
WS