Problem In Etabs Design Summary

[khalid]

Salaikom dear professionals,

First of all I would like to express my sincere thanks to the initiators of this forum for establishing such an exceptional atmosphere for knowledge/experience sharing, I it is really useful, In fact since I have found the forum I am mostly online and busy reading the posts. I would also like to thank the members for their professional comments and advice.

As my first post in this forum I would like to ask the following queries:

1-After running the analysis and design when I check the DESIGN DATA through Display >> Show Tables >> DESIGN DATA >> Concrete Frame Output, there is no specific message in Column Summary Data and Beam Summary Data, but in Joint summary data it is showing that “Joint B/C check not done”. Does anyone has any idea? I am sharing the ETABS model for your information and easy reference.

ETABS MODEL.zip

2- ETABS provides greater area of steel in the upper column than the column at BASE, perhaps due to higher moment. Could someone explain why this is so? In practice should we maintain maximum steel in both stories? Or we shall follow what the software suggests?

3- Beside considering the minimum thickness required for deflection control of Beam as per Table 9.a Chapter 9 ACI-318 , using concrete crack behavior in ETABS and checking story drift, Do we have to check the deflection of beams for the serviceability propose elsewhere in ETBAS? If yes, Could anyone explain it?

Regards, and look forward to any explanation

[WR1]

2- Upper columns will have higher moment and less axial force so more area of steel. Yes you could provide different reinforcement in upper and lower stories as per your design.

 

3-yes you have to check serviceability for beams. Code minimum thicknesses are "deemed" to be okay but its independent of loading magnitude. You can check long term deflection on actual loading in SAFE.

[khalid]

2- Upper columns will have higher moment and less axial force so more area of steel. Yes you could provide different reinforcement in upper and lower stories as per your design.

 

3-yes you have to check serviceability for beams. Code minimum thicknesses are "deemed" to be okay but its independent of loading magnitude. You can check long term deflection on actual loading in SAFE.

 

 Thank you very much Sir Rana,

 

 

3- Mean that I should also check the beams deflection for (1D.L + 1L.L) load combination in SAFE and compare the calculated deflection with MAXIMUM PERMISSIBLE COMPUTED DEFLECTIONS as in table 9.5.b of ACI 318.

 

Please correct me sir if I am wrong.

[WR1]

1DL + 1LL is for short term linear elastic deflection. You should multiply this by a factor given in ACI chapter 9 for long term deflection. Or perform actual long term deflection in SAFE by using creep and shrinkage coefficients (time taking and boring..right?

 

Okay! just a trick here, make ETABS combination of 1.0 Dead cases + 1.0 Live cases and apply cracked modifiers. Check this deflection, it should be nearly similar to long term deflection. Increase it by some percentage to be on safer side.

 

Compare this by L/240 or L/480 and L should be in the direction of deflected shape contour. 

[khalid]

Thanks Sir Rana,  your detailed and helpful explanation has cleared my doubts on two questions (2,3)....... PERFECT 

 

 

In question No.1 there may be some problem in modeling which I couldn't find out Please could check the model and give a brief explanation on that too.

 

 

Thank you very much again for having spent your time in making me more correct. 

[UmarMakhzumi]

Khalid,

 

You might want to look at this thread too for long term deflection: http://www.sepakistan.com/topic/85-long-term-deflection-multiplier/

 

Regarding 1), I am not sure as I have been out of commercial building engineering and using ETABS for more than 5 years. Other people may be better able to answer you on that.

 

Thanks.

[WR1]

1. Joint B/C check is not done for low seismic zones. Its beam column joint capacity. 

[khalid]

Thank you sir Umar for providing the link.

 

Sir Rana, The building which I am designing is located in seismic zone III (S_S=1.28 S₁=0.51), thus Beam Column Joint Capacity is a must.

[EngrJunaid]

Dear Khalid !

 

i go through your model and upto my understanding your model is OK but 2 corrections are required.

 

1) As you mention that the zone of the building is ZONE 3.So you must have to check the Beam Column Joint Ratio which you are saying that it is not done.it is because you have overwrite the sway property from Sway Special to Sway Intermediate.So change your sway type from Sway Intermediate to Sway Special then you will get the required B/C Ratio.

 

2) In defining the slab you have define slab as a MEMBRANE and membrane takes in-plane stresses only. And in reality the slab also take out of plane stresses by deflecting/bending. So define slab as a SHELL THIN then it will take both in-plane and out-plane stresses.

 

Have a look to the snaps and the model with the two corrections i have made.

Sari Pul_V13.rar

[khalid]

Dear Engineer Sahib Junaid, Thank you very much for your valuable inputs.

 

I revised the model accordingly, but when I am defining the slab type as Mesh, the following problems in the model appear:

 

1- The value of designed reinforcement is far away from manual calculation and the values which are calculated for Membrane type slab.

2- In Shell type slab there is no specific message and error on Beam/Column joint capacity ratio, but when I am changing the slab type to Membrane some it is different.

 

Regards 

Khalid 

[WR1]

This is slab on beams, use membrane for modelling and also for beam design. Ignore the out-of-plane stiffness.

[khalid]

Rana Bahai, …. I am wondering that how changing the type of slab to Shell and Membrane can change the design result this much.

 

In Shell type slab there is no specific message and error on Beam/Column joint capacity ratio except the very less amount of steel bar of Beams.

 

As you advised and In order to be on safe side I defined the slab as Membrane and ignored the out-of-plane stiffness of the slab, but after running the analysis amount of designed reinforcement is remarkably changing and the “Beam Column Capacity Ratio Exceeding the limits” is also appeared, while this message was not there in Shell type slab.

 

I got confused with this  , your help is highly appreciated.

[WR1]

Membrane is modelling is correct.

[EngrJunaid]

Load which is applied to MEMBRANE objects transfers directly to supporting structural objects i.e beams, whereas SHELL objects have bending stiffness and therefore resist a portion of the load through flexural deformation. As a result, less load will be available to transfer to beams located under a shell, while 100% of the load will transfer through a membrane.

 

SO in membrane type slab case your B/C joint capacity exceeds the limit because you need more strong joint than that of the SHELL type slab case.

If you are assigning the slab as Membrane and want to overcome the joint capacity failure so either increase the column size or reduce the beam size.....