Etabs Frame Force Moment And Design Moment Differs !!

[Juli]

Q 1. Hello, The +ve and -ve moment of a beam from it's member force Diagram of Etabs (M33)  are 75.89 and -94.23, After designing, When I see the summary report, I see Etabs designed that beam for counting exactly neg moment -94.23, but why taking +ve moment 47.11 in stead of 75.89 for designing? And Etabs is counting the half of total neg moment as design positive moment I see, but why not directly considering the positive moment shown on the frame force diagram?  See the image please..

 

Q 2. Why serviceability checks (Drift, Story Displacement ) are only done manually for only considering EQx or EQy values? Why the values aren't checked for load combinations rather than just EQ?  

[UmarMakhzumi]

1- Can you please attach your design summary results. PDF would do.

2- Serviceability requirements are different for different cases. For lateral case, if seismic is governing, we do use factored load combos to check inter-story drift.  See the post here: http://www.sepakistan.com/topic/1341-ubc-seismic-drift-limits/

If you have wind governing, you need to use the appropriate wind load.. Generally, building codes recommend using 1 in 10 max wind for serviceability checks and 1 in 50 max wind for design checks.

 

Thanks.

[Juli]

The general model and Envelope for the frame result I am attaching. You can check after running design that from frame moment and design moment varies. 

jtjytyj.EDB

Envelope.pdf

[sameersaleem]

this is due to the pattern live load. In etab default pattern live load  factor is 0.75 (for +ve moment only) in concrte design preferences. if you change the value to 1 then both moments will be same.

[Juli]

 Thanks, But I have tried with Pattern Live load factor as 0, but same result and also again tried with 1 , but same result. But see the image, Etabs designing moment for the neg most at support, it's ok, but taking positive moment at that end, not on middle moment yet it is greater, See the image please and does this not occur to your models you create? Have you matched the moments  that are taking etabs for designing are same with the moment diagram or is anything fault with me with Etabs?

 

[UmarMakhzumi]

this is due to the pattern live load. In etab default pattern live load  factor is 0.75 (for +ve moment only) in concrte design preferences. if you change the value to 1 then both moments will be same.

 

Pattern Live Load would use a design moment greater than analysis moment. If design moment is less than analysis moment, analysis moment is used.

 

 

 Thanks, But I have tried with Pattern Live load factor as 0, but same result and also again tried with 1 , but same result. But see the image, Etabs designing moment for the neg most at support, it's ok, but taking positive moment at that end, not on middle moment yet it is greater, See the image please and does this not occur to your models you create? Have you matched the moments  that are taking etabs for designing are same with the moment diagram or is anything fault with me with Etabs?

 

I don't have ETABS but I do have SAP2000 installed. I can't open your model in SAP2000. Can you upload the .s2k file and perhaps I would be able to open it in SAP2K.

 

Thanks.

[WR1]

Which version is this 13.1 or 13.2

 

Please use the latest one, as initial versions of ETABS 13 had lot of bugs which is normal for any new mega release of any software!

[sameersaleem]

etab design a beam on many locations if you check for max reinforcement for top then you will find your correct value of moment at support and if you select for max bottom reinforcement you will find middle span value for moment at middle.the attached screen shot is for -ve moment at support for +ve you should select max bottom reinforcement location.

 

see the attached pdfmoment.pdf

[Ussama Waseem]

This is a very critical topic that you have brought to notice. Interpretation of results is always very tricky for a rooky like myself. To avoid such confusion, I export the beam forces on excel and look for maximum and minimum moments manually for a beam, regardless of the design combos. This gives me an opportunity to go for the maximum moments rather than looking for a specific combo or location. It can be a little conservative approach, but as Mr. Umer said somewhere that you gotta have a conservative approach if you have doubts. Although, I have always wondered if I should do this with Beam Design Forces or Beam forces for Analysis(As the later also include individual combo eg. Dead, Live, EQx etc.). I would really appreciate other members to comment on this style and enlighten me.

[saleem khan]

Dear Juli, Etabs has designed the beam for the reqiured moment of 75 kip-ft. You are comparing middle positive moment with end positive value which of course will be different because of SMRF reqiurements. Etabs either reports max. Positive value and min. negative or vice versa for beam moment. There shoud be no confusion now.

[Engr. Kamrul Hasan]

etabs why design beam shear force  different  from analysis