Auto-Generated Load Combinations ETBAS & Building Codes for Pakistan

[Omer Ahmed]

I am so happy to finally find this phenomenal platform to ask questions from professionals! So I used to work in a private company in Lahore where we would use auto-generated load patterns in ETABS v9. After my MSc from UK I just had an interview at another firm last week where I was embarrassed to know that we should manually generate load combinations instead of using system generated combos for frame design! Now this got more confusing when I saw some YouTube videos where they defined load patterns manually using UBC97 (which is an easy case since it have just 5 load cases I think) but since ETABS v18 doesn't come with UBC-97and ACI load cases have like 19 load cases, (1) should I put all 19 cases manually? (2) What is really wrong with auto-generated frame combinations? why some people on YouTube and interviewer said we have to put them manually? (3) Suppose I need to design a foundation and for that I display Base Reactions after analyzing the structure, against which load combination I should be seeing the values of base reactions? Will it be the one with the maximum value of Pz, calculated from Load combination provided/ generated before? (4) Can we mix multiple design codes in 1 model? I mean what if I use UBC-97  (say in an older version), can I use ACI as a design preference and for Load combination as well? because it confuses me that older versions of ETABS doesn't come with UBC-97 as a design preference code but surprisingly it still have UBC-97 in defining load patterns where we put Dead, Live, Earthquake and Wind load data etc. Since the system allows us to pick different codes at different staged, does this mean there will be no serious effect on results?

Many thanks for reading this. Please let me know if there is a need of screenshots to answer. 

Edited March 4, 2021 by UmarMakhzumi
Changed topic title to make it more relevant to discussion.

[Badar (BAZ)]

Fundamentally speaking, there is nothing wrong with autogenerated load combinations, provided you know the what is the difference. It is not at all difficult to figure out what will be the difference (load combos have just one variable, some factor x specified loads), just read the relevant code.

Your foundation will have a certain value of flexural and shear capacity measured in terms of k-ft and kipps or whatever the system of units you are using at different locations. Why is it difficult to decide what to do with the results of Etabs? You need to compare the results of all ultimate limit state load combinations with the capacity at these locations.

You can use different codes within the same family of codes. But, you should keep in mind that the definition of DBE level earthquake is different in UBC 97 as compared to the ones that arrived after it. Having said that, the design methodology (strength reduction factored and loads factors) is the same.

 

[Ayesha]

@Badar (BAZ) is totally correct. I will just add that some people prefer manual Load Combinations to address vertical seismic effects which you can also do with Auto, if you specify the correct parameters. See the thread below.

https://www.sepakistan.com/topic/1307-etabs-load-combinations/

 

[Omer Ahmed]

@Badar (BAZ) @Ayesha thanks for your replies.

So I am guessing auto-generated load combos are fine as long as we double-check factors from the codes. I have one more question here, so If I use ACI-318 as a design preference code and UBC-97 for Wind and Earthquake loads,  What do I do with the earthquake load factors in Load combinations? I read it somewhere that we need to multiply all the load factors of combos with the Earthquake load in it with 1.1 (as per UBC-97). Can someone please confirm if that's right?

As for the Foundation design i.e. Isolated footing, what I mean to ask is that in order to design it (after finding the area of footing), I am going to need a Load and Moment value from super structure which it will be designed to resist. I have attached a screenshot below ( please ignore the values/results) what I want to know is that there are different Load Cases and Combos in Base reaction table, Should I pick the maximum value from of FZ from the table? assuming that we have a Fix base, which Moment value I should pick from the table which it will be designed to resist?

Many Thanks.

Edited February 20, 2021 by Omer Ahmed

[Muhammad Hashmi]

Dear Muhammad,

There is no difference between adding the combinations manually or use default combinations.

For vertical seismic effects you need to modify the value of seismic design category and SDs. 

The program will automatically change the factors of load cases once changed.

[Omer Ahmed]

@Muhammad Hashmi Hi, I am not sure if you understand my question asked in the replies section above.

[Ayesha]

On 2/20/2021 at 6:44 AM, Omer Ahmed said:

so If I use ACI-318 as a design preference code and UBC-97 for Wind and Earthquake loads,  What do I do with the earthquake load factors in Load combinations? I read it somewhere that we need to multiply all the load factors of combos with the Earthquake load in it with 1.1 (as per UBC-97). Can someone please confirm if that's right?

You should use IBC with ACI as UBC is not compatible. If you still want to use UBC, this is grey area and engineering judgement should be used. Discuss with the people in your office and pick something conservative.

On 2/20/2021 at 6:44 AM, Omer Ahmed said:

As for the Foundation design i.e. Isolated footing, what I mean to ask is that in order to design it (after finding the area of footing), I am going to need a Load and Moment value from super structure which it will be designed to resist. I have attached a screenshot below ( please ignore the values/results) what I want to know is that there are different Load Cases and Combos in Base reaction table, Should I pick the maximum value from of FZ from the table? assuming that we have a Fix base, which Moment value I should pick from the table which it will be designed to resist?

There are two approaches. Either you design based on Envelope (Consider the max of all forces, Fx, Fy, Fz, Mx, My, Mz) and design for that. Or you can each load combination one by one. You can develop an excel macro to do that.

 

[Omer Ahmed]

@Ayesha Pardon my ignorance, so I used to opt for UBC since it have earthquake zones of Pakistani cities. Does IBC or ACI have that as well?  The problem with UBC is that it doesn't come in every version of ETABS. I am familiar with Eurocode and ACI but do I need to feed earthquake zone data in Etabs as well if I go with them?

Edited February 25, 2021 by Omer Ahmed

[Ayesha]

On 2/19/2021 at 10:38 AM, Ayesha said:

https://www.sepakistan.com/topic/1307-etabs-load-combinations/

The seismic data for IBC for major cities of Pakistan is provided in this thread that I posted above.