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How to design building with soft story in etabs?
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binaya shahi
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PostPosted: Fri Nov 25, 2016 11:45 am    Post subject: How to design building with soft story in etabs? Reply with quote

According to IS code soft story should be design with 2.5 times the base shear.

So, I multiplied the mass source factor by 2.5 in etabs.
but base shear is not increasing twice.
Maybe I should increase the mass source factor until the base shear of soft story building is 2.5 times that of normal building.

I have many questions?
Am I doing it right?
Is my understanding correct.
Is there more appropriate method to do it?
Are columns only structural elements I should separately design for for soft story? or I should design beams and slabs too for soft story.

Please somebody clarify my questions and suggest best way to design building with soft story. and also common errors and misunderstanding regarding such design?
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sakumar79
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PostPosted: Sat Nov 26, 2016 4:47 am    Post subject: Reply with quote

I think the methodology you adopt is not correct. I think the following is simpler methodology.

1. Create new set of load combinations (1.5 DL+1.5 LL, 1.5 DL+/- 1.5 EL, etc) in addition to existing set of load combinations.

2. For the new set, in all places where you have EL load, multiply the existing factor by 2.5. For example, in place of 1.5 DL+1.5 EL, use 1.5 DL + 3.75 EL.

3. Design only soft storey members for the new load combinations.

4. Beams also have to be designed for soft storey combinations. Slab design is not affected.

Yours sincerely
Arunkumar
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binaya shahi
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PostPosted: Sat Nov 26, 2016 6:31 pm    Post subject: Reply with quote

I think as mass source in etabs is the earthquake load. multiplying mass source by 2.5  or multiplying earthquake load in load combination should give the same result.

The main problem is that multiplying mass source by 2.5 or earthquake load in load combination by 2.5 in not increasing the base shear much.

To get 2.5 times the base shear, I had to multiply the mass source by 6.
the reinforcement for one of the column for normal analysis was 1200 sq. mm. where as that for the 2.5 times base shear(or 6 times earthquake load) was 6200 sq. mm.

The reinforcement from soft story analysis is more than 5 times that of normal analysis. I want to know whether the difference in reinforcement between two analysis is plausible of not.

for normal case I used the mass source : 1 (dead load), 1(wall load), 1(floor finish), 0.25(live load)

whereas for soft story analysis, i used the mass source: 2.5(dead load), 2.5(wall load), 2.5(floor finish), 0.625(live load)

when I checked the m2 value for the same column in two analysis.
the m2 of the column for soft story analysis was about 3 times the m2 value for normal analysis.

My understanding is that according to IS code the base shear and the moment in beams and columns should be increased by 2.5 times for the soft story analysis.

My doubt is whether I should just use (2.5 times the earthquake load that resulted in not much increase in base shear) or (6 times the earthquake load that resulted in 2.5 times base shear).

Please help me out?
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saikiran gone
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PostPosted: Tue Nov 29, 2016 1:23 pm    Post subject: Reply with quote

binaya shahi wrote:
I think as mass source in etabs is the earthquake load. multiplying mass source by 2.5  or multiplying earthquake load in load combination should give the same result.

The main problem is that multiplying mass source by 2.5 or earthquake load in load combination by 2.5 in not increasing the base shear much.

To get 2.5 times the base shear, I had to multiply the mass source by 6.
the reinforcement for one of the column for normal analysis was 1200 sq. mm. where as that for the 2.5 times base shear(or 6 times earthquake load) was 6200 sq. mm.

The reinforcement from soft story analysis is more than 5 times that of normal analysis. I want to know whether the difference in reinforcement between two analysis is plausible of not.

for normal case I used the mass source : 1 (dead load), 1(wall load), 1(floor finish), 0.25(live load)

whereas for soft story analysis, i used the mass source: 2.5(dead load), 2.5(wall load), 2.5(floor finish), 0.625(live load)

when I checked the m2 value for the same column in two analysis.
the m2 of the column for soft story analysis was about 3 times the m2 value for normal analysis.

My understanding is that according to IS code the base shear and the moment in beams and columns should be increased by 2.5 times for the soft story analysis.

My doubt is whether I should just use (2.5 times the earthquake load that resulted in not much increase in base shear) or (6 times the earthquake load that resulted in 2.5 times base shear).

Please help me out?



Hi binaya shahi,
you need to increase the forces i.e 2.5 X Ah X Vb as per IS 1893  not base shear as seismic weight is constant.

It better and easy to follow procedure told by arun kumar.

With best,
Saikiran.
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rahul.leslie
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PostPosted: Mon Dec 05, 2016 6:19 pm    Post subject: Reply with quote

There's no point in boosting the mass source by 2.5, since it changes the mass-stiffness ratio of the building, that shifts the time periods, which again shifts the spectral acceleration. Maybe the mode shape also changes which again changes the mass participation factor too. Finally you doesn't end up with what you expected, as you have ended up with.

Rahul Leslie


Last edited by rahul.leslie on Fri Dec 09, 2016 5:52 pm; edited 1 time in total
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Dr. N. Subramanian
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PostPosted: Thu Dec 08, 2016 7:15 pm    Post subject: Reply with quote

Dear All,

I agree with Er Rahul Leslie,

You do the analysis for the actual code stipulated loads. While doing the designs, multiply the forces (AF, BMs) by 2.5 times and do the design. Note that there is no scientific explanation for the factor 2.5 and it was inserted as an Ad-hoc solution.

But the best solution is not to have soft storey! This can be done by providing shear walls or increasing the size of columns in the storey and making M.I. of members in this storey equal to M.I. of members in other stories.

Best wishes,
NS  

rahul.leslie wrote:
There's no point in boosting the mass source by 2.5, since it changes the mass-stiffness ratio of the building, that shifts the time periods, which again shifts the spectral acceleration, maybe the mode shape also which again changes the mass participation factor too -- finally doesn't end up with what you expected, as you have ended up with.

Rahul Leslie
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prof.arc
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PostPosted: Sat Dec 10, 2016 2:00 am    Post subject: How to design building with soft story in etabs? Reply with quote

That factor of 2.5 is just STUPIDSince the intention is to discourage SOFT STOREY it should be explicitly stated as such


if the system is properly modelled [which is rarely done], there is no need for arbitrary values


it has been suggested to incorporate shear wall panels at the corners. this will cause least disturbance to parking and simultaneously increase performance to resist lateral loads


ARC

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saikiran gone
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PostPosted: Tue Dec 13, 2016 7:40 am    Post subject: Reply with quote

saikiran gone wrote:
binaya shahi wrote:
I think as mass source in etabs is the earthquake load. multiplying mass source by 2.5  or multiplying earthquake load in load combination should give the same result.

The main problem is that multiplying mass source by 2.5 or earthquake load in load combination by 2.5 in not increasing the base shear much.

To get 2.5 times the base shear, I had to multiply the mass source by 6.
the reinforcement for one of the column for normal analysis was 1200 sq. mm. where as that for the 2.5 times base shear(or 6 times earthquake load) was 6200 sq. mm.

The reinforcement from soft story analysis is more than 5 times that of normal analysis. I want to know whether the difference in reinforcement between two analysis is plausible of not.

for normal case I used the mass source : 1 (dead load), 1(wall load), 1(floor finish), 0.25(live load)

whereas for soft story analysis, i used the mass source: 2.5(dead load), 2.5(wall load), 2.5(floor finish), 0.625(live load)

when I checked the m2 value for the same column in two analysis.
the m2 of the column for soft story analysis was about 3 times the m2 value for normal analysis.

My understanding is that according to IS code the base shear and the moment in beams and columns should be increased by 2.5 times for the soft story analysis.

My doubt is whether I should just use (2.5 times the earthquake load that resulted in not much increase in base shear) or (6 times the earthquake load that resulted in 2.5 times base shear).

Please help me out?



Hi binaya shahi,
you need to increase the forces i.e 2.5 X Ah X Vb as per IS 1893  not base shear as seismic weight is constant.

It better and easy to follow procedure told by arun kumar.

With best,
Saikiran.


Hi binaya shahi,

Sorry the typo mistake in my previous post:
you need to increase the forces i.e 2.5 X Ah X W as per IS 1893  not base shear as seismic weight is constant.

Or else you can you can multiply the factor (2.5/1.5) 1.67 with the seismic forces (BM,SF)for all seismic resisting beams and columns.It is always better to avoid the soft story by increasing the stiffness of that floor, by a providig shear wall or by increasing the column size.

With Best,
Saikiran.
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rahul.leslie
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PostPosted: Tue Dec 13, 2016 8:58 am    Post subject: Reply with quote

I am afraid, the above needs correction, I feel. What Er. ArunKumar said ...
Quote:
For example, in place of 1.5 DL+1.5 EL, use 1.5 DL + 3.75 EL.
... is what is to be done. Somebody please clarify my opinion.

Rahul Leslie
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Abishek_Siingh
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PostPosted: Wed Dec 14, 2016 5:02 am    Post subject: Reply with quote

We have been doing what Er. Arun Kumar has proposed.
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