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Soft Storey effect
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Bilal Lakdawala
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PostPosted: Mon Dec 03, 2012 5:08 am    Post subject: Soft Storey effect Reply with quote

Dear All

Does any one takes into account the effect of soft-storey for RCC high rise buildings. Because generally we have parking at lower levels and residential or commercial at upper levels, and a drastic change in stiffness between these levels.

If we want to consider then what is the easiest or shortest method to compute the stiffness taking into account all RCC, masonry and openings.

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Bilal Lakdawala
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partha
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PostPosted: Mon Dec 03, 2012 5:37 am    Post subject: Soft Storey effect Reply with quote

To avoid this mostly people use shorter height for ground floor compared to floors above it and keep machine room / driver rooms to increase stiffness of ground floor so that it is at least 80% of first storey or 75% of average of upper stories.
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Partha  On Dec 3, 2012 10:59 AM, "Bilal Lakdawala" <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear All

Does any one takes into account the effect of soft-storey for RCC high rise buildings. Because generally we have parking at lower levels and residential or commercial at upper levels, and a drastic change in stiffness between these levels.

If we want to consider then what is the easiest or shortest method to compute the stiffness taking into account all RCC, masonry and openings.

Regards
Bilal Lakdawala
     



     


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ANANDSM
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PostPosted: Mon Dec 03, 2012 5:37 am    Post subject: Soft Storey effect Reply with quote

Hi
I have completed my thesis on soft storey buildings, soft storey at ground floor is more critical , so we have design the soft storey columns for 1.8 times the moment and beam for 1.4 times the moment


On Mon, Dec 3, 2012 at 10:38 AM, Bilal Lakdawala <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear All

Does any one takes into account the effect of soft-storey for RCC high rise buildings. Because generally we have parking at lower levels and residential or commercial at upper levels, and a drastic change in stiffness between these levels.

If we want to consider then what is the easiest or shortest method to compute the stiffness taking into account all RCC, masonry and openings.

Regards
Bilal Lakdawala
     



     


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Manoharbs_eq
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PostPosted: Mon Dec 03, 2012 5:40 am    Post subject: Reply with quote

Dear sir,

we have to consider soft storey effect if A soft storey is one in which the lateral stiffness
is less than 70 percent of that in the storey above
or less than 80 percent of the average lateral stiffness
of the three storeys above, we do consider it, for all the soft stories we design the elements in soft storey to increased forces of magnitude as per 7.10.3
7.10.3 Alternatively, the following design criteria are
same building with separatiolljoint in between shall
to be adopted after carrying out the earthquake
be separated by a distance equal to the amount R times
analysis, neglecting the effect of infill walls in other
the sum of the calculated storey displacements as per
a) the columns and beams of the soft storey are floor levels of two similar adjacent units or buildings
to be designed for 2.5 times the storey shears are at the same elevation levels, factor R in this
and moments calculated under seismic loads requirement may be replaced by R/2.
specified in the other relevant clauses: or. 7.12 Miscellaneous
b) besides the columns designed and detailed
for the calculated storey shears and moments,
shear walls placed symmetrically in both
directions of the building as far away from
the centre of the building as feasible; to be
designed exclusively for 1.5 times the lateral
storey shear force calculated as before,


Rgds
Manohar
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cckeshav
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PostPosted: Mon Dec 03, 2012 6:19 am    Post subject: Soft Storey effect Reply with quote

Dear SEFIans:

For the stilt floor, we must continue the shear walls and core walls. As far as the columns are concerned, we can increase the size of the columns in the stilt floor to offset the soft storey effect.

C.Channakeshava


Subject: [E-CONF] Re: Soft Storey effect
From: forum@sefindia.org
Date: Mon, 3 Dec 2012 11:18:02 +0530
To: econf34289@sefindia.org

           To avoid this mostly people use shorter height for ground floor compared to floors above it and keep machine room / driver rooms to increase stiffness of ground floor so that it is at least 80% of first storey or 75% of average of upper stories.
Regards
Partha On Dec 3, 2012 10:59 AM, "Bilal Lakdawala" forum@sefindia.org)> wrote:       --auto removed--

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Bilal Lakdawala
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PostPosted: Mon Dec 03, 2012 7:36 am    Post subject: Soft Storey effect Reply with quote

Dear All

I am thankful for your valuable replies. But majority are considering the soft-story effect at lower levels only, might because i had given the example of parking and usable levels. But storey can be soft even at upper levels due the fact that mostly there are large openings at upper levels (eg., for duplex, double living room ht) in highrises.
Would like your opinions on that front also.

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Bilal Lakdawala
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thirumalaichettiar
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PostPosted: Mon Dec 03, 2012 7:51 am    Post subject: Reply with quote

The silt storey is common in many apartments due to the open space required for car parking or shops where the openings are most part of the space between the column.

In addition to  suggestions by others that is to be considered while designing stage  I add the following as:

1. If it is a steel building frame it is wise to provide bracing in such a manner not to obstruct the passage of vehicles.


2. If the frame is R.C.C. in order to increase the stiffness of column, propose a composite column with steel I or other section and take the section at least to sill level of the first floor or provide necessary anchorage.

3. A rough idea is to build brick wall on all ot possible sides of column and if possible provide reinforcement from column to tie this pillared walls. The idea is to have composite action.
Other way is first build the walls around the reinforcement of column and cast concrete when the brick wall is set.

4. FRP or even add jacketing with mesh or required reinforcement if already built.


T.RangaRajan.
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srijith
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PostPosted: Mon Dec 03, 2012 9:25 am    Post subject: Soft Storey effect Reply with quote

How about increasing the size of the column width wise with additional reinforcement only in the parking area.(my opinion)
On Dec 3, 2012 11:58 AM, "thirumalaichettiar" <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           The silt storey is common in many aprtments due to the open space required for car parking or shops where the openings are most part of the space between the column.

In addition to suggestions by others that is to be considered while designing stage I add the following as:

1. If it is a steel building frame it is wise to provide bracings in sucha manner not to obstract the passage of vehicles.


2. If the frame is R.C.C. in order to increase the stiffness of column, propose a composite column with steel I or other section and take the section at leat to sill level of the first floor or provide necessary anchorage.

3. A rough idea is to build brick wall on all ot possible sides of column and if possible provide reinforcement from column to tie this pillastered walls. The idea is to have composite action.
Other way is first build the walls around the reinforcement of column and cast concrete when the brick wall is set.

4. FRP or even add jacketing with mesh or required reinforcement if already built.


T.RangaRajan.[/color:dc72889472]
     



     


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cvrm
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PostPosted: Mon Dec 03, 2012 2:20 pm    Post subject: Soft Storey effect Reply with quote

Dear Colleagues

Early in the eC, Professor Swaminathan warned us of the use of flexible lower storeys in Tall Buildings. But, a few posts, especially in the last two days, prompted me to write some thoughts...

If one were to list top ten key requirements for Tall Buildings, the need for eliminating flexible storeys will be one of them. The flexible storey cannot be present at any level, not just at the base of the building. Of course, the problem is most severe when the flexible storey is at the base of the Tall Building, which is the support of the vertical cantilever. Capturing abnormalities of stiffness in any storey of a Tall Building is relatively easy (than abnormalities in storey strength); floor displacements and storey shears of the building subjected to equivalent static EQx or EQy latreal loads can be used to estimate the stiffness of each storey in each plan direction.

What is worrysome is the interpretation of the IS:1893(1) clause of allowing a reduction in stiffness of lower storeys by about 20-30% to be applicable for Tall Buildings. In low rise buildings, that have a dominant lateral SHEAR behaviour, the reduction may be allowed as a compromise to admit some archiectectural features, even though it is known to cause harm to the building. But, in Tall Buildings, especially those that have a dominant lateral FLEXURAL behaviuour, significant axial load is induced in the lower storey vertical members (columns or walls) owing to large overturning effects. This icnreased axial load makes the vertical members behave in a brittle manner; this is a serious matter.

For tall buildings, it would be prudent to not allow any reduction in stiffness or strength in any storey in comparison to stiffness or strength in the storey above.

with warm regards...
C.V.R.Murty
..=

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Manoharbs_eq
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PostPosted: Tue Dec 04, 2012 4:05 am    Post subject: Reply with quote

Dear Murthy sir,

It is evident that soft storey at lower level is not a good structural system to adopt that we have learnt from past failures. But Architect never bother to change anything and we are forced to design to their need. so soft storey is unavoidable.

The only solution left is to drop down the shear walls to accompany and increase the stiffness. In a commercial building this doesnt pose much a problem but residential buildings its always a problem to handle since upper floors generally have walls.


Rgds
Manohar
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