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Welcome Note From Prof Swaminathan Krishnan to the Econferen
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ishacon
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PostPosted: Fri Nov 23, 2012 7:53 am    Post subject: Welcome Note From Prof Swaminathan Krishnan to the Econferen Reply with quote

Sir,

While agreeing with all your observations, let us not forget that R factors in ACI are
correspondingly higher than in IS code (R=5 for DBE = 1/2 MCE ).

Also in Indian conditions it is mostly observed that the residents in multi storeyed
buildings after taking possession remove the partition walls which are 115 thick.
Under such situation, how wise would it be to consider these partition walls for
stiffness?

V.P. AGARWAL
ISHA CONSULTANTS (P) LTD
NEW DELHI 110074

PH : 011-2630 1158 ;
08010071749 ; 093 1345 2180
Quote:
-- Original Message --
From: swamikrishnan (forum@sefindia.org)
To: econf34289@sefindia.org (econf34289@sefindia.org)
Sent: Friday, November 23, 2012 11:51 AM
Subject: [E-CONF] Re: Welcome Note From Prof Swaminathan Krishnan to the Econferen


1. Dual system proportioning question by Er. Rangarajan: If you are performing linear elastic static or dynamic analysis of the structure, you can ensure that the moment frame is resisting 25% of the forces, by comparing the sum of the column shear forces in each story against the total story shear force in that story (sum of column shear forces and shear wall shear forces).

2. FRAME3D can only be accessed through the Caltech Virtual Shaker site: http://virtualshaker.caltech.edu (Er. Rangarajan's question).

3. Er. Rangarajan's question on UBC: As I mentioned in my posting, the International Building Code is a conglomeration of all the former regional codes in the US, one of which is the Uniform Building Code (UBC). The IBC supercedes UBC and the other regional codes.

4. Dr. N. Subramanian: Thank you for your contributions to this conference and this forum in general as well.

5. Er. Gautam's query on DBE (475 year return period) Vs MCE (2475 year return period):

US: DBE = 2/3*MCE
India: http://www.iisc.ernet.in/currsci/mar102007/639.pdf

6. Er. Gautam's comment on partitions: I am not familiar with partitions used in residential towers in India. From my experience in Indonesia, Taiwan, and S. Korea, hollow concrete blocks seem popular for residential units in highrise buildings; gypsum boards partitions are common in office towers. The stiffness from partitions may be significant at low levels of shaking when they are intact; under intense shaking they may be damaged/separated (decoupling/sliding) and, perhaps, cannot offer much by way of stiffness. The elastic design base shear must be computed with the true fundamental natural period (which by definition is a characteristic of the elastic dynamic response). This must account for partitions which do contribute some stiffness at low-amplitude excitation that induces elastic response.

Swami






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PostPosted: Fri Nov 23, 2012 1:30 pm    Post subject: Idealisation in Software Tools and Structural Irregularities Reply with quote

When it comes to the analysis and design of Tall Building it is extremely important to idealise the correct mathematical model while using any software tool like STAAD, ETABS, ROBOT etc. It is often noticed engineers prepare a frame model consisting of Beams and Columns and perform a First Order Linear Analysis. These leads to following issues which becomes very critical and leads to inaccuracy.  

Modelling of Floor Slab:

  1. Residential administrative, office, educational and many other buildings and structures are made of continuous floor slabs without any opening. Industrial buildings, on the other hand, in many cases are having different sizes of cut-outs in the floor.  

  2. Presence of Slab not only contribute vertical loading but also impart a horizontal diaphragm action in the structure due to large in-plane stiffness of floor slabs.  

  3. Dynamic behaviour of a structure modelled taking into account the stiffness of the floor slab shows wide differences to that modelled without floor slab.  

  4. A floor slab can be modelled in two ways which are available in almost all software tools:

    1. By including plane stress elements

    2. By using master slave command


Soil-Structure Interaction:

  1. Generally engineers assume that the foundation is fixed in space. But, depending on the nature of soil, the base of the structure undergoes deformations. The soil can be seen as a spring with certain stiffness.  

  2. In almost all software tools, besides “Fixed”or “Pinned” options, user can also assign soil-springs by the User Defined or Fixed But option. Tools like Autodesk's Robot Structural Analysis even allow user to model soil strata at different levels with all parameters including its stiffness and damping ratio. Also utilities like Automatic soil-spring generation (i.e. MAT) can also be availed .

  3. In seismic analysis problems, research works has shown that a certain mass of soil vibrates along with the foundation. But no soil-mass is included in the model, as no specific guideline is yet available. Only stiffness and damping ratios of the soil-springs are generally included in a mathematical model.

  4. The spring constant of soil springs depends on factors like shear modulus of soil, Poisson's ratio of soil, foundation dimensions, and shear-wave velocity through the soil-strata.

  5. During dynamic analysis the damping ratios of the soil-spring may be obtained by summing two quantities: material damping and geometric damping of the soil-spring.

  6. Formulations of soil-spring stiffness and damping ratio are available in text books on machine foundations.  

  7. Once can analyse the same Building Frame using the same Software Tools with two different Spring Stiffness (say, 10E+10 kN/m^2/m run for Hard Soil and 10E+04 kN/m^2/m run for Soft Soil) it may be observed that the response is very much sensitive to soil-spring stiffness. Higher soil-spring stiffness increases overall stiffness of the structure, resulting in lowering of fundamental periods and attract more seismic force (increasing base shear).  

  8. The participation of soil in vibration is also altered which may be reflected by the values of composite modal damping.


Soft Storey and Modelling of Shear Walls

  1. According to IS 1893-2002 (Part 1), there are two types of stiffness irregularities as given below:

    1. A soft storey is one in which the lateral stiffness is less than 70% of that in the storey above or less than 80% of the average lateral stiffness of the three storeys above.

    2. An extreme soft storey is one in which the lateral stiffness is less than 60% of that in the storey above or less than 70% of the average lateral stiffness of the three storeys above.

  2. The lateral stiffness of a vertical element (column or wall) of a building frame is equal to (12*E*I/L^3). The lateral stiffness of a storey is the summation of the lateral stiffness of all columns and walls in that storey.  

  3. It is important to check Soft Storey and thus the modelling of Shear Wall is very much important as it will affect stiffness of the particular floor.  

  4. Some engineers even model compression only member or strut to resemble the presence of Brick Walls or Non-structural Partition Walls. However no specific guideline is available about formulation of equivalent strut members.
  
- Partha Pratim Roy, B. E. (Civil); M. E. (Structure); A. I. E.
Vice President (Technical), ADAPT International (pproy76@gmail.com)

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PostPosted: Sat Nov 24, 2012 7:22 am    Post subject: Welcome Note From Prof Swaminathan Krishnan to the Econferen Reply with quote

Friends,
I have two questions on design of tall buildings:

1. As a the column depth is increased it should slowly transition to a structural wall. While modelling, till what storey height/ column depth ratio should one model the member as a column ( frame) element? Literature seems to indicate a value between 2 & 3. OR are there any other considerations? I want to know current practices & your views. This has a lot of implication on the design as well.

2. With the above grey area, what happens to the 25% shear being resisted by frame members?

3. Where Can I get the design procedure for coupling beam between two shear walls?

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PostPosted: Sat Nov 24, 2012 5:17 pm    Post subject: Welcome Note From Prof Swaminathan Krishnan to the Econferen Reply with quote

Dear Mr. Agarwal,

1. My note about accounting for the partition stiffness should be viewed in the context of the "Alternate Design Guidelines" in the US where there is no minimum limit on the fundamental period. Even there, it should be used ONLY in the calculation of the design base shear, so as to NOT UNDERESTIMATE the design base shear.

2. One should NEVER include the stiffness or strength of partitions for drift or stress checks for precisely the reasons that you mentioned. This applies always, be it a code design or an alternate design.

Swaminathan Krishnan

On 11/23/12 1:20 AM, ishacon wrote:

[quote]    Sir,

While agreeing with all your observations, let us not forget that R factors in ACI are
correspondingly higher than in IS code (R=5 for DBE = 1/2 MCE ).

Also in Indian conditions it is mostly observed that the residents in multi storeyed
buildings after taking possession remove the partition walls which are 115 thick.
Under such situation, how wise would it be to consider these partition walls for
stiffness?

V.P. AGARWAL
ISHA CONSULTANTS (P) LTD
NEW DELHI 110074

PH : 011-2630 1158 ;
08010071749 ; 093 1345 2180



Read this full topic [thread] online here:
http://www.sefindia.org/forum/viewtopic.php?p=52805#52805

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PostPosted: Sun Nov 25, 2012 1:04 pm    Post subject: Welcome Note From Prof Swaminathan Krishnan to the Econferen Reply with quote

Thanks Prof Krishnan for replyinhg in such details. One vital issue is whether or not to include stiffness of partiction walls (an in my perception the external claddings also) in structural stiffness matrix while constructing the global dynamic stiffness matrix of the structure we are analysing. Iam here referinfg to shear model only for simplicity. whether we can assimilate the brick masonry structure (walls) with RCCcolumns and shear walls considering the system as composite and Eb/Ec as modular ratio (b is for brick). As this morning I went through Er. Ahuja's note that many apartment owners switch over to lighter partitions of aluminium, teak ply blocks etc, which certainly will lower the overall stiffness of the floor wheresuch change has been made.
While not considering stiffness of walls (both partition and external cladding) we are lowering the floor stiffness, we are considering their dead loads to increase the floor mass. Since time period is directly proportional to square root of stiffness and inversely to square root of mass, we are in fact reducing the design time period. This is from a SDOF system but a MDOF result will not vary much from this observation.

In Indian code we adopt the stipulate less is time period, more is spectral acceleration. Hence more is the spectral acceleration more the storey forces we shall arrive at. I feel the design as per IS 1893 is conservative, may be a bit illogically.




On Thu, Nov 22, 2012 at 12:17 PM, gautam chattopadhyay <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            how to simulate hystogram for 475 yrs return period and 2475 yrs return period? do brick masonry walls offer any effective resistance during oscillation of a structure? particularly the partition walls? In modern internal decoration people are more and more adopting artificial timber partitions which do not have any structural stiffness at all.

On Wed, Nov 21, 2012 at 5:27 AM, swamikrishnan forum@sefindia.org (forum@sefindia.org))> wrote:
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PostPosted: Sun Nov 25, 2012 7:23 pm    Post subject: Re: Welcome Note From Prof Swaminathan Krishnan to the Econf Reply with quote

gautam chattopadhyay wrote:
Since time period is directly proportional to square root of stiffness and inversely to square root of mass, we are in fact reducing the design time period. This is from a SDOF system but a MDOF result will not vary much from this observation.

In Indian code we adopt the stipulate less is time period, more is spectral acceleration. Hence more is the spectral acceleration more the storey forces we shall arrive at. I feel the design as per IS 1893 is conservative, may be a bit illogically.


Dear Mr.Gautam Chattopadhyay,

Time period is NOT directly proportional to square root of stiffness and NOT inversely proportional to square root of mass. It is the other way around. Hence, the time period calculated will be longer. However, as far as I know, IS 1893 does not allow to use the base shear force calculated using modal analysis if it is less than the one calculated using the empirical formula for time period.

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PostPosted: Mon Nov 26, 2012 12:25 am    Post subject: IS:1893 must be revised for use in TALL Buildings Reply with quote

Revision is necessary
for the following compelling reasons
1. Anchoring the base shear to an empirical formula for fundamental period
2. use of time history rather than the approximate shape of spectra [flattened too much & erring on shape for so called soil sites as the foundation of very tall buildings would go to hard strata]
3. use of high value of R as 5.
the reduction factor for different type of ductility definitions like
inter story displacement (called displacement ductility); strain ductility,
moment-curvature ductility
any recommendation be based ob reduction factor vs ductility for tall buildings rather on single degree of freedom models first used by Newmark & Blume in their studies
further experimental data on tall building models be used to obtain what order of ductility [for deflection/strain/moment-curvature] can be achieved
4. use of flat slabs - how the frame is restrained by shear slab as opposed to beams restraining the columns in conventional 3D frame analysis
5. must ensure DUCTILE behaviour in all zones

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PostPosted: Mon Nov 26, 2012 4:08 am    Post subject: Re: Alternate Design Guidelines vs Prescriptive Code Reply with quote

swamikrishnan wrote:

(a) The ADG eliminates height limits on all structural systems. It permits the use of any structural system for any tall building as long as the Engineer of Record performs three checks -- a serviceability check [no damage in an earthquake with a 43-year return period (50% probability of being exceeded in 30 years), i.e., structure should remain elastic], a life-safety check [this is the code-level seismic evaluation, some damage is acceptable, but intent is to protect life in an earthquake with a 475-year return period (10% probability of being exceeded in 50 years)] and a collapse prevention check [safe guard against collapse in an earthquake with a 2475-year return period (2% probability of being exceeded in 50 years)]. The collapse prevention check must be done using nonlinear time history analysis of the building subjected to 7 or more appropriate ground motion time histories. The design must be reviewed by a recognized panel of experts. Prescriptive codes do not require the serviceability check or the collapse prevention check or peer review.

(a) Eliminating the hard lower bound on the fundamental period (1.3Ta) in the base shear computation:  Can we believe the natural periods predicted by our structural models that typically do not include stiffness associated with "non-structural elements" such as partitions and facades as well as the stiffness contributed by the partial restraint inherent in simple (non-moment) connections of the gravity system?


Dear Prof.Swaminathan Krishnan:

Eurocode 8 part 1 allows to use the time period obtained from the modal analysis for the computation of base shear. This is due to the fact that there is significant damage of the non-structural elements under the design seismic action (life safety check).

The following is an excerpt from the book 'Seismic Design, Assessment and Retrofitting of Concrete Buildings: Based on Eurocode 8', by Prof.Michael Fardis, which explains the approach of Eurocode 8 part 1 on the calculation of time period.

Quote:
Codes (CEN 2004a, BSSC 2003, SEAOC 1999) give also empirical expressions for T1, representing lower bounds (mean minus standard deviation) from measurements on buildings in California in moderate earthquakes. Such measurements reflect also the influence of non-structural elements on the response. So, the empirical expressions underestimate the period compared to Eq. (4.7). The empirical expressions may give values for T1 that lie in the constant spectral acceleration region even for flexible buildings. So, they are sometimes used to obtain a safe-side estimate of Sa,d(T1) for force-based design. In the light of the upcoming displacement-based design and assessment, where realistic estimation of displacement demands is of prime importance, the empirical expressions for T1 are not just inaccurate and misleading, but unsafe as well. So, given that Eq. (4.7) gives accurate estimates of T1 at no additional effort, further use of the empirical period formulas in seismic design seems unwarranted.

Unlike Eurocode 8, which tries to emulate in linear static analysis a modal response spectrum one through a mechanics-based value of T1 (e.g., from Eq. (4.7)) and a value of meff,1 in Eq. (4.6) which under certain conditions is less than the total mass m, US codes (BSSC 2003, SEAOC 1999) seem to have more confidence in the empirical expressions for T1 than in Eq. (4.7). So, if the designer applies Eq. (4.7) or any alternative mechanics-based approach, he/she should respect a lower limit on the pseudo-acceleration Sa,d(T1) from the design spectrum. In SEAOC (1999) the limit is 80% of the Sa,d(T1) value determined from the spectrum at the empirical T1-value. In BSSC (2003) a lower limit is set to the value of T1 from Eq. (4.7) or other mechanics-based expressions: the T1-value to be used cannot exceed the empirical period times 1.41.7.

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PostPosted: Mon Nov 26, 2012 1:36 pm    Post subject: Welcome Note From Prof Swaminathan Krishnan to the Econferen Reply with quote

Is ZPA a pragmatic approach to design? what the records of 100 yrs return period earthquakes say? What was duration of worst pulse? can that not be simulated in time history?


On Sat, Nov 24, 2012 at 10:51 PM, krishnan_caltech <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            Dear Mr. Agarwal,

1. My note about accounting for the partition stiffness should be viewed in the context of the "Alternate Design Guidelines" in the US where there is no minimum limit on the fundamental period. Even there, it should be used ONLY in the calculation of the design base shear, so as to NOT UNDERESTIMATE the design base shear.

2. One should NEVER include the stiffness or strength of partitions for drift or stress checks for precisely the reasons that you mentioned. This applies always, be it a code design or an alternate design.

Swaminathan Krishnan

On 11/23/12 1:20 AM, ishacon wrote:

[quote] Sir,

While agreeing with all your observations, let us not forget that R factors in ACI are
correspondingly higher than in IS code (R=5 for DBE = 1/2 MCE ).

Also in Indian conditions it is mostly observed that the residents in multi storeyed
buildings after taking possession remove the partition walls which are 115 thick.
Under such situation, how wise would it be to consider these partition walls for
stiffness?

V.P. AGARWAL
ISHA CONSULTANTS (P) LTD
NEW DELHI 110074

PH : 011-2630 1158 ;
08010071749 ; 093 1345 2180



Read this full topic [thread] online here:
http://www.sefindia.org/forum/viewtopic.php?p=52805#52805

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