[SEFI] Time period of rc frame buildings

[naveen]

Dear All,
I am structural engineer working on the design of buildings. At present i am working on the design of an rc framed building of about 14 stories height. This building is nearly square in plan and is not having any vertical irregularities. Also there are no shear walls. This is a residential building with brick infill walls.
For calculating the time period of rc buildings IS 1893 gives two formulas. One is given in section 7.6.1 which gives time period for rc frames without infill walls and the other is in section 7.6.2 which gives time period for rc frames with infill walls. Until now, i was using the the formula given in 7.6.2 for calculating the time period of buildings with brick infill panels.
For modelling the building the a bare frame is considered and the infill panels were not modelled. Recently i was instructed to use the formula given in section 7.6.1 i.e., the formula for frames without infill panels, as we are not modelling the infill panels. But i am of the opinion that even though we are not modelling the infill panels, the real structure will have brick infill panels and will be stiffer compared to the bare frame and will attract more forces in an earthquake, so i prefer to use the brick infill frame formula.

Can anybody please elaborate or explain about which formula have to be taken for calculating time period of the rc moment resisting frame building modelled as a bare frame without brick infills.

Waiting for reply and thanks in advance for any replies.

Regards

Naveen.

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[sakumar79]

Dear Naveen,

You are correct. For buildings with infill wall, you have to use the formula in section 7.6.2. The main aim of the clause is to ensure that even if the building is modelled without the brick walls, a certain amount of improved accuracy will be provided by making use of the said equation... Only if there are no brick walls can one make use of section 7.6.1 formula...

Arun

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[drnsmani]

Dear Mr. Naveen,

I also would suggest you to use the the formula given in 7.6.2 for calculating the time period of buildings with brick infill panels.

Some engineers are using the formula given in section 7.6.1 (time period for rc frames without infill walls), since they are not sure about the location of brick infills- they may be removed in future.

Best wishes
Subramanian



--- On Wed, 8/20/08, naveen wrote:

Quote:

From: naveen Subject: [SEFI] Time period of rc frame buildings To: gen...@sefindia.org Date: Wednesday, August 20, 2008, 4:34 PM

http://www.sefindia.org/forum/viewtopic.php?p=8147#8147

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[akbar.civil]

Dear Mr. Naveen,

You are right that the time period of the structure is more exact with the equation 7.6.2 which gives time period for rc frames with infill walls.

But I fell your case is little different. You have already modeled the building without brick infill walls and is entering an approximate time period of the building for further dynamic analysis of the building, Isnt it?. Please correct me if I am wrong.

In that case I feel that you should enter the time period corresponding to equation 7.6.1 (without infill walls) because the model you already have is without brick infill wall. In doing so you are entering a wrong time period compared to the actual model but correct time period for the one you have already modeled. In doing so the dynamic forces you get on structural elements like beams, columns, etc will be more exact.

All comments are welcome as this is purely my opinion.

Regards,

*******************************
M. A. Akbar
M-Tech: Structural Engg.
Calicut, Kerala
India
*******************************

----- Original Message ----
From: naveen
To: gen...@sefindia.org
Sent: Wednesday, August 20, 2008 4:04:01 AM
Subject: [SEFI] Time period of rc frame buildings

http://www.sefindia.org/forum/viewtopic.php?p=8148#8148

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[anujdalvi]

Naveen
7.6.2 should be used,but also assign the dead weight of infill walls in your model even if you do not assign them,we had a similar case using Etabs,practically giving infills makes the model clumsy and crowded to work on.
To what extent do infill walls add to stiffness,i suppose when you are modelling your building at all levels you have excluded walls so chances of geting a soft storey is minimal right,Besides when you consider the stiffness of structure to resist earthquakes it would have been appreciable in case shear walls were included in this building.
If possible suggest for shear walls around the lift that would add to the seismic capacity.

Regards

Anuj.


On 8/20/08, drnsmani fo...@sefindia.org)> wrote: [quote] Dear Mr. Naveen,

I also would suggest you to use the the formula given in 7.6.2 for calculating the time period of buildings with brick infill panels.

Some engineers are using the formula given in section 7.6.1 (time period for rc frames without infill walls), since they are not sure about the location of brick infills- they may be removed in future.

Best wishes
Subramanian



--- On Wed, 8/20/08, naveen wrote:

[b]--auto removed--

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[sangramiitk]

Why you are calculating the time period manually bu hand has the answer.
You are going to calculate the lower limit of seismic force/base shear

For which structure you are going to calculate. If in practical your structure is with brick wall then simply use to formula that is used to calculate first natural time period for brick in fill wall.

Regards
-Sangram


On Wed, Aug 20, 2008 at 10:20 PM, akbar.civil fo...@sefindia.org)> wrote:

Quote:

Dear Mr. Naveen, You are right that the time period of the structure is more exact with the equation 7.6.2 which gives time period for rc frames with infill walls. But I fell your case is little different. You have already modeled the building without brick infill walls and is entering an approximate time period of the building for further dynamic analysis of the building, Isnt it?. Please correct me if I am wrong. In that case I feel that you should enter the time period corresponding to equation 7.6.1 (without infill walls) because the model you already have is without brick infill wall. In doing so you are entering a wrong time period compared to the actual model but correct time period for the one you have already modeled. In doing so the dynamic forces you get on structural elements like beams, columns, etc will be more exact. All comments are welcome as this is purely my opinion. Regards, ******************************* M. A. Akbar M-Tech: Structural Engg. Calicut, Kerala India ******************************* ----- Original Message ---- From: naveen To: gen...@sefindia.org (gen...@sefindia.org) Sent: Wednesday, August 20, 2008 4:04:01 AM Subject: [SEFI] Time period of rc frame buildings

http://www.sefindia.org/forum/viewtopic.php?p=8153#8153

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[prof.arc]

"But i am of the opinion that even though we are not modelling the infill panels, the real structure will have brick infill panels and will be stiffer compared to the bare frame and will attract more forces in an earthquake, so i prefer to use the brick infill frame formula".

Please stick to your preference - the one that gives stiffer period. The period you obtain ignoring infill walls (that is considering bare frame only) would give a fictitious elongated period.

ARC


On 8/20/08, naveen fo...@sefindia.org)> wrote:

Quote:

http://www.sefindia.org/forum/viewtopic.php?p=8160#8160

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[ajeetkokil]

You are absolutely correct on using the said formula (pertaining to the brick infills) to calculate time period for str under consideration.
However, behavior of your mathematical model should match with that of the prototype.
I mean, you must simulate brick infill walls, may be with imagnary bracings with infinite stiffness and zero density, so that the force distribution is in line with your thought process.

Hope this helps


Ajit  

Quote:

On 8/20/08, naveen wrote: Quote:

Dear All, I am structural engineer working on the design of buildings. At present i am working on the design of an rc framed building of about 14 stories height. This building is nearly square in plan and is not having any vertical irregularities. Also there are no shear walls. This is a residential building with brick infill walls. For calculating the time period of rc buildings IS 1893 gives two formulas. One is given in section 7.6.1 which gives time period for rc frames without infill walls and the other is in section 7.6.2 which gives time period for rc frames with infill walls. Until now, i was using the the formula given in 7.6.2 for calculating the time period of buildings with brick infill panels. For modelling the building the a bare frame is considered and the infill panels were not modelled. Recently i was instructed to use the formula given in section 7.6.1 i.e., the formula for frames without infill panels, as we are not modelling the infill panels. But i am of the opinion that even though we are not modelling the infill panels, the real structure will have brick infill panels and will be stiffer compared to the bare frame and will attract more forces in an earthquake, so i prefer to use the brick infill frame formula. Can anybody please elaborate or explain about which formula have to be taken for calculating time period of the rc moment resisting frame building modelled as a bare frame without brick infills. Waiting for reply and thanks in advance for any replies. Regards Naveen.

Ajit Shankar Kokil
Engineer,
Strl. and Arch. Deptt.
Engineers India Limited,
New Delhi
call me at 99112 02590

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[pat_vchoudhary]

Dear all

I have been modelling infill walls and compared it with bare frame models. The results are surprising.

1. First, Static load at column bases changes as compared to bare frame. some column has nore load and some less, but the total load on all columns remains same in both cases. It must be due to transfer of forces through bracings, which are modelled to represent infill walls. Which load to consider - from bare frame or from infill model.

2. What should be the value of E for brick infill walls. A lot depends on the value of E for correct modelling.

3. Shoud brickpanels with openings also be modelled? If yes, upto what size and location of openings?

In my opinion, modelling of bare frames should be stopped, as it gives unrealistic results compared to actual behaviour of structure during earthquake. And we need a lot of discussion on this instead of partial factor of safety1.5/1.2/.9

Thanks

Er Sudip kumar


[quote] ----- Original Message -----
From: ajeetkokil (fo...@sefindia.org)
To: gen...@sefindia.org (gen...@sefindia.org)
Sent: Thursday, August 21, 2008 1:45 PM
Subject: [SEFI] Re: Time period of rc frame buildings

You are absolutely correct on using the said formula (pertaining to the brick infills) to calculate time period for str under consideration.
However, behavior of your mathematical model should match with that of the prototype.
I mean, you must simulate brick infill walls, may be with imagnary bracings with infinite stiffness and zero density, so that the force distribution is in line with your thought process.

Hope this helps


Ajit

[b]--auto removed--

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[suku82in]

Dear all,



Lot of discussion is going on brick infill topic. I want to to know while modeling equivalent strut for replacing brick infill ,how to account for opening in the wall.



Regards,

S.adhikari


--- On Thu, 8/21/08, pat_vchoudhary wrote:

Quote:

From: pat_vchoudhary

Subject: [SEFI] Re: Time period of rc frame buildings

To: gen...@sefindia.org Date: Thursday, August 21, 2008, 6:23 PM

Dear all I have been modelling infill walls and compared it with bare frame models. The results are surprising. 1. First, Static load at column bases changes as compared to bare frame. some column has nore load and some less, but the total load on all columns remains same in both cases. It must be due to transfer of forces through bracings, which are modelled to represent infill walls. Which load to consider - from bare frame or from infill model. 2. What should be the value of E for brick infill walls. A lot depends on the value of E for correct modelling. 3. Shoud brickpanels with openings also be modelled? If yes, upto what size and location of openings? In my opinion, modelling of bare frames should be stopped, as it gives unrealistic results compared to actual behaviour of structure during earthquake. And we need a lot of discussion on this instead of partial factor of safety1.5/1.2/.9 Thanks Er Sudip kumar

Quote:

----- Original Message ----- From: ajeetkokil (fo...@sefindia.org (fo...@sefindia.org)) To: gen...@sefindia.org (gen...@sefindia.org) (gen...@sefindia.org (gen...@sefindia.org)) Sent: Thursday, August 21, 2008 1:45 PM Subject: [SEFI] Re: Time period of rc frame buildings You are absolutely correct on using the said formula (pertaining to the brick infills) to calculate time period for str under consideration. However, behavior of your mathematical model should match with that of the prototype. I mean, you must simulate brick infill walls, may be with imagnary bracings with infinite stiffness and zero density, so that the force distribution is in line with your thought process. Hope this helps Ajit [b]--auto removed--

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[ibarua]

23 Aug 2008


The formula to be used depends on the thickness of the infill walls. I the NE Region, we usually have 4.5" thick infill walls (to reduce dead load, seismic shears and cost). With such walls the formula without contribution of infill walls may be used. However, if infill walls are 10" or more thick, then these will definitely contribute to story stiffness and have to be accounted for in the time period computations.

Indrajit Barua.

On Wed, 20 Aug 2008 sakumar79 wrote :

Quote:

Dear Naveen, You are correct. For buildings with infill wall, you have to use the formula in section 7.6.2. The main aim of the clause is to ensure that even if the building is modelled without the brick walls, a certain amount of improved accuracy will be provided by making use of the said equation... Only if there are no brick walls can one make use of section 7.6.1 formula... Arun

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[ishacon]

Yes, I agree with this approach, since there is no certainty when the 4.5" wall may be removed by the owner.
In any case because of little or no anchorage with the bare frame, these 4.5" walls could possibly collapse at the first major jolt.

VP Agarwal

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ish...@bol.net.in (ish...@bol.net.in)
ish...@rediffmail.com (ish...@rediffmail.com)
[quote] ----- Original Message -----
From: ibarua (fo...@sefindia.org)
To: gen...@sefindia.org (gen...@sefindia.org)
Sent: Saturday, August 23, 2008 10:58 AM

Subject: [SEFI] Re: Time period of rc frame buildings

23 Aug 2008


The formula to be used depends on the thickness of the infill walls. I the NE Region, we usually have 4.5" thick infill walls (to reduce dead load, seismic shears and cost). With such walls the formula without contribution of infill walls may be used. However, if infill walls are 10" or more thick, then these will definitely contribute to story stiffness and have to be accounted for in the time period computations.

Indrajit Barua.

On Wed, 20 Aug 2008 sakumar79 wrote :

[b]--auto removed--

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[taylordevicesindia]

Dear All,

There has been considerable debate on this topic as also a number of posts. However none of the responses is even close to what is the implication of this clause and how we need to understand it and use it while undertaking Dynamic analysis. What is sad is that the gentry of engineers who are members of SEFI are amongst the brighter lot existing in our country and even they have not been able to understand the single most important clause in IS-1893, even though the code has been around now for over 6 years.

Below I have endeavored to give a step by step simple clarification/ explanation which should be easily understood by all. I recommend that readers download the excel sheet (web-link below) so as to follow the explanation given with greater clarity.

http://www.taylordevicesindia.com/IS_1893_Static_Time_Period.xls

1. This Static Time Period clause is the most important as it has the greatest influence on the design of structural members. This implies that it also has the greatest implication on the cost of the structural system, so if you can play around (use this clause wrongly) you can actually show great cost savings and impress your clients. Today many are doing this deliberately whereas there are some who are doing through ignorance and lack of understanding. The three IS-1893 clauses that have the highest bearing on structural costs are:
(a) Zone factor (Z)
(b) Importance factor (I) and
(c) Static Time Period
The code has not left any levy on selection of Zone factor as it actually gives a table of cities and the zone factor. However the guidance on importance factor and static time-period is far from satisfactory thereby giving sufficient freedom to engineers to play around with these clauses and thus design unsafe buildings. For example the code suggests an importance factor of more than one may be used for multi-storey buildings housing multiple residential units and that’s about all. What we need to ask is should a clause having such severe bearing on design be left open ended, could it not have been qualified further by connecting it to the total area of building and/or the building height. Similarly the wording of the clause on static time period is most atrocious and far from satisfactory thus causing avoidable confusion.

2. Firstly and most importantly –THE STATIC TIME PERIOD CLAUSE HAS GOT ABSOLUTE NOTHING TO DO AND HAS NO INFLUENCE ON THE BUILDING RESPONSE WHEN WE UNDERTAKE DYNAMIC ANALYSIS (in our case RESPONSE SPECTRUM).

3. The sole purpose of this clause in our analysis is to arrive at the MINIMIUM BASE SHEAR VALUE that a building is to be mandatorily designed to.

4. When we undertake Dynamic analysis there can be two cases w.r.t. base shear:-

Case-A - Dynamic Base Shear value is more than Static Base Shear value.
Case-B - Static Base Shear value is more than the Dynamic Base Shear value.

If the response spectrum analysis is of type Case-A then the static time period clause has absolutely no meaning or bearing on analysis as the building structure design would be governed by the value of Dynamic Base Shear. However this would seldom be the case and on most occasions while undertaking response spectrum analysis we will have the Dynamic Base Shear to be less than the Static Base Shear. In such cases i.e. Case-B we assign a scale factor to the response spectrum case (Scale Factor = Static Base Shear/Dynamic Base Shear) such that the value of Dynamic Base Shear becomes either equal to or slightly more than the Static Base Shear value.

5. The implications of using the wrong Static Base Shear formula are extremely severe. For example incase a mid-height building is designed to confirm to Zone-V standards and all of the code provisions have been followed but for using the wrong static time-period (used the bare frame static time-period formula instead of the formula for frames with brick infill panels) in actual THE BUILDING DESIGN WILL ONLY CONFORM TO ZONE-3 STANDARD. An excel sheet showing the base shear comparison can be downloaded and you can play around with the input values shown in colored cells to see how that static base shear varies. Once you manipulate the input parameters you would realize that the percentage variation between the two methods of calculating base shears is dependent on the building base dimension (footprint) and the building/storey height.

6. Having debated the static time-period clause for buildings with brick infills and its implications I would also like to bring out that this clause in our code is flawed as after a certain height the minimum base shear just remains constant. The graph showing the static base shear values by different methods should be referred. For RCC framed buildings with brick infills I would recommend the use of the static time period formula which is in line with the US codes i.e. T = 0.05 * H ^ 0.75

7. It is also recommended that a study be undertaken to see how the minimum base shear value using Indian codes compares with other international codes. I am of a opinion that we are using minimum base values that are much less that what the other codes are recommending.

Sincerely.

Taylor Devices India Pvt. Ltd.

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[prof.arc]

Quote:

On Sun, Aug 24, 2008 at 1:04 AM, taylordevicesindia fo...@sefindia.org)> wrote:

It is presumptuous for anyone to think that one can predict true behaviour of RC-MS-Buildings.
The assumptions involved in the analysis are too many - the loads and modelling of structural behaviour. The purpose of the code is to ensure that there is no gross misuse in arriving at forces and that clause on minimum base shear was meant that there would be no "free for all".

It is abundantly clear to me that in the event of mega events of 1897 & 1950 reoccurring, there will be substantial damage even to engineered buildings, particularly stilt type buildings. The preamble to the code states explicitly that it is not EARTHQUAKE PROOF DESIGN.

The main factor of underestimation is in intensity of ground motion. Since most designers fondly hope that they can achieve R=5, for a structure deemed as I=1, the so called seismic coefficient works out to 0.09 in 2002 version as opposed to 0.08 in 1984 version [assuming all other factors for soil, foundation, etc as unity]. It implies a zero period acceleration of 0.036g ONLY.

The code only provides for an UNDEFINED degree of protection for mild to moderate earthquake
and expects damage without complete collapse in case of severe event.

In conventional [non-stilt type] buildings there is a lot of factor of safety as live loads existing at the time of actual event is always smaller. Contribution of a number of structural elements are ignored in bare frame design.

Assuming no foundation failure, it would be a good idea to overdesign stilt floor deliberately - provide shear walls at corners / utilise lift wall by providing RC shear wall at stilt level

If stilt floor stands, there will be no complete collapse.

IT WILL BE A GOOD IDEA IF PERSONS CAN PROPOSE DRAFT REVISIONS TO THE CODE INSTEAD OF ONLY LAMENTING THAT THOSE WHO THREW THE HAT IN THE RING ARE FOOLISH !!. No doubt the code can be improved but it can never be a perfect document.

What is good in this business is taking a statistical view (not emotional), the total loss to buildings all over the world to those damaged due to environmental causes is small, say, as compared to loss due to factors like traffic accidents, public health, etc. That is why people continue to stay in severe earthquake zones hoping that the event would not occur in their life time. Or in dilapidated chawls of Mumbai in monsoon season.

Prof. ARC

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[ajay]

Dear All,
I think in all discussion some main points are missing.
As per my knowledge,
Earthquake is noting but series of shocks.(Fore shocks-Main Earthquack-Aftershocks)
So Practically,
1)Period of vibration T by Modeling ( Or considering) the non-structural infilled walls will also not present the correct value of T when the Infilled Partition(Brick Partition) walls get damaged during EARTHQUAKE EPISODE.

2)During use of structure there is always possibility that owner may remove the partitions( as already mentioned by Subramanian Sir)

3)Strength of Non structural elements (like Infilled walls) we consider as a part of OVERSTRENGTH( Buffer strength).Due to overstrength & Ductility( or Ductility reduction factor)we are designing structure for much lower seismic force that what may actually sustain at maximum shaking intensity experienced.

4)Response Spectrum Method will give much lager value of natural period of vibration T for frame with Infilled partition as compared to its real natural period of vibraion that can be obtain after properly modelling the infills( but practically not possible because it is very laborious& also not represent correct value as mention in point 1) )

So it is always better to anlyse the structure with bare frame( without Non-structural infill walls).

If I am wrong please correct.

Regards
Ajay S. Palwe
ajay_st...@yahoo.co.in

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[ibarua]

25th Aug 2008

I couldn't agree more with Prof. ARC. We can never design & build eq. proof buildings -- society will just not pay the price. What we can design are eq. resistant buildings, in which there will be no fatalities from building collapses if a major eq. (like the ones of 1897 and 1950)hits.

Indrajit Barua.
On Sun, 24 Aug 2008 prof.arc wrote :

Quote:

Quote: On Sun, Aug 24, 2008 at 1:04 AM, taylordevicesindia

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[drnsmani]

Dear Mr. Ajay S. Palwe,

Your points are correct. In this connection I would second the opinions expressed by Prof. ARC. Though we may be using sophisticated software packages, Structural design is approximate and is as good as the design assumptions. Concrete design is still approximate than steel design as the material itself is not homogenous and we make so many assumptions in the design.

Seismic response of structures is an evolving field as evidenced by the large number of articles appearing in several journals. As pointed out by Prof. ARC, we use a smoothed elastic design acceleration response spectrum (SEDRS). It is important to recognize that the response spectra approach does not consider the duration effects.

The Z values given in the code represent a reasonable estimate of PGA in the respective zones. Thus a value of Z equal to 0.36 in Zone V implies that a value of 0.36g is reasonably expected in Zone V. But it does not imply that acceleration in Zone V will not exceed 0.36g. For example, during the 2001 Bhuj earthquake, a PGA of about 0.6g was recorded at Anjar, 44 km away from the epicenter. Similarly the recorded acceleration values in Northridge (USA, 1994) and Kobe (Japan, 1995) values were much higher.

The term R gives an indication of the level of overstrength and ductility that a structure is expected to have. The seismic design factors (Response modification coefficient R, System over-strength factor, and Deflection amplification factor Cd) are given in Table 12.2.1 of ASCE/SEI 7-05, which runs to 3 pages, which is referenced by the 2006 International Building Code (IBC). It has to be noted that these factors cannot be directly calculated based upon analysis and cyclic test data. The reason for this is that code-defined seismic design factors were developed over a period of many years based largely upon observed seismic performance of systems subjected to actual earthquake demands, limited test data, and comparison with other code-defined systems. However, because the seismic design factors were considered subjective, very detailed code provisions were developed to limit displacement and provide adequate strength and ductility for elements and subassemblies specific to each defined structural system.

Regarding your question on considering the effect of masonry walls, it is still in the research stage, though some equations have been proposed to consider them as bracing members with hinged ends. The width of the bracing to be taken in the analysis may be debatable. The average value of E may be taken as Em = 550fm (Hemant B. Kaushik, Durgesh C. Rai and Sudhir K. Jain,2007). However, as already discussed the 4.5 inch (115mm) thick walls may not provide sufficient stiffness and may be ignored. Even the 9 inch walls, if having openings near the edges, may not possess sufficient stiffness. Large openings in walls may also reduce their strength and stiffness. Moreover 9 inch walls may be provided only along boundary of multi-storey buildings mainly for providing safety against thieves (some promoters provide these 9 inch walls only in GF). Hence in my opinion the brick walls may be omitted in the analysis and the equation given in section 7.6.1 may be used for many practical frames.

As pointed out by Prof. ARC the failure is mainly due to provision of irregular plan, mass, stiffness, and soft storey in the building in addition to poor and improper detailing at beam- column junctions and insufficient column ties with 135 degree bends. Moreover in my opinion moment resistant frames should not be adopted in Zone 4 and above.



Best wishes and regards,


Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Mon, 8/25/08, ajay wrote:

Quote:

From: ajay

Subject: [SEFI] Re: Time period of rc frame buildings

To: gen...@sefindia.org Date: Monday, August 25, 2008, 1:07 AM

Dear All, I think in all discussion some main points are missing. As per my knowledge, Earthquake is noting but series of shocks.(Fore shocks-Main Earthquack-Aftershocks) So Practically, 1)Period of vibration T by Modeling ( Or considering) the non-structural infilled walls will also not present the correct value of T when the Infilled Partition(Brick Partition) walls get damaged during EARTHQUAKE EPISODE. 2)During use of structure there is always possibility that owner may remove the partitions( as already mentioned by Subramanian Sir) 3)Strength of Non structural elements (like Infilled walls) we consider as a part of OVERSTRENGTH( Buffer strength).Due to overstrength & Ductility( or Ductility reduction factor)we are designing structure for much lower seismic force that what may actually sustain at maximum shaking intensity experienced. 4)Response Spectrum Method will give much lager value of natural period of vibration T for frame with Infilled partition as compared to its real natural period of vibraion that can be obtain after properly modelling the infills( but practically not possible because it is very laborious& also not represent correct value as mention in point 1) ) So it is always better to anlyse the structure with bare frame( without Non-structural infill walls). If I am wrong please correct. Regards

Ajay S. Palwe ajay_st...@yahoo.co.in (ajay_st...@yahoo.co.in)

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[ishacon]

Dear All,

This is in reference to mail to SEFI wherein, Taylor devices has quoted as under :


QUOTE :

However none of the responses is even close to what is the implication of
this clause and how we need to understand it and use it while undertaking
Dynamic analysis. What is sad is that the gentry of engineers who are
members of SEFI are amongst the brighter lot existing in our country and
even they have not been able to understand the single most important
clause in IS-1893, even though the code has been around now for over 6 years.

and

" 6. Having debated the static time-period clause for buildings
with brick infills and its implications
I would also like to bring out that this clause in our code is flawed as
after a certain height the minimum base shear just remains constant.
The graph showing the static base shear values by
different methods should be referred. For RCC framed buildings with brick
infills I would recommend the use of the static time period formula

which is in line with the US codes i.e. T = 0.05 * H ^ 0.75 . " UNQUOTE

I believe that this coefficient is from Algerian code, because even UBC 1997 had
this coeff as 0.0731 and not 0.05 for RCC frame buildings.

Pls correct me if I am in error.

I am still of the opinion that the half brick walls with openings that we normally have in
apartment complexes, are invariably improperly attached to the main concrete frame and
they would perhaps collapse at the first seismic jolt and at subsequent stages the
owner may even remove them. Under this situation, it would seem to be inappropriate to consider them as permanent features.

Yes, for 230 thk brick walls, the case could be different.

So I also do agree with Dr. Subramaniam's statements in his last post.

In the meantime, I think making such sweeping remarks about gentry of engineers on SEFI site
by the gentleman are rather uncalled for and I hope that in future these are not made since evryone has
a right to express his views of his / her understanding.


.

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ish...@bol.net.in (ish...@bol.net.in)
ish...@rediffmail.com (ish...@rediffmail.com)
[quote] ----- Original Message -----

From: drnsmani (fo...@sefindia.org)
To: gen...@sefindia.org (gen...@sefindia.org)
Sent: Monday, August 25, 2008 11:37 PM

Subject: [SEFI] Re: Time period of rc frame buildings

Dear Mr. Ajay S. Palwe,

Your points are correct. In this connection I would second the opinions expressed by Prof. ARC. Though we may be using sophisticated software packages, Structural design is approximate and is as good as the design assumptions. Concrete design is still approximate than steel design as the material itself is not homogenous and we make so many assumptions in the design.

Seismic response of structures is an evolving field as evidenced by the large number of articles appearing in several journals. As pointed out by Prof. ARC, we use a smoothed elastic design acceleration response spectrum (SEDRS). It is important to recognize that the response spectra approach does not consider the duration effects.

The Z values given in the code represent a reasonable estimate of PGA in the respective zones. Thus a value of Z equal to 0.36 in Zone V implies that a value of 0.36g is reasonably expected in Zone V. But it does not imply that acceleration in Zone V will not exceed 0.36g. For example, during the 2001 Bhuj earthquake, a PGA of about 0.6g was recorded at Anjar, 44 km away from the epicenter. Similarly the recorded acceleration values in Northridge (USA, 1994) and Kobe (Japan, 1995) values were much higher.

The term R gives an indication of the level of overstrength and ductility that a structure is expected to have. The seismic design factors (Response modification coefficient R, System over-strength factor, and Deflection amplification factor Cd) are given in Table 12.2.1 of ASCE/SEI 7-05, which runs to 3 pages, which is referenced by the 2006 International Building Code (IBC). It has to be noted that these factors cannot be directly calculated based upon analysis and cyclic test data. The reason for this is that code-defined seismic design factors were developed over a period of many years based largely upon observed seismic performance of systems subjected to actual earthquake demands, limited test data, and comparison with other code-defined systems. However, because the seismic design factors were considered subjective, very detailed code provisions were developed to limit displacement and provide adequate strength and ductility for elements and subassemblies specific to each defined structural system.

Regarding your question on considering the effect of masonry walls, it is still in the research stage, though some equations have been proposed to consider them as bracing members with hinged ends. The width of the bracing to be taken in the analysis may be debatable. The average value of E may be taken as Em = 550fm (Hemant B. Kaushik, Durgesh C. Rai and Sudhir K. Jain,2007). However, as already discussed the 4.5 inch (115mm) thick walls may not provide sufficient stiffness and may be ignored. Even the 9 inch walls, if having openings near the edges, may not possess sufficient stiffness. Large openings in walls may also reduce their strength and stiffness. Moreover 9 inch walls may be provided only along boundary of multi-storey buildings mainly for providing safety against thieves (some promoters provide these 9 inch walls only in GF). Hence in my opinion the brick walls may be omitted in the analysis and the equation given in section 7.6.1 may be used for many practical frames.

As pointed out by Prof. ARC the failure is mainly due to provision of irregular plan, mass, stiffness, and soft storey in the building in addition to poor and improper detailing at beam- column junctions and insufficient column ties with 135 degree bends. Moreover in my opinion moment resistant frames should not be adopted in Zone 4 and above.



Best wishes and regards,


Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Mon, 8/25/08, ajay wrote:

[b]--auto removed--

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[prof.arc]

It is good to see a vigorous debate on this issue.

Mr. Palwe has advocated analysis as a bare frame. There is a clause 7.10.3 (which I believe is still in force) which imposes a stiff penalty for ignoring infills which I hope is not lost sight of by designers doing bare frame analysis.

For the 1893-1984 version, an explanatory handbook was published by BIS. BIS ought to have published a handbook for 2002 version also. I feel the publication from IIT-K by Prof. SKJ, et.al could be deemed as the official handbook and everyone must read and digest it for effective use of IS:1893-2002. Almost all points raised in this discussion has been answered in that excellant publication.

Having said thus, I feel the code now needs revision. I am for simplified rules for buildings in Zone III & II - use equivalent static forces only and use bare frame model for analysis with the stipulation of enforcing MINIMUM BASE SHEAR. Suggested values are 0.20*I/R and 0.125*I/R

SEFI members may write to BIS requesting for revision of 1893.

ARC


On Mon, Aug 25, 2008 at 1:07 AM, ajay fo...@sefindia.org)> wrote:

Quote:

So it is always better to anlyse the structure with bare frame( without Non-structural infill walls).

Ajay S. Palwe

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[sdec.in]

Dear Mr VP Agarwal
Thanks for the rejoinder; I agree with you and was in the process of writing back strongly myself.When we participate in SEFI and share our opinions, such comments are totally unnecessary and must be avoided.
regards
Sangeeta Wij

Quote:

----- Original Message ----- From: ishacon (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org) Sent: Tuesday, August 26, 2008 6:19 AM Subject: [SEFI] Re: Time period of rc frame buildings

ish...@bol.net.in (ish...@bol.net.in) (ish...@bol.net.in (ish...@bol.net.in)) ish...@rediffmail.com (ish...@rediffmail.com) (ish...@rediffmail.com (ish...@rediffmail.com)) Quote:

----- Original Message ----- From: drnsmani (fo...@sefindia.org (fo...@sefindia.org)) To: gen...@sefindia.org (gen...@sefindia.org) (gen...@sefindia.org (gen...@sefindia.org))

Sent: Monday, August 25, 2008 11:37 PM Subject: [SEFI] Re: Time period of rc frame buildings Dear Mr. Ajay S. Palwe, Your points are correct. In this connection I would second the opinions expressed by Prof. ARC. Though we may be using sophisticated software packages, Structural design is approximate and is as good as the design assumptions. Concrete design is still approximate than steel design as the material itself is not homogenous and we make so many assumptions in the design. Seismic response of structures is an evolving field as evidenced by the large number of articles appearing in several journals. As pointed out by Prof. ARC, we use a smoothed elastic design acceleration response spectrum (SEDRS). It is important to recognize that the response spectra approach does not consider the duration effects. The Z values given in the code represent a reasonable estimate of PGA in the respective zones. Thus a value of Z equal to 0.36 in Zone V implies that a value of 0.36g is reasonably expected in Zone V. But it does not imply that acceleration in Zone V will not exceed 0.36g. For example, during the 2001 Bhuj earthquake, a PGA of about 0.6g was recorded at Anjar, 44 km away from the epicenter. Similarly the recorded acceleration values in Northridge (USA, 1994) and Kobe (Japan, 1995) values were much higher. The term R gives an indication of the level of overstrength and ductility that a structure is expected to have. The seismic design factors (Response modification coefficient R, System over-strength factor, and Deflection amplification factor Cd) are given in Table 12.2.1 of ASCE/SEI 7-05, which runs to 3 pages, which is referenced by the 2006 International Building Code (IBC). It has to be noted that these factors cannot be directly calculated based upon analysis and cyclic test data. The reason for this is that code-defined seismic design factors were developed over a period of many years based largely upon observed seismic performance of systems subjected to actual earthquake demands, limited test data, and comparison with other code-defined systems. However, because the seismic design factors were considered subjective, very detailed code provisions were developed to limit displacement and provide adequate strength and ductility for elements and subassemblies specific to each defined structural system. Regarding your question on considering the effect of masonry walls, it is still in the research stage, though some equations have been proposed to consider them as bracing members with hinged ends. The width of the bracing to be taken in the analysis may be debatable. The average value of E may be taken as Em = 550fm (Hemant B. Kaushik, Durgesh C. Rai and Sudhir K. Jain,2007). However, as already discussed the 4.5 inch (115mm) thick walls may not provide sufficient stiffness and may be ignored. Even the 9 inch walls, if having openings near the edges, may not possess sufficient stiffness. Large openings in walls may also reduce their strength and stiffness. Moreover 9 inch walls may be provided only along boundary of multi-storey buildings mainly for providing safety against thieves (some promoters provide these 9 inch walls only in GF). Hence in my opinion the brick walls may be omitted in the analysis and the equation given in section 7.6.1 may be used for many practical frames. As pointed out by Prof. ARC the failure is mainly due to provision of irregular plan, mass, stiffness, and soft storey in the building in addition to poor and improper detailing at beam- column junctions and insufficient column ties with 135 degree bends. Moreover in my opinion moment resistant frames should not be adopted in Zone 4 and above. Best wishes and regards, Subramanian Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI, Computer Design Consultants: www.cdcstruct.com Maryland, USA See my books at: www.multi-science.co.uk/subramanian-book.htm www.oup.co.in/search_detail.php?id=144559 --- On Mon, 8/25/08, ajay wrote: [b]--auto removed--

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[ajay]

Thank you Prof. Arc, Subramanian Sir & Others for comments
“There is a clause 7.10.3 (which I believe is still in force) which imposes a stiff penalty for ignoring infills which I hope is not lost sight of by designers doing bare frame analysis. “- Prof ARC.
Sir, please mention the Code/book so I can take review.

As per my understanding I think BIS allow both type of analysis through
1)IS 1893:2002 cl.7.10.2 allow analysis with infilled frame structure.
2)IS 1893:2002 cl.7.10.3 allow analysis with bare frame only. We have to take care in design as per cl.7.10.3 a) & b)
Actually this cl. are regarding soft story. If my implication is wrong, then please correct.

There are practical difficulties with Infill brick walls (size, position, existence, and opening sizes) as no one is taking it seriously. In general, Structural Designer not checking whether contractor has constructed Brick walls or not. Architect never comes for revision of structural design for any changes in Infilled bricks walls (I know tendency of Architects better as my wife is a Architect. ). Interior designers don’t take care of infilled walls, chajja, lintel, and loft. (I am also a Interior Designer & I saw many times owner have removed even Tie beams for good returns.) Owners always do what they want. This is my personnel opinion.

There is another doubt in my mind as there is nothing mention about infilled/ without infilled frame in Table7, IS1893 (Part1)-2002. We know Brick infill walls in RC frame reduce structural drift, increases strength & stiffness that means ductility reduces but overstrength increases.

Suppose we have to design two simple buildings (OMRF). Plans & other things are same.
One with infill walls & another without infill walls. For both building Table7, IS1893-2002 suggest
Response Reduction Factor R = 3. But R is related to Ductility Reduction Factor & Overstrength, and these two factors get changed with infill walls. So how for both type buildings, R is same?

Regards
Ajay S. Palwe
ajay_st...@yahoo.co.in

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