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Accounting Infill Walls in Seismic Evaluation of RC building
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Vinay Jayaram
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PostPosted: Thu Oct 17, 2013 7:58 am    Post subject: Accounting Infill Walls in Seismic Evaluation of RC building Reply with quote

Dear Sefians,

While analysing a 4 storied RCC-OMRF building with abundant infill walls for lateral loads such as seismic and wind, we tend to ignore the stiffness contribution of infill walls. Though the walls are permanently built 200 thick solid block-works, due to the ignorance of stiffness in analysis model, the performance of the structure due to lateral load cannot be improved.

Referring to a paper published by Vishal P. Jamnekar and D.J.Chaudhari in ICJ Aug13 issue, on “Effect of brick masonry infill in seismic evaluation of RC building” it is eminent that there is a significant improvement in energy absorption of the structure with infill walls and reduction in deflection, time period and thereby economical design.

Simulating the infill walls in RCC framed structural analysis, I have done a building model of 4 storied structure with cross bracing (wherever 200 thick solid block masonry is there as per Architectural plan) of 200x200 RCC member released at both the ends. Typically these bracings are designated as compression members only as STRUT. The behaviour of the structure is totally different from un-braced structure.  We are having a stiffer frame up to plinth level from top whereas below plinth it behaves as a soft storey as it is not braced below plinth. But the overall behaviour of the structure has improved drastically with less force in columns above plinth for lateral loads and thereby an economical design is achieved.

I have been doing this for past many years for structures up to 5 to 6 storey and results found to be good and economical. When IS:1983 talks about infill walls and flexible diaphragm, why not we consider this? Whether IIT or IISC approves this proposal?

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sakumar79
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PostPosted: Thu Oct 17, 2013 1:47 pm    Post subject: Reply with quote

Dear Sir,
      Please note the following constraints which are (in my humble reason) why code enhances time period for calculation of base shear but generally avoided in structure modelling


1. Location and size of openings will affect the effect of the infill. Even if this is studied and modelled somehow, there is no way we can anticipate the changes that may be done to it (either during construction after the frame is in place, or during the life of the structure)

2. Dynamic load and inelastic behaviour - It is well known that behaviour of the infill is brittle in nature and once it cracks, the frame stiffness will drop drastically. Considering that we are reducing the design eq load by a factor of R considering the ductile nature of the design, we will have to consider a smaller R value if we are to incorporate the infill stiffness.

3. Infill material property variation - There are many infill material available these days - clay bricks, flyash bricks, solid concrete blocks, hollow concrete blocks, Aerated Concrete (AAC) Blocks, Cellular Lightweight Concrete (CLC) blocks, Perforated Clay Blocks, Compressed Stabilized Earth blocks, etc. Each has different types of material. We need to do lot of research to identify their behaviour under cyclic loading from earthquake and the effects of openings, etc.

4. Quality of material and workmanship - In material, some of the blocks like AAC, CLC, etc which are coming from limited established manufacturers have better QC than others like clay bricks and flyash bricks. Very poor quality bricks may be used in construction under the excuse that the frame is taking the load and the brick is not taking it. Similarly, workmanship may vary considerably.

5. Identifying wall locations - We cannot go for generic formula here to increase the stiffness of the frame, but we need to model the walls as struts since the location of the walls can affect the frame behaviour both locally as well as globally.

6. Wall removal - When we build a structure with columns and beams, the owners take it for granted that the structure transfers load through the RCC members only. Hence, during the life of the building, they dont think twice about removing a wall as they deem fit in such structures.

     It may however be noted that code does offer some relaxation by allowing us to use model calculated time period in estimating story drift, seismic separation joint width, etc...

    Under the above circumstances, I would prefer to avoid wall strut modelling to cut construction cost. Other sefians may share their views.

Yours sincerely
Arunkumar
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thirumalaichettiar
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PostPosted: Thu Oct 17, 2013 1:57 pm    Post subject: Reply with quote

Dear Er.Vinay Jayaram,

I fully endorse the many practical aspects as explained by Er.Arunkumar. His method of explanation speaks well on the way it shall be modeled and anlysed without brick infill but to account for it IS 1893-2002 has given separate Time period.

T.Rangarajan.
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Dr. N. Subramanian
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PostPosted: Thu Oct 17, 2013 3:25 pm    Post subject: Re: Accounting Infill Walls in Seismic Evaluation of RC buil Reply with quote

Dear Vinay Jayaram,

I do not think any other code suggests to include the stiffness of infill brick walls, unless they are reinforced. As you say, by including these stiff walls, the time period and deflection will be reduced and there will be economy. But during the first few cycles of earthquake,  the infill brick wall will collapse (infact many injuries are attributed to the flying debris of brick wall)

The points to be noted are:
1. Most of the multi-storey buildings use  half – brick thick (115 mm thickness) walls, except for the outer walls, which  will collapse immediately in EQ.
2. Even in outer walls, we will have window openings, which will weaken the walls unless RC bands are provided.
3.  In fact the openings in brickwalls, if not properly placed, will even result in short column effect and affect the RC column.
4. Richer cement-sand mortar of 1:4 mixture (1 part cement by 4 parts of sand) makes the masonry stronger against earthquake shaking as compared with the usual 1:6 mortar used in such construction, by a factor of 2.5 to 3.0. Also 1:6 mortar is stronger than lime cinder or lime-surkhi mortar.
5. Infill walls must be uniformly distributed in the building and should not be discontinued at any intermediate storey or the ground storey level, causing undesirable effect on load paths.
6. In several buildings, especially commercial ones, the infilled brick wall may be removed for several reasons, which will not be considered in the analysis.

Due to these reasons, inclusion of brickwork is not recommended.

Please note that the stiffening effect of brick wall was recognized as early as 1960's (see Stafford-Smith, 1962).


A good review of the effect of infill frames is provided by Kaushik et al 2006.

A latest article by Surendran and  Kaushik on the effect of opening in infilled wall is enclosed for your reference.

Best wishes,
NS

References:
B. Stafford-Smith, "Lateral stiffness of infilled frames", J. Struct.
Div. ASCE, vol. 88, no. ST6, pp. 183-199, 1962.
H.B. Kaushik, D.C. Rai, and S.K. Jain, “Code approaches to seismic design of masonry-infilled reinforced concrete frames: a stateof- the-art review”, Earthquake Spectra, vol. 22, no. 4, pp. 961-983, 2006  


Vinay Jayaram wrote:
Dear Sefians,

While analysing a 4 storied RCC-OMRF building with abundant infill walls for lateral loads such as seismic and wind, we tend to ignore the stiffness contribution of infill walls. Though the walls are permanently built 200 thick solid block-works, due to the ignorance of stiffness in analysis model, the performance of the structure due to lateral load cannot be improved.

Referring to a paper published by Vishal P. Jamnekar and D.J.Chaudhari in ICJ Aug13 issue, on “Effect of brick masonry infill in seismic evaluation of RC building” it is eminent that there is a significant improvement in energy absorption of the structure with infill walls and reduction in deflection, time period and thereby economical design.

Simulating the infill walls in RCC framed structural analysis, I have done a building model of 4 storied structure with cross bracing (wherever 200 thick solid block masonry is there as per Architectural plan) of 200x200 RCC member released at both the ends. Typically these bracings are designated as compression members only as STRUT. The behaviour of the structure is totally different from un-braced structure.  We are having a stiffer frame up to plinth level from top whereas below plinth it behaves as a soft storey as it is not braced below plinth. But the overall behaviour of the structure has improved drastically with less force in columns above plinth for lateral loads and thereby an economical design is achieved.

I have been doing this for past many years for structures up to 5 to 6 storey and results found to be good and economical. When IS:1983 talks about infill walls and flexible diaphragm, why not we consider this? Whether IIT or IISC approves this proposal?



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PostPosted: Thu Oct 17, 2013 5:21 pm    Post subject: Check with NICEE of validity of your idea. What they say? Reply with quote

Dear Er. Vinay Jayaram,
I know in USA you can not use URM (Unreinforced Masonry) in Seismic Zones. You have to use Reinforced Masonry. But I suggest you get blessings from NICEE of your idea of using URM Infill Walls stiffness for Seismic Lateral Load Analysis.
  
National Information Centre of Earthquake Engineering - IIT-Kanpur.

NICEE: what  their research say on using URM in Seismic Zones. In USA use of URM is prohibited by law in Seismic Zones like California. If  NICEE also agrees with your idea than we are on a solid ground to use your method of using URM Infill Walls stiffness in Analysis and Design.  My recommendation is, run your idea through NICEE. Let us see what they say.

California , USA Work:  
--------------------------------------------------------------------------------------------------------
In California,construction of new unreinforced masonry buildings were prohibited in 1933, and state law(enacted in 1986) required seismic retrofiting of existing structures. Retrofits are relatively expensive, and may include the building being tied to its foundation, tying building elements(such as roof andwalls) to each other, so that the building moves as a single unit, rather than creating internal shears during an earthquake, attaching walls more securely to underlying supports, so that they do not buckle and collapse, bracing or removing parapets and other unsecured decorative elements.[3][4]Retrofits are generally intended to prevent injury and death to people, but notto protect the building itself.[3]Accordingto the 2006-04 CA seismic safety commission report, there are still 7800 URM buildings with no retrofitting in CA. 1100 in the city of Los Angeles.
The California law left implementation and standards, upto local jurisdictions. Compliance took many years.
[5]As of 2008, most (but not all) of the unreinforced masonry buildings have  undergone retrofitting.[6]
-------------------------------------------------------------------------------------------------

Regards.

Vasudeo Pandya P.E. ; S.E.
Sttructural Engineer
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vishaljamnekar
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PostPosted: Mon Oct 21, 2013 11:16 am    Post subject: REPLY TO YOUR POST Reply with quote

Dear,
    I think you are new to handle "Analysis of RC buildings using SAP2000" because you have written-" RCC-OMRF building with abundant infill walls" also you must refer IS 1893-1987 and not IS1983.
Regarding IIT or IISC approvals- Now this is part of research.
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mehul bhavsar
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PostPosted: Fri Oct 25, 2013 7:50 am    Post subject: Reply with quote

Dear Er.Arunkumar sir and N. Subramanian sir,
Thank you for your valuable notes on effect of Infill walls in seismic analysis..
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srkandula
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PostPosted: Sat Oct 26, 2013 3:36 am    Post subject: Reply with quote

Dear Mr.Jayaram,
I fully agree with Mr. Arunkumar and Mr.Subramaniyam. As they rightly said that the quality of infill and size and location of infill matters a lot. In India construction practices are not fully systematized and hence one can not assure the desired/designed parameters.


srinivasa rao
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Abishek_Siingh
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PostPosted: Sat Oct 26, 2013 9:08 am    Post subject: Reply with quote

I have a doubt here.

If we are using infill time period, then we should model the struts. If we are NOT modeling the struts then we should NOT use infill period.

Using infill time period without modeling struts can give wrong results. We should use bare frame time period in that case... Am I right in my thinking? Please explain.

What Arun and Dr. Subramanian said is very clear and lucid, only on this part some clarity is needed...

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knsheth123
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PostPosted: Sun Oct 27, 2013 7:36 am    Post subject: Reply with quote

The results obtained either by Strut model or fem analysis with plane stress model of brick wall definitely raise temptation to take an advantage that apparently looks analytically acceptable.  

Most of the experimentation for MRF with infill is done for concrete blocks not clay bricks. The books on seismic design by Manish Shrikhande etc. provides information and equations required for modelling. Modelling brick infill as a Strut in RCC moment resisting frame is highly empirical.

The equations derived for brick strength more than 50 MPa can not be directly used for clay bricks with comp. strength 5 MPa and negligible tensile strength. Moreover the infill is made to have shear connection with RCC Frame to consider them to act as a unit to resist lateral forces. Such practice is uncommon in our practice.

If we want take the advantage of Clay Brick Infill, It is there in the code for vrtical loads. If  a blind infill wall exist above RCC beam for 0.6 X L height the maximum bending moment reduces from  w.l2/8  due to deep beam action to lesser than wl2/24 or even lower. Hence for blind infilled wall continuous for a no. of storey, infill offers advantage.

K. N> Sheth
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