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AXIAL DEFORMATION OF COLUMN IN TALL STRUCTURES
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P.K.Mallick
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PostPosted: Fri Nov 23, 2012 4:32 pm    Post subject: AXIAL DEFORMATION OF COLUMN IN TALL STRUCTURES Reply with quote

Concrete members experience time-dependent behaviour caused by creep and shrinkage .During the past,much research has been made in this area providing means for good understanding of the effect of creep and shrinkage on concrete and processes through which they evolve. Basically,two types of behaviour are distinct because of creep and shrinkage:
1)Creep and shrinkage lead to increased deformations in plain concrete.
2)In reinforced concrete ,creep and shrinkage cause stress redistribution between the compressive zone in concrete and steel reinforcement.The direction of stress transfer in reinforced concrete column is normally from concrete section to reinforcement,leading to an increase in steel stress and decrease in concrete stress with time.
A reinforced concrete column also undergoes axial shortening due to creep and shrinkage and this phenomenon is known as time-dependent shortening of column.With the increase in height of buildings,the importance of time-dependent shortening of columns and shear walls become more critical owing to cumulative nature of such shortening.It is known that column with varying percentage of reinforcement and varying volume to surface ratio will undergo varying strains due to creep and shrinkage under similar stresses.
In a multistoried building ,adjacent columns may have different percentage of reinforcement due to different tributary areas or different wind loads.As a result,the differential elastic and inelastic shortening will produce moments in the connecting beams or slabs and will cause load transfer to the element that shortens less. As number of stories increase,the cumulative differential shortening also increases,and the related effect become more severe.A common example is the case of a large,heavily reinforced column attracting additional loads from adjacent shear wall which has higher creep and shrinkage due to lower percentage of reinforcement and lower volume to surface ratio. Significance differential shortening may also occur due to a time gap between a slip-formed core and the columns. In this case the columns are subjected to full amount of creep and shrinkage ,while the core may have had the bulk of its inelastic shortening occurring prior to casting of adjacent columns.  

------------------------------------
TO BE CONTINUED--
-----------------------------------

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PostPosted: Sat Nov 24, 2012 2:10 am    Post subject: Re: AXIAL DEFORMATION OF COLUMN IN TALL STRUCTURES Reply with quote

EFFECT OF TIME DEPENDENT SHORTENING OF COLUMNS- CASE HISTORIES AND REAL ISSUES ASSOCIATED WITH CALCULATION OF AXIAL SHORTENING:

It is customary ,at present,to neglect the effect on the frame of elastic and inelastic shortening of columns and walls.For a low and intermediate height structures this may be acceptable : however,neglecting the differential shortening in ultra-high-rise building may lead to distress in the structure and in a non-structural elements of the building.

In a number of tall buildings in the United States built in the early sixties,structural cracking and partition distress were observed as a result of differential creep between shear walls and highly reinforced columns in close proximity to each other. Another example of the reality of differential creep and shrinkage of vertical elements is of fifty story building in Australia in which the measured differential shortening at the roof level between the concrete core and peripheral column was 27.94mm after about four and half years. Fortunately , no problems were experienced,the long span of about 11m between the core and peripheral columns caused only small slab rotations.The elevator rails had to be adjusted twice over the years to accommodate the shortening of elevator shafts.
Building up to 30 stories with flexible slab systems ,such as flat plate slabs of average spans or long span joint systems,are not adversely affected structurally by differential shortening of supports.In those cases the knowledge of the total shortening is needed to make allowance in architectural details to avoid further distress of partitions,windows,cladding,and other nonstructural elements.

Differential shortening can be minimized by proportioning adjacent columns or walls to have similar stress of the transformed section and similar percentage of reinforcement. The volume to surface ratio has a lesser effect on differential shortening.

Although a large amount of research information is available on shrinkage and creep strains,it is not directly applicable to columns of high-rise building. The available shrinkage data must be modified since they are obtained on small standard prisms or cylinders stored in controlled laboratory environment.The available creep research is based on application of loads in one increment and such creep information ,therefore,is applicable to flexural elements of reinforced concrete and to elements of prestressed concrete.

In the construction of high-rise building,however columns are loaded in as many increments as there are stories above the level under consideration. If a 50 storied building is constructed in 50weeks ,then the first story columns receive 2% of their design load every week during construction period.Incremental loading over a long period of time makes considerable difference in this magnitude of creep too.

---------------------------------------------------
TO BE CONTINUED-----
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PostPosted: Sat Nov 24, 2012 4:22 am    Post subject: Reply with quote

In response to Er.P.K.Mallick's article on the above topic I could find a presentation  on

Presentation on Column Shortening in Tall Buildings from the internet. By Shapour Mehrkar-Asl

Since it is not stated as copy righted one I am posting for the benefit of all.

CONTENT:About the presentation:


The Event was held on 26 September 2012 at the Abu Dhabi Menís College, Abu Dhabi, UAE.


The design of tall buildings normally leads to columns in the exterior with a higher average compressive stress compared to that of the interior columns/cores  elements.  In buildings  of normal  height,  30 to 40 floors,  this would not normally cause any issues. However, in taller buildings this difference leads to a different vertical elevation for the exterior columns compared to interior columns/cores, normally referred to as Column Shortening even though it is actually a relative shortening.

The consequence of that are sloping of the floor towards the edges of the building, cracking in the partitions and cladding. Other non-structural elements such as piping may get affected.
Column shortening calculation on steel structures is slightly different to concrete structures mainly because of the properties of the steel and concrete.

Concrete is  subject  to  creep  and  shrinkage.  In  addition,  age  of  concrete  affects  its properties such as Elastic Modulus, basic Creep and Shrinkage behaviors. As these parameters are functions of time then movements become function of time  bringing  the  construction  sequence  into  play.  In  steel  structures  even though the construction sequence is important the time intervals in the construction sequence become less important as material properties are relatively constant.

The presenter explains the effects as described by the authors of the original publication of PCA by Fintel et al. In addition he shows the results of such an application on a 71 floor building with the help of a computer program he had developed in 2005 based on the approach given in the publication.

T.RangaRajan



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PostPosted: Sat Nov 24, 2012 11:09 am    Post subject: Reply with quote

thirumalaichettiar wrote:
In response to Er.P.K.Mallick's article on the above topic I could find a presentation  on

Presentation on Column Shortening in Tall Buildings from the internet. By Shapour Mehrkar-Asl

Since it is not stated as copy righted one I am posting for the benefit of all.

CONTENT:About the presentation:


The Event was held on 26 September 2012 at the Abu Dhabi Menís College, Abu Dhabi, UAE.


The design of tall buildings normally leads to columns in the exterior with a higher average compressive stress compared to that of the interior columns/cores  elements.  In buildings  of normal  height,  30 to 40 floors,  this would not normally cause any issues. However, in taller buildings this difference leads to a different vertical elevation for the exterior columns compared to interior columns/cores, normally referred to as Column Shortening even though it is actually a relative shortening.

The consequence of that are sloping of the floor towards the edges of the building, cracking in the partitions and cladding. Other non-structural elements such as piping may get affected.
Column shortening calculation on steel structures is slightly different to concrete structures mainly because of the properties of the steel and concrete.

Concrete is  subject  to  creep  and  shrinkage.  In  addition,  age  of  concrete  affects  its properties such as Elastic Modulus, basic Creep and Shrinkage behaviors. As these parameters are functions of time then movements become function of time  bringing  the  construction  sequence  into  play.  In  steel  structures  even though the construction sequence is important the time intervals in the construction sequence become less important as material properties are relatively constant.

The presenter explains the effects as described by the authors of the original publication of PCA by Fintel et al. In addition he shows the results of such an application on a 71 floor building with the help of a computer program he had developed in 2005 based on the approach given in the publication.

T.RangaRajan


Respected RangaRajan Sir
I had a glance on the presentation by Shapour Mehrkar-Asl. Useful certainly. Though the presentation is as recent as the year 2012,the methodology described is quite old. I wish to go beyond that. Let me try. Warm regards.

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PostPosted: Sat Nov 24, 2012 11:23 am    Post subject: Reply with quote

Dear Er.P.K.Mallick,

Whether old or new the basic principle is same I think.

As you are writing this topic in detail you can write up  the difference between old and new methods so that I and others can learn form your write up.

Wish you to continue further.

T.RangaRajan
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PostPosted: Sun Nov 25, 2012 12:08 am    Post subject: Reply with quote

Dear Er. P.K.Mallick,

In continuation of my posting on 24-11-12, is it possible to put an example of calculations for the above topic using the new method with the required calculation and equations so that it will be beneficial to members to make use of the above subject in addition to my previous request of listing the difference between the old and new methodology.

With Warm regards,
T.RangaRajan.
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PostPosted: Sun Nov 25, 2012 2:17 am    Post subject: Reply with quote

It has already been high lighted that though we have lot of research data on creep and shrinkage,those are not directly applicable for prediction of inelastic shortening of column.
An effort has been made here for comprehensive review and comparison of method of prediction of inelastic shortening including that developed recently. The purpose of this review is to comment on theoretical validity and to compare them in terms of their efficiency .accuracy and practical value. The methods are considered in more or less,their chronological order of development.Considered roughly in their order of sophistication,the methods reviewed are:

1)Method developed by Mark Fintel & Fazlur .R.Khan.  

2)Method developed by Mark Fintel , H.Iyenger & S.K.Ghosh.

3)Method developed by Raed M.Samra.


Though the method developed by Raed M.Samra ,I call it as most recent ,but it in reality it was first published in the year 1995 in Journal of Structural Engineering.As far as I know there is no significant development after the work of Raed M. Samra. But if it has happened,I would like to be updated on that.

METHOD DEVELOPED BY M.FINTEL & F.R.KHAN:


Though it is long recognized fact that in reinforced concrete columns,creep result in gradual transfer of load from concrete to reinforcement,the procedure for prediction of the amount of creep and shrinkage strains was first outlined in the late 1969."Effect of creep and shrinkage in tall structures -prediction of inelastic column  shortening " was perhaps the first paper on this subject to be published in ACI Journal December 1969 issue and credit goes to M.Fintel and F.R.Khan for this publication.

The procedure takes care:
a) Loading History of Columns.
b) Volume to Surface Ratio of Sections.
c)Effect of percentage of Reinforcement.


For Structural Engineering practice,the specific creep has been considered. The specific creep Σc is defined as the ultimate creep strain per unit of sustained stress. Since creep decreases with age of concrete at load application,each subsequent incremental loading contributes a smaller specific creep to the final average specific of the column.

Determination of Specific Creep, Σc :

There are two ways to determine the value of specific creep.It can be obtained by extrapolation from number of laboratory samples prepared in advance from actual mix to be used in structure.It is obvious that sufficient time for such tests must be allowed prior to start of construction,since the reliability of the prediction improves with length of time over which creep is actually measured.
An alternative method to predict basic creep is from elastic modulus of elasticity. In the mentioned article a curve(we call fig-1) is shown which give the creep magnitude as related to initial modulus of elasticity for different load durations. For design purposes ,the 20 year creep can be regarded as the ultimate creep.Thus from the specified 28days strength,the basic specific creep for loading at 28days can be determined and then modified for construction time,member size and percentage of reinforcement.

Effect of construction time on creep:


To determine the effect of construction time on creep,this method takes the help of curve(we call fig-2) giving relationship between creep and age at loading.The total creep strain for an incrementally loaded column "N" stories below the roof will be

Σc = ΣNi fci Σci

Where fci Σci are creep strains produced by the stress increment fci .Individual value for specific creep can be obtained from fig-1 or from the creep of a test specimen loaded at 28days and then modified for various age at loading using fig-2.
The procedure gives formula for weighted average of specified creep where load increments are unequal. Another formula is given for where load increments are equal. Then the procedure gives formula for total creep strain.
The procedure explained above has been further simplified and a curve (we call fig-3) has been developed which gives relationship between "Time of Construction" and "Coefficient for incremental loading".
The Coefficient for incremental loading plotted in figure-3 is used to convert the 28day creep into average specific creep for a column load with equal load increment at equal time intervals.
----------------------------------------------------
TO BE CONTINUED
----------------------------------------------------



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Last edited by P.K.Mallick on Sun Nov 25, 2012 6:28 am; edited 3 times in total
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PostPosted: Sun Nov 25, 2012 2:19 am    Post subject: Reply with quote

thirumalaichettiar wrote:
Dear Er. P.K.Mallick,

In continuation of my posting on 24-11-12, is it possible to put an example of calculations for the above topic using the new method with the required calculation and equations so that it will be beneficial to members to make use of the above subject in addition to my previous request of listing the difference between the old and new methodology.

With Warm regards,
T.RangaRajan.


Sir,I do not know how to write Engineering formula here.Still I will try.
Warm Regards.

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PostPosted: Sun Nov 25, 2012 4:50 am    Post subject: AXIAL DEFORMATION OF COLUMN IN TALL STRUCTURES Reply with quote

To add to the response:

The presentation by Dr. Shapour is based on his works on a project in Dubai and the method of assessing column shortening is adopted from PCA publication by Mark Fintel, S.K. Ghosh and Hal Iyengar titled “Column shortening in tall structures – Prediction and compensation” published in 1986. This book is still by far one of the most comprehensively covered paper on the subject. The complete methodology of assessing columns shortening is explained in detail with examples and refers to ACI publications for many of the parameters. I am not sure if a computer programme is available but one can be easily produced. A spread sheet can also be developed with macros to do a few iterations required in the analysis.

Another easy to use tool to assess column shortening is the spread sheet TCC55 by Reinforced Concrete Council (RCC, BCA), UK which is based on Euro Code. The main limitation is it can only be used for 12 levels (updated to 24 levels perhaps now). However, for a high rise building, floors can be clubbed together in to 12 or 24 groups in order to use this yet get a fairly good assessment of axial and differential shortening of columns. RCC spread sheets are shareware and can be distributed freely but should not be used commercially.

Ranjith Chandunni
Buro Happold



From: P.K.Mallick [mailto:forum@sefindia.org]
Sent: 24 November 2012 16:40
To: econf34289@sefindia.org
Subject: [E-CONF] Re: AXIAL DEFORMATION OF COLUMN IN TALL STRUCTURES



thirumalaichettiar wrote:
In response to Er.P.K.Mallick's article on the above topic I could find a presentation on

Presentation on Column Shortening in Tall Buildings from the internet. By Shapour Mehrkar-Asl

Since it is not stated as copy righted one I am posting for the benefit of all.

CONTENT:About the presentation:[/color:e03acba3a1]


The Event was held on 26 September 2012 at the Abu Dhabi Men’s College, Abu Dhabi, UAE.


The design of tall buildings normally leads to columns in the exterior with a higher average compressive stress compared to that of the interior columns/cores elements. In buildings of normal height, 30 to 40 floors, this would not normally cause any issues. However, in taller buildings this difference leads to a different vertical elevation for the exterior columns compared to interior columns/cores, normally referred to as Column Shortening even though it is actually a relative shortening.

The consequence of that are sloping of the floor towards the edges of the building, cracking in the partitions and cladding. Other non-structural elements such as piping may get affected.
Column shortening calculation on steel structures is slightly different to concrete structures mainly because of the properties of the steel and concrete.

Concrete is subject to creep and shrinkage. In addition, age of concrete affects its properties such as Elastic Modulus, basic Creep and Shrinkage behaviors. As these parameters are functions of time then movements become function of time bringing the construction sequence into play. In steel structures even though the construction sequence is important the time intervals in the construction sequence become less important as material properties are relatively constant.

The presenter explains the effects as described by the authors of the original publication of PCA by Fintel et al. In addition he shows the results of such an application on a 71 floor building with the help of a computer program he had developed in 2005 based on the approach given in the publication.

T.RangaRajan[/color:e03acba3a1]



Respected RangaRajan Sir
I had a glance on the presentation by Shapour Mehrkar-Asl. Useful certainly. Though the presentation is as recent as the year 2012,the methodology described is quite old. I wish to go beyond that. Let me try. Warm regards.




P.K.Mallick
pk_mallick1962@rediffmail.com (pk_mallick1962@rediffmail.com)

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PostPosted: Sun Nov 25, 2012 7:45 am    Post subject: Reply with quote

METHOD DEVELOPED BY M.FINTEL & F.R.KHAN-Continued.

In continuation to explanation of above method let us look into the rest of the issues associated with the method.

Effect of Member size on Creep:

Creep is less sensitive to member size than shrinkage since only the  drying creep component of total creep is affected by size and shape of members,where as basic creep is independent of size and shape. It appears from a laboratory investigation that drying creep has its effect only during the initial three months.Beyond 100days,the rate of creep is equal to basic creep.

Shrinkage Strains-Adjusted For Column Size:


Shrinkage of concrete is caused by evaporation of moisture from the surface. Similar to creep,the rate of shrinkage is high at early ages,decreasing with increase of age,until the curve becomes asymptotic to final value of shrinkage.Since evaporation occurs only from the surface of members the volume to surface ratio of a member has a pronounced effect on the amount of its shrinkage.
The amount of shrinkage decreases as the size of specimen increases. Much of the shrinkage data available in the literature is obtained on 27.9 cm long prisims of a 7.6* 7.6 cm section. Obviously ,such data can not be applied to usual size columns without considering side effect.  The relationship between the magnitude of shrinkage and the volume-to-surface ratio has been plotted in a curve(we call fig-4). The size coefficient for shrinkage shown in fig-4 is used to convert shrinkage data obtained in 6inch cylinders to any other size columns.

Effect of relative humidity on shrinkage:

The shrinkage specimen should be stored under conditions similar to those for actual structures. If this is not possible ,the shrinkage results of a specimen not stored under field humidity conditions of structure must be modified to account for humidity conditions of structure. The curve developed by C.L.Freyermuth  showing relative humidity percentage and shrinkage humidity correction factor must be used in this regard.

Progress of Creep and Shrinkage with Time:

Both creep and shrinkage have similarity regarding the rate of progress with respect to time. A curve  (we call fig-5) is developed to show ratio of creep or shrinkage at anytime to final value at time infinity. This curve can be used to extrapolate the ultimate creep and shrinkage values from laboratory testing of certain duration time.

Effect of Reinforcement on creep and shrinkage:

Long term test has shown that on columns with low percentage of reinforcement the stress in steel increased until yielding while in highly reinforced columns after entire load had been transferred to steel ,further shrinkage actually caused some tensile stresses in the concrete. It should be noted that despite the redistribution of load between concrete and steel ,the ultimate steel capacity of the columns remains unchanged.

The total creep and shrinkage strains of a non-reinforced column are

Σ = fc Σc + Σs

where
fc =Initial elastic stress in the concrete.
Σc = ultimate specific creep strain of plain concrete
ΣS = Ultimate shrinkage strain of plain concrete.

A curve (we call fig-5) has been developed to determine residual creep and shrinkage strains of reinforced column from the total creep and shrinkage strain of identical column without reinforcement for various percentage of reinforcement,varying specific creep and modulus of elasticity of concrete.

THEREFORE,THE TOTAL STRAIN IN COLUMN DUE TO CREEP AND SHRINKAGE IS SUM TOTAL OF STRAINS CALCULATED DUE TO VARIOUS FACTORS AFFECTING SHRINKAGE AND CREEP.
--------------------------------------------------------------------
TO BE CONTINUED
-----------------------------------------------------


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