www.www.buonovino.com

[P.K.Mallick]

[gautam chattopadhyay]

Effect of creep on columns may appear to be an academic query but I feel it has got a practical sense in case of multi storeyed buildings, 50 and above. By definition creep is deformation under sustained loading. A column is always under compression whose magnitude varies with change of live load at different floors. In case of a column we know axial deformation is the only deformation a column can undergo. In case of small and medium high buildings we do not pay any attention to the axial deformation of columns but in a high rise building this phenomenon may affect the entire structural system. So long I have not seen any formula for assessing effect of creep categorized for structural elements like column, beam or slab. We consider concrete loses strength due to creep. We can start from this premises and consider that after 100 years the column will function in its residual strength only. On Mon, Nov 26, 2012 at 9:00 PM, Dr. N. Subramanian forum@www.buonovino.com)> wrote:

[P.K.Mallick]

The beauty of Prof.Samra�s method is that it separates the issue of the prediction of creep coefficient and ultimate shrinkage from the main problem of calculation of axial shortening. Therefore, one can use the codal provision of his own country or the output of latest research findings to calculate creep coefficient and ultimate shrinkage. This flexibility is not available other two methods discussed (Method developed by M.Fintel & F.R.Khan and the Method developed by M.Fintel, H.Iyenger & S.K.Ghosh.)
For solving axial deformation problem by Prof.Samra�s method, I prefer to use the code of ACI 209R-92(Reapproved in 1997)-PREDICTION OF CREEP, SHRINKAGE AND TEMPERATURE EFFECTS IN CONCRETE STRUCTURES for calculation of creep coefficient and ultimate shrinkage.

Hence let me explain the provisions of ACI 209R-92(Reapproved in 1997).

CALCULATION OF CREEP COEFFICIENT BY ACI 209R � 92 (REAPPROVED IN 1997) METHOD

This code expresses the creep coefficient � (t, t o ) as a function of time
� (t, t o) = X/Y (�∞ (t o ))

X= (t- t o ) 0.6

Y= 10 + (t- t o )0.6

Where creep coefficient is the ratio of specific creep C (t, t o ) at age �t� due to a unit stress applied at the age �t� to a unit stress applied at the age �t o� , where age �t o � is measured in days.
Since the initial elastic strain under a unit stress is equal to the reciprocal of the modulus of elastically Ec (t0)

� (t, t o ) = C (t, t o ) x Ec(t0)

(t, t o ) is the time since application of load and �∞ (t, t o ) is the ultimate creep coefficient, which is given by

�∞ (t, t o ) = 2.35 K1 K2 K3 K4 K5 K6

For age at application of load greater that 7 days for moist curing, or greater that 1 to 3 days for steam curing, the coefficient �K1� is estimated from:

For moist curing:
K1 = 1.25 t o -0.118
For steam curing:
K1 = 1.13 t o -0.095

The coefficient �k2� is dependent upon the relative humidity �h� (percent)
K2 = 1.27 � 0.006h
For h ≥ 40

The coefficient �K3� allows for member size in terms of volume/surface ratio, V/s which is defined as the ratio of the cross sectional area to the perimeter exposed to drying. For values of V/s smaller that 37.5mm, K3 is given below.

Value to Surface Ratio (mm) Coefficient (K3)
12.5 1.3
19.0 1.17
25.0 1.11
31.0 1.04
37.5 1.0

When V/s is between 37.5 and 95mm, K3 is given by:
For (t-to) ≤ 1 year:
K3 = 1.14 � 0.00364 v/s
For (t-to) > 1 year:
K3 = 1.1 � 0.00268 v/s
When v/s ≥ 95mm
K3 = 2/3 [1+1.13e-0.0212(v/s) ]

The coefficients to allow for composition of concrete are K4, K5.
Coefficient K4 is given by :

K4 = 0.82 + 0.00264S
Where S = slump of fresh concrete.

Coefficient K5 depends on the fine aggregate/total aggregate ratio, Af/A, in percent and is given by :
K5 = 0.88 + 0.0024(AF/A)

Coefficient K6 depends on the air content �a� (percent)
K6 = 0.46 + 0.09a ≥1

弹性应变和蠕变变形下unit stress is termed the creep function �, which is given by:
� (t, t o ) = [1/Ec(t0)]* [1 + � (t, t o )]

Where Ec (t0) is related to the compressive strength of test of cylinders. If the strength at age �t0� is not known, it can be found from the following relation
Fcy (t0) = (t0/(X+Yt0)) * (fcy28)

Where �fcy28� is the strength at 28 days and �X� and �Y� are given below in table:

Type of cement Curing condition Constant
X Y
Ordinary Portland Moist 4 0.85
水泥

Steam 1 0.95



Rapid hardening
Portland cement Moist 2.3 0.92
Stream

Steam 0.7 0.98


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TO BE CONTINUED

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[P.K.Mallick]

I would request ADMIN to transfer this topic to General discussion Forum

Thanks and Regards.

[Dr. N. Subramanian]

Dear Er Mallick,

Why do you want to transfer it to General discussion Forum? Axial deformation of columns is a problem only in Tall Buildings!

Best wishes & Regards,
NS

[P.K.Mallick]

[admin]

Dear Sirs,
Glad to inform we have planned to extend the e-conf till Dec 5. We expect to make formal announcement shortly.
Regards
Admin

[JayakumarK]

Hi everybody,

As it is mentioned below by Mr. Ranjith, the spread sheets may have some restrictions regarding the height of the building, but there is a software called MIDAS(GEN) to model and predict the column shortening effects. I have used this software to model a 70 storeys building in Dubai.


Regards,
Jayakumar K
email:ejk.iitb@gmail.com(ejk.iitb@gmail.com)




On Sun, Nov 25, 2012 at 8:09 AM, Ranjith.Chandunni forum@www.buonovino.com)> wrote:

[P.K.Mallick]

CALCULATION OF ULTIMATE SHRINKAGE BY ACI 209-R-92(REAPPROVED IN 1997) METHOD


According to AC I 209.R � 92, Shrinkage Sh (t,τo) at time t(days), measured from the start of drying at τo (days) is expressed as follows:

For moist curing

Sh (t, τo ) =(( t- τo)/(35 + (t � τo))) Sh∞

For steam curing

Sh (t, τo) = ((t- τo)/(55 + (t- τo))) Sh∞

Where Sh∞= Ultimate shrinkage and

Sh∞ = 780 x 10-6 K1 K2 K3 K4 K5 K6 K7

For curing times different from seven days for moist cured concrete,

the age coefficient K1 is given below:

Period of moist curing shrinkage coefficient (K�)
1 1.2
3 1.1
7 1.0
14 0.93
28 0.86
90 0.75

and for steam curing with a period of 1 to 3 days

K1 = 1

The humidity coefficient K2 � is

K2 = 1.4 � (0.01)h, where 40 ≤ h ≤ 80

K2 = 3.0 � 0.3h, where 80 ≤ h ≤ 100

Where h = Relative humidity (Percent)

Coefficient K3 allows for the size of the member in terms of the

Volume / surface ratio V/S.

For values of the V/S <37> 50)

Where Af/A = Fine aggregate / total time aggregate ratio by mass

K6 = 0.75 + 0.00061τ, Where τ = Cement content (Kg/m3 )

K7 = 0.95 + 0.008 A, Where A = Air content (Percent)



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TO BE CONTINUED.
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[Dr. N. Subramanian]

Dear Er Mallick,

I wish to inform you that the current issue(Dec 2012) of ICJ is a spl. issue on Creep and shrinkage and contains the following articles:

• Factors affecting creep and shrinkage of hardened concrete and guide for modelling
  Mario A. Chiorino and Domingo J. Carreira
  
• Evaluation of the structural response to the time-dependent behaviour of concrete: Part 1 - An internationally harmonized format
  Mario A. Chiorino and Carlo Casalegno
  
• Evaluation of the structural response to the time-dependent behaviour of concrete: Part 2 - A general computational approach
  Mario Sassone and Carlo Casalegno
  
• Costanera Center - Shortenings due to elastic deformation, creep and shrinkage of concrete in a 300-m tall building
  Ren� C. Lagos, Marianne C. Kupfer, Sim�n T. Sanhueza and Francisco V. Cordero

Regards,
NS