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maulesh SEFI Member
Joined: 09 Mar 2009 Posts: 17
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Posted: Thu Jun 26, 2014 2:56 am Post subject: stiffness modifiers |
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Dear All,
we have received G+13 strorey building having structural system of Flat slab + shear wall. we have used ETABS for modelling of building. In etabs we need to consider stiffness modifiers to cater effect of cracked concrete. I think this modifiers are used in second order Analysis (p-delta) protion only (Gravity + Seismic).
As per indian code, such parapmeters are not mentioned.
I have concern wheteher we need to consider modifiers for designing of different elements (columns/beams/shear walls).
Please guide us for above mentioned query.
Thanks lot,
Maulesh Shah |
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Manoharbs_eq General Sponsor
Joined: 17 Jul 2012 Posts: 423
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Posted: Thu Jun 26, 2014 12:29 pm Post subject: |
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Dear Maulesh Shah.
If the structure is too complex and second order effects play a major role then P-delta analysis has to be performed.For instance if the structure has too many floating columns, then p delta is necessary.
However, for regular configuration my opinion is p delta analysis is not required.
It is true that IS code is silent regarding the modifiers.However, till now i have referred to ACI modifiers for P delta analysis.
Rgds
Manohar
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zubairmeer1 General Sponsor
Joined: 27 Feb 2010 Posts: 88 Location: Hyderabad
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Posted: Fri Jun 27, 2014 12:48 pm Post subject: |
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Dear Er. maulesh..
sorry for being off the topic, could you please let me know the 'R' value being considered by you for base shear calculations for this type of structure ( Shear Wall + Flat slab)..
regards
Meer |
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VPandya General Sponsor
Joined: 09 Nov 2009 Posts: 842
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Posted: Fri Jun 27, 2014 12:59 pm Post subject: We use Modified Properties even if we are not doing P-Delta. |
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Dear Er. Manohar ,
Just a clarification on ACI 318-11 Code: We use reduced properties (Cracked Section) for Concrete Frame analysis for Service Loads and at Factored (Limit State) Loads . Even if we are not doing P-Delta (Second Order) analysis, we have to use Modified Properties (Cracked Section) for analysis. I had an interesting discussion on this with some engineers who insisted that we use Cracked Section properties only when we do P-DELTA analysis. I got that checked from ACI. Concrete Section cracks when there is even a small Flexure Tension at Service Load level. For what values of Cracked Section properties based on ACI 318-11 to be used see my SEFI Post dated 11th June 2014:
11th June 2014 post:
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Dear Er. Mohit Dhiman,
If you are designing a Concrete High-Rise Building based on ACI 318-11 Code you have to use Cracked Section properties for Beams and Columns. This cracked section properties are clearly defined in ACI 318-11 for both at Service Loads and at Limit State (Strength Design). I myself run in to this problem while reviewing a fellow Engineers Concrete Frame Analysis and Design. Being an ACI member, to make sure I send this query ACI technical help and they said my understanding of this section of ACI 318-11 and its interpretation is correct.
See my SEFI post dated 20th March 2014 on Title SAP 2000 properties. Reproduce here:
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Regards.
Vasudeo Pandya P.E. ; S.E.
Structural Engineer |
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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5538 Location: Gaithersburg, MD, U.S.A.
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Posted: Sat Jun 28, 2014 3:14 am Post subject: Re: We use Modified Properties even if we are not doing P-De |
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Dear Er Manohar,
As Er Pandayi has rightly said we need to consider cracked member section for beams and columns in the analysis also, as we consider cracked member only in limit state design.
But, in most of analysis we do not consider T section properties and take only rectangular section properties. Hence some people argue that this will cater to the cracked section properties for beams. However we need to consider cracked section properties for columns also.
Best wishes,
NS
VPandya wrote: | Dear Er. Manohar ,
Just a clarification on ACI 318-11 Code: We use reduced properties (Cracked Section) for Concrete Frame analysis for Service Loads and at Factored (Limit State) Loads . Even if we are not doing P-Delta (Second Order) analysis, we have to use Modified Properties (Cracked Section) for analysis. I had an interesting discussion on this with some engineers who insisted that we use Cracked Section properties only when we do P-DELTA analysis. I got that checked from ACI. Concrete Section cracks when there is even a small Flexure Tension at Service Load level. For what values of Cracked Section properties based on ACI 318-11 to be used see my SEFI Post dated 11th June 2014:
11th June 2014 post:
---------------------------------------------------------------------------------------------
Dear Er. Mohit Dhiman,
If you are designing a Concrete High-Rise Building based on ACI 318-11 Code you have to use Cracked Section properties for Beams and Columns. This cracked section properties are clearly defined in ACI 318-11 for both at Service Loads and at Limit State (Strength Design). I myself run in to this problem while reviewing a fellow Engineers Concrete Frame Analysis and Design. Being an ACI member, to make sure I send this query ACI technical help and they said my understanding of this section of ACI 318-11 and its interpretation is correct.
See my SEFI post dated 20th March 2014 on Title SAP 2000 properties. Reproduce here:
-----------------------------------------------------------------------------------------------
Regards.
Vasudeo Pandya P.E. ; S.E.
Structural Engineer |
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Manoharbs_eq General Sponsor
Joined: 17 Jul 2012 Posts: 423
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Posted: Sat Jun 28, 2014 4:14 am Post subject: |
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Dear Dr.NS sir and ER.Vpandya sir,
Thanks for correcting me, but in IS code there is no specific instructions to instate the property modifiers.Hence, while we design the structure using IS codes we do not assign any modifiers, unless p delta is performed.
Why is that IS code silent in instating the modifiers, as we design the structure as cracked section, we have to consider modifiers for LS design.But only available instance that is calculation of long term deflection that the code has any specific instructions about crack.Or else there is no mention of crack or modifiers.
However, in BS8110 and ACI 314 there is detailed explanation.
My question is while designing as per IS codal provisions should we use modifiers.Because, no one would do so since it is not in coe.
Rgds
Manohar |
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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5538 Location: Gaithersburg, MD, U.S.A.
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Posted: Sat Jun 28, 2014 4:48 am Post subject: |
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Dear Er Manohar,
Please note that IS 456, though it was revised in 2000, most of the revisions pertain to durability clauses, may be because the team responsible for revision is from Cement and concrete research. Hence it contained most of the provisions from the earlier version of the code.
Also note that any code provides only the minimum requirement and the designer should not say that he has not considered any condition just
because the code does not contain the provision. Taking cracked section properties is a must, especially for high rise buildings that too in EQ zones!
In the recent July Aug issue of ACI journal I read a paper written by Rajesh P. Dhakal, et al from New Zealand which says that the eqn for effective width of T Beam that is given in ACI code significantly underestimates it. In Reality, the significant increase in strength of beam could easily render the column to be weaker than beams thereby leading to catastrophic consequences ( because plastic hinges will now form in columns). Hence giving the correct value of MI is important in the analysis. As the cracking pattern of beams is different in different sections, ACI has given approximate values based on tests. The 2011 version also gives a more accurate equation based in research, to predict the cracked MI of RC beams.
Regards,
Subramanian
Manoharbs_eq wrote: | Dear Dr.NS sir and ER.Vpandya sir,
Thanks for correcting me, but in IS code there is no specific instructions to instate the property modifiers.Hence, while we design the structure using IS codes we do not assign any modifiers, unless p delta is performed.
Why is that IS code silent in instating the modifiers, as we design the structure as cracked section, we have to consider modifiers for LS design.But only available instance that is calculation of long term deflection that the code has any specific instructions about crack.Or else there is no mention of crack or modifiers.
However, in BS8110 and ACI 314 there is detailed explanation.
My question is while designing as per IS codal provisions should we use modifiers.Because, no one would do so since it is not in coe.
Rgds
Manohar |
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Manoharbs_eq General Sponsor
Joined: 17 Jul 2012 Posts: 423
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Posted: Sat Jun 28, 2014 5:47 am Post subject: |
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Dear DR.NS sir,
I was under wrong impression these days,thanks for enlightenment.
I really hope all our counterparts and clients/consultants will also understand the importance of MI in RC structure and adopt the same.
Since, i have failed to enforce this in many projects, due to several reasons, I my self am doing wrong knowingly.
In this age of computing ability, we can almost predict the behavior close to reality using FEM.However, we must assign proper parameters provided.
Thanks and regards
Manohar |
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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5538 Location: Gaithersburg, MD, U.S.A.
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Posted: Sat Jun 28, 2014 1:13 pm Post subject: |
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Dear Er. Manohar,
Hope that the next edition of IS 456 will contain provisions for cracked MI of beams and columns.
Another provision that is missing is with regards to reinforcement in slab ends, so that they will effectively function as diaphragms.
Regards,
Subramanian |
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vegad ...
Joined: 25 Dec 2013 Posts: 138
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Posted: Mon Jun 30, 2014 3:22 am Post subject: |
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Dear Maulesh,
Though IS code does not provide section modifiers, it definitely calls for it in 22.3.1 (c).
The impression of using stiffness modifiers during P-Delta only is due to both being categorized into secondary effects.
This further calls for performance based patch up, hence the use of this.
Hence less evolved codes calls for considering these effects at complex situations only (P-delta/ Crack width calculations. .etc.) where the secondary effects are more pronounced.
The degree of complexity can be decided by the designer on the basis of various complexities of geometry, scale of load paths (high rise), material, functional performance (liquid retention) and alike.
But, be sure that the you can use the modification only on the "affected parts of the elements" and not even the complete element. Basic aim it to incorporate the effects of redistribution due to weakening of section due to cracks.
Thank you. |
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