www.sefindia.org

STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

 Forum SubscriptionsSubscriptions DigestDigest Preferences   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups  RegisterRegister FAQSecurity Tips FAQDonate
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log in to websiteLog in to websiteLog in to websiteLog in to forum 
Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.

stiffness modifiers
Goto page 1, 2  Next
 
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion
View previous topic :: View next topic  
Author Message
maulesh
SEFI Member
SEFI Member


Joined: 09 Mar 2009
Posts: 17

PostPosted: Thu Jun 26, 2014 2:56 am    Post subject: stiffness modifiers Reply with quote

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
Back to top
View user's profile Send private message
Manoharbs_eq
General Sponsor
General Sponsor


Joined: 17 Jul 2012
Posts: 423

PostPosted: Thu Jun 26, 2014 12:29 pm    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
zubairmeer1
General Sponsor
General Sponsor


Joined: 27 Feb 2010
Posts: 88
Location: Hyderabad

PostPosted: Fri Jun 27, 2014 12:48 pm    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
VPandya
General Sponsor
General Sponsor


Joined: 09 Nov 2009
Posts: 842

PostPosted: Fri Jun 27, 2014 12:59 pm    Post subject: We use Modified Properties even if we are not doing P-Delta. Reply with quote

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
Back to top
View user's profile Send private message
Dr. N. Subramanian
General Sponsor
General Sponsor


Joined: 21 Feb 2008
Posts: 5538
Location: Gaithersburg, MD, U.S.A.

PostPosted: Sat Jun 28, 2014 3:14 am    Post subject: Re: We use Modified Properties even if we are not doing P-De Reply with quote

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
Back to top
View user's profile Send private message
Manoharbs_eq
General Sponsor
General Sponsor


Joined: 17 Jul 2012
Posts: 423

PostPosted: Sat Jun 28, 2014 4:14 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
Dr. N. Subramanian
General Sponsor
General Sponsor


Joined: 21 Feb 2008
Posts: 5538
Location: Gaithersburg, MD, U.S.A.

PostPosted: Sat Jun 28, 2014 4:48 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
Manoharbs_eq
General Sponsor
General Sponsor


Joined: 17 Jul 2012
Posts: 423

PostPosted: Sat Jun 28, 2014 5:47 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
Dr. N. Subramanian
General Sponsor
General Sponsor


Joined: 21 Feb 2008
Posts: 5538
Location: Gaithersburg, MD, U.S.A.

PostPosted: Sat Jun 28, 2014 1:13 pm    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
vegad
...
...


Joined: 25 Dec 2013
Posts: 138

PostPosted: Mon Jun 30, 2014 3:22 am    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion All times are GMT
Goto page 1, 2  Next
Page 1 of 2

 

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


© 2003, 2008 SEFINDIA, Indian Domain Registration
Publishing or acceptance of an advertisement is neither a guarantee nor endorsement of the advertiser's product or service. advertisement policy