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clause 7.2 Table 6 cracked section properties
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This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-conference on IS 16700 13th March to 18th March 2018
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hemalmistry
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PostPosted: Wed Mar 14, 2018 5:04 pm    Post subject: clause 7.2 Table 6 cracked section properties Reply with quote

For factored (seismic) loads for shear walls Ieff=0.7 Ig, is it irrespective of extent of cracking in walls?

ACI 318 suggests 0.35Ig for cracked walls.
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Gangisetty Krishna
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PostPosted: Thu Mar 15, 2018 5:26 am    Post subject: Reply with quote

Alok sir,

In Table 6 of the code, it is specified that to do the crack section analysis for un-factored and factored load. What is the importance of the un-factored crack section analysis? when will be this analysis governs the section size and reinforcement in the design.

For the crack section is it required to do analysis 4 times with un-factored and factored properties with 2% damping for wind load, 5% damping for Earthquake load. The analysis becomes bit complex or not?

Thank you sir,

With regards,

G V Krishna
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GARA SAI CHAND
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PostPosted: Thu Mar 15, 2018 6:19 am    Post subject: clause 7.2 Table 6 cracked section properties Reply with quote

Hello Sir,

            It is glad to participate in this discussion on IS-CODE. Could anyone of you send me the copy of this "Tall Building Code". Hoping to receive it earlier and eagerly waiting to actively participate in this.  






Thanking you








Enjoy the MomentG.S.Chand

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bsec
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PostPosted: Thu Mar 15, 2018 1:45 pm    Post subject: clause 7.2 Table 6 cracked section properties Reply with quote

Dear Mr G V Krishna

Thanks for your query. Though I am not an expert in Tall Buildings and have never designed one, my understanding in this regard, as a structural engineer is as follows :


a) On cracked and uncracked moment of Inertia :


Though Alpa clarified in one of her email (while clarifying clause 5.4.1) that 'factored' load pertains to earthquake load and 'unfactored' load pertains to wind load combination, I am not sure whether the same logic will hold good in case of Table 6. Here I think the concept of 'factored' and 'unfactored' loads refers to structural analysis for 'SLS' or 'ULS' checks respectively. I would submit here that this is my own understanding and not sure whether the intent of code makers is same. Alpa can clarify.


So in my view, analysis needs to be carried out twice ....once for SLS loads with uncracked properties and once for ULS load combination with cracked properties. Distribution of moments between beams and columns at a joint will be different in these cases.


b) On 2% Damping (for Wind) and 5% Damping (for Seismic)


Yes, you have to have two separate set of analysis. One for wind and one for seismic.  


Best Wishes


Alok Bhowmick


On Thu, Mar 15, 2018 at 10:56 AM, Gangisetty Krishna <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Alok sir,

In Table 6 of the code, it is specified that to do the crack section analysis for un-factored and factored load. What is the importance of the un-factored crack section analysis? when will be this analysis governs the section size and reinforcement in the design.

For the crack section is it required to do analysis 4 times with un-factored and factored properties with 2% damping for wind load, 5% damping for Earthquake load. The analysis becomes bit complex or not?

Thank you sir,

With regards,

G V Krishna
     



     


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hemalmistry
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PostPosted: Thu Mar 15, 2018 2:13 pm    Post subject: Reply with quote

TABLE:6 have stiffness modifiers for slab for service and factored level analysis. I understand it is for one-way or two-way slab and not for flat slab as required by code. If we consider 25% or 35% stiffness of slab as allowed by code in lateral load analysis, slab will contribute to resist lateral load by some amount and for which it shall be designed.

Is it compulsory to consider code specified slab stiffness in analysis or it can be neglected. Neglecting slab stiffness may result in over conservative drift and lesser base shear under response spectrum analysis. However, we are scaling dynamic base shear to match with static one, so strength design will be safe.
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hemalmistry
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PostPosted: Thu Mar 15, 2018 2:26 pm    Post subject: Re: clause 7.2 Table 6 cracked section properties Reply with quote

According to me

Two models:

(1) wind load : service level modifiers (table:6), 2% damping, strength design with codal load combination, drift check for unfactored load combination (<hi/500)

(2) seismic load : factored level modifiers (table:6), 5% damping, strength design with codal load combination, drift check for unfactored load combination (<hi/250)








bsec wrote:
Dear Mr G V Krishna

Thanks for your query. Though I am not an expert in Tall Buildings and have never designed one, my understanding in this regard, as a structural engineer is as follows :


a) On cracked and uncracked moment of Inertia :


Though Alpa clarified in one of her email (while clarifying clause 5.4.1) that 'factored' load pertains to earthquake load and 'unfactored' load pertains to wind load combination, I am not sure whether the same logic will hold good in case of Table 6. Here I think the concept of 'factored' and 'unfactored' loads refers to structural analysis for 'SLS' or 'ULS' checks respectively. I would submit here that this is my own understanding and not sure whether the intent of code makers is same. Alpa can clarify.


So in my view, analysis needs to be carried out twice ....once for SLS loads with uncracked properties and once for ULS load combination with cracked properties. Distribution of moments between beams and columns at a joint will be different in these cases.


b) On 2% Damping (for Wind) and 5% Damping (for Seismic)


Yes, you have to have two separate set of analysis. One for wind and one for seismic.  


Best Wishes


Alok Bhowmick


On Thu, Mar 15, 2018 at 10:56 AM, Gangisetty Krishna <forum> wrote:
Quote:
            Alok sir,

In Table 6 of the code, it is specified that to do the crack section analysis for un-factored and factored load. What is the importance of the un-factored crack section analysis? when will be this analysis governs the section size and reinforcement in the design.

For the crack section is it required to do analysis 4 times with un-factored and factored properties with 2% damping for wind load, 5% damping for Earthquake load. The analysis becomes bit complex or not?

Thank you sir,

With regards,

G V Krishna
     



     


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alpa_sheth
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PostPosted: Thu Mar 15, 2018 4:32 pm    Post subject: clause 7.2 Table 6 cracked section properties Reply with quote

Dear Alok and All,

Let's just say all roads lead to Rome. I think we are possibly both saying the same thing. 


When we say unfactored loads or service level loads we are talking of loads for which the structure is designed to be  in the elastic range ie design wind loads.


When we talk of factored loads or ultimate level loads we are talking of loads for which we are designing the structure to go into the inelastic range - which is the design earthquake loads. 


I hope this clarifies the issue. So one has to  design for wind loads with  unfactored RC cracked section coefficients and 2% damping and for design earthquake loads we use factored RC cracked section coefficients and 5% damping. 


Hemal has asked a question regarding extent of cracking and appropriate cracked section properties. Hemal, let me get back to you on that as a long clarification on the last day. 


Best regards,
Alpa




On 15-Mar-2018 7:16 PM, "bsec" <forum@sefindia.org (forum@sefindia.org)> wrote:[quote]            Dear Mr G V Krishna

Thanks for your query. Though I am not an expert in Tall Buildings and have never designed one, my understanding in this regard, as a structural engineer is as follows :


a) On cracked and uncracked moment of Inertia :


Though Alpa clarified in one of her email (while clarifying clause 5.4.1) that 'factored' load pertains to earthquake load and 'unfactored' load pertains to wind load combination, I am not sure whether the same logic will hold good in case of Table 6. Here I think the concept of 'factored' and 'unfactored' loads refers to structural analysis for 'SLS' or 'ULS' checks respectively. I would submit here that this is my own understanding and not sure whether the intent of code makers is same. Alpa can clarify.


So in my view, analysis needs to be carried out twice ....once for SLS loads with uncracked properties and once for ULS load combination with cracked properties. Distribution of moments between beams and columns at a joint will be different in these cases.


b) On 2% Damping (for Wind) and 5% Damping (for Seismic)


Yes, you have to have two separate set of analysis. One for wind and one for seismic.  


Best Wishes


Alok Bhowmick


On Thu, Mar 15, 2018 at 10:56 AM, Gangisetty Krishna forum@sefindia.org (forum@sefindia.org))> wrote:
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Ranjith.Chandunni
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PostPosted: Thu Mar 15, 2018 4:32 pm    Post subject: clause 7.2 Table 6 cracked section properties Reply with quote

Dear Mr. Alok Bowmik,

Thank you very much for the clarifications which are quite helpful.

I would like to add to what you have clarified regarding cracked and uncracked moment of inertia. Cracked section properties under un-factored loads from Table 6 are used for SLS checks however only for wind loads. These properties are not to be used for ULS design of wind loads. Seismic loads are considered as factored and while checking the drift ratio and for ULS design, cracked section properties under factored loads are to be used.

Regarding the issue of 2% and 5% damping, 5% damping should be used for seismic loads in dynamic analysis. 2% damping is used for gust wind calculations which is normally done outside the 3D analysis model and input as user defined loads. 2% damping is also used in wind tunnel assessment while estimating the structural wind load on buildings.

So, in the 3D models we normally use for analysis, only 5% damping need to be applied, and two sets of analysis, with un-factored and factored cracked sections properties, have to be carried out.

A still lower damping is used for wind acceleration calculations but its serviceability damping which is less than 2% and depends on the structural system.

Best regards,

RANJITH CHANDUNNI


From: bsec <forum@sefindia.org>
Sent: 15 March 2018 19:16
To: econf@sefindia.org
Subject: [E-CONF] Re: clause 7.2 Table 6 cracked section properties



Dear Mr G V Krishna

Thanks for your query. Though I am not an expert in Tall Buildings and have never designed one, my understanding in this regard, as a structural engineer is as follows :


a) On cracked and uncracked moment of Inertia :


Though Alpa clarified in one of her email (while clarifying clause 5.4.1) that 'factored' load pertains to earthquake load and 'unfactored' load pertains to wind load combination, I am not sure whether the same logic will hold good in case of Table 6. Here I think the concept of 'factored' and 'unfactored' loads refers to structural analysis for 'SLS' or 'ULS' checks respectively. I would submit here that this is my own understanding and not sure whether the intent of code makers is same. Alpa can clarify.


So in my view, analysis needs to be carried out twice ....once for SLS loads with uncracked properties and once for ULS load combination with cracked properties. Distribution of moments between beams and columns at a joint will be different in these cases.


b) On 2% Damping (for Wind) and 5% Damping (for Seismic)


Yes, you have to have two separate set of analysis. One for wind and one for seismic.


Best Wishes


Alok Bhowmick


On Thu, Mar 15, 2018 at 10:56 AM, Gangisetty Krishna forum@sefindia.org (forum@sefindia.org))> wrote:
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Ranjith.Chandunni
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PostPosted: Thu Mar 15, 2018 5:17 pm    Post subject: Re: clause 7.2 Table 6 cracked section properties Reply with quote

bsec wrote:
Dear Mr G V Krishna

Thanks for your query. Though I am not an expert in Tall Buildings and have never designed one, my understanding in this regard, as a structural engineer is as follows :


a) On cracked and uncracked moment of Inertia :


Though Alpa clarified in one of her email (while clarifying clause 5.4.1) that 'factored' load pertains to earthquake load and 'unfactored' load pertains to wind load combination, I am not sure whether the same logic will hold good in case of Table 6. Here I think the concept of 'factored' and 'unfactored' loads refers to structural analysis for 'SLS' or 'ULS' checks respectively. I would submit here that this is my own understanding and not sure whether the intent of code makers is same. Alpa can clarify.


So in my view, analysis needs to be carried out twice ....once for SLS loads with uncracked properties and once for ULS load combination with cracked properties. Distribution of moments between beams and columns at a joint will be different in these cases.


b) On 2% Damping (for Wind) and 5% Damping (for Seismic)


Yes, you have to have two separate set of analysis. One for wind and one for seismic.  


Best Wishes


Alok Bhowmick


On Thu, Mar 15, 2018 at 10:56 AM, Gangisetty Krishna <forum> wrote:
Quote:
           Alok sir,

In Table 6 of the code, it is specified that to do the crack section analysis for un-factored and factored load. What is the importance of the un-factored crack section analysis? when will be this analysis governs the section size and reinforcement in the design.

For the crack section is it required to do analysis 4 times with un-factored and factored properties with 2% damping for wind load, 5% damping for Earthquake load. The analysis becomes bit complex or not?

Thank you sir,

With regards,

G V Krishna
     



     


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hemalmistry
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PostPosted: Fri Mar 16, 2018 6:04 pm    Post subject: Re: clause 7.2 Table 6 cracked section properties Reply with quote

Respected alpa mam,

According to ASCE 7 for service level loads (unfactored loads) stiffness modifiers shall be 1.4 times that of factored level loads. In IS 16700, for unfactored loads stiffness modifier for beam is 0.7 which is double of that for factored load. Slab, column and wall values are comparable.

[quote="alpa_sheth"]Dear Alok and All,

Let's just say all roads lead to Rome. I think we are possibly both saying the same thing.


When we say unfactored loads or service level loads we are talking of loads for which the structure is designed to be  in the elastic range ie design wind loads.


When we talk of factored loads or ultimate level loads we are talking of loads for which we are designing the structure to go into the inelastic range - which is the design earthquake loads.


I hope this clarifies the issue. So one has to  design for wind loads with  unfactored RC cracked section coefficients and 2% damping and for design earthquake loads we use factored RC cracked section coefficients and 5% damping.


Hemal has asked a question regarding extent of cracking and appropriate cracked section properties. Hemal, let me get back to you on that as a long clarification on the last day.


Best regards,
Alpa




On 15-Mar-2018 7:16 PM, "bsec" <forum> wrote:
Quote:
            Dear Mr G V Krishna

Thanks for your query. Though I am not an expert in Tall Buildings and have never designed one, my understanding in this regard, as a structural engineer is as follows :


a) On cracked and uncracked moment of Inertia :


Though Alpa clarified in one of her email (while clarifying clause 5.4.1) that 'factored' load pertains to earthquake load and 'unfactored' load pertains to wind load combination, I am not sure whether the same logic will hold good in case of Table 6. Here I think the concept of 'factored' and 'unfactored' loads refers to structural analysis for 'SLS' or 'ULS' checks respectively. I would submit here that this is my own understanding and not sure whether the intent of code makers is same. Alpa can clarify.


So in my view, analysis needs to be carried out twice ....once for SLS loads with uncracked properties and once for ULS load combination with cracked properties. Distribution of moments between beams and columns at a joint will be different in these cases.


b) On 2% Damping (for Wind) and 5% Damping (for Seismic)


Yes, you have to have two separate set of analysis. One for wind and one for seismic.  


Best Wishes


Alok Bhowmick


On Thu, Mar 15, 2018 at 10:56 AM, Gangisetty Krishna forum@sefindia.org (forum@sefindia.org))> wrote:
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