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[ali.pendhari@gmail.com]

Respected all,                I am student of m.tech structure so  i am having  doubts in software sap-2000 is how to model cracked section and uncracked section. i got the notes from tutorial of sap-2000 but in that they not mentioned the detailed procedure to model the cracked and uncracked section please help me and also help me theoretically what is the difference between design of cracked and uncracked section.
thank you


Mohammedali Pendhari
student M.tech Structure  

Posted via Email

[VPandya]

Dear Pendhari,

I can help you with this problem of  "CONCRETE CRACKED SECTION PROPERTIES" to be used in Concrete Frame Analysis in STAAD/PRO or SAP-2000.  Recently I myself run in to this problem while reviewing a fellow Engineers Concrete Frame Analysis and Design. This Frame Analysis and Design for an International Project was based on
ACI 318-05 .  Properties of Concrete Frame analysis were based on Gross Section ( Ig = 1/12X bxd**3) and not on reduced properties of ACI 318-05 section
10.11.1. This was corrected later. A new analysis and design was done.
Point is there still exist some confusion for many engineers on International Projects as to how to meet requirements of ACI 318 code for  Concrete Frame Analysis. What is the intent of  ACI 318-11 Section 8.7, 8.8.1 and 8.8.2.?

Here is my understanding of Concrete Cracked Section Properties based on ACI 318-11 Section 8.8.1 and 8.8.2 (a).
Cracked Section properties from Section 10.10.4.1:
Modulus of Elasticity Ec = Ec  section 8.5.1 (No reduction)
Moment of Inertia of Columns (Iyy and Izz) = 0.7Ig  (Ig = Gross Ig = 1//12bd**3)
                                     No reduction in Ixx (Torsion) and Ax, AY , AZ    

Moment of Inertia of Beams (Iyy and Izz)  = 0.35Ig (Ig = Gross Ig = 1//112bd**3)
                                      No reduction in Ixx (Torsion) and Ax, AY , AZ  

     
There  are two seperate analysis you will be doing of your 3D Concrete Frame:
1) At Service Load Combinations:----For Frame Drift, Member Deflections and Foundation Pressures (Soil Bearing Stress):  Based on section 8.8.1:
a) For Columns Use Iyy and Izz = 1.4X0.7Ig = 0.98Ig = Ig (at Service Load
                  combinations you can use Ig (uncracked) for Columns.
b) For  Beams (Flexure) Use Iyy and Izz = 1.4X0.35Ig = 0.49Ig (say 0.5Ig or 50% of
    gross Moment of Inertia for beams.  Think of this even at Service Loads analysis
    ACI Code wants you use only 50%Ig for Beams in Frame Analysis. This will
    change your frame Drift, Member Deflections and may impact your Frame Member
     Sizes to get to the allowable Drift, Member Deflections Values.  STAAD/PRO Tech.
     Reference Manual Section 3.3 has the same values of Concrete Cracked Section
     Properties for Beams (50% Ig) for many years.  

2) At  Factored Load Combinations:-- For Member Design Forces and Reinforcement
                                                           Design.
    a) For Columns use  Iyy and Izz = 0.7Ig  
         No reduction in Ixx (Torsion) and Ax, AY , AZ       
    b) For Beams (Flexure) use Iyy and Izz = 0.35Ig
        No reduction in Ixx (Torsion) and Ax, AY , AZ    
        If you want Code also allows you to use  Iyy and Izz = 0.5Ig for both Columns
        and Beams based on  section 8.8.2(b).

Point is using Gross Moment of Inertia Ig for Concrete Frame Analysis at Factored
Load Combinations is simply wrong. Some of us engineers still have not cought up
with this change and use Gross Moment Of Inertias (Ig) for beams at Service Load and Factored Loads. Based on ACI 318 Code we can not and should not do that.

I hope this answers your question.     

Regards.

Vasudeo Pandya




     
  

[VPandya]

Some corrections.

Vasudeo Pandya


From: vasudeopandya@hotmail.com
To: econf@sefindia.org
Subject: RE: [ECONF] modelling in sap2000-14. Use ACI 318-11 Section 8.7, 8.8.1 , 8.8.2 for Concrete Cracked Sections.
Date: Tue, 20 Mar 2012 17:05:14 +0000

.ExternalClass .ecxhmmessage P {padding:0px;} .ExternalClass body.ecxhmmessage {font-size:10pt;font-family:Tahoma;}  Dear Pendhari,

I can help you with this problem of "CONCRETE CRACKED SECTION PROPERTIES" to be used in Concrete Frame Analysis in STAAD/PRO or SAP-2000. Recently I myself run in to this problem while reviewing a fellow Engineers Concrete Frame Analysis and Design. This Frame Analysis and Design for an International Project was based on ACI 318-05 . Properties of Concrete Frame analysis were based on Gross Section ( Ig = 1/12X bxd**3) and not on reduced properties of ACI 318-05 section 10.11.1. This was corrected later. A new analysis and design was done.

Point is there still exist some confusion for many engineers on International Projects as to how to meet requirements of ACI 318 code for Concrete Frame Analysis. What is the intent of ACI 318-11 Section 8.7, 8.8.1 and 8.8.2.?

Here is my understanding of Concrete Cracked Section Properties based on ACI 318-11 Section 8.8.1 and 8.8.2 (a).
Cracked Section properties from Section 10.10.4.1:
Modulus of Elasticity Ec = Ec section 8.5.1 (No reduction)
Moment of Inertia of Columns (Iyy and Izz) = 0.7Ig (Ig = Gross Ig = 1/12xbd**3)
No reduction in Ixx (Torsion) and Ax, AY , AZ  

Moment of Inertia of Beams (Iyy and Izz) = 0.35Ig (Ig = Gross Ig = 1/12xbd**3)
No reduction in Ixx (Torsion) and Ax, AY , AZ  


There are two seperate analysis you will be doing of your 3D Concrete Frame:
1) At Service Load Combinations:-For Frame Drift, Member Deflections and Foundation Pressures (Soil Bearing Stress): Based on section 8.8.1:
a) For Columns Use Iyy and Izz = 1.4X0.7Ig = 0.98Ig = Ig (at Service Load
combinations you can use Ig (uncracked) for Columns.
b) For Beams (Flexure) Use Iyy and Izz = 1.4X0.35Ig = 0.49Ig (say 0.5Ig or 50% of
gross Moment of Inertia for beams. Think of this even at Service Loads analysis
ACI Code wants you use only 50%Ig for Beams in Frame Analysis. This will
change your frame Drift, Member Deflections and may impact your Frame Member
Sizes to get to the allowable Drift, Member Deflections Values. STAAD/PRO Tech.
Reference Manual Section 3.3 has the same values of Concrete Cracked Section
Properties for Beams (50% Ig) for many years.  

2) At Factored Load Combinations:-- For Member Design Forces and Reinforcement
Design.
a) For Columns use Iyy and Izz = 0.7Ig  
No reduction in Ixx (Torsion) and Ax, AY , AZ  
b) For Beams (Flexure) use Iyy and Izz = 0.35Ig
No reduction in Ixx (Torsion) and Ax, AY , AZ  
If you want Code also allows you to use Iyy and Izz = 0.5Ig for both Columns
and Beams based on section 8.8.2(b).

Point is using Gross Moment of Inertia Ig for Concrete Frame Analysis at Factored Load Combinations is simply wrong. Some of us engineers still have not cought up with this change and use Gross Moment Of Inertias (Ig) for beams at Service Load and Factored Loads. Based on ACI 318 Code we can not and should not do that.

I hope this answers your question.  

Regards.

Vasudeo Pandya


Subject: [ECONF] modelling in sap2000-14
From: forum@sefindia.org
Date: Tue, 20 Mar 2012 09:57:16 +0530
To: econf@sefindia.org

--  Respected all, I am student of m.tech structure so i am having doubts in software sap-2000 is how to model cracked section and uncracked section. i got the notes from tutorial of sap-2000 but in that they not mentioned the detailed procedure to model the cracked and uncracked section please help me and also help me theoretically what is the difference between design of cracked and uncracked section.
thank you


Mohammedali Pendhari
student M.tech Structure

Posted via Email

[rahul.leslie]

Dear Vasudeo Pandya,

That was nice... very informative in capsule form. Thanks

Rahul Leslie

[Engineer]

Mohammedali Pendhari

SAP 2000 has option to modify section properties, Once You Give t3 and t2 dimensions, software will compute geometrical parameters like area, moment of inertia , torsion constant, shear area etc.



You can now specify modification factors to this properties.



For Beam Cracked Moment of Inertia is 0.5 times Gross Moment of Inertia ( Un-cracked), i.e Modification factor of I33 in SAP will be 0.5 , similarly for columns the factor is 0.7. ( See UBC 97)



Hope above answers your queery.



Regards,

Hemant, gorhemant@G



From: ali pendhari (forum@sefindia.org)
Sent: Tuesday, March 20, 2012 8:27 AM
To: econf@sefindia.org (econf@sefindia.org)
Subject: [ECONF] modelling in sap2000-14




Respected all, I am student of m.tech structure so i am having doubts in software sap-2000 is how to model cracked section and uncracked section. i got the notes from tutorial of sap-2000 but in that they not mentioned the detailed procedure to model the cracked and uncracked section please help me and also help me theoretically what is the difference between design of cracked and uncracked section.
thank you


Mohammedali Pendhari
student M.tech Structure

Posted via Email