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There are two types of cracked section analysis i.e., Immediate Cracked Deflection and Long-Term Cracked deflection accounting the creep and shrinkage effects.
In SAFE v12, cracked section analysis can be performed by using two different methods which are described as follows:

Method-1:
Apply a single load pattern in load case and start another case continues From State at End of Nonlinear Case.... For example, adding SELFWEIGHT, SIDL and LIVE load case for performing cracked section analysis creates the following three cases:
• Add SELFWEIGHT Load case using Nonlinear (Cracked) started from Zero Initial Condition.
Where SELFWEIGHT: Presents the own weight of structure itself.
• Add SIDL Load case using Nonlinear (Cracked) started from using From State at End of Nonlinear Case "SELFWEIGHT".
Where SIDL: Presents the additional dead load due to finishing … etc.
• Add LIVE Load case using Nonlinear (Cracked) started from using From State at End of Nonlinear Case "SIDL".
Where LIVE: presents the additional live load.

SELFWEIGHT load case predicts the cracking from zero initial condition when no load was present and computes the cracking due to application of SELFWEIGHT load pattern. Adding SIDL in other case staring From State at End of Nonlinear Case "SELFWEIGHT" uses the stiffness at the end of SELFWEIGHT load case and computes the additional deflection due to SIDL case. The reported deflection shows the total deflection due to SELFWEIGHT plus SIDL case. However, this method is unable to recognize the increase in SELFWEIGHT load deflection due to increase in cracking when SIDL load is added. Therefore this method is not recommended.


Method-2:
The recommended method by CSI SAFE 12's developers for computing cracked section analysis is to apply all load patterns in a single load case and use the Crack analysis or Cracked Long-Term Analysis Option as discussed below:

Immediate Cracked Deflection:
Apply all loads (i.e. SELFWEIGHT + SIDL + Live) in a single load and use the Crack Analysis option.

Long-Term Cracked Deflection:
The creep and shrinkage effects are only applicable for sustained type loading i.e., SELFWEIGHT, SIDL and a portion of LIVE load (in case of Warehouse, Stadium etc.) if applicable. The analysis is divided into two category cases i.e. a cracked section analysis which determines the incremental deflection due to non-sustain portion of LIVE load and the long-term cracked analysis which includes the sustained type of loading including creep and shrinkage effects.

A 25% Live load is assumed to be of sustained type in the example shown below:

Case 1: Short term load with short term concrete modulus (SELFWEIGHT + SIDL + Ψs LIVE) where Ψs = 1.0 (i.e. Crack analysis)
Case 2: Permanent load with short term concrete modulus (SELFWEIGHT + SIDL + ΨLLIVE) where ΨL = 0.25 (i.e. Crack analysis). Use ΨL = 0 if 100 percent of LIVE load is non-sustained type of load.
Case 3: Permanent load with long-term concrete modulus plus creep and shrinkage (SELFWEIGHT + SIDL + ΨLLIVE) where ΨL = 0.25 (i.e. Long-Term Cracked analysis with creep and shrinkage)

The long term deflection is the combination of Case 3 + (Case 1- Case 2)

The difference due to Case 1 and Case 2 represent the incremental deflection due to non-sustained loading without accounting creep and shrinkage for fully cracked structure.


Conclusion:
For Nonlinear Cracked Section Analysis for both Short & Long Term Conditions, It seems that SAFE 12 Doesn't store the tangent stiffness / deformation at the end of Nonlinear Case, Thus CSI SAFE 12's developers always recommend to use method-2 as mentioned above.

Hussein Rida.
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(02-14-2012 04:10 PM)sea2007 Wrote: You are not allowed to view links. Register or Login to view.معظم المهندسين تستخدم الطريقة الاولى

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فما رايك فى ذلك؟

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كلتا الطريقتيين تعطى نفس النتائج تقريبا وبدقعه عاليه
تقبل تحياتى
First of all, there is quite difference between the results obtained from method-1 and method-2 that described above.
It is strongly recommended to follow method-2 as advised by CSI SAFE 12's developers, since it seems the only realistic analysis way that reflects the actual development of cracks under certain sequence/history of applied loadings. Also, for Nonlinear Cracked Analysis, SAFE 12 doesn't store perfectly the tangent stiffness/deformation at the end of previous nonlinear case, otherwise both method 1 & 2 shall be equally recommended by CSI developers.

I went through a lot of papers that talking about the conceptual principals of calculating immediate and long term deflection taking in consideration the effects/history of developed cracks in reducing the flexural rigidity and then increasing the deflection curvature under certain sequence of applied loading.

Below figures are extracted from the one of most interesting papers talking about the methodology of developing the deflection from initial/immediate stage to the long term one. And how the degradation in flexural rigidity is occurred because of gradually increasing in degree of cracking under a certain time-history of applied loads, and then decreases the effective moment of inertia.

Regarding the overestimated long term deflection values obtained from SAFE, I would like to mention the followings:
  • One of the main players in this game (deflection) is the concrete modulus of rupture “fr” which is the value of tension stresses that the cracks start being developed in concrete after reaching this value “fr”.
    In ACI318 Code, this value equal to 7.5 * sqrt(f’c) in psi units. However the default value taken in SAFE is almost equal to half of this value: 4 * sqrt(f’s).
    This default value of “fr” as taken in SAFE will give overestimated values of deflection.
    As many of you know that “fr = 4 * sqrt(f’s)” is recommended by ACI committee 435 for two way slab system for many of reasons mentioned there, however using this value in SAFE analysis algorithm will produce an overestimated value of deflection which is very far from those measured in realistic field. Thus, I believe that using the full value of “fr=7.5 * sqrt(f’c)” as given by ACI318 is much more realistic. Here I would like to mention that when we asked CSI SAFE 12's developers about this issue, they didn’t give a clear answer and kick the ball to the playground of the designer.
  • Pay attention for values of creep coefficient and ultimate shrinkage strain that used in SAFE nonlinear long term cracked analysis.
  • Also, for nonlinear cracked analysis, special attention shall be given for reinforcement used in calculating Crakcs width.

(For more details refer to below figures).


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(12-06-2010 01:07 PM)hussein.rida Wrote: You are not allowed to view links. Register or Login to view.
Method-2:
The recommended method by CSI SAFE 12's developers for computing cracked section analysis is to apply all load patterns in a single load case and use the Crack analysis or Cracked Long-Term Analysis Option as discussed below:

Immediate Cracked Deflection:
Apply all loads (i.e. SELFWEIGHT + SIDL + Live) in a single load and use the Crack Analysis option.

Long-Term Cracked Deflection:
The creep and shrinkage effects are only applicable for sustained type loading i.e., SELFWEIGHT, SIDL and a portion of LIVE load (in case of Warehouse, Stadium etc.) if applicable. The analysis is divided into two category cases i.e. a cracked section analysis which determines the incremental deflection due to non-sustain portion of LIVE load and the long-term cracked analysis which includes the sustained type of loading including creep and shrinkage effects.

A 25% Live load is assumed to be of sustained type in the example shown below:

Case 1: Short term load with short term concrete modulus (SELFWEIGHT + SIDL + Ψs LIVE) where Ψs = 1.0 (i.e. Crack analysis)
Case 2: Permanent load with short term concrete modulus (SELFWEIGHT + SIDL + ΨLLIVE) where ΨL = 0.25 (i.e. Crack analysis). Use ΨL = 0 if 100 percent of LIVE load is non-sustained type of load.
Case 3: Permanent load with long-term concrete modulus plus creep and shrinkage (SELFWEIGHT + SIDL + ΨLLIVE) where ΨL = 0.25 (i.e. Long-Term Cracked analysis with creep and shrinkage)

The long term deflection is the combination of Case 3 + (Case 1- Case 2)

The difference due to Case 1 and Case 2 represent the incremental deflection due to non-sustained loading without accounting creep and shrinkage for fully cracked structure.


Conclusion:
For Nonlinear Cracked Section Analysis for both Short & Long Term Conditions, It seems that SAFE 12 Doesn't store the tangent stiffness / deformation at the end of Nonlinear Case, Thus CSI SAFE 12's developers always recommend to use method-2 as mentioned above.

Hussein Rida.

السلام عليكم و رحمه الله

قيم الهبوط المسموح بها
حسب الكود الامريكي
ACI318-08
TABLE 9.5(b) — MAXIMUM PERMISSIBLE COMPUTED DEFLECTIONS

يأخذ الكود الامريكي سهم الهبوط اللحظي تحت الاحمال الحيه فقط و ليس كل الاحمال و هذه القيمه هي ما يتم مقارنتها بالمسموح به في الجدول السابق

هل عند تطبيق الشرط التالي
Immediate Cracked Deflection
Apply all loads (i.e. SELFWEIGHT + SIDL + Live) in a single load and use the Crack Analysis option.
تم أخذ تأثير كل الاحمال عند حساب الهبوط اللحظي و لم يؤخذ تأثير الحمل الحي فقط؟؟
و ما هو الحل للتأكد من الهبوط اللحظي المسموح تحت تأثير الحمل الحي فقط حسب الجدول السابق بالكود الامريكي بواسطه برنامج
safe v12?????
Dear hussein.rida

Thank you very much for your clear explanations, in your comment i read that you think modulus of rupture fr = 4 * sqrt(f’s) recommended by ACI committee 435 will produce an overestimated value of deflection which is very far from those measured in realistic field. Can you please specify where do you get data those measured in realistic field (or maybe you get that information from journal, can you please specify detail information about that journal?). In my opinion, ACI 435 recommended that value because report from Tam and Scanlon (1986) -->ACI 435 page 54. Tam and Scanlon say it is very difficult to consider restraint crack and shrinkage strain in two way slab, and the easy way to do that is to reduce modulus of rupture. SAFE v8 also still used effective inertia (Bronson method) to calculate crack in slab (effective inertia method just consider cracking because of flexural load, not membrane load), but we now, since SAFE v.12 has already used moment curvature method (by Ghali) to calculate crack analysis and long term crack analysis, membrane force in slab has already considered when calculate degree of cracking. But in Ghali book, he still reduce modulus of rupture to consider uniform shrinkage strain in edge restraint slab. ACI 209 for shrinkage strain did not consider it, so the easy way is to reduce modulus of rupture. In Ghali book (example 9.7) he reduce modulus of rupture until near 5 * sqrt(f’s) for restraint slab. so in my opinion, reduced modulus of rupture is a good way since in practical case continuous slabs is enough to restraint the slab. Any comment ?

Thank you very much :)
dear eng. hussein

can you provide the link of the reference book that you use it to capture the image.

thanks in advance
Reference URL's