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Crack width

 
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Anand _Ekbote
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PostPosted: Thu Jun 25, 2015 12:58 pm    Post subject: Crack width Reply with quote

Dear sefiens,

It is not clear from code IS 456, whether Crack width is to be checked only for combinations of the Permanant loads ( i.e. DL, DL+LL etc.) or also for combinations with seismic or wind loads ( DL+SL or DL + WL).

I believe, Crack width calculations are more relevant with permanant sustained loads. Is it mandatory to meet the provisions of cl 35.3.2 of IS 456  under combinations with seimic or wind loads?

If protective coating is done on the concrete surface, will it help to relax on the crack width requirements.

Experts are requested to comment.

Regards,

Anand R Ekbote
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suresh_sharma
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PostPosted: Sun Jul 05, 2015 5:13 pm    Post subject: Reply with quote

Clause 35.3.2, cracking, falls under the heading 'Limit State of Serviceability' of the code. It is therefore implied that checking has to be done for the loads laid down in the code for limit state of serviceability.
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Anand _Ekbote
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PostPosted: Tue Jul 07, 2015 7:28 am    Post subject: Reply with quote

Thank you Er.Suresh.

I put folloing points to get the further clarifications,

1) SP24 & IS456-1978 was mentioning crack width as 0.004 times cover thickness.
2) The above clause is not there in IS456-2000.
3) American codes however mentions xxx times cover as crack width.
4) SP 24 mentions basis for crack width as 25mm cover.

In this context, IS456-2000 Amd 4 mentions surface crack width not to exceed 0.1mm for Very Severe or Extreme exposure condition. Evidences show that corelation with rebar corrosion & crack width is conflicting.

Currently for higher severity of exposure, the provided cover thickness is also more. However to meet the crack width requirements, the overall cross-section increases.

Thus what should be the base cover thickness for working out crack width? If 25mm is the basis as per SP24, ( i.e.0.1mm crack for 25mm ccover) then for 75mm thk cover , crack width of 0.3mm should serve the purpose?
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nimish.khanolkar
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PostPosted: Wed Jul 08, 2015 11:48 am    Post subject: Reply with quote

I think you can safely remove earthquake loads from crack width considerations, irrespective of where the section is placed.

For the simple reason that the structure is not expected to remain elastic even under the serveceability earthquake of 1 EQ, as you must have applied a response reduction of 3 atleast!

If the structure is responding inelastically, after yielding, the crack width theory no longer applies, as the mechanics are different.

For your other query, please follow cover requirements from durability. A higher cover places steel bars further away from the edge, and results in a larger crack width. Hence, place the bars as close to the edge as the appropriate durability and fire requirements permit.

There are different approaches to calculate crack width, but they do have a common thread. The common thread is stress in steel. All approaches, whether ACI 350, or EN, or IS first calculate stress in steel, and then via empirical equations(ACI uses GErgely Lutz), this is related to crack-width.
For our latest and consistent approach, id recommend IS 3370 for liquid retaining structures which is consistent with intl codes.
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Anand _Ekbote
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PostPosted: Thu Jul 09, 2015 4:21 am    Post subject: Reply with quote

Thank you Er.Nimish.
You hit the point regarding the seismic load combinations.
Will review your other suggestions & comeback.
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haridevender
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PostPosted: Wed Jan 20, 2021 7:19 am    Post subject: Load combinations required for crack width calculation Reply with quote

Dear All

In continuation to above replies, can anybody let us know any further recent developments mentioned in technical papers or codes regarding the load combinations to be considered for crack width calculations in RC elements.

Whether we shall use only DL+LL combination or we shall include lateral loads due to wind or Earthquake?
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mrbabu77
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PostPosted: Thu Jan 21, 2021 12:38 pm    Post subject: Crack width Reply with quote

Dear Anand,
Following is my view on the queries on crack width-
1-Is it mandatory to meet the provisions of cl 35.3.2 of IS 456  under combinations with seismic or wind loads?
Simple answer is yes, because we have to follow IS 456-2000 for design. However as Engineers we need to fully understand the provisions of the Code and its implications on the design. For estimating crack width, the load combinations shown in table 18 shall be used. In this table, for dead and imposed load combo, partial safety factor of 1.0 is used. This means that we have to apply full characteristic load when checking crack width (or serviceability). Recognizing the fact that the characteristic value is the maximum value of load, we can reduce partial safety factors when combining with other effects such as wind or seismic. That is why we have 0.8 factors with imposed and wind as per table 18.
Now in my view, we should not be considering wind and seismic loads when estimating crack width. Main reason is that these loads are transient (not static) and mechanics are different. Seismic is life safety issue rather than serviceability issue. Wind load based on 50 year return period is again based on life safety. In a case where we have to use wind load for serviceability checks then the return periods shall be reduced to 10 years.
I would like to draw your attention to footnote 2 of table 18. This states that the load combos given in table 18 is for SHORT TERM effects. The Code further states that " while assessing the long term effects due to creep of the dead load and that part of the live loads likely to be permanent may only be considered". The Code is silent on how to calculate long term effects and what portions of the live load to be considered etc. For guidance we can use ACI 318 or Eurocode code for  assessing creep effects. The sustained loads (part of live load that is permanent) requires engineering judgement and is usually taken as 20 to 40% of the live load. If the live loads are permanent in nature -eg-ware houses- full live load shall be used.
Apart from the direct load effects, there are other effects to be considered for crack width estimation- early thermal cracking and temperature effects. These also to be estimated, and combined with load effects. IS code is not providing any advise on this.
2-If protective coating is done on the concrete surface, will it help to relax on the crack width requirements?
Protective coating on concrete surface surely help in mitigating crack width requirements. However in my experience no Code/authority is allowing us to relax crack width requirements. The protective coating is thus considered as second line of defense.
The crack width requirements are actually to be correlated with tables 3&4 of IS 456-2000. Unfortunately this is not available in the present Code and hope that future revisions will address crack width requirements more rigorously.

Regards,

Babu M Raghavan
Chartered Structural Engineer
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