Effective thickness of Grouted Joint in Fatigue Analysis

Dear Sir,

I'm running a spectral fatigue analysis for a platform with grouted legs. These are my section input for my joint cans at leg

SECT LEGCAN    TUB                               111.765.08091.4403.810

SECT LEGCAN1   TUB                               111.765.50091.4403.810

SECT LEGCAN2   TUB                               111.766.00091.4403.810

Below are my input in fatigue input file that I use ISO 19902 for the effective thickness calculation for grout.

FTOPT 7    1.0           1.0  SMIJ2  SK  MNSK                            K LPEFT 

FTOPT2   PTPTVC    PV    AP      2.0   -3.0    5.5   -69.70069.7000  31.75

SCFLM       1.5  

However, after I ran the analysis and check the fatigue output listing file, the results shows that it consider the thickness of outer member which are 5.08mm, 5.5cm and 6.0cm only. 

From my calculation of Te = (T^2 + Tp^2)^0.5 in formula 14.5-1, it should be 6.35cm, 6.69cm and 7.107cm.

Please help to let me know it there any input I missed for grouted joint, or there is other consideration that not allow me to consider effective thickness of grout. I can share my input file for further discussion if it's required.

Thanks,

Best Regards,

Pei Shing

  • Hi,

    After I checked the manual, I found that I missed G in column no.6 of FTOPT, so now the fatigue input become

    FTOPTG7    1.0           1.0  SMIJ2  SK  MNSK                            K LPEFT  

    However, what surprise me is the results all totally same except the chord thickness has been changed. Please see below one of my joint result.

    With G

    1306 1233-1306 H04  TUB  106.70  5.000 Y   BRC  12.00         3.50  1.70  2.85  1.50   .25724-2  T   388.7482

    1306 1230-1306 L14  TUB  111.76  9.464 Y   CHD  12.00         4.94  2.19  1.68  1.95   .95773-2  T   104.4136

     Without G

    1306 1233-1306 H04  TUB  106.70  5.000 Y   BRC  12.00         3.50  1.70  2.85  1.50   .25724-2  T   388.7482

    1306 1230-1306 L14  TUB  111.76  6.000 Y   CHD  12.00         4.94  2.19  1.68  1.95   .95773-2  T   104.4136

    Is my analysis correct? It's a bit weird when the result is no different with or without consideration of grout effective thickness.

    Appreciate your help and advice. 

    Thanks,

    Pei Shing

  • Hi Pei Shing Ooi

    As my understanding, option G in FTOPT is applicable for 'Single Skin Grouted' which is the member fully filled with grouted concrete.

    However, your case is 'Double Skin Grouted' which is one member (pile) inserted inside another member (sleeve) and grouted in between those skin. 

    As per the code, for 'Fatigue Analysis'

    Double Skin Grouted - It is recommended in both ISO and DNV, however, it is not mentioned in API

    - Effective Grouted Thk = Toutside + 0.45*Tinside

    Single Skin Grouted - It is recommended in both ISO and DNV, however, in API, the equation is found in Section B.8.3.4. Nevertheless, it is not clear in API that it will be applied to Single or Double Skin. 

    - Effective Grouted Thk = (5*Doutside + 134*Toutside) / 144 = 0.0035*Doutside + 0.93*Toutside  -- > This shows in API

    Your Equation Te = (T^2 + Tp^2)^0.5 is applicable for 'Strength Check of Double Skin Grouted'. Then, do not confuse with 'Fatigue Check'. You cannot use effective thickness equation recommended for strength analysis to use with fatigue analysis.

    As a result, when you use G in FTOPT, you will found that effective grout thickness is followed 'Single Skin Grout' which is (5*111.76 + 134*6) / 144 = 9.46cm which is not corrected as you wish.

    Your effective thk should be 6+0.45*3.81 = 7.71cm which is 'Double Skin Grouted'

    I would suggest to use 'JNTOVR' to override chord thickness to be 7.71cm rather than use those option G.

    For the result of option G, you show that it is the same for with and without G even the chord thickness is changed i.e. 6cm vs 9.464cm. I need the answer from Bentley as well !!

    Regards,

    Kasiphon

    Answer Verified By: Pei Shing Ooi 

  • Hi Kasiphon,

    Thank you very much for your explanation in details.

    Could I know from which ISO standard and section you get this formula for effective thickness? 

    "Double Skin Grouted - It is recommended in both ISO and DNV, however, it is not mentioned in API

    - Effective Grouted Thk = Toutside + 0.45*Tinside"

    My client accept ISO standard. I tried to search around ISO 19902:2007 but cannot get it.

    As per recommended by you I removed G and added this line to my ftginp file

    JNTOVR 1306                      7.71 

    and this is the result I get

    1306 1233-1306 H04  TUB  106.70  5.000 Y   BRC  12.00         3.40  1.61  2.66  1.50   .20203-2  T   494.9716

     1306 1230-1306 L14  TUB  111.76  7.710 Y   CHD  12.00         3.75  1.60  1.50  1.50   .38179-2  T   261.9239

    I actually did a sensitivity check with 1 to 5 input in FTOPT2 column 70, it seems that when the input is 3(ISO) or 5 (API), I get similar result with when there is no G, except the chord thickness changed. For other cases, the results changed but not as good as compared to use JNTOVR, even the thickness are thicker.

    As what you said, in FTOPT2 line the column 70 and 71-74 is actually not function at all for double skin grouted? Are you engineer in Bentley, I think I need to raise this in service ticket so Bentley is aware about it.

    From our discussion, I believe it's better to use JNTOVR as recommended by you instead of using the G and column 70 in FTOPT2, however I need to convince my client that SACS having error on this first, so I really need the ISO to show them.

    Thanks again for your help.

    Best Regards,

    Pei Shing

  • Hi

    Actually, I read this long time ago. Then, I just looked in the code again.

    I also could not found T+0.45Tp in ISO19902. However, this equation is very clear in DNVGL-RP-C203, see picture below.

    You will found that the equation (5D + 134T) / 144 shown in ISO and API was mentioned clearly that it is used for 'Single Skin Grouted'.

    Read the link below for more information

    https://www.google.nl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&uact=8&ved=0ahUKEwiAp7a7xIPWAhWNalAKHZ0rCjkQFghEMAg&url=http%3A%2F%2Fscholarbank.nus.sg%2Fbitstream%2Fhandle%2F10635%2F20901%2FShenwei.pdf%3Fsequence%3D1&usg=AFQjCNH0bQZpDkbksM-283sMUVEkBJLEuQ

    Btw, I am just user and not bentley staff.

    The weird result of G option should be raised to Bentley.

    Regards,

    Kasiphon

    Answer Verified By: Pei Shing Ooi 

  • Hi Kasiphon,

    Yes, I also found that in DNV. As compared to formula (A.16.10-4) in ISO, I think it should be sufficient to explain to my client about this. I will also tried to get justification from Bentley with service ticket. I think your explanation is better, faster and more accurate than Bentley engineer sometime, you might send your CV to Bentley and request a high pay from them...haha...

    Thanks again for your help, it really save my time a lot for my work.

    Best Regards,

    Pei Shing