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Cracking Stress Reported per AASHTO 5.7.3.4. Calc for Spacing Required?

Brandon Neill
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I need to check that the Service III stress for the bent cap is under 24 ksi.

From RC-Pier's Cap Design Report, I get the following:

CRACKING/FATIGUE CHECK:
=======================
Cracking check as per AASHTO LRFD (6th Edition, 2012)

...

                           Span 9: From 66.00   ft  To 75.00   ft
 
                      <------ Cracking ------>     <------ Fatigue ------>
 Loc    AbsLoc   H   Comb   fs-t  dc  Srqt  Sprt   fs-t   ratio fs-t  Comb
 Loc    AbsLoc   H   Comb   fs-b  dc  Srqb  Sprb   fs-b   ratio fs-b  Comb
  ft      ft     in         ksi   in   in    in    ksi
------------------------------------------------------------------------------
  0.00   66.0   42.0   8   18.1    4.1  25.2   6.8    0.0     0.00        0
                       0    0.0    4.3  48.0  13.5    0.0     0.00        0
  1.00   67.0   42.0   8   11.6    4.1  43.9   6.8    0.0     0.00        0
                       0    0.0    4.0  48.0   4.6    0.0     0.00        0
  2.75   68.8   42.0   8    0.5    4.1  48.0   6.6    0.0     0.00        0
                       1    1.7    4.0  48.0   4.4    0.0     0.00        0
  6.25   72.3   42.0   0    0.0    4.1  48.0   6.6    0.0     0.00        0
                       1   24.8    4.0  16.6   4.4    0.0     0.00        0
  6.73   72.7   42.0   0    0.0    4.1  48.0   6.6    0.0     0.00        0
                       1   29.1    4.0  12.9   4.4    0.0     0.00        0
  8.00   74.0   42.0   0    0.0    4.1  48.0   6.8    0.0     0.00        0
                       1   25.1    4.0  16.3   4.6    0.0     0.00        0
  9.00   75.0   42.0   1    1.0    4.1  48.0   6.8    0.0     0.00        0
                       0    0.0    4.3  48.0  13.5    0.0     0.00        0

I understand that the 29.1 stress is based upon the spacing required per AASHTO 5.7.3.4, but I'm not understanding where the required spacing comes from.  Is the reported stress a function of the required spacing or vice versa?  Is the stress reported just a limit, per AASHTO 5.7.3.4?  How is RC-Pier determining the required spacing?

  • 0 Offline
    Generally speaking At different locations along your cap bent you can see the spacing required decreasing when stress is increasing.
    The formula used for required spacing will decrease spacing for increasing stress until eventually spacing will become a negative value.

    Please check the AASHTO eqn. 5.7.3.4-1 where in it makes it quite clear that the spacing and stresses are interrelated.
    RCPier is determining the required spacing based on AASHTO eqn. 5.7.3.4-1
  • 0 Offline in reply to Vinay mysore
    So, RC-Pier finds the spacing based upon the fss in AASHTO Eq. 5.7.3.4-1. and not vice-versa. Is the fss calculated from fs = Ms/(As j d)? When the area of steel is 4~#11 + 6~#6 = 8.88 in2, the stress is 29.1 ksi with a Srqb = 12.9". When the rebar is changed to 7~#10 = 8.89 in2, the stress is 32.3 ksi with a Srqb = 10.0". How can the stress change so much with just a slight change in reinforcement? Is the stress found per bar? Thanks.
  • 0 in reply to Brandon Neill

    Even though this is an old request, I also am looking for the equations RCPier uses for calculating fss.

  • 0 Offline in reply to Brandon Neill

    i'm trying to give a generic answer but if you are still unable to find the right information then please do let me know I shall further look into it.

    Once you have designed the Pier cap then please click on PRINT and then click on Detailed Design Report.

    Then select the required Report you want to generate and then click SUBMIT.

    Then at every POI the program will show the equation, Article number and units the program is using to calculate the Spacing.

    I hope it helps.

    Please see the attached screen shot.

  • 0 in reply to Vinay mysore

    Thank you for the screenshots. This is very nearly what I am looking for. However, I can't see in the design report what the equation is for fss. 

    I have the same question as Brandon, which is:

    Brandon Neill said:
    Is the fss calculated from fs = Ms/(As j d)?

    If that is the case, is it possible to see the values for Ms, As, j, and d that result in 1.49 (from your example). I can understand that answering in a broad sense helps the most people, but in this case, I am wondering what the specific values in the equation for fss, as well as how (and where) the program chooses those values. Could you send a screenshot of the Analysis tables for Service Load State 3, if the result is pulled from there, or the moment diagram?

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