How is the single bolt shear capacity P_{ss} calculated in a BS column base connection?

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Tekla Connection Designer

Question

For the BS design code, how is the single bolt shear capacity P_{ss} (for compression) or P_{tt} (for tension) calculated in a column base connection?

Answer

The program follows the guidance of the SCI Moment Connections "Green Book" for British Standards Page 97.

For bolts set as Non-adjustable, Column Base will use 6d2fcu to calculate the capacity for the bolt bearing on concrete for the compression case.

For the concrete strength fcu, the minimum of that of the concrete base and grout is used.   

For the Adjustable bolts option, the factors set in Column Base for the bearing length ("Factor * d") and the concrete strength ("Factor * fcu") are used.   The defaults for both of these factors are set = 1.0, therefore the bolt bearing capacity on concrete will by default be d2fcu. Both formulas are derived below.

Tension

Note that for the tension case, following the SCI Moment Connections "Green Book" for British Standards, the shear capacity is the lesser of the bolt bearing on concrete value and 0.4* the bolt shear capacity = 0.4*ps*As and the latter will commonly govern.

Non-Adjustable bolts

  • Bearing length of bolt in concrete = 3*d
  • Average bearing stress = 2*fcu
  • Therefore bearing area of bolt in concrete As= 3*d*d = 3d2
  • Pss = (3*d2)*(2*fcu) = 6*d2*fcu

Known Issue

Please note that there is a further issue when using Non-Adjustable bolts that requires a workaround to obtain correct results.  Currently the second "Factor * fcu" value under the option Horizontal Shear Transfer > "Friction and Bearing on Shear key" is used to calculate the concrete bearing stress for the the shear check, even when this option is not selected.  This is shown in the picture at the bottom of the article.  Since the default for this factor is 0.8, this results in a very low bolt shear capacity. 

To workaround this issue do as follows:

  1. Edit the connection and select the "Bolts" page
  2. Enable the option "Friction and Bearing on Shear key"
  3. Set the value of "Factor * fcu" = 2.0
  4. Re-enable the previous Horizontal Shear Transfer option and OK

Adjustable

  • Bolt bearing length = bearing length factor*d
  • Bearing stress = bearing stress factor*fcu
  • Pss = bearing length factor * bearing stress factor * d2fcu

 

Image
TCD-BS-wrong-factor-fcu
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