2021 SP3: Steel column base plate design - USA head code - design for shear lugs added

Tekla Structural Designer
2021
Tekla Structural Designer

2021 SP3: Steel column base plate design - USA head code - design for shear lugs added

The scope of Steel Column Base Plate Design checks is further enhanced in this release for the USA Head Code with the option to specify Shear Lugs to resist shear loads. When specified shear lugs are designed for the following loading conditions and associated checks:
Applied Load Conditions New / Enhanced Checks
(Major) shear load with or without negative or positive vertical load

Shear

  • Concrete bearing strength of shear lug
  • Concrete breakout strength of shear lug
  • Shear lug bending strength
Negative vertical load with or without major shear load*

Rods and Anchorage:

  • Shear lug dimensional checks
  • Rods in Tension
  • Concrete Breakout (in Tension)
  • Concrete Pull Out (in Tension)

*The new/ enhanced Rods and Anchorage checks only apply when the model Resistance Code for Concrete Design is set to ACI 318-2019.

  • The picture below shows how the new/ enhanced checks appear in the base plate connection design details dialog.

    • Note that shear lug welds are not currently considered in the model or the current weld checks.


  • To specify a shear lug, first select one of the new Horizontal Shear Transfer options “Bearing on Shear Lug” or “Friction and Bearing on Shear Lug”. The dimensions of the shear lug can then be specified and it will be displayed in the base plate connection object, as shown in the picture below.

Below are briefly listed some notable aspects of the new and enhanced checks together with pictures showing examples of the reported check details in the column base connection design results dialog. For fuller details, please see the updated Help Topic Column base plate design to AISC 360.

  • Shear:

    • The checks for the Shear Lug are divided into three separate parts and consider the full (major) shear force: 1) Concrete bearing strength of shear lug, 2) Concrete breakout strength of shear lug and 3) Shear lug bending strength.

      • The checks undertaken are ostensibly the same for both options “Bearing on Shear Lug” or “Friction and Bearing on Shear Lug”, the principal difference for the latter option being that the checks consider a net shear force i.e. the remaining (major) shear force not taken by frictional resistance alone.

        • The check details for this option thus include an additional Friction calculation, at the start of each of the three Bearing on Shear Lug parts, that derives the net (major) shear force.

      • All three separate parts vary in design approach according to the ACI 318 design code year selected in the model, with demarcation between pre-2019 (i.e. 2008, 2011 or 2014) and 2019.

        • When ACI 318 pre-2019 is selected, the concrete strength checks are calculated in accordance with guidance from AISC Design Guide 1 (American Institute of Steel Construction. Design Guide 1: Base Plate and Anchor Rod Design (2nd Edition, 2nd Printing, revised). AISC, March 2014) and Gomez et al (Gomez, I.R., et al. Shear Transfer in Exposed Column Base Plates. AISC, 2009). Otherwise calculations are in accordance with ACI 318-19. Since concrete design only takes account of LRFD requirements, these concrete strength checks are not performed for ASD load combinations.

    • Concrete Bearing Strength of Shear Lug

      • When ACI 318 pre-2019 is selected this check is carried out under AISC DG1 guidance, which references ACI 349 Appendix Section D.4.6.2. The nominal concrete bearing strength is based on an embedded depth of shear lug equal to actual depth of lug less the grout pad thickness.

      • When ACI 318 2019 is selected, this check is carried out in accordance with the provisions of ACI 318-19 Section 17.11.2, and the nominal concrete bearing strength is based on an effective embedded depth of shear lug equal to the lesser of the embedded depth of lug and twice the lug thickness. The 318-19 design method also includes a bearing factor, Ψbrg,sL, which varies between 1.0 and 2.0 for applied axial compression, and between zero and 1.0 for applied axial tension.

      • For all ACI 318 code years, if the embedded depth of the shear lug is less than 2 inches (or 50 mm in metric design) then a Warning is displayed in the results viewer.

      • The picture below shows an example of the details for this check displayed in the design details dialog.
        • Note that this is for ACI-14 and that results and details for ACI-19 can be quite different.

    • Concrete Breakout Strength of Shear Lug

      • When ACI 318 pre-2019 is selected this check is carried out to recommendations by Gomez et al whose research report was presented to AISC in 2009. The research report recommends the nominal concrete breakout strength of shear lug is based on the minimum of that found from a 35 degree fracture failure plane with a design equation adapted from a Concrete Capacity Design (CCD) method given by Fuchs et al (Fuchs, W., et al. Concrete Capacity Design (CCD) Approach for Fastening to Concrete. ACI Structural Journal, V. 92, No. 1, Jan.-Feb. 1995, pp. 73-94), and that found from a 45 degree tension failure plane, per ACI 349 Appendix Section D.11.2.

      • When ACI 318 2019 is selected, this check is carried out in accordance with the provisions of ACI 318-19 Section 17.11.3, and the nominal concrete breakout strength of shear lug is based on an effective area of shear lug and a 35 degree fracture failure plane. Note in the calculation of the nominal concrete breakout strength of shear lug, Vcb,sL, the modification factor for cracked concrete, Ψc,V is set to 1.0 and only shear perpendicular to the concrete edge is considered.

      • The picture below shows an example of the details for this check displayed in the design details dialog.

        • Note that this is for ACI-14 and that results and details for ACI-19 can be quite different.
    • Shear Lug Bending Strength

      • The shear lug bending strength is calculated in accordance with AISC 360 (American Institute of Steel Construction. ANSI/AISC 360-05. -10, -16 Specification for structural steel buildings. AISC, 2005, 2010, 2016) and AISC Design Guide 1. AISC, March 2014) for both LRFD and ASD load combinations. The choice of ACI 318 design code year determines only the lever arm distance for bending.

        • When ACI 318 pre-2019 is selected the lever arm for bending of the lug is taken as half the embedded depth of the lug plus the grout pad thickness.

        • When ACI 318 2019 is selected the lever arm for bending of the lug is taken as half the effective embedded depth of the lug plus the grout pad thickness.

        • For all ACI 318 code years, following the guidance of AISC DG1 Section 3.5.2, if the thickness of the shear lug is greater than the thickness of the base plate then a Warning is displayed in the results viewer.

      • The picture below shows an example of the details for this check displayed in the design details dialog.
        • Note that this is for ACI-14 and that results and details for ACI-19 can be quite different.
  • Rods and Anchorage:

    • Shear Lug Dimension Check

      • The shear lug dimensional checks are calculated in accordance with ACI 318 when the following conditions apply; negative vertical load (i.e. axial tension / uplift) is present; the ACI 318 code year selected is 2019.

        • Since the presence of a shear lug can influence anchor rod tension, the dimensional checks are performed for both LRFD and ASD load combinations.

        • The checks are calculated using the provisions of ACI 318-19 Section 17.11.1.1.8


    • Rods in Tension, Concrete Breakout in Tension and Concrete Pullout in Tension checks

      • Again this applies only when the following conditions apply; negative vertical load (i.e. axial tension / uplift) is present; the ACI 318 code year selected is 2019. In this case, for all these checks there are additional calculations (reported in the design details) to determine the additional tension load due to forces on the lug, per the requirement of ACI 318-19 Section 17.11.1.1.9 (reference was also made to Section 3.5 of Cook and Michler (Cook, R.A., and Michler, H. Behavior and Design of Anchorages with Shear Lugs. 3rd International Symposium on Connections between Steel and Concrete, 2017, pp. 560-571).)

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