By: Javier Encinas, PE | April 4, 2017

Shear lugs are steel elements welded to the underside of base plates to resist shear loads. The design of shear lugs is covered by the ACI 349 anchorage provisions. This post is an overview of the structural design process. Our software ASDIP STEEL will be used to support our discussion.

How do you calculate the shear force in a shear lug?

The lateral forces acting on a structure will produce a horizontal reaction at the foundation level. For steel frames supported on base plates, a small horizontal force can be resisted by the friction between the plate and the underlying concrete. However, as the shear force increases, the remaining portion of the load will be transferred directly to the shear lug which is bearing laterally against the concrete support, as shown below.

shear-lug-plan-and-elevation

What are the failure modes in a shear lug?

Once the shear force has been calculated, the shear lug should be checked for the following failure modes:

shear-lug-failure-modes
  • Steel Failure – This is a measure of the capacity of the shear lug material itself. The shear and bending capacities should be evaluated by calculations based on the properties of the material and the physical dimensions of the lug. Typically the shear lug is a plate, but it’s common to see a HSS in case of biaxial shear.
  • Concrete Bearing – The shear lug will apply a lateral bearing pressure against the concrete support. This pressure should be smaller than the concrete bearing strength.
  • Concrete Breakout – The design shear strength is determined based on a uniform tensile stress of 4φ√f’c acting on an effective stress area defined by projecting a 45-degree plane from bearing edges of the shear lug to the free surface. The bearing area of the shear lug shall be excluded from the projected area.

ASDIP STEEL accurately calculates, for any support conditions, the breakout area Avc, and provides a graphic view, as shown below.

shear-lug-breakout-area-avc

The calculation of the breakout failure mode in shear is particularly important since a concrete failure would be non-ductile, and therefore it should be avoided. To prevent this kind of failure, the Code allows the use of reinforcing steel across the failure surface. This anchor reinforcement, however, must be designed and detailed carefully so that the strength of the rebars can be developed at both sides of the failure surface.

Are there any additional dimensional requirements?

The grout pad between the base plate and pier top is considered to be ineffective for transfer of shear. In addition, as a good practice, the shear lug should be embedded a minimum of 2 in (5 cm) into the concrete pier. Therefore the minimum lug height equals grout thickness + 2″. The required notch at the top of the concrete support is sometimes seen as a construction problem, so well detailed drawings are necessary.

The weld of the shear lug to the base plate should be sized for the design loads, and to comply with the minimum requirements of the AISC. The image below shows the typical shear lug calculations by ASDIP STEEL.

shear-lug-calculations

Detailed information is available about this structural engineering software by visiting ASDIP STEEL. You are invited to download a Free 15-Day Software Trial or go ahead and Place Your Order.

Best regards,

Javier Encinas, PE
ASDIP Structural Software

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