Shear lugs are steel elements welded to the underside of base plates to resist shear loads. The design of shear lugs is now included in the ACI 318-19 anchorage provisions, with some important modifications to the old guidelines in ACI 349.2R-07. This post is an overview of the current structural design process. Our software ASDIP STEEL will be used to support our discussion.
For small to moderate shear loads, the anchor rods can be designed to resist the full load. However, for high shear loads the anchor rods may not be the best solution, since the anchor rods are normally in tension and the tension-shear interaction may affect adversely the capacity to resist shear. In those cases the use of shear lugs is justified.
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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.
It should be noted that the anchor rods will not resist any portion of the shear load, unless the washers are welded to the base plate. This is to account for the fact the base plates are fabricated with oversize holes, and it's unlikely the the anchor rods bear laterally against the plate.
What are the failure modes in a shear lug design?
Once the shear force has been calculated, the shear lug should be checked for the following 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 HSS sections in case of biaxial shear.
- Concrete Bearing – The bearing depth is limited to two times the shear lug thickness. 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 an effective stress area defined by projecting a 35-degree plane from bearing edges of the shear lug to the free surface, both in plan and in elevation. The bearing area of the shear lug should be excluded from the projected area.
- Weld Strength – The shear lug will be welded to the underside of the base plate. The weld strength should be checked against the shear and bending reactions.
It should be noted that the sign of the applied shear becomes important for base plates that are placed eccentrically on the support, since the breakout area will be different in each direction. ASDIP STEEL accurately calculates, for any support conditions, the breakout area Avc in X and Z, and provides a graphic view as shown below.
The calculation of the breakout failure mode 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 shear transfer. In addition, as a good practice, the shear lug should be embedded a minimum of 2 in (5 cm) into the concrete support. 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.
The design of shear lugs may be time consuming since it includes the check a number of limit states per the ACI 318-19. ASDIP STEEL includes the design of shear lugs, with multiple options to optimize the design in less time.
For our collection of blog posts about base plate and anchorage design please visit Anchor Rods Design.
Javier Encinas, PE
ASDIP Structural Software