Shear lugs are steel elements welded to the underside of base plates to resist shear loads. ASDIP STEEL is a structural engineering software for the design of steel members. It includes the design of shear lugs based on the ACI 349 anchorage provisions. This document is a step-by-step shear lug design example using ASDIP STEEL software.
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Shear lug design example
As an example, consider the base plate shown below, which supports a column in an industrial building. The column is not heavily loaded but it's subject to the action of lateral loads. The column is exposed to dead, live, and wind loads, whose reactions are shown below. Design the shear lug.
Enter the given geometric dimensions of the base plate and pedestal, as well as the applied loads. In this case, the shear load is high enough to justify the use of a shear lug under the plate. Usually the anchor rods are in tension and their capacity in shear is limited, but for simplicity in this example let's assume a downward load without moment.
The grout under the base plate is considered to be ineffective for transfer of shear, therefore this should be taken into consideration when calculating the bearing stresses. In this case the grout thickness is 1-1/2".
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For illustration purposes the design will follow the LRFD methodology. The controlling load combination is 0.9D+W since the shear will be resisted by a combination of friction + shear lug, and the friction force depends on the vertical load. Small shear forces 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. In this case the maximum shear resisted by friction is 13.5 kips, therefore the shear lug will be designed for the remaining load 50 - 13.5 = 36.5 kips.
The design of the shear lug itself implies checking the limit states of steel bending, steel shear, weld capacity, concrete bearing, and concrete breakout. The shear lug dimensions must be selected in such a way that all these failure modes are covered. The concrete breakout failure mode is particularly important since a concrete failure would be non-ductile, and it should be avoided if possible.
Note that the higher the lug the larger the bending moment it must resist, so the designer has to play with the numbers to get a good balance. In this case a 4" deep shear lug will keep the bearing stresses under the limits, which means that 2-1/2" are effective after deducting the grout. The lug thickness will be governed by the shear and moment requirements, and it should be specified in increments of 1/4". The bending moment of 100 k-in can be resisted by a 1-1/4" thick shear lug. Based on this lug thickness the fillet weld size is specified as 5/16" to comply with the minimum weld size per AISC. The Condensed tab shows a more complete set of results grouped by topic, great for a more detailed overview of your design.
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Once the steel failure modes are checked, we check the concrete breakout based on a uniform tensile stress of 4φ√f’c acting on an effective stress area defined by projecting a 45-degree plane, and excluding the bearing area of the shear lug. For irregular concrete supports this calculation may be tricky and time-consuming. ASDIP STEEL calculates the breakout area and shows graphically the meaning of the calculation, as shown below. For the data of this example the breakout area Avc=308 in2.
At the end of the design, we can see that the design ratio is 0.79, and the controlling failure mode is the steel bending, which produces a ductile design, therefore the design is satisfactory as shown.
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ASDIP STEEL includes the design of shear lugs, with multiple options to optimize the design easily. The design of base plates with shear lugs may be cumbersome and error-prone. This shear lug design example shows that the task can be completed and optimized within minutes.
For engineering background, please read the blog post Shear Lug Design: Overview of the ACI Provisions. For our collection of blog posts about base plate and anchorage design please visit Anchor Rods Design.
Javier Encinas, PE
ASDIP Structural Software