Structural Engineers have a number of options up their sleeves to enhance the punching shear capacity of a floor slab. This article will explore three common options, examine each and compare against one another to determine which is best for your project. So lets take a deep dive into a comparison of Shear Cap vs Drop Panel vs Column Capital.
Considering each one of these elements goal, in part, is to increase the punching shear capacity of a slab, lets first take a look at what punching shear is…
What is Punching Shear
Each concrete extrusion mentioned in our introduction (Shear Cap vs Drop Panel vs Column Capital), enhances a slabs punching shear capacity to some degree.
Punching shear is a phenomenon where a concentrated force on a slab causes a supported element or a supporting element to physically “punch” through the full depth of the slab. If the slab is not proportioned thick enough and/or the vertical concentrated load is too high, this results in punching shear failure.
Punching shear failure is a relatively sudden failure mechanism (with little warning of pending collapse) and is caused by in-sufficient slab thickness and/or in-sufficient shear reinforcing at the localise concentrated force.
The punching shear capacity of a slab can be increased by introducing one or more of the following elements to your slab design:
- Increasing the cross-sectional area of the concrete shear interface by increasing the slab thickness locally at the column.
- Increasing the cross-sectional area of the concrete shear interface by increasing the shear perimeter length (in plan view)
- Provide shear ligatures through either stud rails or enclosed ties locally at the column.
Punching shear is very similar in nature to one-way shear (which occurs predominantly in one-way slabs and beams). For an in-depth article exploring one-way shear in beams and a comparison between the previous and current versions of the Australian Concrete Design Code (AS3600-2009 and AS3600-2018 respectively), take a look at THIS article.
What is a Shear Cap
A shear cap is less common in Australia and is used more frequently in the United States as well as other regions…
A shear cap is a projection below the slab used to increase the slabs shear strength.
That was a direct quote from the US Concrete code ACI 318-08. A shear cap is constructed along-with the adjacent slab. ACI 318-08 provides guidance on the dimensional requirements of a shear cap…
Based on this proportioning, a shear cap may resemble the following form…
The primary purpose of shear cap is to increase the punching shear capacity of the slab at support columns. This is achieved through an increase in the shear perimeter length. The shear perimeter is the line in which the slab may fail due to punching shear. A longer shear perimeter length results in a larger concrete shear interface area which results in a higher overall shear capacity.
What is a Drop panel
A drop panel is similar in nature to a shear cap, however its plan dimension is larger.
A drop panel is a projection below the slab used to reduce the amount of negative reinforcement over a column or the minimum required slab thickness, and to increase the slab shear strength.
That was another quote taken straight out of ACI 318-08. You will now notice that a key difference between a Shear Cap vs Drop Panel is that a drop panel not only enhances the shear capacity of a slab but also allows for a reduction in negative reinforcement over a column and a reduction in thickness of the adjacent slab. For a discussion on how bending works in slabs, refer to THIS article which also goes into detail about post-tensioning within slabs.
The increased thickness over the column, over a larger plan area, increases the bending stiffness and therefore reduces the slabs deflection. This then allows the adjacent slab zones to be reduced.
ACI 318-08 also provides guidance on minimum dimensions for a drop panel…
Converting the guidance in clause 13.2.5. into a visual format, we achieve the following minimum arrangement…
What is a Column Capital
A column capital differs from a shear cap and a drop panel in that the column capital is poured integral with the support column rather than the slab.
A structural column capital is a widening of the top of a support column where it meets the soffit of a slab. The widening of the column produces a larger shear perimeter within the slab which increases its punching shear capacity. Structural column capitals are not to be mistaken with architectural column capitals which are more aesthetic in their application rather than providing structural enhancement.
When should a Shear Cap or Drop Panel or Column Capital be Used
Each option between Shear Cap vs Drop Panel vs Column Capital aims to increase a slabs shear capacity through modification of either the slab itself or the support column.
The question of which individual one to choose is a little less cut and dry and a little more project dependant…
- Shear Cap: Is ideal for projects where the only concern is shear capacity of the slab. The introduction of a shear cap does not add thickness over a large area so it allows ceiling heights to be increased. A shear cap allows larger areas with higher ceiling heights.
- Drop Panel: A drop panel is ideal for reduction of steel tonnage and improvement of punching shear capacity of the slab. Due to its enhancement of the slabs stiffness it generally allows an overall reduction in concrete volume as well. This is ideal in scenarios where your project is heavily cost driven.
- Column Capital: If a structural column capital can be dressed to also serve an architectural purpose this is the perfect solution to solve form and function. Depending on your region, modifications to the column head may be a more costly option. In Australia for example, column capitals are rarely used these days as it is deemed more efficient to provide a drop panel or stud rails.
Which is better, Shear Cap vs Drop Panel vs Column Capital
The table below compares the Shear Cap vs Drop Panel vs Column Capital against different assessment criteria
Behold the Ultimate Drop Panel and Column Capital Combination
One of my most favourite slab soffits within a car park structure is located right here in my home town of Melbourne. It is also an iconic film location which made an appearance in a great Aussie classic movie, Mad Max.
The location is Melbourne University in Parkville, Victoria. It is an extreme case of a column capital morphing into a drop panel. The end result is a quasi arch, column capital, drop panel arrangement. Built in 1971, its a great example of impressive form-work, especially considering the time it was constructed.