Recently I posted an article about whether a pile cap is required for a single pile foundation (you can see the article via THIS link). This has prompted me to think about other redundancies I often see in foundations which some engineers are unsure are required or not. Another such example is… is top reinforcement required in isolated footings?
Top reinforcement is generally not required in isolated footings (or pads) unless tension in the top surface of the footing is present. There are allowances for this in most concrete codes around the world.
To discover how to properly design and detail an isolated footing for full code compliance take a look at THIS article. I also provide a design spreadsheet for isolated pad footing design at the same link.
In this article we will go through the code requirements and identify instances where top reinforcement may or may not be required in your isolated footing. Before that, lets take a look at how an isolated footing behaves…
How does an Isolated Footing Work
Isolated footings are also often referred to as a spread footings or pad footings. An isolated footing spreads the load from a single column or wall over a given surface area therefore reducing the applied stress on the founding support soil. The less the bearing capacity of the soil, the larger the area of the isolated footing needs to be.
Think of snow shoes… ski goers wear snow shoes which prevents them from sinking into the snow as they walk. The snow shoes are usually flat and wide with a footprint much larger than your regular shoe size. This larger surface area spreads your weight across a larger area of snow allowing it to better support you.
The isolated footing uses bending and shear to spread the load from the column or wall it supports. The image below shows a cross-section of this behaviour…
Provided that the isolated footing supports a single column/wall which has a continuous net downward force, the bottom face is always in tension while the top face is always in compression. Under this condition, top reinforcement is not required in isolated footings…
When do you Need top Reinforcement in an Isolated Footing?
Now that we have covered a little on how an isolated footing works, lets look at some scenarios where reinforcement is required in the top layer of isolated footings.
Top reinforcement is required in Isolated Footings with Shear Reinforcement
The thickness of your footing and the loading applied to it may require you to introduce shear reinforcement (often called ligatures, stirrups or fitments depending on where you are located). If this is the case, you will require top reinforcement for the top hook of the shear reinforcement to wrap around. Shear reinforcement is only effective when it works in conjunction with reinforcement in the top and bottom layers.
You may have very good reasons to proportion your isolated footing to require shear ligatures however it is highly recommend it avoid it if you can for the following reasons…
- The cost comparison between shear reinforcement versus a slightly thicker footing usually ends up cheaper with a thicker footing (slightly more concrete volume and excavation but less steel). Therefore deepening the footing to gain more shear capacity out of the concrete component is an option that should be explored.
- Introducing shear reinforcement in your pad foundation doesn’t just add additional steel due to the shear ligatures themselves but also the additional top layer of reinforcement which you may not otherwise have needed.
- Introducing shear ligatures will prevent the reinforcement cage for the footing being fabricated and lowered into place in one piece. It is very difficult to fabricate the top, bottom and shear reinforcement in a single unit and install it on-site. Therefore you aren’t just adding material costs but also time and labour costs.
Top Reinforcement is required where Tension exists in the top face of the Footing.
Of course if you are experiencing tension in the top face of your isolated pad footing, this is a case where reinforcement will be required. This is determined during your analysis and is dependant on what elements your footing supports.
If your isolated footing supports multiple columns for example, you may develop tension in the top face in the zone between the columns due to moment reversal. This would then trigger the requirement to add top reinforcement…
A great way to check these types of footings supporting multiped columns or walls is with an FEA software like RAM Concept. To see a step-by-step guide on how to design footings/foundations using RAM Concept, take a look at THIS article.
What about Minimum Reinforcement Requirements in Isolated Footings?
There is a bit of a journey to go through in the Australian concrete Code AS 3600 to confirm that reinforcement is not required in the top surface of your isolated footing. Lets run through this journey together (other concrete codes are very similar)…
The first station of the journey is to refer to Chapter 21 which is “Slab-on-Ground floors, pavements and footings”. This is a very short chapter (one page). Clause 21.3.1 refers us to chapter 9 to design and detail a reinforced footing (this is the slabs chapter in AS3600).
Now we arrive at Chapter 9 (design of slabs) in AS3600 and refer to the crack control section (this generally sets out the minimum reinforcement requirements for crack control in slabs). This clause is found in section 9.5.1
The most important part of this clause which allows removal of top reinforcement in your isolated footing is highlighted in yellow above. This first sentence outlines the intent of the crack control section of the code…
Cracking shall be limited to an extent that does not impair the durability or serviceability of the slab, both in terms of function and appearance.
Lets identify the key points that this sentence outlines and discuss the requirements when it relates to a footing application (not a slab):
- Appearance (serviceability): One requirement for serviceability is appearance. If a slab is cracked and visible to the general public, this may cause alarm and be an indication that the structure is not performing adequately. This may also not be in line with the architectural intent of the structure. In the case of a buried isolated footing however, no one is likely to see if the footing is cracked or not.
- Function: If shrinkage cracking occurred in the footing during construction, these cracks would soon close up when the footing is loaded due to the top surface remaining in compression and the bottom surface remaining in tension.
- Durability: Similarly with the comments above, any top surface cracks will close up as the footing is loaded. Also, since we are concerned with protecting the bottom reinforcement from corrosion (the bars which are contributing to the bending strength of the footing) the cover from these bars to the top of footing will be more than adequate for corrosion protection in accordance with the code.
The clause then goes on to apply two requirements (a) and (b) that need to be satisfied. Item (b) is self explanatory, item (a) refers back to clause 9.1.1…
This clause sends you further back to chapter 8 (design of beams). The only clause between 8.1.1 and 8.1.8 which influences minimum strength requirements is clause 8.1.6. This clause requires a little bit of interpretation to justify the top reinforcement away from an isolate footing…
The first sentence starts off by mentioning “critical sections”. By Structural Engineering principles, the top layer of reinforcement in an isolated footing is not considered a critical tension if it is constantly in compression.
The clause finishes off by saying that the clause can be totally waved if it can be demonstrated that:
- Not satisfying this clause will not lead to sudden collapse
- Not satisfying this clause will not lead to reduced collapse load
On both accounts, not providing a top layer of reinforcing steel in an isolated footing with top layer in compression will not cause sudden collapse nor reduce the collapse load. This clause would definitely need to be satisfied for the bottom reinforcement requirements however!!
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