Residential Isolated Brick Pier Foundations (No Concrete Pad under) - AS2870

[Ben_StructEng]

Hi All,

I've recently migrated from Large commercial Structures to small residential structures and I have come across a few different properties recently where a sub-floor series of Brick piers supporting tiber bearers & Joists above are not founded on a concrete pad footing, but rather just embedded a couple of courses of brickwork onto direct strata.

I am told through word of mouth that this was typical in the 1950's and prior, and that provided you're not trying to add new loads/justify the capacity in anyway that it is still a compliant design due to the 'test of time' theory.

My Questions therefore are:

1. If I am designing for a new load path to these piers and need to assess the pier/foundation capacity, I am needing to do this to new codes, such as AS2870 which requires a min 200-300 deep concrete pad depending on the strata. Due to the fact that the pier was likely compliant back to old standards, am I able to justify the design for new loads based on the bearing applied from the surface area of Brickwork directly to strata? Or do I need to specify a new pad underneath to meet the AS2870 requirements?

2. If I have a scenario where there has been some soil disturbance due to water, or a tree root for example, which is undermining the brick pier due to soil erosion with no concrete pad under, am able to:
- provide a solution to mitigate those issues, and then, provided that there are no new load paths/design items to consider in terms of loading,
- Provide certification that the Pad footing is adequate (Even though it doesn't have a concrete footing in accordance with newer code requirements)

Thanks All. Hope that was clear enough!

Ben

 

[Enable]

We come across similar questions in the Canadian restoration industry quite frequently. Unfortunately, while in some situations it is obvious what one's obligation is, in most that is not the case. It's just the nature of the work when dealing with existing structures. I suspect Australia to be the same.

In general our approach is this:

1) When significantly modifying structures or replacing structural elements one has to bring things up to code as much as is "reasonable"
For Example:
a) Replacing an existing non-compliant stairwell off of an underground parking garage one has to make the new stairs comply with code even if the previous one did not.

b) When removing areas of slab to facilitate a new entrance into an underground parking garage one has to make sure that the new support / structure all complies even if the adjacent walls/columns/etc do not.

2) When performing a significant repair (even if not modifying the structure) one has to bring things up to code in the repaired area as much as is "reasonable"
For Example
a) When repairing several bays or more of a suspended slab one has to re-design the steel to comply with current code.

3) When performing a light repair to simply maintain the structure as-is one does not have to bring this up to current code unless to do so is less work than to not comply
For Example
a) When repairing only a single bay of the same suspended slab, one can reuse the existing design without requiring a new analysis with current codes.

How do I think this applies to the situations you described
1. These brick "footings" are typical in Canada as well. Though we usually see a thickening near the bearing location. In the case of changing the load path / adding load I would consider this to fall under my situation (1) above. And therefore you would need to find a way (probably underpinning of sorts) to make the pier comply with current codes.

2. Here I would think you are really just trying to maintain structural integrity. More of a repair to maintain "as-is" conditions rather than anything substantial. In this circumstance, I would say that you would not have to worry as much about bringing this up to current standards. That would change depending on the breadth of your repair, however. If the damage is limited to a section, no problem. If the damage extends for near the entire perimiter...it gets harder to justify not looking at bringing this up to current code.

Like I said, this is a real grey area and you are not likely to find a definite yes/no. It will also depend on how much your building official wants to get involved. Sometimes they ask for impossible things and you would have to demolish a building and start new to facilitate their wishes. That's always fun! But usually as long as you can justify what you are doing in the context of the three situations I described above, you will be fine.

Hopefully that definite non-answer helps some

 

[Ben_StructEng]

Hi Enable,

Thank you very much for your thorough response! And apologies for the delay. I've been on a break.

I think that approach seems sensible for me and was in alignment with my thinking.

Thanks again!

Ben