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A Cautionary Tale - Laying a good foundation

Foundations can be complicated to design and build. In this article, we outline a recent project we completed as a cautionary tale to highlight some of the problems we faced, and how you can avoid them for your next project.

The Project

For this project in the Wellington Region, we were engaged to design the beams and lintels of a new residential property. We were not engaged to design the foundations because, usually, an engineer is not required to design a RibRaft slab if the site conditions fit the parameters/guidelines of the RibRaft manual. In this project, the site conditions did fit these guidelines. However, early on during the construction phase of the project, the Council completed its first round of inspections and notified the Client that they weren't comfortable inspecting the standard non-engineered RibRaft slab. The Council also informed the Client that they weren’t going to return to the site to inspect any other areas until an engineer assessed the foundation slab's build.

What is RibRaft? The RibRaft system, developed by Firth, is a popular foundation style in New Zealand. It can be ideal for residential or light commercial builds because the foundation sits ‘on’ the ground rather than ‘in’, which helps make construction easier because there is less excavation and waste on the building site. However, a few key components in the RibRaft system ensure the foundation is structurally secure. These are:

The polystyrene pods

Which reduces the amount of concrete and distributes the load evenly across the ground

The steel reinforcing rods

Which reinforces the concrete poured over the polystyrene pods

The plastic chairs

Which separates the top layer of reinforcing rods from the polystyrene pods.

With this outline in mind, let's get back to the problems we faced.

The Problems

When the Council asked for an engineer's assessment, we were engaged to look at the plans the builder had followed, compare them to what we could see on site, and assess whether the foundation slab design was structurally sufficient. We noted some issues on site that did not meet the minimum requirements of the Firth RibRaft Guidelines. The two main issues were:

1. Inadequate steel mesh laps covering the reinforcing

2. Inadequate height of the plastic chairs

After our site visit, we issued a report with the recommendations before the concrete was poured. Usually, this would be the end of the story. However, we could not verify whether the changes had occurred because we weren't there when the pour happened. In our correspondence, the builder stated that they had made these changes, but no photos were taken to prove this.

Our solution

For the concrete slab to pass the inspection, and because there were no photos to back up

the builder’s claim, the Council required us to investigate what the effects were of the original foundation design, assuming that the builder had not made the changes.

This added complexity to the project. We had to thoroughly understand the effects of the originally designed reinforced mesh laps and whether they affected the structural strength of the foundation slab.

As mentioned previously, the steel mesh laps and plastic spacer height were less than the minimum required per the Firth RibRaft guidelines. However, we calculated that the slab was still code compliant as it fit the minimum requirements per the Ministry of Business and Innovation (MBIE) Exposure Classification for a ‘slab on grade’.

Our calculations noted that the mesh lapping did not affect the structural strength of the slab, but instead could lead to shrinkage cracks on the top of the slab.

Because the structural strength of the slab was not compromised, we left it up to the client to decide whether they were happy with the increased risk of aesthetic shrinkage cracks. We also gave them flooring options to best cover any cracks that may occur in future.

Our final step in this project was to convince the Council that the foundation slab did pass the minimum requirements for the code of compliance. They accepted it, even with the higher risk of aesthetic shrinkage cracks, because we showed that our recommendations would help alleviate the issues.

What can we learn?

There are three main things builders, architects, and homeowners can learn from this cautionary tale:

1. Councils are becoming more risk averse

This issue came about because the Council was uncomfortable assessing a non-engineered RibRaft Foundation specified by the architect. We've noticed a trend in Councils wanting engineers assessments for foundations that in the past could be specified by an architect. Take this into consideration for your next project, and involve an engineer early!

2. Make sure the engineer sees any reinforcing before a concrete pour

If an engineer gets involved with any sort of concrete reinforcing design or assessment, make sure they see the final reinforcing before the pour is happening, either in person (site visit), or with photos.

3. Take photos and videos (and more photos)!!

Take photos and videos as often as possible throughout the project's construction. This is especially the case if concrete reinforcing is involved. Many structural parts of a building (whether designed by an engineer or not) are covered up once construction is completed, and photos will be your only proof of what was built underneath.

More information on building foundations in New Zealand

You can find more information on different foundation types, what constitutes 'good ground' and how they could affect your building project by navigating to several sites:

All this can become complicated and time-consuming. If you're an architect, building owner, or builder and need some help designing or assessing a foundation, give us a call. Whether it's a suspended floor or a concrete slab, we can guide you through the process!


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