Cyclone Damage

Cyclone Damage

We are structural engineers. We can help you if your building was damaged by the cyclone.

We can assist with structural engineering and make-safe work after the cyclone. If it is important that you get the right beam size and repairs done quickly, call us for structural engineering in Ayr, Townsville,  Bowen and Mackay.

The best number to call us on?

07 3102 2835

Site photos a big hit

If a picture tells a thousand words, our construction photos tell even more. Watch some great Brisbane renovation projects being built and learn what goes into a building to make it strong.

Our photo of the week this week is Julian Kajewski on site in Banyo, Brisbane where a house extension is well underway by Ascot builder James Maguire Construction. Designed by clever building designer Roger Cook of Roger Cook Design, the double storey extension is ready for cladding and roof sheeting.

Matthew Cornell of Cornell Engineers Brisbane did the timber frame inspection this week – aided by Julian who is studying construction management in Brisbane.

Julian Kajewski doing a frame inspection.
Julian Kajewski on site.

The photo generated a flurry of likes and comments on Cornell Engineers FaceBook page. It reinforces Cornell Engineers’ philosophy that engineering isn’t about buildings – it’s about people working with people.

Matthew Cornell said,”Like our FaceBook page! We’re always updating the page with photos of our jobs under construction. It’s a great way to take the mystery out of the building process. Structural engineers needn’t be the only one that get to appreciate the beauty of the structure under the skin of houses.”

Head over to Cornell Engineer’s FaceBook page here.

Roger Cook Design

I’m pleased to introduce my friend and colleague, Mr Roger Cook, of Roger Cook Design. Roger Cook

Roger is one of the first building designers I worked with when I moved to Brisbane. With an office in the next suburb, Hendra, I have already worked with Roger on a number of projects.

As a building designer, Roger works hard to turn his client’s visions into reality. He takes the time to work with his clients and understand their motivation and needs. Then, with a firm understanding of structure, design and aesthetics, Roger creates a 3-d model and works with Cornell Engineers to create a blueprint for construction.

Roger complements our commitment to “people working with people”, so I am proud to recommend Roger as an exceptional designer for your next project.

Please follow this link to visit Roger’s website:

http://rogercookdesign.com.au/

 

 

 

Slab Cracking – Waffle Slab vs Conventional Raft Slab

Slab Cracking

The age old question has been raised again this week. Should you go with a waffle slab or stick to a conventional raft footing and slab when building a new house?

Terminology

For those unfamiliar with the terms, refer to our page of Structural Engineering Terminology

IMG_3487

Today I’d like to address just the issue of slab cracking.

Waffle Slab vs Raft Slab

Which slab is more likely to crack? The waffle slab or the conventional raft footing?

Check out this post: Why does concrete crack?

The Good News

Concrete cracks are very common. How do you know if the cracks in your slab are bad cracks?

The good news is that a crack in a house slab doesn’t mean the concrete has failed.

Concrete does crack. It’s rare to see a new house slab that doesn’t have some cracks in it.

Hairline cracks generally won’t affect the strength of your house slab because they often don’t penetrate right through the concrete. They are often surface cracks and are controlled by proper placement of the slab reinforcement (around 30mm to 40mm below the surface).

Hairline cracks in older slabs tend to fray and might appear wider at the surface but when I have inspected core samples taken through older cracks, once again the crack stops at the reinforcement.

Shrinkage Cracking

Concrete Shrinkage Crack
Typical Shrinkage Crack in Concrete

Of the slabs that I have inspected, the majority exhibited shrinkage cracking. (What is shrinkage cracking?) Either no curing was used, it was a hot day and the bleed water evaporated or the slab was over-worked and the bleed water was pushed away during screeding.

The same cracks will occur regardless of whether is is waffle slab or conventional slab. So no clear winner yet.

Plastic Shrinkage Cracks

The other slab cracking that we see in residential slabs, but less frequently, is parallel lines following the mesh at around 20 centimetre centres in both directions.

This is caused by poor

Structural engineer - photo of plastic slab cracks
Plastic shrinkage cracks caused by concrete draping over the steel reinforcement.

compaction of wet concrete and the concrete slumping over the mesh reinforcement. Sometimes, in hot weather, it is also caused by the concrete drying out quicker around the hot mesh.

Again this cracking can occur on both types of slab if the mesh isn’t cooled or the slab concrete isn’t vibrated. Still no winner in the raft slab vs waffle slab shoot out!

Pure Shrinkage

The only type of cracking that might be different between waffle

slabs compared to conventional slabs would be pure shrinkage caused by the concrete trying to shrink in volume as it cures.

These cracks don’t follow the mesh and sometimes start in internal corners. You will also see shrinkage in long, thin slabs where there are no control joints.

In waffle slabs the slab can shrink freely because there is less restraint by the the ground to the slab contracting. In conventional slabs, the edge beams in the ground stop the slab shrinking in overall length. Engineers use heavier mesh in larger house slabs to counter these shrinkage forces. So waffle slabs just took the lead!

Overloaded Slabs

Concrete slabs will crack when they are overloaded. The steel reinforcement in concrete slabs is there to control the width of cracks under normal conditions. When a slab is overloaded, the steel stretches and cracks become visible.

A stronger slab system can take more load before it cracks. In theory there’s no real winner here because waffle slabs and raft slabs are designed for similar loads and will behave similarly when overloaded.

However raft slabs are cast against the ground whereas waffle slabs are cast onto polystyrene void formers and strips of concrete. The raft slab edges back a point. An overloaded raft slab is less likely to crack because it is cast onto the ground.

Who’s the Winner?

So, are waffle slabs less likely to crack than conventional raft slabs? My opinion is a reserved yes. The problems that cause cracks in slabs affect both slab types, but there should be less shrinkage stresses and fewer cracks in a waffle slab but a raft slab is less likely to crack if it is overloaded.

When to Worry

The Australian Standard AS2870-2011 gives advice on when slabs cracks are bad enough to cause concern – and often this is when you’ll need an engineer to help solve the problem.

Distinct cracks: around  2mm wide and accompanied by 10mm to 15mm change in offset from a 3m straightedge centred over the defect.

Wide cracks: 2-4mm cracks and accompanied by 15mm to 25mm change in offset from a 3m straightedge centred over the defect.

Gaps in slab: 4mm-10mm wide cracks and more than 25mm change in offset from a 3m straightedge centred over the defect.

Need More Advice?

Need more advice about cracking in waffle slabs and raft slabs? Contact Us or Get a Quote.

Residential Structural Engineers

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