Waffle Slab Inspection With Engineer’s Commentry

G’day. This is Matt Cornell from Cornell Engineers. This is a waffle slab inspection that we did a little while ago and I wanted to go through and put some words to it to let you know what we’re looking at during this inspection.

So here we go coming in from the front I’ll just pause it there and explain some of these things.

The white things that you can see are the polystyrene waffle pods they come in about a metre by metre squares they’re sort of semi- hollow underneath. They’re not solid polystyrene but on top they’re solid. The space between waffle pods is about 110mm wide.

In between these are the ribs of the waffle. So at the moment the space when this concrete slab gets poured this will become concrete the top of the waffle pods all will be covered in
concrete and that’s the strength these beams or these ribs that occur at bout a 1 meter centres.

You can also see a few other things that are interesting in this shot around the outside edge there’s an extra heavy bar – much bigger
or thicker than all the rest of the bars in the middle – so this is some crack control around the edge of the slab.

These things are bar chairs they keep the mesh up off the slab and the timber in the front is the formwork that’s the
the side of the concrete when the concrete gets poured and this
timber gets removed how that’ll be the outside edge of the concrete.

So let’s get a little bit closer and have a look here. OK. So now we can see the bar chairs closely we can see that really looking right up the middle of that 110mm wide wide rip and we know it’s that wide because there’s some pod spaces down in the bottom holding these pods apart.

So let’s zoom around this way so looking round to the front maybe this is the entry in behind here.

These pipes sticking into the air – they’ll be toilets and sinks and basins. They’re connected to the pipes are in
the ground that are running underneath the waffle slab.

So you can see here there’s a gap in waffle pods and you can see there’s a gap around the front between the formwork
and the first waffle pod. So this one’s about 300 millimeters wide this is the strong part where the walls going to be built on. The code in Australia, AS2870, is the code for Australian residential footings and slabs and it says that you have to have a certain amount of strength and thickness in underneath the outside walls so that’s what this is through here.

This big area through here is the stiffness that’s required to support a little bit of footing that comes out in front of us here to the right. So the space where there’s no waffle pods will be concrete.

So in this waffle slab this area will all be concrete and it’ll continue out through the middle here.

Okay so coming up to the left-hand side we get an opportunity to have a good close look at what’s in the bottom of around the outside edge.

So there’s these three bars, one two three. There’s that extra bar upon the top don’t forget it’s hanging off the slab mesh but these three bars in the bottom are the bottom reinforcement. Waffle slab design and the slab mesh is actually the top reinforcement so this is the strength in one direction when there’s load is bearing and pushing down and this is the strength when there’s an uplift load or hogging we call it when part of the slab is cantilevering.

So it’s coming around this way. So in this area you can see the shape of the waffle. The waffles been cut. It didn’t fit exactly within the slab and this is what it looks like when it’s cut through it so you can see there’s thin legs of polystyrene.

This area even though this is meant to be void obviously concrete’s going to be able to flow into this space. That’s okay. It’s a little bit extra concrete. They probably didn’t allow for it but it’s okay it’s good. It’s really hard. You can’t get waffles to fit every house so they get adjusted on-site by the concreters or by the carpenters.

Jjust stepping back here so through this area is where the slab goes from inside to outside So this would be the outside area. There’s a
strong beam through here but the extra reinforcement on the top and then this is probably the patio slab in the back.

So it’s a little bit lower these pods are actually set a little bit lower and this is part of the problem that we have with waffle slabs that the slab the waffle pods have to be the same height for strength but the fact that they’re set a little bit lower.

So on our website we talked a lot about drainage under waffle slabs and this is the area
that we’re talking about where the pods are actually set lower it’s actually allows water to get in underneath the house and can’t get away because it’s in there like a swimming pool.

Coming around the back edge of the house of patio so we’ve got some cut waffle pods again, some ribs, and then a series of ribs all at one metre centers because the shape of the pod.

Bar chairs holding the mesh up and they’re doing a great job.

Okay and some more ribs. So coming to the end of the inspection we’ll see as we scroll past we’ll see some of these pipes they’ve been set up for termite protection to stop termites coming up beside them they’ve got discs which are installed by the termite installer.

I don’t think I’ve mentioned already that’s the
bottom reinforcement that runs through the ribs. So the engineering drawings tell the builder how big these bars need to be. So a couple of things that control how strong this slab is is the height of the waffles of the waffle pods so the higher they are the thicker the concrete is the higher the mesh is from the ground and the bottom reinforcement. The higher they are the stronger the slab is so in this case I can’t remember how high this was maybe 310 millimeters high overall and the waffle pods are set up to account for about 85 to 100mm thick slab and they sit straight onto the dirt.

Okay so that takes us to the end of our waffle slope inspection. I hope you’ve picked up something from this. A couple of things that we actually look for structurally as structural engineers
is to make sure that these bars, the steel bars, aren’t touching the formwork and this is not a bad shot you can see that there’s a bit of space there between the formwork and the first bar and you can see there’s a bit of space there between the reinforcement and the waffle pods. That means when this gets filled with concrete the concrete is going to be all around these bars – protected from the moisture and the outside environment by the concrete and that’s what stops them from rusting. So that’s good .

There’s one thing missing that we haven’t spoken at all about and that is the fact that there’s a step between the where the bricks are going to sit so there is going to be a rebate or a step in here and that’s going to be installed in the next day or so after this video. So they won’t actually put in a horizontal piece of
of formwork they’ll probably put in a vertical piece that
cantilevers out over this outside formwork that becomes the top part of the step and this is the vertical face that you can see on the outside when the job is all done and the bricks will sit on this space in here.

All right so thank you very much for joining us. That makes up the end of our waffle pot inspection. I hope you took something away from it. All the best. We’ll catch up with you later.

Cornell Engineers Anzac Day Thursday Morning Briefing

Hi. This is Matt Cornell from Cornell Engineers. This is Barlee Cornell from my place and this is the Cornell Engineers
Thursday morning briefing.

Today is Anzac Day. We’ve paused to pay our respects to
those who have fallen protecting our country we’d like to remember the Anzacs, Would like to remember those who’ve who’ve sacrificed their lives defending our country and our way of life and say thank you.

At Cornell Engineers this week we have had two engineers away next week and the next couple of weeks we’ve got two engineers away so we’ll be working hard to keep up with their projects and
just give us a call. We’re still happy to help. We still have plenty of engineers available.

Coming up on Monday I have a trip to central New South Wales. So just a quick reminder of that yes we do service New South Wales. I’m flying to Newcastle and flying out of Sydney. So a little bit of driving in between. A couple of inspections in between so yes we handle New South Wales and we do insurance inspections so if you are in need of those sorts of things let us know. Maybe we can help you out.

And the shout out today is to Barry from Indianapolis in the USA, which is unusual, but I just read an email from Barry this
morning. He was asking about frame construction, beg your pardon balloon construction which is lightweight timber construction “as light as a balloon” something a terminology that not we’re not really familiar with here in Australia but something that we are pretty familiar with as a as a way of construction which is lightweight timber construction. We’re doing it all the time and we actually do it pretty well so can’t really help you unfortunately Barry – a little bit too far out of our zone of capability but it’s great to
hear from you.

Thank you very much for sending – you set an inquiry on our
website and thank you to everyone who sends us inquiries on our website. Where we can help we certainly will and if we can’t we’ll do our best to refer you to someone who can.

So that’s it for today’s Thursday morning briefing. Hope you’ve
had a great week so far. Have a good weekend coming up and we’ll see you next Thursday.


Good morning. I'm Matt Cornell. This is Barlee Cornell and this is the Cornell Engineers Thursday Morning Briefing. 
So I just want to say today thank you very much to everyone who supports us. We've got an amazing group of staff, we've got to say thank you to our friends and thank you to our family for supporting us.
I've been very busy and we're working hard trying to keep up with all the jobs.
Thank you very much for trusting us with your work.
Pretty short and sweet today so we'll go straight to the shoutouts.
So we want to say a big hello to the girls who work for the company whose name starts with "S". Very secretive. (Thank you Barlee.)
So hello to Mari, Emily, to Kerrie Kylie, Michelle, Jacquie, Sarah, Saara, Joe... and Paul. Hang on. No. Joe's not a girl - Joe and Paul thank you for your support guys. Love working with you. Thank you very much.
As far as keeping the busy (Barlee's left me already) so this week we've already had a pretty large week, Thursday morning included. I've got staff engineers in Griffin, Bald Hills, Murrarie, Narangba, Coorparoo, Wavell Heights Brighton and Clayfield this week and have had been this week.
So thanks for tuning in. We'll see you next Thursday. Have a great week!