Overkill spec on reinforcement- suspended slab, basement wall

[Tony C]

Hi, I am just arguing with S.E over the spec on reinforcement schedule, I feels that its a overkill !

 

Suspended slab 175mm - H12 200cc top and bottom (double layer)

2m deep Basement wall 200mm - H12 200cc double layer

 

S.E is reasoning the soil is clay and it is near the large tree which is taken down about a year ago, but me and Ground work contractor are struggling to see why we can not use mesh instead specially its single storey timber frame build ...Its too expensive to use rebar, as it takes much more labour to connect them all.

The slab size range between 5x5m to 9x6m.

Groundwork contractor did similar project and used A252 for the ground floor slab, A393 double on basement wall.

What was the spec for your build?

 

[Conor]

Our basement slab is 200mm 40n with single layer of mesh. Thickened to 300mm around the external load bearing walls, and extends out from walls between 350 and 600mm, depending on loading. Walls are 200mm ICF, not sure on the amount of reinforcement.

 

She originally had it as a 100mm slab with lots of rebar, asked her to change it for mesh + more concrete as labour costs are much lower, and concrete is cheap here so works out £1000+ saving.

 

 

[Mr Punter]

Ask them to look again.  The A393 is fairly hefty.  Maybe it could be beefed up locally if required.

[Bitpipe]

We had 16 tonnes of rebar in our 120m2 basement  - all hand tied, no mesh at all.

 

But we were sited over chalk which is like Swiss cheese so the whole structure was bomb proof.

 

Ultimately SE is legally liable for your design, groundworker is not. Who signed off their previous job using mesh?

 

 

[Tony C]

1 hour ago, Bitpipe said:

We had 16 tonnes of rebar in our 120m2 basement  - all hand tied, no mesh at all.

 

But we were sited over chalk which is like Swiss cheese so the whole structure was bomb proof.

 

Ultimately SE is legally liable for your design, groundworker is not. Who signed off their previous job using mesh?

 

 

Both job had SE involved and its signed off by them.

I looked at re bar schedule, we have about 9 tonnes of rebar in 135m2 ground floor slab + 25m2 basement wall+slab.

 

By searching similar project in planning portal, I can see for the basement wall and slab TOP and Bottom A393 is common spec.

 

Same for the Suspended Slab (there will be cellcore 160mm below, as the soil is clay),  TOP and Bottom A393 seems to be commonly specified by SE.

I will try again to speak to our SE and see where I get there!

 

 

[Gus Potter]

Hope this helps, a bit lengthy but...

 

You maybe can use mesh but if you want to achieve the same equivalent bar area to 12mm diameter H12 bars then you may need to look at the B type structural meshes, perhaps a B785 mesh? The rub here is that the bars are different sizes in each direction with different spacing.

 

Has the slab (maybe basement walls too) been designed as a two way spanning slab. Roughly meaning that all the edges of slab are fully supported all round so that the main reinforcement bars act in two directions perpendicular to each. If so, then you may need four layers of mesh (2 top + 2 bottom) as opposed to your 12mm diameter H12 loose bars top and bottom as the secondary bars in the mesh may be too small.

 

Although mesh can be great you can have a problem with “nesting” if using lots of layers. Were four sheets come together it’s difficult to lap them properly and keep the concrete “cover” to the reinforcement. You can get a flying end mesh but on a small project this can add to the cost, difficulty in sourcing and you may need to detail it up so it fits. Also think about how you reinforce the corners of the basement walls as you can get congestion here too.

 

Although mesh can be appealing if you have congested reinforcement it’s harder to compact the concrete properly and that can cause problems later on.

 

Perhaps have a look again at using loose bars. An H12 is not a bad bar, not too floppy and not too heavy. They come in various stock lengths, easy to source and price match. Also, if you run out you can nip to the stock holder and grab a few more. Any off cuts are great for garden stakes or using as dowels etc.

Once you get going with tying loose bars you’ll get along fine I’m sure. If you make a small mistake then all you need to do is remove the odd bar or two rather than sheets of mesh that you may have cut.

 

Just remember that when rebar is tied together it is very heavy so make sure it is properly braced and shuttering is supported.

 

Clay soil (say when you have cut down a tree) can exert a significant load on the walls of a basement. The soil can take a number of years to readjust to the new ground water conditions.

 

There are a good few ways of designing concrete basements / floor slabs.  When you don’t have other buildings /sewers etc close by then broadly some key areas considered are;

 

strength (so it does not collapse), deflection (so it does not bend too much and damage other parts of the structure) cracking (to control water ingress and again damage to other components and finishes) and buoyancy / drainage... it’s not a boat so you don’t want it to float if the ground water rises.

 

When a floor slab is say simply supported at each end only and it is loaded from above you will get tension in the bottom of the slab. Steel is good for resisting tension hence your main bottom steel.

 

If you have a load bearing internal wall in the basement then you have a two span beam. You still get tension in the bottom of the slab as you approach the middle of each of the two spans. However, you usually get tension in the top of the slab over the internal wall. Hence your main top steel.

 

It may be that the basement has been designed as a continuous box. In effect the concrete and reinforcement work together at the corners (often called a moment connection) as opposed to say the basement floor and walls acting together with the suspended slab only designed to prevent the basement wall heads moving inwards and to carry the loads from above this is more of what is called a pinned connection.

 

If this is the case then you also have tension in the top of the suspended slab and the outside of the vertical basement wall at and near this junction thus you need some steel in the top and outside of the vertical walls and this can be a congested area.

 

You can ask the Engineer how the design works, often they are more than happy to explain. It’s worthwhile to know how something stands up, especially when you are finished the project and enjoying the fruits of you labour in front of the fire on a windy winters night.

[Bitpipe]

12 hours ago, Gus Potter said:

Hope this helps, a bit lengthy but...

 

Excellent description - our basement design was an open top box (as I wanted a suspended timber ground floor) so the design was a bit more complex.

 

The rebar design was quite intricate, especially at the wall slab interfaces.I was most impressed (as was the BC) at how well and how fast the crew implemented it - they even spotted a few errors in the drawings (misplacement of some elements) around corner details which the SE corrected immediately, plus suggested their own details such as diagonal bars at the corner of basement window and door openings to reduce cracking.

 

Could it be that your groundworker is just not that confident or experienced in working to the spec you've been given? Our crew were lightening fast in doing the bar tying and really took their time with the shuttering also, even detailing grooves for the water bar etc - but they did this day in and out and that's why the GW had subbed them in. His team focused on demo, excavation and backfill, plus laying services.

[Tony C]

On 27/06/2020 at 21:04, Gus Potter said:

Hope this helps, a bit lengthy but...

Thank you for all the info, very helpful. Our ground worker is talking to SE and currently our plan is to go for H12 bars for the basement slab and walls. (yes the basement is open box)

For the ground floor slab, we are planning to dig more trial pit, and identify the ground condition further. We did carry out 5 trial pit but the result was varied. There is a possibility some slab is sitting on the sand then we don't need the void former, we can just use mesh. Raft foundation and slab.

If the area turns out to be on clay soil then we need Strip foundation and slab on void former. This is suspended slab so we need to use H12 bars on top and bottom.

I will have a zoom meeting with SE next week, so I might have some more update!

I don't think our Groundwork contractor is not use to the bars, but they just think its quicker and cheaper to use mesh specially on the ground floor slab.

 

[Gus Potter]

Hope all goes well / has done with your zoom meeting. Most Engineer's are delighted to be asked how something works and will often be very helpful and keen to explain to an enthusiastic Client. The motto is don't be afraid to ask how for a layman's explanation of how their design works.

As an aside, if you have a basement that is going say 3.0 m into the ground you need to know what is under the basement floor a good bit below. If you are doing trial pits an average JCB will go down say 3.5 m tops and you can’t get in the hole safely to inspect the ground in situ in its undisturbed state. Once the soil is out the ground it’s disturbed. I’m not referring to the music scene here and the band Disturbed..

You can investigate deeper by using a technique called “window sampling” –  tubes knocked into the ground to collect samples. You can find out more on the internet. So don’t always assume you need a big drilling rig to do boreholes. Window sampling can be cost effective in the right, but not all types of ground. Food for thought.