Basement Slab Reinforcement

[Internet Know How]

Hey, I am building a new home with basement. My structural engineer has specified a 400mm thick sl ab, and the slab reinforcement is 100% H20 rebar. This is to support a 5 bed property over ground, 1st, 2nd floor, meaning the basement walls are structural.

 

I assumed that the slab would have been mainly made up of steel mesh, and H20 starter bars for the walls. Instead, over a 240m2 base there will be 8259 linear meters of H20 top and bottom. The wall starter bars are specified H25. Does this H20 and H25 sound overkill? £16k + vat worth of steel in the slab alone.

 

If you know groundworks well, I would be interested in your thoughts

 

Thanks 

[Ronan 1]

Does look overkill unless there is some specific driver that's not clear, I'd expect to often see 20mm closing the perimiter but not the whole slab and 20mm for the walls definitely looks over kill. 

I've put up alot of 4, 5 ,6 story RC frames with nothing heavier than 16mm in the walls but there may be something I cant see ie loading on the walls from the side as opposed to from the building above etc.. but yes it's definitely heavier than normal.

Definity worth asking your designer  whats driving the heavy steel ? 

[Conor]

Is that a 240m² slab? As in you're building a three story, 720m² house?! If so I'm not surprised by the spec.

 

It's a normal amount of steel but the 20mm bar is unusual. It will be very heavy and difficult to work with. Definitely ask if you can use two layers of mesh in the slab and 16mm starters.

[tonyshouse]

Way OTT,  hopefully a printing/nomenclature error - can’t see what the toe is doing either.

 

how will you insulate it? 

[Bitpipe]

Our basement slab was 120m2 and sat over chalk / gravel so was designed with the expectation of 'solution features' aka holes. Was 300mm thick but nothing bigger than H16 bars.

 

Basement walls (linear 45m x 3mhigh ) were also cantilevered off the slab as there was no basement lid (steel / timber GF). Similar 300mm thick.

 

We also had no toe, just a clean corner.

 

Friends built a basement in Thames tributary and were underwater from the get go - de watering equipment for the build etc. Their basement design had a massive slab and toe to stop it getting pushed out of the ground by the hydrostatic pressure. 

[Internet Know How]

6 hours ago, Conor said:

Is that a 240m² slab? As in you're building a three story, 720m² house?! If so I'm not surprised by the spec.

 

It's a normal amount of steel but the 20mm bar is unusual. It will be very heavy and difficult to work with. Definitely ask if you can use two layers of mesh in the slab and 16mm starters.

Hey Conor, its 650sqm over all floors. Slab is 240sqm, with 208sqm internal for basement and ground floor. 1st floor internal is about 144sqm and 100sqm 2nd floor

 

i will ask him as you suggest as i feel the same amount of steel is fine, just reduce the grading and use mesh in the slab

[Internet Know How]

11 hours ago, Ronan 1 said:

Does look overkill unless there is some specific driver that's not clear, I'd expect to often see 20mm closing the perimiter but not the whole slab and 20mm for the walls definitely looks over kill. 

I've put up alot of 4, 5 ,6 story RC frames with nothing heavier than 16mm in the walls but there may be something I cant see ie loading on the walls from the side as opposed to from the building above etc.. but yes it's definitely heavier than normal.

Definity worth asking your designer  whats driving the heavy steel ? 

I think he is just over specifying. The basement walls will hold inner leaf block and beam at ground level and outer leaf for facing brick work. Then on the 1st floor  the block and beam will sit on the internal block work right up to 2nd floor. 
 

i agree that 16mm is ample for the walls and maybe 20mm starter bars for closing the perimeter walls to the base as you mentioned

[PeterW]

Isn't this being built on sand ..? Or at least a loose substrate ..? If so, not surprised at the spec, but that is all going to need to be bent offsite as H25 is bloody hard ..!! 
 

I’m assuming he is expecting you to use a shuttering system for this ..?

[Internet Know How]

2 hours ago, Bitpipe said:

Our basement slab was 120m2 and sat over chalk / gravel so was designed with the expectation of 'solution features' aka holes. Was 300mm thick but nothing bigger than H16 bars.

 

Basement walls (linear 45m x 3mhigh ) were also cantilevered off the slab as there was no basement lid (steel / timber GF). Similar 300mm thick.

 

We also had no toe, just a clean corner.

 

Friends built a basement in Thames tributary and were underwater from the get go - de watering equipment for the build etc. Their basement design had a massive slab and toe to stop it getting pushed out of the ground by the hydrostatic pressure. 

Seems a common theme thar h16 are very much standard. Our basement walls will be 4.2m high

[Internet Know How]

11 hours ago, Ronan 1 said:

Does look overkill unless there is some specific driver that's not clear, I'd expect to often see 20mm closing the perimiter but not the whole slab and 20mm for the walls definitely looks over kill. 

I've put up alot of 4, 5 ,6 story RC frames with nothing heavier than 16mm in the walls but there may be something I cant see ie loading on the walls from the side as opposed to from the building above etc.. but yes it's definitely heavier than normal.

Definity worth asking your designer  whats driving the heavy steel ? 

Our basement walls will be 4.2m high, however we did start with 3m high walls and the Steel grading was the same. If you have done so much of this you clearly know the norm. Im no expert but it even jumped out at me 

[Internet Know How]

4 hours ago, tonyshouse said:

Way OTT,  hopefully a printing/nomenclature error - can’t see what the toe is doing either.

 

how will you insulate it? 

 

4 hours ago, tonyshouse said:

Way OTT,  hopefully a printing/nomenclature error - can’t see what the toe is doing either.

 

how will you insulate it? 

The SE soecified some foam type boards to help with ground heave. This basement slab isnt the internal basement floor because we will have a 1m void across the entire footprint which was mainly to accommodate the pool area and avoid a split level slab. We will have UFH in the basement which will use celotex or similar

 

 

[Internet Know How]

14 minutes ago, PeterW said:

Isn't this being built on sand ..? Or at least a loose substrate ..? If so, not surprised at the spec, but that is all going to need to be bent offsite as H25 is bloody hard ..!! 
 

I’m assuming he is expecting you to use a shuttering system for this ..?

We will be at clay 4.8m down where the slab will be formed. For the walls we will use aluminium formwork. There is about £20k of steel in the base using the as is spec. 

[PeterW]

Ok so that starts to explain the structural design and the choices of the engineer. And as it will be his insurance on the line, you can see why he’s choosing the steel he is. 

[Conor]

Ok, that design is very reasonable and I wouldn't push him too hard on it. Just make sure the rebar is all bent and cut to spec at the factory as it'll be a nightmare to work with on site. I am assuming you will have a contractor doing this all and not yourself!!!

[Ronan 1]

56 minutes ago, Internet Know How said:

Our basement walls will be 4.2m high, however we did start with 3m high walls and the Steel grading was the same. If you have done so much of this you clearly know the norm. Im no expert but it even jumped out at me 

A rule of thumb for a basement slab would be circa 120kg of steel per m3 of concrete so you should be in and around 12 Ton of steel and this is the norm for basement slabs for RC building 10 to 20 times your size. 

What's fairly common is people run it through a software aligned with euro code and then because there unsure they just jump the bar sizes up one but at H20 everywhere your running circa 25 Ton which is way way outta the norm.

 

Ask what the driver is for this ? It's your money. What's the tonnage that's spec'd as is? Also you steel prices seem very high are you talking about fixed or just bought ??

[Trw144]

1 hour ago, Internet Know How said:

 

The SE soecified some foam type boards to help with ground heave. This basement slab isnt the internal basement floor because we will have a 1m void across the entire footprint which was mainly to accommodate the pool area and avoid a split level slab. We will have UFH in the basement which will use celotex or similar

 

 

what is your floor build up above the void? Beam and block/concrete panels? We are also looking at a 200m2 plus basement with pool so interested to see what you are doing.

[PeterW]

1 hour ago, Internet Know How said:

was mainly to accommodate the pool area

Ok so you’ve just added another point load without giving any context ..!!

 

11x5m pool, 1m deep and we now have another 60+ tonnes of concrete and water not evenly spread on to a concrete stress member. And a floor suspended from the wall around the edges too..?? That changes the dynamic of the edge of the slab again. 
 

So ... I think your SEng is working from a set of plans, and we are working from a set of guesses ..!! Where does the car lift and the 10,000 litre water tank fit in all this so we can have all the picture...??

 

1 hour ago, Conor said:

that design is very reasonable and I wouldn't push him too hard on it.

I agree ..! At this point I wouldn’t be cutting corners on what will be a £1.5m build. 

[Ronan 1]

1 hour ago, PeterW said:

11x5m pool, 1m deep

Oh I missed that... There in probably drives the 20mm bars. That's an aweful unbalanced load and wouldn't make the previous detail outrageous at all

[Internet Know How]

Just now, Ronan 1 said:

Oh I missed that... There in probably drives the 20mm bars. That's an aweful unbalanced load and wouldn't make the previous detail outrageous at all

The pool is 10x3.5m. I am going to drop in some exact figures in a table shortly in response to your earlier comment buddy

[Internet Know How]

3 hours ago, Ronan 1 said:

A rule of thumb for a basement slab would be circa 120kg of steel per m3 of concrete so you should be in and around 12 Ton of steel and this is the norm for basement slabs for RC building 10 to 20 times your size. 

What's fairly common is people run it through a software aligned with euro code and then because there unsure they just jump the bar sizes up one but at H20 everywhere your running circa 25 Ton which is way way outta the norm.

 

Ask what the driver is for this ? It's your money. What's the tonnage that's spec'd as is? Also you steel prices seem very high are you talking about fixed or just bought ??

Hey Ronan, I have pulled this table together from the SE rebar schedule. Slab is 137kg/m3, Basement walls 144.79kg/m3, Pool walls 133kg/m3 of concrete. Overall I am happy with the rebar in the walls, but for the base slab I just expected to see  2 layers of maybe A393 mesh, but even using 90 sheets that would be 6300kg sheet or 65kg/m3 of concrete....half what you suggested. A larger basement build nearby used 2 layers of mesh, but they didnt have a 2nd floor and they did install on sand.

 

 

 

 

 

Edited April 4, 2021 by Internet Know How

[Internet Know How]

3 hours ago, Conor said:

Ok, that design is very reasonable and I wouldn't push him too hard on it. Just make sure the rebar is all bent and cut to spec at the factory as it'll be a nightmare to work with on site. I am assuming you will have a contractor doing this all and not yourself!!!

Yeah I will have a contractor come in and install. All I am doing up front is working with sika, formwork company and arranging other materials as needed. Check out the tables I just dropped into this thread too. 

[Ronan 1]

1 hour ago, Internet Know How said:

Hey Ronan, I have pulled this table together from the SE rebar schedule. Slab is 137kg/m3, Basement walls 144.79kg/m3, Pool walls 133kg/m3

Hey theses weight per m3 look about right with the pool on the slab and the wall heights. I'd expected circa 120kg per m3 without knowing about the pools 137kg wouldn't alarm me. And 144kg for 4.2 retaining walls is about right also the usual range would be in between 120kg and 150kg.

 

FYI steel is currently running at £600 per ton, You probably be paying a bit more as we have a good bit of buying power.

You should find it easy to get a good rate per ton for fixing also, fixers really like 20mm when on price.

[Internet Know How]

1 hour ago, Ronan 1 said:

Hey theses weight per m3 look about right with the pool on the slab and the wall heights. I'd expected circa 120kg per m3 without knowing about the pools 137kg wouldn't alarm me. And 144kg for 4.2 retaining walls is about right also the usual range would be in between 120kg and 150kg.

 

FYI steel is currently running at £600 per ton, You probably be paying a bit more as we have a good bit of buying power.

You should find it easy to get a good rate per ton for fixing also, fixers really like 20mm when on price.

ok thats good to know. Would you say that mesh is not good enough for the slab and it should be 20mm rebar? I am waiting on some quotes for fixing. Any ideas what I should expect to pay going rate wise? For example is this type of job priced per m3 of concrete labour and materials. From others I have seen done similar to this, it looks about 8-10 weeks work starting with the hole ready dug out. I reckon 8 weeks no problem 5-6 men

 

Appreciate your replies

 

[Ronan 1]

36 minutes ago, Internet Know How said:

ok thats good to know. Would you say that mesh is not good enough for the slab and it should be 20mm rebar? I am waiting on some quotes for fixing. Any ideas what I should expect to pay going rate wise? For example is this type of job priced per m3 of concrete labour and materials. From others I have seen done similar to this, it looks about 8-10 weeks work starting with the hole ready dug out. I reckon 8 weeks no problem 5-6 men

 

Appreciate your replies

 

I wouldn't think given what I've seen that mesh would be sufficient.

 

Regards price it would depend on where you are and wether and how you are breaking up the works. Steel fixing alone is usually priced per tonne, concrete flat work usually by the m3 with extra over for finish by m2, walls dependant on finish spec can be either by m2 or m3

 

If I know where you are I MIGHT be familiar with the rates and could give you some idea.

[Internet Know How]

1 hour ago, Ronan 1 said:

I wouldn't think given what I've seen that mesh would be sufficient.

 

Regards price it would depend on where you are and whether and how you are breaking up the works. Steel fixing alone is usually priced per tonne, concrete flat work usually by the m3 with extra over for finish by m2, walls dependant on finish spec can be either by m2 or m3

 

If I know where you are I MIGHT be familiar with the rates and could give you some idea.

This is in Liverpool, north west. With regards to the works, at the moment I have broken it up for people to quote as follows:

 

1.) excavation to required depth ready to start slab

2) installation of slab and walls (does not include tanking, as already covered by others)

·       Base Slab: 96m3 

·       Main Walls: 116m3 

·       Pool Walls: 9.5m3

·       TOTAL: 221.5m3 

 

Thanks