Modern concrete buildings - technical question please - I want to get this right

[TEAMYAKIMA]

Hello

I am modelling a 2005 apartment block under construction. I have been given advice on the look of concrete columns and that they should have steel reinforcement rods sticking out of the top. Is there a minimum or maximum length for the sticking out bit?

 

I have been sent this photo, but with no info on how far those rods are sticking out.

 

 

This is a trial one on my building - will need to cut them down and straighten them later when I know more about the real thing.

 

 

I assume that whatever length I eventually choose, all the lengths on all the columns will be the same

[Nick Holliday]

In the UK, in my days on site, columns were usually cast in single storey lifts, it may be different in China.  Here, the rebar would extend beyond the top of the cast concrete to allow for:-

Slab or beam thickness + Allowance for a kicker (roughly 4 inches) + Lap length  

Lap length is roughly 45 times the diameter of the reinforcement bar, say 45 x 25 mm = 1.1m.  Beam thickness could be anything from 300mm to 1m.  Theoretically, columns using larger bars would have them extending higher above the new slab level to achieve the necessary lap.

Don't forget that this only applies where the column or wall continues directly above.  Where you've come to the top of the column there will be L bars which only protrude into the slab zone.

Edited February 4, 2022 by Nick Holliday

[TEAMYAKIMA]

Many thanks. Time to get started, Glasgow show is in less than 3 weeks! 

[2ManySpams]

Hi, you currently have 2 wires sticking out of the centre of your column. What you probably need for what looks like a square column are 8 around the perimeter (one in each corner and one half way along each side). The bars are likely to be somewhere between 20 and 40mm diameter, so that gives you and idea of the sort of wire you should be using. The bars will be set in something like 50-75mm from the edge, depending on how thick the horizontal loops are that go around the vertical bars.

 

The bars are likely to protrude around 1m from the top of the last concrete pour. Depending on how far up the column the join is, some of the protruding bars may well be bent in an L shape to tie in to the next floor. There's loads of photos on the net for inspiration.

 

The above is an approximation and there's quite a bit of variation between columns in the real world depending upon column size, loadings, other connections, design codes etc. Hopefully will be of use though.

[TEAMYAKIMA]

59 minutes ago, 2ManySpams said:

Hi, you currently have 2 wires sticking out of the centre of your column. What you probably need for what looks like a square column are 8 around the perimeter (one in each corner and one half way along each side). The bars are likely to be somewhere between 20 and 40mm diameter, so that gives you and idea of the sort of wire you should be using. The bars will be set in something like 50-75mm from the edge, depending on how thick the horizontal loops are that go around the vertical bars.

 

The bars are likely to protrude around 1m from the top of the last concrete pour. Depending on how far up the column the join is, some of the protruding bars may well be bent in an L shape to tie in to the next floor. There's loads of photos on the net for inspiration.

 

The above is an approximation and there's quite a bit of variation between columns in the real world depending upon column size, loadings, other connections, design codes etc. Hopefully will be of use though.

 

Hello Chris

 

Yes,  thanks, I am increasing the number of rods to four and I have started now and so there's no turning back. From now on it's a case of "It is what it is" in the sense that I have reached the limit of my abilities and considering that the average exhibition viewer will give the whole building a look of approx 5 seconds I think I have got the balance of accuracy against time taken, just about right.

 

As long as I achieve something which will not have architects and construction workers queuing up to point out obvious (to them) errors, I will be happy. This model was originally built at an exhibition demonstration stand at the CMRA exhibition back in 2016 by a friend of mine - he got so far with it and the passed it back to me to finish it off. I put it on the back burner and didn't do anything to it for six years! Actually things got worse. Because it wasn't finished, it never got permanently fixed down on the layout. Consequently, it was constantly getting moved in and out out carrying boxes and on at least three occasions we forgot it wasn't fixed down and it fell off the baseboard onto the floor when we turned the board on its side.

 

In short, it deteriorated and was not maintained/repaired. All I can say is that I think that it will be a lot better than it was at my last show in November.

 

Photo taken just after my last show in November 2021 ..................

 

 

 

Photo taken this morning - see left hand side for the first upgraded column.

 

Edited February 4, 2022 by TEAMYAKIMA

[Nick Holliday]

17 minutes ago, TEAMYAKIMA said:

Photo taken this morning - see left hand side for the first upgraded column.

 

I like the formwork/shuttering for the walls.  However, it should have the reinforcement fixed before the second side is shuttered up.  Often the rebar in walls is now delivered as a prefabricated mesh, so if you can find a suitable square mesh etch or fabric, with spacing around 3-4mm, then that could be slipped in between the two shutters.  You might also want a stack of such mesh for the floor slabs, and bundles of straight and bent wire to represent the unfixed rebar.

[F-UnitMad]

12 minutes ago, Nick Holliday said:

I like the formwork/shuttering for the walls.  However, it should have the reinforcement fixed before the second side is shuttered up.  Often the rebar in walls is now delivered as a prefabricated mesh, so if you can find a suitable square mesh etch or fabric, with spacing around 3-4mm, then that could be slipped in between the two shutters.  You might also want a stack of such mesh for the floor slabs, and bundles of straight and bent wire to represent the unfixed rebar.

In other words, it's currently far too tidy for a building site!!

[Nick Holliday]

1 minute ago, F-UnitMad said:

In other words, it's currently far too tidy for a building site!!

Yes!

Although in the UK the outside of the building would be sheeted in, so difficult to see what might be happening inside, but there should also be evidence of the formwork for the columns, waiting for the next lift, as well as materials for the next floor of decking.  Sometimes this takes the form of flying tables, but they have to either stored somewhere on site, or repositioned immediately at the next level. Buildings tended to work from end to end, so one section might be at level 2, say, whilst part of the decking for level 4 will be in place, as can be seen in the prototype photo posted earlier.  The only time you might see all the vertical work (columns and walls) in place and no slab might be if there were material shortages, or the type of construction at that level changed, such as going for a steel roof to complete the building (or Spanish Hotel Syndrome).

[TEAMYAKIMA]

52 minutes ago, Nick Holliday said:

I like the formwork/shuttering for the walls.  However, it should have the reinforcement fixed before the second side is shuttered up.  Often the rebar in walls is now delivered as a prefabricated mesh, so if you can find a suitable square mesh etch or fabric, with spacing around 3-4mm, then that could be slipped in between the two shutters.  You might also want a stack of such mesh for the floor slabs, and bundles of straight and bent wire to represent the unfixed rebar.

 

As I just said in my last post, I did not build the original, but I have gone back to look in the 'bits box' . I think there is the mesh you talked about and some other items. There are more than these, but this is the selection.

 

 

So, should I paint the mesh and have sections laying about and some sticking up through the wooden shutters - please note that the instructions were lost years ago and so I have/had no idea where these went

Edited February 4, 2022 by TEAMYAKIMA

[Nick Holliday]

23 minutes ago, TEAMYAKIMA said:

 

As I just said in my last post, I did not build the original, but I have gone back to look in the 'bits box' . I think there is the mesh you talked about and some other items. There are more than these, but this is the selection.

 

 

So, should I paint the mesh and have sections laying about and some sticking up through the wooden shutters - please note that the instructions were lost years ago and so I have/had no idea where these went

That would be a good idea for using the mesh.  The pink mouldings look as if they are cantilever brackets which would be under the edge beams to support some scaffold boards (perhaps the white mouldings top left, or use planked plastic sheeting) to provide external access for beam and column construction. The triangular pink items might be "Acrow" props - screw jacks to support slab and beam formwork, although you should need a lot more of them, but a few stacked around the place would do. The lattice beam on the left would be used to stiffen the wall formwork, normally being used vertically to resist the enormous hydraulic pressure wet concrete exerts.

[TEAMYAKIMA]

3 hours ago, Nick Holliday said:

That would be a good idea for using the mesh.  The pink mouldings look as if they are cantilever brackets which would be under the edge beams to support some scaffold boards (perhaps the white mouldings top left, or use planked plastic sheeting) to provide external access for beam and column construction. The triangular pink items might be "Acrow" props - screw jacks to support slab and beam formwork, although you should need a lot more of them, but a few stacked around the place would do. The lattice beam on the left would be used to stiffen the wall formwork, normally being used vertically to resist the enormous hydraulic pressure wet concrete exerts.

 

Hello again Nick

 

Firstly, many thanks for taking an interest in my project.

 

Secondly, please remember that I know nothing (NOTHING) about how such buildings are built in real life and so sometimes I have difficulty understanding your references and terms.

 

Lastly, I am no great modeller, but I want to make the best possible model using the components at hand.

 

If I have understood you correctly, the mesh should fit as in this photo, I can trim it so that it extends the whole length at the back across to the left hand edge.

 

 

I am a bit concerned that the flat back to the shuttering (correct term?) looks in complete contrast to the other side of the molding. So, should I put more shuttering (?) in the space inked in in this photo and that will then hide the flat plastic back. 

 

I can repair these brackets

 

 

Have you any other ideas of how I can use the components at my disposal i.e. without me buying/making anything else (except I have loads of plastic sheet).

 

In that context, I have always worried that the section at the back may not comply with 'real life'. There is the makings of window spaces, but no top section. I have no idea if that fits in to the scheme of things in real life i.e. did we simply lose the bit which should go there?

 

 

The building is two kits knitted together with a front and back showing - does that make sense?  so, in this photo you can see a front - on the right and a back on the left. And so the section I'm talking about is actually a 'front' even though it's at the back. You can see that then front on the right does have that missing top section. I can easily make a section out of plastic sheet if you think it appropriate.

 

I am genuinely sorry to ask so many questions, but I want to get this right before I glue it permanently on the layout.

 

 

[Nick Holliday]

4 hours ago, TEAMYAKIMA said:

 

Hello again Nick

 

Firstly, many thanks for taking an interest in my project.

 

Secondly, please remember that I know nothing (NOTHING) about how such buildings are built in real life and so sometimes I have difficulty understanding your references and terms.

 

Lastly, I am no great modeller, but I want to make the best possible model using the components at hand.

 

If I have understood you correctly, the mesh should fit as in this photo, I can trim it so that it extends the whole length at the back across to the left hand edge.

 

 

You are right about the mesh, being extended to the end. There should also be a short length along the central return. Theoretically there would be two layers of mesh, front and back, but I think that would be over the top. The mesh shouldn’t really be rust red - it would be condemned if too rusty - but a steel grey. There should also be bars protruding from the cast walls below, if they are to be extended.

4 hours ago, TEAMYAKIMA said:

I am a bit concerned that the flat back to the shuttering (correct term?) looks in complete contrast to the other side of the molding. So, should I put more shuttering (?) in the space inked in in this photo and that will then hide the flat plastic back. 

 

Formwork or wall shuttering consists of a flat surface, usually plywood, against which the concrete is cast, backed with a grillage of timber or aluminium beams to give it rigidity. So the flat side of the moulding is quite correct.

4 hours ago, TEAMYAKIMA said:

I can repair these brackets

 

You could do that, as access will be required to build the next storey. There is rather a lack of scaffolding - even the Chinese use it these days, but adding it properly is rather complex, so perhaps omitting it makes its absence less obvious.

4 hours ago, TEAMYAKIMA said:

Have you any other ideas of how I can use the components at my disposal i.e. without me buying/making anything else (except I have loads of plastic sheet).

 

In that context, I have always worried that the section at the back may not comply with 'real life'. There is the makings of window spaces, but no top section. I have no idea if that fits in to the scheme of things in real life i.e. did we simply lose the bit which should go there?

 

 

The building is two kits knitted together with a front and back showing - does that make sense?  so, in this photo you can see a front - on the right and a back on the left. And so the section I'm talking about is actually a 'front' even though it's at the back. You can see that then front on the right does have that missing top section. I can easily make a section out of plastic sheet if you think it appropriate.

I am genuinely sorry to ask so many questions, but I want to get this right before I glue it permanently on the layout.

Since your building is under construction it would be reasonable to have missing sections. For various reasons some parts of the structure lag behind the main build, so perfectly OK. Your addition of starter bars (the reinforcement sticking up from the tops of columns or walls) is indication enough that more work is due to be carried out.

Hope some of this is of help!

[mwrosebury2000]

Rebar can have an iron oxide coating making it look rusty even though it isn't.

 

M

[pH]

On 04/02/2022 at 12:25, Nick Holliday said:

Formwork or wall shuttering consists of a flat surface, usually plywood, against which the concrete is cast, backed with a grillage of timber or aluminium beams to give it rigidity. So the flat side of the moulding is quite correct.

There should also be bolts, with large square plate washers, through these beams and through the shuttering into threaded holes in the steelwork. These hold the shuttering  into place.

 

The plywood surfaces are (or at least were) coated with some hydrophobic substance - we used diesel - to allow easy release of the shuttering after the concrete pour has set. So after the first use, depending on what is used, the plywood surface can be quite dark.

 

1 hour ago, mwrosebury2000 said:

Rebar can have an iron oxide coating making it look rusty even though it isn't.

Rust = iron oxide.

[Nick Holliday]

9 hours ago, pH said:

There should also be bolts, with large square plate washers, through these beams and through the shuttering into threaded holes in the steelwork. These hold the shuttering  into place.

 

The plywood surfaces are (or at least were) coated with some hydrophobic substance - we used diesel - to allow easy release of the shuttering after the concrete pour has set. So after the first use, depending on what is used, the plywood surface can be quite dark.

 

Rust = iron oxide.

I was trying to keep thing simple!  Such detail would be a bit OTT for this project, I feel, and given there are many fancy shuttering systems such as this

anything is possible. 

Although I am sure our chippies occasionally resorted to using (red) diesel, I don't think it did the concrete any favours. There were proper sprays which did a much better job. There were some special ply sheets that had a dark red plasticised finish to give a better finish.  Standard ply would only last a few uses, especially if there was heavy use of a vibrator.

[pH]

1 hour ago, Nick Holliday said:

I was trying to keep thing simple!  Such detail would be a bit OTT for this project, I feel, and given there are many fancy shuttering systems such as this

anything is possible. 

 

Although I am sure our chippies occasionally resorted to using (red) diesel, I don't think it did the concrete any favours. There were proper sprays which did a much better job. There were some special ply sheets that had a dark red plasticised finish to give a better finish.  Standard ply would only last a few uses, especially if there was heavy use of a vibrator.

I suppose I’m talking about practices of 50+ years ago. I’m quite prepared to believe they’re not today’s best practices. Probably weren’t best practice then, either!

[TEAMYAKIMA]

19 hours ago, pH said:

The plywood surfaces are (or at least were) coated with some hydrophobic substance - we used diesel - to allow easy release of the shuttering after the concrete pour has set. So after the first use, depending on what is used, the plywood surface can be quite dark.

 

Yet again, please forgive my ignorance of such matters, but could you find me a photo please to guide me with the colour as I have no idea what colour they start with, let alone finish with

[Nick Holliday]

I wouldn’t worry too much about colour. Timber and plywood come in “timber” colours, varying from almost white to almost black. System formwork as the photo I posted could come in various colour, depending on manufacturer, red, yellow, blue or green, or unpainted aluminium, but with use they might get covered in oil, concrete or rust, as did the adjustable props that were used. Try a Google search on “plywood formwork” or “shuttering systems” and view the images.

[TEAMYAKIMA]

Thanks again to everyone who is offering help and support with this project. I am making some progress, but this shuttering still confuses me.

 

First progress.

 

Every column now has four reinforcement bars with 11mm (1metre) sticking out.

 

 

I thought I would deal with the other stairwell first as it is pretty devoid of interest. I want to make this one different to the first one. I don't want shuttering at all on this one. So, am I correct in saying that before they start to add shuttering the walls will have some mesh sticking up?  I have temporarily posed the mesh behind the structure.  If I am right how tall should the mesh be?  Assuming that I am right I would trim off sections of mesh and glue them on top of the wall.

 

Just to clarify my assumptions.................

 

I am thinking/planning to have some mesh apparently sticking through all walls as below

 

 

Yet again I apologise for my lack of knowledge, please assume that I have just landed from another planet and so you will have to explain things to me in very basic language.

[Fat Controller]

I would have thought the mesh was for the flooring? The load-bearing element of the walls would be the pillars, infilled with blockwork.

[Nick Holliday]

4 hours ago, TEAMYAKIMA said:

Thanks again to everyone who is offering help and support with this project. I am making some progress, but this shuttering still confuses me.

 

First progress.

 

Every column now has four reinforcement bars with 11mm (1metre) sticking out.

 

 

I thought I would deal with the other stairwell first as it is pretty devoid of interest. I want to make this one different to the first one. I don't want shuttering at all on this one. So, am I correct in saying that before they start to add shuttering the walls will have some mesh sticking up?  I have temporarily posed the mesh behind the structure.  If I am right how tall should the mesh be?  Assuming that I am right I would trim off sections of mesh and glue them on top of the wall.

 

Just to clarify my assumptions.................

I am thinking/planning to have some mesh apparently sticking through all walls as below

Yet again I apologise for my lack of knowledge, please assume that I have just landed from another planet and so you will have to explain things to me in very basic language.

 

 

1 hour ago, Fat Controller said:

I would have thought the mesh was for the flooring? The load-bearing element of the walls would be the pillars, infilled with blockwork.

Just to confuse you there is no single way of doing things, as much will depend on the complexity of the structure and the equipment available for materials handing.  The absence of a tower crane alters the way things might be done. I may have upset the apple cart by suggesting using what appears to be mesh as wall reinforcement, just for simplicity .  As @Fat Controller says, mesh is usually used for floor slabs, but where the walls are fairly simple, and the size of the reinforcement bars relatively small, it is quite common to have wall panels welded up off site, and delivered as a mesh, to be simply lifted into place by the crane. In the absence of a tower crane, such mesh panels are a bit unwieldy for man-handling, although mesh could still be used for the floor, as a bundle would be lifted to the level using a mobile crane, and the mesh is light enough to be easily moved horizontally into place.  Normally, the next lift (storey) of wall reinforcement would start only after at least one section of wall shuttering has been put in place, so that line and level can be established, and there is something solid to work from.  As various sections of the reinforcement have been fixed and checked, the other side of the shuttering can then be lifted into position, and that section prepared for concreting. Whilst a single lift shaft might be simple to build - fit the four internal shutters, reinforce, outer shutters - more complex shafts, such as multiple shafts, requires a careful consideration of what goes next, otherwise things get missed out.

Returning to your point, wherever a wall or column is continuing directly above that under construction, there will be some form of starter bars to provide continuation of the steel bars.  Sometimes, for very specific reasons, very expensive couplers are used, but normally, as I have suggested before, the bars forming the vertical reinforcement will be extended beyond the height the concrete is poured to. This extension will be roughly 1m above the level of the floor slab to be cast over the top of the wall, so the visible extension might be much greater, from 1.3m to accommodate a simple floor slab, to over 2m where there might be a metre deep beam, which can occur around the perimeter of a building. So, to answer one of your earlier questions, the columns don't all have to be the same height, although the top of the starter bars probably should be.

What you could do for this second area is to add starter bars to the top of the stair walls (both faces I'm afraid) and then infill part of the floor area with plastic sheeting, to represent the decking (the shuttering to support the the floor slab until the concrete has achieved its correct strength) and use the moulded mesh with workers spreading it out.

Something like this, plus people!

 

Note how the bars have been left down where the stair flight comes up to the slab, and there would only be short lengths, probably u-bars, to tie in with the slab reinforcement.  The decking, shown stippled, should theoretically have panels to suit the normal 8' x 4' (metricated of course) ply sheets, but that might be going too far!

As for @Fat Controller's comment regarding infills between columns, that would all depend on the architectural design and the need for structural stability.  Buildings need to be braced in all directions, and this is often achieved using the core walls, and in a concrete framed building, this means an abundance of solid walls.  Where there are infills, this would often be in curtain walling, with glazed panels, or using lightweight steel framing to support plasterboard forming the internal walls.  Often the brickwork you see on the face of the buildings is completely non-structural, and is, in fact, supported at each floor level, on the outside face of the buildings, with steel bracketry, or concrete nibs formed on the outside face of the structure. 

Apologies for length - over 40 years in the building industry!