Concrete Formwork for Suspended Slabs and Beams

On this page. Slabs with Acrow-V-shore | Slabs,timber only,low tech | A modular panel system. | Ring beams on brickwork |s

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Concrete formwork using Acrow "V-shore" or similar.

Concrete formwork - Acrow "V" shore. "U" screw jacks at the top support heavy timbers which in turn support joists for play

Concrete formwork - Square plated screw jacks at bottom, nailed to timber sole plates. These under all of the steel support legs and props.

On the right is the slab formwork for a suspended concrete slab to a multi story building, but this system of concrete formwork could just as easily be used on an owner builder house.

What you see is a series of steel frames with cross braces holding them together.

They have adjustable screw jacks at the bottom and top. The top jacks carry heavy timber bearers, (150 x 100) at say 1200 centres which in turn carry joists (100 x 75) at 400 centres.

At the base of the frames there are heavy timber pieces called sole plates.

The main purpose of the sole plates is to spread the load of the frame to the ground.

In this instance they are sat on the building pad, which is a stable pad of compacted earth and then road gravel that forms a base for the building work.

On the next floor up, the frames will be sat on the concrete floor, but the sole plates will still be used.

Apart from spreading the load on the frame legs, the plates also have a function of stopping the frames moving around under vibration.

This is particularly important with say a row of single steel props. The sole plate not only spreads the load and provides a firm base for the prop, but a couple of nails through the prop into the timber sole plate keeps the prop steady under vibration.

Concrete formwork - Slab support layout, steel frames and timber

Concrete formwork - beam side layout.

Here is a sketch of the layout. The frames are all the same width and come in about four different heights so that a combination of different frame sizes and screw jack settings will cover any height.

I have put a larger detail of the side form for a beam here. Some times they can be quite deep, so there may be a need for braces.

An important detail, are the provision of vertical spacers between the timbers making up the side form. Say at 600 centres these transfer the load from the top ply down to to next layer. Without them the weight on the upper joists could push the top timber off the ply.

Here's a few points that come to mind when using this sort of system.

• First and foremost, this type of system is no different to building and working on scaffolding, so the same safety procedures apply.
• As you erect a section brace it off.
• Provide good planks for the guys erecting the next stage to stand on.
• If you are building a tall narrow section provide external braces.
• When building separate floor levels, similar to the sketch above with a lower beam section, the cantilevered ends of the joists depend on the beam sides for support. Don't leave these cantilevers unsupported.
• Always provide a good solid base to work on.
• Sole plates should span at least two legs.
• The screw jacks come in solid and tubular form. Don't extend the solid ones more than 600mm and the tubular ones more than 450.

Concrete formwork - cross braces

• The system of two cross braces that are joined at the middle, and that are clipped onto the vertical members with gravity toggles is common on quite a few scaffolding/support systems. The word to remember is GRAVITY. The toggle is held level and the brace end is slipped over it. When it is in the right position the toggle drops down into a secure position by gravity.
• If a frame happens to be put in upside down, the toggles won't drop down. So the cross brace will not be secure. WATCH IT! Make sure the frames are the right way up.
• Frames that are distorted, have broken or bent toggle lugs, or toggles that don't work, don't use them.

Concrete formwork - Early stage set up.

Here's a section of the job in the early stages. You can see temporary planks here and there for the guys to stand on while they are fixing the joists. There will be a laser level set up at the side somewhere and as the frames are fixed the screw jacks will be brought up to level.

Concrete formwork - slab falsework from the top.

Here's a shot of the job from the top.

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A Low Tech Way To Do Concrete Formwork for Slabs and Beams

Concrete formwork - Deck and beam low tech method

Concrete formwork - Timber beam support cross.

Concrete formwork - prop adjustment with fox wedges.

I the sketch above I show the concrete formwork for a beam at the edge of a concrete slab, using all timber formwork, with the exception of adjustable steel floor centres. We could of course have used timber joists as in the details above.

We also could use the timber cross and beam shutters on their own, just to form up beams.

We never left the floor centres supported at the ends only. There was always a least one timber bearer with props supporting the centres also, to take a lot of the weight off the cross props shown above.

The main support for the beam and edge of slab was what we called a "cross" which is sketched out to the right.

These were made up out of sawn hardwood timber, usually, but I have worked on jobs where we used "bush props" with the main prop consisting of 100 to 150m dia sapling trees.
Typically they were set up at 1200 centres. (Depending on loads they could be closer of course.)

At the bottom, we used "fox wedges" to adjust for height. These along with timber packers as needed were quite adequate.

The beam soffits were set up on the cross props between columns. The cross props were nailed at the top to the shutters, they were nailed at the bottom to the sole plates, and unless they were fairly short, we used to nail 75 x 25 braces on them in the middle also.
I once did a job that some comedian nailed a ply sign on the outside, "Bill's Jungle". It didn't bother me at all, I'd sooner be seen as the guy who put too many props and braces in. than remembered as someone who didn't do enough.

Concrete formwork - jig for making wedges

The sketch to the right has my idea of a simple jig for cutting consistent wedges on a bench saw.

Easily made up out of a bit of scrap timber.

• If you are making say 50mm thick wedges for formwork use a bit of 150 x 50 for your jig.Cut a length of say 150 x 50 hardwood into 250 long lengths.
• After the first wedge is cut flip the material length wise to cut the next wedge and so on.
• Always use a push stick , never be tempted to get that last one out of a piece. Keep your fingers AWAY from the blade.
• The saw bench insert should be in good condition, as there is a tendency for the thin end of the wedge to drop and jam at the front edge of the blade.

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A Modular Panel system

Concrete formwork - A modular panel system

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Here is another system for forming slabs.

This type of modular system works well for simple plans, as here where the slab is spanning between concrete blockwork walls, and there is no requirements for forming up beams within the slab.

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Ring Beams on Brickwork.

Concrete formwork - A way of shuttering a ring beam around a masonry wall.

We don't do this sort of construction any more, because of our increased cyclonic coding.

Here is what we used to do at the top of a cavity brickwork wall.

For a small one off job I would use the wire tie as detailed, but if I was doing a couple of repetitions, I would buy some 12mm threaded rod and nuts and washers.

The reason I have shown the wire above the forms is that the concrete is usually not very deep and this way is sufficient, and there is no need to chip the wire back, like it would e if it was embedded in the concrete.

If you decide to put the ties lower, be very careful not to put too much pressure on the top course of bricks. It is very easy to dislodge them, as usually the brickwork has only been laid a few days before.

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