Formwork for demolition of concrete stair 4

[ikka]

I have a concrete stair of span (2450mm+4200mm)=6650mm in plan and over a height of of 4300mm to demolish. I have an intermediate support at 2450 from bottom end of stair. It is in a confined space. The stair top has been close out previously with a slab. The contractor said they would be using jack hammer for demolition. What type of formwork and props are to be designed and for what loads?

 

[dik]

Is this the only exit from the confined space? You may have to prepare a procedure for demolition. That might be tricky. Any hazards? It's a long stair without a landing; I'm surprised. You have to protect any existing mechanical equipment.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SWComposites]

Build a continuous wood formwork support under the stair sufficient to carry entire stair weight + equipment + personnel with a large safety factor.

 

[milkshakelake]

Loading would just be the dead load of the stairs plus light construction live load (25 psf where I practice, but it depends on your code/units). Use two rows of props at about 2' on center along the length of the stairs if there's rebar reinforcement in the stairs. If not, then yeah, continuous wood like SWComposites said. I only disagree with the large safety factor.

 

[SWComposites]

I would make the formwork very stout as one has no idea what the dumbasses with the jackhammers are going to do.

 

[XR250]

That would be a "means and methods" for me. Way too lawsuity for my liking (in the US anyhow).

 

[Enable]

Just as a point of clarity formwork is the molding surface for new concrete. What we're talking about here is falsework or shoring. You would also never place formwork intended for placement directly below the surface to be jackhammered as the hammering will destroy the form. You'd put it on sleepers.

When I demo stairs I usually place props in rows (2 per row for typical width stairs) at 2 - 3 foot intervals up the stairs. Resting against a 2x cleat on the stairs (screw anchored to existing) and braced against angles with blocking at the base. I rely on it being tied together with bar and to the slabs and such (if I cant do that for whatever reason then all props are vertical but this is more annoying for the contractor).

You have a bigger concern with worker safety. Even if they are tied off the open side poses a significant challenge. You'll probably have to design a temp guard rail at the open face. Let it fall off as concrete is chipped back.

 

[milkshakelake]

@Enable Do the posts have to be placed at an angle? The horizontal component would be taken axially by the staircase itself. Seems like you have to add a bunch of additional stuff to do it diagonally.

 

[Enable]

Definitely don't have to be. But as a contractor I've found the angled props easier most of the time. If I go vertical I need more robust scaffolding to get to the underside of the stairs to put solid blocking / angle (non-swivel head props have 6" head or so which needs to be blocked to get sufficient bearing). Safer / easier to put a 2x cleat off a ladder, clean the rubble, then come and place falsework which now doubles as my walking surface); scaffolding twice (once for install of blocking at prop head and then again for forming) is expensive so I like to avoid it. The props also need to be much longer because they no longer go from flight to landing (vertical props are flight to flight and at the top they can be very large - might be hard sometimes to negotiate those into the stairwell). Angles and blocking at base take no time at all since we use those in large quantities for lateral shoring against foundation walls / columns. Just a quick screw anchor or two and you're done.

Shoring is mostly about using what you have and since I have these items I tend to use them. This particular job (in drawing above) I had an issue that I couldn't obtain any more peri props since the rental company was out. So I could only shore with what I had, which was mostly No.2 post shores. Hence why some things look really funny (it was to make the distance short enough so the No2s would work lol).

Below is example of angle/blocking configuration. Have 100s of these guys just sitting around so easy to pick-up for misc stuff.

 

[BAretired]

That is a long stair. Maximum height about 14'-0". Supports shown in dashed red lines would likely be wood posts, braced to prevent buckling. Verticals would have to be recessed into the underside of stair slab to achieve satisfactory bearing. Alternatively, steel bearing plates could be attached to the soffit using suitable anchors.

 

[ikka]

Hi all,

Thank you all for the replies. They all were a lot of help in understanding the process. I still have some doubts.
@Enable "You would also never place formwork intended for placement directly below the surface to be jackhammered as the hammering will destroy the form. You'd put it on sleepers." Please bear with me as I am not at all familiar with this type of work. Did you mean not to provide any type of form board below and allow free fall of concrete that is chipped?
Should the vertical props be connected to as joist/stringer and those to be connected to the stair underside?
@BAretired Please forgive for the misinformation in my thread. The stair does not span till the wall, it stops at the slab. I have attached what i understood and is proposing for any corrections to be made and all guidance.

 

[Enable]

If you can let the concrete fall that is always preferable since it is much faster. Demo is quick and cleanup is similarly so. Chipping onto forms is a massive pain since the debris has to be entirely manhandled, and the concrete doesn't break nearly as easy since it has continuous support. Naturally then if I can at all help it I always chip and let the debris fall. If there is mechanical I protect it (plywood box of some sort), and if the slab below is suspended I shore it for the impact loads. Much cheaper than chipping onto forms.

However, sometimes you are restricted such as when you are chipping overtop a mechanical room or hydro vault. Perhaps you are restricted in your case because you can't shore the slab below or whatever. In these cases, we do chip on to forms as much as I hate doing so (because well we have to). But we lower the forms with a sleeper of sorts (usually 2x4). This does two things:

1 - It protects the form surface from hammer strikes

2 - It allows the concrete to break away more readily since it has somewhere to fall. Concrete in continuous contact with a surface is very difficult to chip with hand tools

Pardon the crude sketches. I'm on the way to the airport and I am poor illustrator at the best of times.

You can see the big guys chizzle is just ripe to go through the form surface. That's bad because you have to either replace the surface after demo (formply is expensive...) or else skin it before chipping (with something less expensive than formply). The skinning practice is cheaper but still leaves the issue that the concrete won't break easy.

Sleepers help with that

 

[BAretired]

If the soffit of the stair is resting on steel angles at an angle of 40.7 degrees, the coefficient of friction between steel and concrete is not sufficient to resist the friction, so the vertical weight of the concrete will add to the lateral force on the false work, particularly with vibration from the jackhammers.

 

[BAretired]

As mentioned by Enable, why not omit the deck under the stair and let the rubble fall directly into bins on the ground? The existing stair slab could be carried by short wood beams supported by wall on one side and posts on the other, drilling through the stair slab and providing two bolts for each short beam.

 

[milkshakelake]

I like Enable's idea because it can protect that piece of mechanical equipment in the corner. The other areas are okay to demolish and let the rubble fall into bins.

I think we can eliminate all the lateral bracing entirely. If you use telescopic support braces (lally columns/screw jacks) that can extend to the correct height, we don't have to worry about Euler buckling. It also means you don't have to field cut wood, and the jacks are reusable. We don't need lateral bracing for movement/vibrations perpendicular to the stairs because we have a solid support at about midspan.

 

[BAretired]

Yeah milkshakelake, getting rid of all that framing seems like a winner to me.

 

[ikka]

Hi all,
Thank you for your time to reply and for information's.
The contractor has informed that they will be taking out the equipment below stair and can allow for free fall on to bins or form boards.
@milkshakelake I couldn't find telescopic support braces for 4.3m spans in Australia(is it available here, I don't know). Can tilt props be used? Because removing bracings would help as it is a bit congested space to work.
@BEretired the adjacent wall is too old and cannot be used to support the short beams. Could you please elaborate the methodology on providing short beams drilled to the other side of stair and supporting them using props on either side.

What would be the appropriate methodology of demolition?

 

[milkshakelake]

@ikka They must be available in some way, because I'm sure you have situations where you need to form concrete for a tall floor, like what was done here. You can repurpose a formwork shoring post for that, and they tend to have high capacity anyway. If it's not available, you can build falsework under the taller areas and put the post on top of that, or do the angle+post like Enable showed in the picture with the diagonal brace.

I personally wouldn't use a tilt prop because it's too "pinned" for my liking. If a piece of the stairs pulls away, the prop will pull away with it. At least with regular screw jacks, they have a tiny amount of fixity. You might be able to use a tilt prop with bracing. You can repurpose some formwork bracing for that. I don't know how it is in Australia, but in USA, Doka and Titan have easily installable and reusable bracing that can clip onto posts. I still don't feel great about using them, but it can be done. I'd double check and triple check their axial load capacity.

Procedure:
1. Install shoring.
2. Protect the area and create a no-access zone. Install the bins. Protect other equipment as necessary.
3. Using hand tools like jackhammers, remove concrete stairs in 0.75m long segments. When one segment is removed, remove the shoring post under the adjacent segment and continue this step until all stairs are demolished.
4. Not sure about your code, but wetting down of flying dust with a water hose/sprayer may be required.

Notes:
1. Workers shall be tied off with fall protection while performing the work. Workers shall wear N95 masks or something similar. Workers shall observe all appropriate site safety measures.
2. Access to the work area shall be limited to workers only.

 

[BAretired]

@BEretired the adjacent wall is too old and cannot be used to support the short beams. Could you please elaborate the methodology on providing short beams drilled to the other side of stair and supporting them using props on either side.

Sounds like baloney to me! Use beam hangers bolted into or through the wall. If the wall is that bad, it should be removed along with the stair.
But it looks fine in the photo.

 

[ikka]

@BAretired was instructed by client like that, will have a word with them.
Could you please explain the methodology for the short beam method?