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?

 

[wrantler]

Instead of putting in safety notes of workers I tend to put a note saying working health and safety plan by others.. I cant be bothered to tell the contractor how to protect their workers. By saying one thing you may be exposing ur risk to other things they dont do..

 

[LittleInch]

One perhaps silly question here, but do you know if those stairs are completely free standing or tied into the wall at some point with rebar or other fixings??

If free standing could you not create a small gap to the wall, break off the top step with it well braced and then lower it to the floor to demolish it?

If its tied in then either the steps will hang there or you'll take part of the wall with you when you demolish it.

Any original drawings of its construction or can you do a bit of investigation on one of the steps?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BAretired]

@ikka,
The methodology should be the responsibility of the contractor after you design the beams and posts to sustain the dead plus live load. The contractor should be able to determine methodology from your drawings, but you asked for it, so here it is.

The stair is likely doweled into the wall, but we do not know that.

Methodology for short beam method:
[ul]
[li]Calculate stair weight per lineal meter. Add 2kPa live load.[/li]
[/ul]
[ul]
[li]Determine short beam position on stair. (every second or third riser is suggested).[/li]
[/ul]
[ul]
[li]Drill vertical hole through stair slab 200mm from each edge of stair.[/li]
[/ul]
[ul]
[li]Cut short beams 200mm longer than stair width, so they can bear on post outside stair edge.[/li]
[/ul]
[ul]
[li]Drill holes through beams to match holes in stair slab.[/li]
[/ul]
[ul]
[li]Install beam hangers, bolted to existing wall and screwed to beam.[/li]
[/ul]
[ul]
[li]Insert two threaded rods through each beam and stair with tapered blocks at bottom. Use large washer and nut top and bottom.[/li]
[/ul]
[ul]
[li]Install one post or column under each beam at stair edge and fasten securely together.[/li]
[/ul]
[ul]
[li]Start concrete removal at the top and work downward, letting rubble fall to bins below.[/li]
[/ul]

 

[ikka]

Thank you for all the replies.
@Littleinch The client said they don't have any drawings. But, will do inspect whether stair is dowelled into wall.
@BAretired Thank you for your time and effort, will change design accordingly.

 

[BAretired]

You are welcome ikka, but I am not happy with the detail through the stair slab. The problem is that the coefficient of static friction between wood and concrete is too low, which means that the tapered block will slide up the slope. I will send a revised detail ASAP.

 

[BAretired]

@ikka,
Further to my last post, I recommend a change in Detail A to prevent slippage of the tapered wood block on the underside of stair when the jackhammers get close to each of the support beams. It is better to drill perpendicular to the stair rather than vertical, allowing a plate and nut parallel to the stair soffit which won't slip. The short beam on top could be timber, but I thought that an angle would be easier to anchor to the wall. It also provides more room for drilling.

If the stair was adequately doweled into the wall, we may not need vertical support on that side, but we don't know that and the possibility of sudden failure is not a risk worth taking.

It appears to be a very narrow stair; I'm guessing less than one metre in width. It is also very steep. It is possible that the original designer intended the width of stair to simply cantilever out from the wall. Again, we don't know that and we cannot reasonably assume that to be true.

Please let me know if you have any comments or concerns about the sketch below.

 

[BAretired]

@BAretired Thank you for your time and effort, will change design accordingly.

Don't change the design unless you agree with it. Discuss it with senior members of your firm before finalizing the design. Remember, you are the EOR, not me. This is an unusual engineering assignment and deserves careful consideration.

I encourage you to review the statics, particularly during jackhammer operations.

 

[BAretired]

@ikka,

I have a new concern today which had not occurred to me earlier. See the sketch and note below. You may have to choose a more conventional method of shoring.