Demolition of existing slab 6

[vlad1981]

Good morning all.
Iam doing some demolition work of some existing slab. Slab is going to be demolished and replaced by a new one. Said slab is shored, but standard shoring is designed for Concrete weight, Forms weight and Live load (ACI recommends 50 psf if not motorized buggies, and 75 psf if motorized carts are used).
To account for the effect of the dynamic loading, I've heard a couple times to increase the concrete self weight for about a 25%. Some have said to use motorized carts LL (75 psf) but it seems to me a bit of an overkill.
Question # 1: Is the Concrete weight * 1.25 a correct assumption? If so, is there any reference or just an "experience" thing.
Question # 2: If not correct, what would be a good approach to take dynamic load due to the demolition of the slab in consideration?
Thanks all in advance.

 

[phamENG]

You didn't mention the demolition method. Will a bunch of guys be running all over it with jack hammers pounding away, or will they be saw cutting small sections and lifting them free? The former could have significant dynamic loading, the latter very little.

 

[vlad1981]

@phamNEG Yeah, thanks and sorry, my bad.
It will be pretty much a mixture of everything you said, they will be cutting, mostly the edge where the main slab will remain in place, and jack hammering the rest.
They may cut small pieces, but as it is all enclosed in an existing structure, they will those small pieces jack-hammered down to be able to manually move it away.
So, yes, the for the most part, it will be done with jack-hammers.

 

[phamENG]

In that case, I'll keep quiet and wait for Enable to show up. I haven't done many of these, and they were all saw cut operations - more building surgery than real demo. Enable sits on both sides of the fence (capable engineer who actually breaks/builds things) and can give better insight than I can.

 

[MotorCity]

You could shore it for the design live load of the slab.

 

[vlad1981]

@phamENG thanks, I still need to present some sort of design for pricing purposes, even though it will or may change later on.
@MotorCity, that seems odd to me, the design LL is 40 psf, which is smaller than ACI 50 psf, or you telling me to add (on top of the shoring LL) the design LL???
any reason or logic behind it?
As mentioned originally, have gotten a couple ideas/ rules of thumb (Like 25% over concrete self weight, or using LL for motorized carts (75 psf)) and now you with the design LL, but I failed to see the logic behind it.

 

[MotorCity]

You did not mention the design live load of the slab. I was assuming it was higher. If it was say 100 psf, then the issue of a 50 psf or 75 psf demolition load would be a moot point. That said, if its just guys with jackhammers and no motorized buggies, I will stand behind using the 40 psf for shoring design.

 

[Enable]

Thanks for the compliment phamENG! I used to spend the majority of my waking hours demolishing parking garage slabs and so we are right in my wheelhouse here. Got out of it because I like to see the sun every now and again. You don't mention where in the structure you are, and that will dictate a few things (e.g. at the roof deck vs second intermediate slab) but in general here is the process if you are hoe-ramming the slab (assuming it's 2-way):

Demolition Shoring (Vertical)
Usually No.2 posts shores at 5' c/c and at all leading edges. This is to ensure that the skidsteer or excavator doesn't punch through more or less. Shore debris field similarly if we are dropping onto suspended slabs below.

Formwork Shoring
Usually concrete self weight + formwork self-weight + 50 psf construction loading. Unless there is some specific piece of equipment or something that needs to be considered the 50 psf is good to go for most circumstances. Though we pretty much pump all our slabs around here. If you are using buggies absolutely the travel path needs to be designed for that but you'll also need to consider the lateral breaking / stopping forces (not simply more vertical load). The loads you mention (1.25 multiplier for concrete DL and 75 psf construction loading) sound factored to me. In Canada our DL factor is 1.25 and our LL factor is 1.5 which would get you to those numbers.

Note: do NOT use the design slab thickness for the slab DL calcs. In existing structures you'll find the variance in slab thickness is massive. Often you'll find that a 8" slab design thickness was really 12" in large areas and 6" in others. You'll try and smooth that out when placing anew but given existing bar elevations into adjacent elements you cant change the thickness as much as you would like. So you want to know what the existing slab thickness variance is and use the upper bound for DL calcs.

Note2: if the slab will see LL applied or heavy construction prior to strength gain or reshoring the shoring system should be designed for that LL as well. But only if it will be applied to the unreleased shores. This need not factor into the calcs if it wont be seen by that shoring system (you may put it into the reshoring system depending on timelines)

BTW there are a ton of other considerations when demolishing a suspended slab (e.g. proximity of hoe-rams to columns, dropping of debris onto suspended surfaces, lateral support to vertical resisting elements, tie-ins to existing to replace shear support, etc) and I'd make sure you enlist the help of a specialist if you have not done this before.

 

[vlad1981]

@phamENG now I know what you meant by wait for Enable to show up, you got me thinking, was a bout to google it....
@Motorcity yeah, design LL is not that big, thats why you surprised me when suggested to use it.
@Enable Formwork shoring I have it clear, yes, agree regarding the differences in slab width, and that is been taken in consideration. Its a second intermediate slab, shoring system is 10K, not individual props, not heavy loads will be applied, so reshoring is not a major concern. and no, the 1.25 is not a factor, just a percentage (25-30%) to increase slab DL to account for the dynamic effect of the demolishing hammers (Not sure if correct, if it is, would like to know where it comes from, mainly).

 

[Enable]

I guess I don't understand the order of events for your project. Usually we have separate systems for when we are demolishing and for when we are placing. So your placement shoring shouldn't receive any loading due to the demolition operations. It kind of sounds like you want to put the falsework up and use that as your shoring system during demolition. But that is only done if we are saw-cutting. It would never be done in the event of hoe-ramming operations.

See the no.2 shores in the picture below? Those were in place during the hoe-ramming above and will be removed by workers before the bobcat picks up the debris. Then the slab shoring (the alumas on beams) is designed for typical slab loads only. Perhaps your situation is different so maybe you can explain the order of operations a bit more so I can better understand what's going on?

 

[vlad1981]

@Enable
Situation is pretty similar to the one you show in the picture.
difference is shoring system, 10K, will remain in place for demolition and pour.
It will be saw cut on the edges, and then, broken into smaller pieces with a chipping hammer. There will not be no bobcats, concrete pieces will remain on the top of the shoring system, and picked up manually, no equipment or heavy equipment involved.
When you mention it will never be done (keeping same support system) in the event of hammering, can you explain further? Will it damage something in the shoring system so it will not be possible to use it for pouring??

 

[Enable]

Aww so you're breaking onto the forms. Well, that sucks but I understand now.

What you're proposing can be done if absolutely required but never with a hoe-ram since the hoe-ram would destroy the forms to bits. When you said chipping/hammering that's what I thought you meant since that's the only way we chip large slabs. Hand chipping large areas onto a form surface is madness but if you have to, I guess you have to. I've done it for small occupied areas but it's slow as all hell and for a large area I'd sawcut the entire thing except around columns or other bar that I need to maintain.

In terms of your loading I would not add anything for dynamic loads. The hammers are A) incredibly small relative to the slab stiffness, B) the slab will still have capacity while you are demoing (assuming it's a 2-way slab) and C) the construction loading allowance of 50psf won't be fully present at the time the workers are jackhammering (who will be spaced at 6+ feet apart I imagine) that can be used to offset any incremental amount of dynamic load. Also, this is a slab we don't care about so who the bloody well cares what happens to it as long as it doesn't drop.

As a practical matter I might consider adding sleepers to the topside of your forms. This will allow the concrete to "drop" which makes it easier to break, and will reduce the number of times the hammers hit the form surface. If you have the form surface immediately under the existing your hammers will mark it every time they strike; basically it'll be ruined and you'll need to add another layer on top (or you can add a sacrificial layer to begin with that you peel off after the concrete has been demo'd).

 

[dhengr]

Vlad1981:
I agree with Enable’s last post and recommendations. The one thing I might add, given what you intend to do, saw cutting at edges, etc. is..., from the entire stability standpoint, start in the middle and work your way toward the edges. Wait to saw cut the edges until you get fairly near them. That way the entire slab, and its shoring system, still has some lateral stability by virtue of its attachment to the rest of the structure, not just due to the shoring posts.