Concrete slab form work removal with limited vertical clearance

[fsosa]

I am working on a concrete platform project that may have a construction issue due to the limited vertical clearance available to remove the form work for the proposed cast-in-place (CIP) slab. The slab will be between 12 ft to 25 ft in width and the span length varies between 10 ft to 20 ft in length. Currently, there is a 4'-3" minimum vertical clearance available under the slab.

Does anybody know the minimum vertical clearance required for form work removal? Would 4'-3" be sufficient?

Many thanks!

 

[SlideRuleEra]

The answer will depend on how the bottom form is constructed. If you post the details of the formwork it should be possible to provide a meaningful answer.

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[BUGGAR]

Use sand jacks below your vertical posts. Put the post bases on a layer of sand, restrained around the edges by a wood (6x6?) dam. After the pour, remove the dams, wash out the sand, and down come the forms.

 

[fsosa]

I do not have any construction information at this point since the project is at the final design phase now and has not gone out to bid yet. From the design point of view, the platform slab will be supported by concrete spread footings and walls that will be poured first. The contractor will design the form work. However, we needed to address a constructability question and verify the proposed design is feasible.

 

[SlideRuleEra]

We addressed situations where constructability was questionable by having bidders submit their proposal for means and methods as part of their bid. Review of this information is part of the bid evaluation process. Of course, treat each bidder's proposal as proprietary.

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[fsosa]

Thanks for your responses, BUGGAR and SlideRuleEra. Would you see any issue with having 4'-3" of vertical clearance to set and remove the form work underneath the concrete slab, if a standard timber or steel form work is used?

 

[SlideRuleEra]

fsosa - I'm sure a qualified Contractor can successfully install and remove forms considering the project's 4' 3" vertical clearance.

IMHO, there is no such thing as a "standard" form. A Contractor looks at this type situation differently than an Engineer:

The Engineer typical investigates cost effective form design. The Contractor does this, too... but the economics are different. Each Contractor's approach will vary.

The Contractor views form design "backwards". That is, "How do I design the forms to make use of materials that I already have (in other words, materials that are 'free')?" The Contractor's result is often different and innovative. An Engineer would have ruled out a Contractor's proposal as not economically viable.

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[fsosa]

I appreciate your input, SlideRuleEra. It is good to know that 4'-3" sounds adequate for construction.

 

[RHTPE]

Access after casting the slab is critical. You say the slab is supported by walls - if the perimeter walls have no openings, how does one get the formwork out?


Ralph
Structures Consulting
Northeast USA

 

[Teguci]

Can you consider the worst case as forms left in place? Its constructible, it just costs more. Put the fake budget number in and move on until bids come in. Who knows? Maybe a contractor might see that as the best way for him to do the job.

 

[BUGGAR]

I don't see any problem with your clearance.
I agree with Teguci that lost in place forms may be the way to go. My experience is that these are frequently cheaper since there are no stripping costs. And what you could strip from your project likely wont be salvageable.
The piers on the Dumbarton Bridge in CA are still full of wood and plywood forms that I designed.

 

[fsosa]

The slab will be accessible after construction, since it will have walls on three sides only.

We considered using metal deck forms to leave the forms in place, but the options are limited for the 20'-0" spans. The cost as Teguci said is much higher, about $14/sq.ft vs. $2/sq. ft for shorter spans. However, we also have an area that will require a 40'-0" span and will need to be cast-in-place (with the 4'-3" vertical clearance limitation as well).

 

[fsosa]

The only disadvantage I see with leaving the plywood forms in place is that we would not be able to inspect/check the slab from underneath. Would you see this as a problem?

 

[RHTPE]

fsosa - You haven't mentioned the slab thickness. Is it a flat plate or are there beams involved (concrete or steel)?

Leaving the forms in place will involve more than just the plywood. I really do not see a problem with clear height under the slab, although I personally wouldn't enjoy stripping out the formwork. As far as being able to see the underside after construction I don't see this as being any different than a slab on metal deck. You can't observe the integrity of a slab on metal deck after construction.


Ralph
Structures Consulting
Northeast USA

 

[fsosa]

I agree that inspecting the concrete slab (1'-3" thick, no beams) from underneath would not be possible with the metal deck forms in place. However, the metal deck forms should be designed to resist the loads without any temporary supports underneath while the plywood forms would be designed considering temporary supports, correct?

 

[RHTPE]

fsosa - This certainly would not be formed with plywood plus a few posts. I would expect that the plywood would be supported by joists 12-16" c/c. The joists in turn would be supported by stringers at perhaps 48-72" c/c, and the stringers supported by posts at an appropriate spacing. The posts would have to bear on mudsill/mudpads whose size would depend on the soil conditions. Spacings of course would be determined by the actual members' material and size. Certainly a lot more material than anyone would want to abandon in place.

Perhaps the use of geofoam blocks might be appropriate if no subsequent access under this slab is anticipated. Provide a reasonably graded/leveled base and fill the space with geofoam blocks.

In either case, the owner will be paying for the material used or abandoned.


Ralph
Structures Consulting
Northeast USA