Precast beams vs. reinforced concrete

[edxgma]

Anyone care to offer some words of wisdom on the following:

As part of a slab to be poured for the floor of a small house, it will be necessary to bridge across the top of a "cellar" in the middle of the house. This hole is about 12 feet wide by 30 feet long. Would precast beams and blocks be a better choice than just rebar across the top of the hole? In either case, the hole will be covered as part of the pour for the entire floor. The area would need to support a ceramic tile floor and interior non-load-bearing 8 ft wood frame & drywall walls.

Any thoughts on the use of metal form decking on this?



Thanks for your thoughts

 

[frv]

In general, precast is economically advantageous due to repetition.. I would be surprised if it would even be in the ballpark, money-wise, to order a precast beam vs. a CIP beam. Frankly, I don't know that you could get a precaster to agree to cast a single beam.

 

[rowingengineer]

Engineering judgement is needed in this call, you say "hole" in floor. Are you able to prop a cast in place beam? What are you boundary conditions for installation? If a cast in place is possible without problems with propping, forming ect, then I would cast-in place, however if there are problems I would consider a different system, one being precast or maybe steel ect depending on the situation.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[paddingtongreen]

Tell us more about this cellar, concrete floor? masonry walls? Headroom problem? If it is concrete floor and masonry or RC walls, I would use some steel beams and metal deck as forms, type etc to depend on loadings.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

If the 'hole' in the floor has a bearing support each side so that a slab spanning 12 feet is all that is required, then precast hollow core slabs would be a good solution.

Other materials may be more economical, however.

BA

 

[JAE]

Precast = higher amounts of creep and camber change over time vs. cast-in-place.

Tile floor = need for minimal movement in the supporting slab.

 

[edxgma]

Thanks for all the input and the questions. I will try to define the project a little better. The "hole" was carved out of the ground (very stable, rocky, hard soil) and 5 foot stone walls (1 ft +/- stones cemented together) were erected around the hole so that it would hold wooden beams across the hole and a floor could be laid on top of it.

The wooden beams and floor have been removed and it is now desired to lay a slab and tile floor across the hole, while maintaining the hole open, with access from the outside, to be used as a cellar.

I have been looking at these precast beam and block systems:

http://www.cube6.co.uk/beam-and-block.htm

and

http://www.supaslab.co.za/

as opposed to using conventional forms or metal form decking, such as:

http://www.metaldeck.com/floor_deck.htm

and

http://www.metaldecksupply.com/

I thank you in advance for your thoughts on this.

 

[concretemasonry]

The precast beam and block (concrete or clay) systems are very commonly used internationally and they are also used in conjunction to create a flooring/roof/framing system for homes.

In the U.S., it is unlikely you would find a producer willing to make the necessary block to use the system since the U.S, construction is one of the few areas where wood and lightweight construction is really common in the total concept.

For such a small DIY area the choice is to use the local materials compatible with your style of construction. - You just have provide adequate support around the "hole", span over it and cover with the materials that are compatible with what you have and what you plan to use as a surface.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[csd72]

One of the rules of construction economy is to minimise the number of different trades required. A beam and block or precast floor would be installed by a different trade.

Also if you change from insitu concrete to precast with topping you are introducing a crack point.

If you need the formwork to be permanent then I would suggest a metal deck as mentioned above but I would avaid using steel beams if possible.

One thing that you need to consider is if you need the slab to restrain the top of the walls, if so then you may want insitu slab rather than deck so it interlocks with the walls.