Concrete Plank vs Precasting ???

[ToadJones]

I'll start off by saying I most of my design work in steel structures & isolated footings

I have a project where I may potentially need to span a ~20' wide utility 'trench' that is to be cut in what is basically solid concrete.

The area may require some tow motor/ vehicular traffic.

I was thinking about cutting a "shelf" into the existing mass concrete (this same mass concrete is being excavated to make the 'trench') and then bridging this with concrete planks or precast beams (panels) that could be removed, if necessary, by cranes in the area. This would allow me to leave an open accessible area for the utilities.

Any pitfalls with this idea?
I have never specified plank

Pitfalls to precast beams/panels?

Hope the post isn't too vague

 

[msquared48]

Boy, I would be concerned with the ability of the mass concrete, hopefully not CDF, to take the load from the planks and vehicles.

Mike McCann
MMC Engineering

 

[ToadJones]

Mike-
CDF ?
Like lean concrete?

 

[msquared48]

Yep - Controlled Density Fill.

Mike McCann
MMC Engineering

 

[ToadJones]

I had concerns with that.
anyway to test the strength or just a bad idea all around?

 

[ToadJones]

I should mention that the CDF appears to be place over structural slab (that itself bears on engineered fill).
Rather than cut a shelf, I guess I could tear the CDF out wider and pour a new wall down to the slab?

 

[msquared48]

That might be a better idea Toad. CDF is not meant to bear any substantial structural weight. Only to be a way to achieve a good compaction for a fill situation more cheaply

Do you know for sure that it is CDF? If not, test with a portable tester it to see what the f'c rating is.

Mike McCann
MMC Engineering

 

[Teguci]

Don't know what environment you are in, but for snow and rain, don't use core slab precast planks. Use solid planks instead. Precast adds at least 15% to the cost for transportation and erection.

 

[ToadJones]

you think casting beams on-site is a viable option?
Do you just design it like a normal beam/slab?

 

[hawkaz]

Agreed- Don't use core slab- for numerous reasons, including pick point issues and cost. I just did a mezzanine slab where core slab came in at $45/S.F., but cast in place was $25/S.F.

They can cast this on site or in a pre-caster's yard. Design as pinned-pinned.

Call for cast in place inserts so when this needs to be removed, they can screw in eyebolts or hooks and lift the slab out. Make sure you have top steel for the lifting process.

 

[Teguci]

Have a look at Dayton precast or tilt up inserts for some ideas for lifting devices. If you go with a cast in place system, you will need to design the slabs for a lifted scenario as well as a simple spanning slab. If you don't have rain and snow, form deck may be an option (with intermediate support).

 

[ToadJones]

good note on top steel.

 

[MiketheEngineer]

How about a simple steel beam structure and deck that spans clear across the whole mess??

 

[ToadJones]

with decking?
Its outside
I was thinking long term
Not much depth to work with so steel beams with concrete deck might be too deep?

 

[Teguci]

Will you be able to exclude truck loading from the slabs (height restriction recommended)? What about emergency vehicles?

 

[ToadJones]

havent given much thought to the vehicles. Owner will have to specify.
I am hoping to keep the system to 12" in height