Suitable concrete decking for 2-axis cantilever?

[MultiVar]

Salutations, my ENGineering cousins! I'm OUT of my field here, but I've designed some of the CAE/CAD/CAM tools you structs use, so pretend I'm an "auxiliary" member of the brotherhood... ;')

Anyway, let's say you've got an International Modernist building (all rectangular sections) with pronounced roof/deck overhangs, on the order of 5' in both X and Y, and located in a coastal HVHZ, so max. wind loadings (and salt spray intrusions) apply.

Are there any precast/pre-stressed products that can span in 2 dimensions, or is this a cast-in-place application? I know for CIP there are several pre-engineered truss/form/pan systems, but which, if any, can fill this particular bill?

Could there be a "combo" solution, with hollowcore plank for the majority of the diaphragm, but something else "hooked in" along the lateral sides and corners?

Yes, I'm a noob, totally "at sea",

Thanks in advance, MV

 

[cntw1953]

Your question is too broad and lack specific information to gather thinking. I couldn't help even you are my blood brother, not to say remote cousins Good luck.

 

[msquared48]

Are you talking of cantilevering in two directions at a building corner? If so, I have done that in wood, steel, and concrete.

Hollow core plank would be a real problem if not impossible, but PT or normal CIP concrete can be done.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[haynewp]

I wouldn't consider precast for spanning 2 dims on a floor. If you use hollowcore with something else hooked in along the typical lateral side, you may run into a problem with getting the hollowcore to work with an edge load (the one that is being hooked into). Hollowcore doesn't take edge loads/twisting so well in my experience.

 

[ishvaaag]

If that is the intent, you can prefab a solid slab for the corner and anchor it with proper methods. Some of the prefab houses would do that.

 

[MultiVar]

Are you talking of cantilevering in two directions at a building corner?

Yes, that's exactly what I'm trying to describe. Would a POST-tensioned solution for this have PT tendons in both axes (implying a rather elaborate "matrix of holes" slab), or tendons in only one axis, with just rebar in the other?

If you use hollowcore with something else hooked in along the typical lateral side, you may run into a problem with getting the hollowcore to work with an edge load (the one that is being hooked into). Hollowcore doesn't take edge loads/twisting so well in my experience.

Yes, I see such mentions in the mfrs. handbooks. Intuitively, hollowcore is such a uni-dimensional entity, and I figured it would be dubious for my application.

OTOH, hollowcore planks are designed to be grouted together, with rebar in the keyway, to make large decks/roofs... but... NOT with 2D cantilevering... hmmm...

What about a configuration where the "TBD" (To Be Determined) custom slab butts up to, but does not tie-in to the HC planks, anchoring primarily to the bond beam?

e.g. 26' x 42' rectangular bldg., CBS/tilt-up/etc. walls, topped with a CIP bond beam having suitable hurricane straps or other approved anchors embedded therein. You place eight std. 4'W x 36'L HC planks spanning the 26' axis, bunched in the middle. So you've got 5' cantilevered overhangs on each end, but you've only spanned 32' of your 42' axis--still need a 10'W x 36'L "TBD" slab on each end of the bldg., to cover the 5' gaps + '5 overhang. But the "TBD" slabs each have a full 26' bond beam plus two 5' side walls to anchor them. Would this result in no significant uplift on HC plank edges?

Is it allowable to have such a roof constructed of 3 largely independent diaphragms???

Feel free to chortle and guffaw at my musings. ;')

 

[ishvaaag]

As you apportion full support to the plank hollow cores, you must also do for any other prefab parts. You can do in any way that satisfy the strength and working requirements. In my view these things better be tied all together, since the main weakness of the prefab things is the one that made the old buildings fail: lack of continuity. The generalization of the redundancy brough by the placement of gluing concrete anywhere is maybe the biggest apport to structural reliability brought by the XX century; and in fact the discontinuities at supports continue be a big issue for the big structures, very well known to fail many times there. Hence go as continuous as you can, always; and where not, use well proven things.

 

[msquared48]

Abandon the hollowcore idea. Use either PT, steel or wood.

The other option, if hollowcore is a must, is to cantilever only in one direction, with columns for the other. (Architects love me).

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MultiVar]

In my view these things better be tied all together, since the main weakness of the prefab things is the one that made the old buildings fail: lack of continuity. The generalization of the redundancy brough by the placement of gluing concrete anywhere is maybe the biggest apport to structural reliability brought by the XX century;

Indeed, I notice these various modular precast decks are often "topped" with 2-4" of concrete, with at least wire mesh reinforcement embedded in the pour.

... use well proven things.

YES, that's why I'm querying you guys (and gals). Definitely not trying to innovate here. Just want to sound out some P.E.'s, before I get a load of (probable) nonsense from South Fla's (typically) "ig-nernt" gen contractors. ;')


Abandon the hollowcore idea. Use either PT, steel or wood.

Done. HCore--OUT.

Wood: (AFAIK-not very far) can't achieve the large overhangs with low profile, AND be strong enough for subsequent attachment of (Miami-Dade) hurricane-proof solar panels/systems. Generally a problematic (ponding, water intrusion) choice for FLAT roofs in the sub-tropics. Can't serve as a pedestrian deck/balcony.

Steel: Strong stuff, but would have to a) be finished to LOOK like concrete; b) be protected from aggressive salt spray.

PT: Now we're talking. Any quick links to suitable PT systems known to be up to the job?

Thanks in advance.