Concrete Floor Systems - State of the Art

[Judge17]

Deltabeam came by my firm recently to promote their system...composite steel beam supporting precast hollowcore planks. Seems like a nice system, though I've never used it. Most of my concrete projects have been skip joist beam systems (66" pans) with post-tensioned girders. 21" depth usually.

So what's everyone seeing out there for concrete office or school construction? I did a biaxial voided slab system recently for a high end architect, which was interesting, but doesn't appear to be economically competitive with two-way flat plate PT or this Delatbeam hollowcore system.

Thought?

 

[jayrod12]

Our office is all over this system. The trick is getting contractor's on board. We've had a few convert over and now they want to do nothing else.

The issue is when using a straight up material cost analysis from conventional steel supporting hollow-core to delta beam. The conventional steel option wins everytime. Delta beams are expensive.

The cost savings are in 2 major areas. 1) Depending on the size of the building you can be significantly shallower floor plates with delta beams when compared to cast-in-place or conventional steel. 2) M+E has nothing they need to bulkhead around or at least minimal bulkheading.

Point 2 is what has really sold the contractors in our area on this system. No holes through beams, no stupid bulkheads (minimal bulkheads).

There are some tricky detailing items you need to keep in mind when using the system but it doesn't detract from the effectiveness. I can get into more detail if you want but it's not required at this time I don't feel.

I should provide a caveat.... Our locale is one of the precast centres of the world. So hollow-core is super prevalent, the support structure is typically the only thing that changes (Cast-in-place, pre-cast, conventional steel, delta beams).

 

[Judge17]

There's is plenty of precast in my area as well (Kansas City). Hollowcore is super cheap here. Seems like the delta beam system also has the benefit of not requiring fireproofing. Many of my architectural clients want to expose the ceiling...and hence want concrete and not steel. I'm wondering if the delatbeam system could give them what they what (exposed concrete ceiling) at a lower cost than skip-joist.

 

[jayrod12]

More things to keep in mind knowing that:
1) yes they can provide the fire rating however the cost of fire rated delta beams are higher than standard delta beams. Which as I indicated are higher than conventional steel in a pound for pound comparison.
The reason for this is they essentially have to neglect any contribution from the bottom flange of the beam and rely solely on reinforcement that has been welded to in the inside of the beam for the tensile resistance of the beam.
2) You still end up with the bottom flange of the delta beam showing as steel, unless the architect is expecting to apply some form of shotcrete finish to the underside of the hollow-core.
3) Architects around here love angles. This doesn't bode too well for exposed hollow-core as the cambers between adjacent panels get all messy when the supporting member is angled. If you've got nice orthagonal framing bays then it's pretty nice.

 

[JAE]

How are frames braced with this system? Moment frames? Concentric Braces welded to gussets welded to the open Delta steel beams?



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[Judge17]

I asked the same questions to salesman. They said by "other methods". Shear walls, braced frames, etc. I don't think they're doing moment frames like we would with a skip-joist system.

 

[JAE]

Well probably not AISC pre-qualified moment connections.

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[jayrod12]

Depends on the system intended for the building. A couple of our projects have been standard braced frames. A couple have been elevator cores and shear walls. Peikko (Delta beam) will design the beams as beam-columns for the bracing loads if you need them to. You just have to specify the design loading and they'll make it work.

We completed a 22 storey tower using this. We're currently in the middle of the design phase for a 40 storey. We've done short building as well, 4 and 5 storey.

 

[Judge17]

@jayrod12, what do you guys do for columns? Peikko was pushing their concrete filled steel tube columns...but the system also appears to work with standard steel columns, concrete columns, and precast columns.

 

[jayrod12]

The two high rise buildings have been wide flange columns. The one of the low rise projects was square tubes, the other was pre-cast concrete columns.

It's pretty good in all cases. Take a look at their PC corbel connectors. Keep in mind that when using wide flange columns you either need to connect to the flange, or provide a plate between flanges for the connection to occur at the outer extents of the column. This is due to the delta beam being wider than the column is deep so it will not fit between the flanges.

The other tricky detail is columns is you need to provide some form of bearing angle for the hollow-core on the side of the column. The delta beam bearing ledge stops at the face of the column.