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Conventional deck & beam systems are difficult to make economical because you need to build extra capacity into the beam to carry the concrete while it is still wet. Once the concrete does set up, you have often times many times what you need for your remaining floor load (some Additional DL and the eventual LL). In most cases, your "worst case" will be your "construction case"!
The "temporary PT" system gives temporary stiffness to the girder/beam while the deck is still wet. Once it sets up and you achieve a composite section, the system then has sufficient strength to carry it's own dead load quite efficiently.
I ran a few numbers to see if I could get a basic concept to work. Take an 18.3 m span (60') x 1.524 m spacing (5') with a 101mm deck (4"), plus 51mm non-structural topping (2"). I tried a few different W's and settled on a W530x74 (meaning 27" deep at 50 lb/ft) which ended up at 91% of flexural capacity during construction (also a small construction LL in place which later is removed). My span-to-depth is 28 w/o the deck, 22 with. Not bad. Oh yes, I'm assuming a 45 MPa (6500 psi) concrete which is common enough these days - esp. in the Pacific NW known for 19,000 psi concretes! The steel is the most common variety 345 MPa (50 ksi)
Now, I run an external PT on a custom built plate girder, although a stock W might be cheaper to use even if it does have a few extra kilos. The section I ended up was effectively a W520x44 (30 kg/m less than the first example). I build the same concrete deck (101mm thick) and let it get to 70% strength (3 days max.) The concrete is compositely bonded to the steel beam so I can release the PT but the stresses imposed in the section are now effectively "locked in". When the PT is released, it is like adding a new axial tension to offset the previously imposed axial compression, so while the two forces negate each other and lead to a state of zero axial force, the flexural and axial stresses are acting on two different sections (one non-comp, the second is fully composite). Long story short = there is still plenty of locked-in stress that has to be overcome by new floor loads before failure occurs. This system also has a max stress of 90% capacity, but it occurs under full D+L, not the construction scenario.
The LL that I used is 2.4 kPa (50 psf), somewhere between a lightly loaded room (bedroom @ 25 psf) and a congested assembly area (theater or mall at 100 psf). As you might expect, the first floor is basically stiffer, so LL deflection at midspan is ~12 mm (1/2"). For the second system, the deflection is greater, ~16mm (5/8"). For the record, neither of these values are especially disturbing and are at least 3x what the code would prescribe as a maximum.
Another question is - will it flex to the point of being annoying to human occupation? The first flexural frequency is 3.1 Hz and under normal walking, a person would expect to feel approx. 0.0025g's of acceleration. The threshold of human comfort varies, but 0.005g's is sometimes considered a reasonable point of tolerance.
The only real difficulty is making this system work is that the girder is very flimsey laterally and the PT force that I need to use is approx. 10x the Euler Buckling criterion. One can always develop methods for restraining the lateral torsional failure mode, so it does become a very intriguing possibility.
For this example then, we could conceivably chop the DL of the beam used from 74 kg/m to 44 kg/m, or 550 kg per 18.3 m span. Just in material cost, I would price that savings as between $600 and $1000. I would wager that it could be an economical solution, eh!
Fireproofing would likely have to be the old fashioned type (spray-on) but that could be done at the factory too (better QA/QC, more efficient productivities, etc.).
Seismic details would be easy enough to incorporate as it is essentially already a very common construction technique (slab on beam) so it could also be integrated into IFC walls or steel/concrete columns too.
Now all I have to do is figure out how much it would cost to retool a old fabrication shop to produce these units to see if the bottom line is workable!
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Member of the SSPIA Senior Committee. Have a question? Go pester Tony.
Last edited by Kelvin; Jan 20, 2007 at 5:33 AM.
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