Jan 15, 2007, 10:04 PM
I build ICF homes (concrete houses), and am looking for an alternative to engineered wood beams for the second and third-floor decks. It has to at least be fire-rated for 1-2 hours, and seismic-rated for C or D1, and be economical.
So far I've studied the Hambro system (red steel joist with poured-in-place deck), hollowcore, and to a degree post-tensioning.
This will be in the Seattle area, so there are seismic considerations, although I want to exceed Code. Homes will be from 40'x50' to 80'x110', and I'm hoping for a clear span all the way, so interior (steel stud) partitions are not load-bearing. If necessary though, I could build an interior ICF wall, but would like to avoid the expense and thickness.
Goal is to reduce the inter-floor space, so there can be high ceilings in this limited building-height state. And yet I want floors to seem substantial, so they don't bounce when a heavy person walks on them like the decks in Sears stores do. Fire-resistance of at least 2 hours would be nice, and hydronic piping will be cast in. I guess I'll need to somehow drop the ceiling below (1st floor) for services and ducting chase.
I saw a 10 story hotel being built in Dallas a while back which was just of the thinnest concrete for walls and floors... looked like the Hollywood Squares show, just a stack if thin boxes. I have got to think they did this with P-T.
Has anyone used P-T upper decks with ICF before? Comments on suitability? Can anyone give a field-guide for installation? Best way to drop the ceiling? Will deck flexure cause drywall cracking downstairs?
Is there another way? What system are the big guys using?
What will I do for fireproof steps? I imagine there is such a thing as precast pretensioned?
Jan 15, 2007, 10:06 PM
isn't there a engineering section on skyscraperpage somewhere?
Jan 16, 2007, 2:00 AM
Fascinating issues related to your proposal Smoke. Outside of my area of knowledge, but was glad for the incentive to look into the basic terms you alluded to. So, is it correct that houses you've built to date have been pre-designed, and that your ambition is to push beyond the limitations of those designs for something more interesting? If you have a website, I wouldn't mind seeing some of the stuff you've been building. Thanks.
Jan 16, 2007, 2:50 AM
I'll look around for an engineering section, thanks Dougall.
Dr, I'm 'Smokie' to friends like you. Yes plans come from Concrete Homes (http://www.concretehomesmagazine.com/) and other sources, but any home plans can easily be revised for ICF by thickening the outer walls out. Only catch is you can't build upper-level ICF walls which are not supported by ICF below.
Concrete in general, and ICF in particular, is a miracle. Nominal wall insulation is R-22, but this doesn't count the thermal mass of 6" of concrete and the fact that the house is practically air-tight. This means an effective insulation value of R-50.
I am moving toward completely fireproof construction, with steel stud interior walls, concrete-fiber exterior siding (50 year warranty), and steel roof trusses with concrete-board sheathing and concrete-fiber shingles. Insoylation (http://www.enduratite.com/ojai_1.html) in the rafters, for structural enhancement. Fireproof and seismic-safe... a 'box of rock'. With care, I could qualify them for LEED Platinum, although LEED is just too expensive in Seattle, as I've bitched in other threads, so sticking with Energy-Star and BuiltGreen.
You have to be careful with ICF brands though, as some are cheep. Also some companies make good product, but have poor support, like Amvic (http://www.amvicsystem.com/ProductCatalog.aspx). Good products are Logix (http://www.logixicf.com/client/LogixICF/Products.nsf/ProductCat?OpenView) and Arxx (http://www.arxxbuild.com/products/arxx_benefits.html).
ICF is something you can build yourself, so no framers needed. It takes less time than framing for an experienced crew, the finished home costs 5%-8% more to build, and should last from 500 to 700 years. As to disaster-proof, put this in your pipe and smoke it (http://www.nbnnews.com/NBN/issues/2005-12-12/Building+Systems/index.html).
More knowledge at +ICF Builder (http://www.icfbuilderinfo.com/current_issue.html)+, the ICF Assoc (http://www.forms.org/index.php) and Concrete Thinker (http://concretethinker.com/).
Also recently solved a technical issue with water, and now have a comprehensive water conservation system plan, but that's a whole 'nother story.
I need the best solution for decks though.
Jan 16, 2007, 4:42 AM
Cast in place concrete slabs are nice and, to get the wafer-thin slabs that you are wanting, you likely will need to use PT. The biggest drawback for your case (but I'll let you crunch costs) is the dollar amount. The next best thing (and almost every major precaster nationwide will have this product in their inventory) is a "hollow core" slab. Basically an extruded slab of some basic width (8', 10', 12') and depth (8", 10", 12"). The weight is reduced by incorporating larger diameter holes in the section and these units can span respectable distances. They are popular, for example, in multi-story office and hotels @ 4 - 8 stories in height. Some designers also PT the hollow cores but again I'll leave that to you.
Seismic-wise these units could perform as well as any other prefab system. As with any seismic-sensitive construction, details at the connections are critical and you would want to hire a PE to develop standardised connections for you. Fire ratings are also likely pretty good, but I don't deal with those types of issues. The final rating would be based on type of concrete, cover, and any other special features.
Jan 16, 2007, 8:43 PM
Ya, I've studied hollowcore (http://concretetech.com/hcbuilding.htm), and it's $4-$6/sqft. For 40' span it's 12" thick, which is a problem given that I'd need to add a drop-ceiling for services. I've looked at using the holes for ducting, but accessing the holes compromises structural integrity. All-in-all hollowcore costs more than the Hambro system, and doesn't solve any of the problems.
Suspicioning PT may be better, particularly if I can master the technical aspects of it to train my crew. But the only info I've found is in two technical guides at post-tensioning.org . I've found the local steel foundries which make fittings and tendons, but where to rent the hydraulic equipment? I doubt it can be done. And I doubt a sub would agree to just tension.
Also hoping there's another alternative I've missed. Is MHays in da house?
Jan 17, 2007, 11:14 PM
What was wrong with the Hambro system in your opinion?
I am looking at building an ICF home here in Calgary and had considered using that system... ?
I too am interested in your final conclusions in this matter.
Care to share your water conservation thoughts? :)
Jan 18, 2007, 4:35 PM
Claeren, the Hambro D500 system is the least expensive I've identified so far, although haven't priced PT. The main problem with Hambro is that for a 43' span the interfloor space is 24". Max height in Seattle is 30' (+5' for roof), and I want high ceilings if possible.
In most cases a 43' span would allow no load-bearing walls inside; and the interfloor space allows lots of room for services, and accommodates hydronic. Hambro steel is recycled, the form wood can be reused, and the concrete can have a high percentage of fly-ash. The system can be installed with a minimum of manpower and shoring, whereas PT requires full shoring. But Hambro is not exactly the most advanced system, in fact it's 80+ years old. And also I'm worried that the bottom chord is not connected to the ICF wall, which reduces seismic strength. Hambro is strong in the running, but I am looking for the most advanced solution.
I am surprised to find that Kelvin seems to be the only one here who knows anything about post-tensioning or any other alternatives. Looks like I may be in the wrong place.
Probably others here wouldn't be interested in water conservation, so I'll PM you about that.
Jan 18, 2007, 8:55 PM
What is your background Smoke?
Did you go to school to get into what you are doing or is it more on the job training?
Jan 18, 2007, 9:48 PM
Please... only conservatives address me as "Smoke". This is how I recognize them.
Jan 18, 2007, 10:31 PM
Do you mean conservatives as in 'those who conserve' or 'those who are conservative (politically)'?
Is this good or bad?
Because in Canada, calling someone a 'conservative' is exactly like calling someone a 'republican' in the US. It is the name of our major (and currently ruling) right-wing party and its supporters (of which i am not one).
Jan 18, 2007, 10:50 PM
You have me right.
I mean to say that 'those who are conservative (politically)', are enemies whom I endeavor to not help.
They know not what they miss. I believe that you would agree, given my private messages.
Jan 18, 2007, 10:55 PM
So..... Dr. Smoke?
Jan 18, 2007, 11:01 PM
So you don't agree?
Jan 18, 2007, 11:05 PM
Oh no, i do agree!
I am just confused about what to call you then?!
Claeren is actually my name so.... :)
Jan 18, 2007, 11:08 PM
But further up in this thread, I said 'friends call me Smokie'. I need some way of distinguishing friends from conservatives.
Jan 18, 2007, 11:12 PM
But further up in this thread, I said 'friends call me Smokie'. I need some way of distinguishing friends from conservatives.
OMG, i am blind!!
I totally read that post too, at least twice, and still didn't 'see it'.
Jan 18, 2007, 11:19 PM
I'm just glad that you keep my secrets.
Jan 19, 2007, 4:58 AM
Another solution is one of European origins but does not have a patented system or trademarked name (so far as I know). You would take a conventional W-section and temporarily PT it so that it cambers by some specified amount (say just enough to take the DL plus some fraction of the LL). Then you either precast a slab segment or do all your deck work as CIP. Once the concrete hardens and you achieve a new stiffer composite section, you may remove the PT and reuse it on the next series of beams. Instead of using bunches of strands braided into cable, you can use high-strength bar such as Dywidag's 105 ksi thread-bar. These bars are much easier to handle and stressing is a bit simpler.
If you precast the deck onto the beams, then you would simply leave a small interstice between each unit so that adjacent panels get grouted together. If you elected to cast the deck over several beams at once, then subsequent grouting is not required. CIP operation would likely give you better seismic capacity because you could integrate the deck directly to the wall. However, you could always develop a grouted connection to the ICF wall that does the trick too.
While it may not prove economical for smaller beams & girders, one can always utilise High Performance Steel (70 ksi) for better performance in the flanges. Therefore build your custom welded plate beams/girders or OWSJ's using 50-ksi through out but then switch to 70-ksi for the central portion of the bottom flange. What you pay in premium for the HPS more than saves you gross tonnage and additional deck DL.
You can combine these ideas together for even greater effect.
Jan 19, 2007, 8:15 PM
Wow, reusable PT. Nice.
Although the W would be cambered, it seems that removing the PT would compromise strength? Unless the deck is cast thick enough and over short enough span to be self-supporting. But this seems to compromise the economics.
Would certainly CIP.
Forwarding some of my ideas in backchannel.
Jan 20, 2007, 5:27 AM
>>> message(s) received <<<
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!
Jan 20, 2007, 3:18 PM
I am glad to find people who know their stuff, here.
An 18.3m span would cover any of my requirements, although 6,500psi concrete could not have a high percentage of recycled materials. (LEED-R)
I am not clear what you mean by:
"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."
This seems to be a W form system with concrete beams, as part of the W form? If so, they are prestressed up, so as you say when PT is released they are in tension at the bottom. This seems necessary, and like it would reduce the tension on the bottom of the beams, while maintaining a flat floor above, as opposed to no camber? If so, seems like we could put alot of camber in it, to simultaneously increase strength and reduce weight?
I presume there'd be a plate on the ends, to spread the PT forces, as the ICF walls would not be cast at this point. Idea is to cast the walls and deck together, to save time. Would it be possible to cast the deck monolitically? IOW, why do you recommend 51mm topping? Hm, maybe a 6" ICF wall couldn't support such a deck system. Likely need to be 8" or 10" of concrete.
I'm a bit concerned that this custom solution would need to be stamped, and that plan review may be delayed. Are there prescribed methods for non-bonded PT? (Best to fly under the radar, as you know, my brother)
Jan 20, 2007, 4:54 PM
I'm not sure what you need to make your concrete LEED, but a 6500 psi mix (or at least my preferred batching) is an 8-10% Silica Fume blended cement and it could also include some Fly Ash as well. Both these are otherwise "waste" materials from the steel industry, so there is a basic recycled component.
The beam I'm using is a steel wide-flange, or W. These are commonly available and easy to fabricate to desired lengths etc. Concrete could just as easily be used, but then I would suggest using permanent prestressing as opposed to temporary pre-tensioning. I'm not sure how much recycled steel makes its way back into the system these days, but I know it is not insignificant. Use of rolled steel shapes, plate girders, or just common OWSJ's are likely to have some "green" value too.
The pre-tensioning would be carried out externally - that is to say, without actually connecting to the beam (kind of like a bow and string). I only grab hold of the ends and then wrench a tension into the bar(s) which are below the beam. During the tensioning operation, the beam would camber up 152mm (6"), but under the weight of the concrete placed the expected deflection would be 122mm (4-3/4"). You would therefore have a net camber (after the concrete hardens) of 30mm/1.25". Barely noticeable on a 60' span! Another factor not discussed at this point, is long-term creep & shrinkage effects in the system. Shrinkage will occur in this system, so that small camber may eventual disappear, but creep will be almost non-existant. Creep is the process whereby materials "creep" under sustained loading - meaning that they actually get a little weaker over time. What we do to account for this process is to keep the material properties constant but add a pretend load to achieve similar deformations (believe a lot of guess work goes into this one!!). In our case, the pre-tension disappears around day-3, so it's not a real concern.
The deck could be poured semi-monolithically. I would see units being installed in 10' widths (easy to carry on the highway) and then, once installed, you simply grout a small preformed gap between adjoining panels and you're done. The topping is sometimes used to get prefab units all with the right elevation in a floor system. For example, if all the units have some large unwanted camber, you then place a monolithic topping in the field to build a nice flat level floor. Or perhaps it's a roof and you want to build drainage lines into the surface. It's entirely optional and could be taken out just as easily.
I'm not sure what kind of loads you can carry with IFC walls, but additional reinforcement could be placed, or perhaps build a pilaster (rather than thickening the whole wall) to carry the extra load. If you don't use 60' spans (perhaps you are at 20' spans), and still use 5' beam spacing, then end reactions might only be 20 - 30 kN (6700 lbs max.) at each end. Placed on a 6" x 6" bearing, it would not amount to anything unworkable.
The floor units would also perhaps a end-regions left without concrete so that they would slip right into your wall and then the floor and wall develop some fixity (this may not actually be structurally desirable - so some thought would be needed here). But you are right, there would be some end plates or anchorages at the ends of the beams (left of from the tensioning op's) that could work to become anchorages in the wall system too.
And also, you are right about the requirement for a PE stamp. This little scheme is a bit unorthodox, but certainly not without precedent. I wouldn't advocate renting a buch of hydraulic jacks and experimenting on your next project, but with some planning it is doable! Precast prestressed concrete girders (for bridges) are very common and externally post-tensioned systems have been used as well.
Apr 3, 2007, 12:48 AM
From what I've seen (and I'm not an architect yet, nor an engineer) from an existing project U/C here in Portland using the Hambro system, it allows slabs of only 4" thickness, which allows you to embed all of your radiant floor heating and other cables in it. However, it can be tough to build, and some sections crack and need to be fixed during construction... apparently it's not a big deal, however.
The problem is that it needs a 24" deep area in the ceiling for the joists... requiring a drop ceiling for residential, which isn't too bad, since most of the high-rises in Portland do that anyway to run their air ducts and the other mechanical through it.
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