Technical Question for Architects, Engineers, Project Managers

[Dr. Smoke]

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?

[Dougall5505]

isn't there a engineering section on skyscraperpage somewhere?

[Drmyeyes]

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.

[Dr. Smoke]

I'll look around for an engineering section, thanks Dougall.

Dr, I'm 'Smokie' to friends like you. Yes plans come from Concrete Homes 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 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. Good products are Logix and Arxx.

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.

More knowledge at +ICF Builder+, the ICF Assoc and Concrete Thinker.

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.

[Kelvin]

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.

[Dr. Smoke]

Ya, I've studied hollowcore, 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?

[Claeren]

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?



Claeren.

[Dr. Smoke]

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.

[Claeren]

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?


Claeren.

[Dr. Smoke]

Please... only conservatives address me as "Smoke". This is how I recognize them.

[Claeren]

haha...

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).





Claeren.

[Dr. Smoke]

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.

[Claeren]

haha...

So..... Dr. Smoke?


eek...


Claeren.

[Dr. Smoke]

Hm, OK.

So you don't agree?

[Claeren]

Oh no, i do agree!

I am just confused about what to call you then?!


Claeren is actually my name so....


Claeren.

[Dr. Smoke]

I see.

But further up in this thread, I said 'friends call me Smokie'. I need some way of distinguishing friends from conservatives.

[Claeren]

Originally Posted by Dr. Smoke

I see.

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'.

Sorry (Smokie!)!!




Claeren.

[Dr. Smoke]

It's cool.

I'm just glad that you keep my secrets.

[Kelvin]

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.

[Dr. Smoke]

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.