how to build?

[slatta]

i though this topic sounded familiar


http://www.chieftalk.com/showthread.php?t=25992

YOU NEED AN ENGINEER TO DESIGN A SYSTEM FOR YOU

[tompapa]

I've seen this done with steel but then you have to get into waterproofing the decks, ventilation, etc. Not to mention, I don't think I'd be able to have portions of the deck sitting on grade and the whole thing gets kind of thick. I figured poured in place concrete with the right rebar, reinforcing steel and that zypex additive in the concrete mix to seal the water out might do the trick. I guess I have to find a structural engineer with the necessary expertise. Maybe they specialize somewhat.

[hawkmoon36]

I've seen designs for spaces under garages. The amount of steel needed was pretty impressive. But a deck doesn't have the same load requirements.

[tompapa]

i though this topic sounded familiar


http://www.chieftalk.com/showthread.php?t=25992

YOU NEED AN ENGINEER TO DESIGN A SYSTEM FOR YOU

Heh, heh, yep, I'm stubborn and still trying to figure this out.

[Joe Carrick]

I've seen designs for spaces under garages. The amount of steel needed was pretty impressive. But a deck doesn't have the same load requirements.

Actually, a deck has a greater load requirement(about 2.5 times as much). Decks may need to be designed for 100 lbs/sq.ft (especially in snow country) while garage floors are only designed to support about 40 lbs./sq.ft. It's true that a garage may have "point" loads of 500-700 lbs, but the area taken by a 2500 lb vehicle is approximately 200 sq.ft. so the average load is about 12-15 lbs/sq.ft.

I typically design garages with 1-1/4" T&G plywood supported on 4x10's @16"o.c. (12' span) and they are totally solid.

If you do decide to do this with concrete, strongly consider using stainless for reinforcing. I've done a lot of restoration work on concrete structures that were reinforced with conventional steel reinforcing and I can guarantee you that given time, steel rebars will rust and cause severe structural damage whenever they are subjected to sun, wind, & water. exposed horizontal traffic surfaces are extremely vulnerable.

A concrete slab cantilevering 8' will probably need to be at least 12" thick at the support even if it's post-tensioned. Don't think for an instant that you don't need to waterproof the concrete deck.

[tompapa]

Are you familiar with xypex Joe? It's an add mix for concrete. According to the manufacturer the stuff seals concrete from water penetration. It's added at the plant.

I can see tapering the decks from the support wall outward and I'm also thinking the placement of the steel would have some effect. Boy, it'd be great if someone that comes on here was able to say, "Oh yeah, we do this all the time and here's how it's done.", heh, heh.

[Joe Carrick]

I'm not familiar with xypex per se, but if it's an epoxy based admixture then it might work. Basically epoxy changes the concrete from stone to a very strong plastic material. Anything that is just a surface sealant will eventually break down, while admixtures become an homogenous part of the concrete itself.

BTW, stay away from anything that's silicon based.

[mack]

...I'm just a dumb architect and not a structural eng., but how about actually hiding engineered trusses into the solid railing (or even exposing them) and spanning the deck floor joists between them? I assume you'd have the means of supporting the railing truss itself (or hangering it to something)... Just think of the railing as your structural element. The approach will considerably vary depending on actual building.

[Joe Carrick]

...I'm just a dumb architect and not a structural eng., but how about actually hiding engineered trusses into the solid railing (or even exposing them) and spanning the deck floor joists between them? I assume you'd have the means of supporting the railing truss itself (or hangering it to something)... Just think of the railing as your structural element. The approach will considerably vary depending on actual building.

You could do this if the distance between the railings was small enough and there was support available. In this particular design, it appears that the deck must be the supporting element.

[mack]

this is what I'm thinking of...

[tompapa]

This is the stuff Joe:

http://www.xypex.com/specs/specs.php

The truss idea sounds good if I had somewhere to hang them off of. Unfortunately that elusive sky hook has never materialized for me yet.

[tompapa]

I'm certain I can do this with steel I-beams, even further out if I want to, but there are some issues that just aren't there if it can be done by pouring this thing in place.

[ChiefChris]

...I'm just a dumb architect and not a structural eng., but how about actually hiding engineered trusses into the solid railing (or even exposing them) and spanning the deck floor joists between them? I assume you'd have the means of supporting the railing truss itself (or hangering it to something)... Just think of the railing as your structural element. The approach will considerably vary depending on actual building.

Mack - that's the difference between an architect and an engineer....the Architect will use the structural componant as part of the aesthetic design - while the engineer anticipates is being hidden behind finish materials...

That's a great idea....the steel lateral trusses could be the railing.....

[mack]

Not true, think of Calatrava, he's a Structural Eng.

The problem is, I can't think of any Prairie-style architecture with trusses exposed for aesthetic reasons... abomination, they call it.

Anyway, with such large overhangs, I'd definitely try to include the railing as a structural beam of some kind, even using concrete.

[dstig1]

Wow. This thread generated a bunch of heat!

Ya know - a post at those corners would make all this go away. And it isn't like you are trying to cantilever over Nicolett Mall here for the artistic statement of the century. The posts would detract nothing from the design, and make it go from the engineering challenge of the century to no-brainer. We use cantilevered structures in my day job - when we want a weak structure (yes there are good reasons to want those for certain non-structural applications).

You are cantilevereing a super long distance for a very low rise. Put the posts in. You won't even see them. And you won't end up building FallingDown 2...