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#21 Atavist

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Posted 27 July 2011 - 02:59 PM

I'll check on a photo bucket account so I can post some pics...

The cellar is 10x10 9 courses high plus the header blocks. got the deck built to pour on this morning, we used industrial palets for hauling engines that are made out of 2x6's with 4x4 skids, and put in a total of 49 posts alternating 2x4 and 4x4s every 16 inches... at 8 inches thick the total load is about 12000lbs which will put a measily 244lbs on each post... we capped the palets with OSB, then shingles around the edge and 6 mil plastic as a final measure to make sure ther are no leaks... we of course also added a rebar star coming together in the center and then a grid on 2foot squares... the concrete truck will be here at 6am tomorrow morning...
... I'll let you know how it goes... hopefully nothing catastrophic will happen but I'm still a bit leary about floating 12000lbs of concrete...

Edited by Atavist, 27 July 2011 - 03:01 PM.


#22 EtdBob

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Posted 27 July 2011 - 04:57 PM

8" thick and 10' x 10' is about 2.5 yards, maybe 150 sixty pound sacks or so.
Yeah, that would be a bit much to try and mix yourself!

Good luck and let us know how it turns out!
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#23 Tobus

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Posted 27 July 2011 - 05:40 PM

Without knowing the member spacing on those pallets, it's hard to say how they will hold up, but I tend to think you'll be fine. The shoring should be fine too. A 4x4 can take thousands of pounds as a simple post strut, depending on height.

Your rebar sounds light, though. You didn't say what size bars you are using, but assuming they are #4 bars (1/2 inch diameter), you're only at about half the required minimum per ACI design practices. If you used #3 bars (3/8 inch), it's even worse.

The slab may not collapse, but you may end up with deflection issues down the road.

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#24 EtdBob

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Posted 22 August 2011 - 02:53 PM

Hey Tobus, I wonder if you could recommend some specifics on rebar for this roof?
I'd like to get the rebar setup this week.

It will span some 79 inches both ways. The slab will be about 4-1/2 inches thick.
I expect a foot of soil on most of it with maybe more in the back, and our usual snow loads in winter, maybe call that as much as a maximum of I dunno, 100 lb/sf snow load.
I'd really appreciate it!
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#25 Tobus

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Posted 29 August 2011 - 08:46 AM

Bob, I apologize profusely for not getting to this sooner. Nidrah and I had to attend to some emergency family business that took up the second half of last week, and I've been away from my computer until this morning. All I could do is check the board with my phone.

So I'm back in my office today, where I can get to my engineering books and such. Here's what I've come up with on your slab.

For a two-way slab design, ACI minimum slab thickness is 5", and I wouldn't go any thinner than that. It's only 1/2" thicker than what you were looking at anyway. So I'm going to approach this using that minimum thickness.

I took some reasonable guesses at the load conditions in your area. The way we design concrete slabs is to put load factors on all the anticipated loads, in varying combinations, to arrive at a design load. These equations are intended to come up with a reasonable worst-case load combination, and are based on actual performance data from case studies. Some combinations place a higher importance on dead loads while some factor up the live loads or snow loads, etc. But I won't bore you with the specifics.

I took a stab at the anticipated loads as follows:

Concrete dead load: 62.5 PSF (this is what a 5" thick slab weighs)
Live load: 30 PSF (this accounts for people walking across the slab, wheelbarrows, and other reasonable transient loads)
Snow load: 20 PSF (this comes from the generic snow load map for your area in the ASCE 7-05 manual)
Rain load: 15 PSF (this accounts for about 3" of standing water, which may never happen if you have good drainage, but let's include it anyway)
Soil load: 188 PSF (this uses a rather heavy number for saturated clay, about 18" thick)

So as you can see, I'm trying to be conservative here. The last thing you ever want to fear is that your slab is gonna crater on you. So when I apply the load combinations to those loads, I'm coming up with a composite design load of about 372 PSF that we should design for.

That load, spanning 79", will yield a design moment of about 24.2 in-k per foot of width, and a shear force of about 1.23k per foot of width.

Because your slab will have soil in permanent contact with the top, we need to maintain a minimum cover of 3" over the rebar. And it wouldn't hurt to maintain 1-1/2" coverage on the underside to account for moisture. Exterior exposure like this is pretty hard on concrete slabs, so it's best to keep the rebar protected from moisture intrusion which will make it rust, causing spalling of the concrete and other nasty side-effects. So I've designed the slab with the rebar at least 1-1/2" from the bottom, and 3" from the top. This actually works out very well, giving us a 1/2" window to put our rebar.

I ran the numbers using #3 bars at 12" centers, but we were coming up shy. So I stepped it up to #4 bars at 12" centers and it works fine. For reference, #4 bars at 12" centers will give you a moment strength (including a 0.9 resistance factor) of 32.98 in-k. This is well above the design moment of 24.2 in-k. The factored shear strength is fine at 3.2k (well over the 1.23k design value). This gives you a nice conservatively designed slab with plenty of reserve strength.

However, if you want to run it down close to the design minimum, you can use #4s at 15" centers. This gives you a factored moment strength of 26.72 in-k, just over the design moment of 24.2 in-k. If for some reason you only have #3 bars, you'd need to use them at a maximum spacing of about 8" centers. That's a lot of bars. (For reference, in case you're not aware, a #3 bar is 3/8" diameter and a #4 bar is 1/2" diameter.)

Therefore, my recommendation is to go with the following at a minimum:

5" thick slab, using minimum 3000psi concrete, ensuring good consolidation during pouring (no voids, bubbles, etc. in the forms when placing).
#4 reinforcing bars at 15" centers (both ways), sitting on 1-1/2" chairs so that the depth from the top of slab to the center of the bars is 3-1/4".


You'll want to ensure full bearing on support walls. Ideally, you'd use hook bars doweled up from the tops of your walls that lap well into your slab at the same elevation as your slab steel. But perhaps you could give me a little more info on just exactly what your slab is bearing on, etc.? As I recall, you're using dry-stacked CMU walls. You may end up needing to pour a bond beam across the top of your wall to ensure good load distribution for proper bearing strength.

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#26 EtdBob

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Posted 29 August 2011 - 05:57 PM

Tobus, thank you very much for taking the time to calculate this for me!

Whatever your emergency was I do hope all is well, or at least as well as everything can be.

I was getting ready to do a criss-cross of # 4 rebar on essentially 16” centers, ( one per block ) right in the middle of the slab which would have been a bit on the light side so I do appreciate your input.

Rather then try and describe the hairbrained way I was planning on doing the top of the wall I’ll simply post a photo or two of it all tomorrow.

Thanks again!
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#27 LowKey

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Posted 20 September 2011 - 02:52 AM

Tobus, thank you very much for taking the time to calculate this for me!

Whatever your emergency was I do hope all is well, or at least as well as everything can be.

I was getting ready to do a criss-cross of # 4 rebar on essentially 16” centers, ( one per block ) right in the middle of the slab which would have been a bit on the light side so I do appreciate your input.

Rather then try and describe the hairbrained way I was planning on doing the top of the wall I’ll simply post a photo or two of it all tomorrow.

Thanks again!


This may be a bit late for your project, but have you looked into ferrocement for your roof?
It's something that I've just started reading up on, but it looks promising.

Basic premise is that you prop up some extruded metal lathe, force concrete into it with a trowel, then add another layer of lathe, another of concrete, ect until you reach the required thickness.
Apparently this stuff has characteristics similar to plate steel.

Tobus, feel free to correct me if I have any of that wrong. :D

Edited by LowKey, 20 September 2011 - 02:52 AM.


#28 EtdBob

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Posted 20 September 2011 - 06:33 PM

I've played around with ferro-cement a little bit and built a tool shed of the stuff -

http://www.freestead...?showtopic=6641

But my problem with making a roof out of it - especially roofs that need to support exceptionally heavy loads as a root cellars roof does - Is the difficulty of troweling in all the concrete all in one go so you don’t get “cold joints” in the finished shell.

I should think that it would be very weak indeed if it were layered up little by little, but I could be wrong – Seems some folk do it that way.

Seems to me it’s just rebar–reinforced concrete done on a small scale, and with the big stuff I know getting it poured all at once it critical for strength.

I dunno about adding successive layers of wire mesh.
Dunno how you’d tie ‘em into the shell.
In my tool shed I used a cattle panel frame backed on the inside with light stucco lath and covered on the outside with several layers of chicken wire.

The inner fine mesh of stucco lath is essential to keep the stuff you trowel on from plopping through and onto the ground. I experimented with burlap stiffened with portland cement and gave it up and used the commercial stucco lath.

Then with a crew of volunteers we made a go of it and got the whole thing coated in one day.
The finished shell was no where near waterproof so my wife and I added two more layers after that with no wire, just a stucco of cement to try and seal the structure. We eventually used a concrete coating with latex to seal it up.
Well, you can see my post on the subject for full details.

I reckon with a big crew or a stucco pumping machine a feller could coat a pretty big wire structure in one go, but I generally work with little help and on a shoe string budget and don’t always have much help or coin to spend on a project.

That’s why I like concrete block for a root cellar; it’s easy for a feller to stack ‘em bit by bit.

Another problem is you are entirely on your own when it comes to ferro-cement building. No engineer can tell you if it will hold up or cave in on you when you back fill the earth over your root cellar.
Thanks to advice Tobus gave above I’m pretty certain my root cellar will last longer then I ever will.

Eh, I like ferro-cement and my play with it further down the road – I designed a nifty ferro-cement yurt using a simple cattle-panel framework, and I’d like to make a barrel vault out of it someday. But for a root cellar I decided to stick with a ( mostly ) conventional slab.
We poured our slab last Saturday.
Anyone know how long do I have to leave the props in and forms around it anyway??
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#29 LowKey

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Posted 20 September 2011 - 09:53 PM

I've played around with ferro-cement a little bit and built a tool shed of the stuff -

http://www.freestead...?showtopic=6641

But my problem with making a roof out of it - especially roofs that need to support exceptionally heavy loads as a root cellars roof does - Is the difficulty of troweling in all the concrete all in one go so you don’t get “cold joints” in the finished shell.

I should think that it would be very weak indeed if it were layered up little by little, but I could be wrong – Seems some folk do it that way.

Seems to me it’s just rebar–reinforced concrete done on a small scale, and with the big stuff I know getting it poured all at once it critical for strength.

I dunno about adding successive layers of wire mesh.
Dunno how you’d tie ‘em into the shell.
In my tool shed I used a cattle panel frame backed on the inside with light stucco lath and covered on the outside with several layers of chicken wire.

The inner fine mesh of stucco lath is essential to keep the stuff you trowel on from plopping through and onto the ground. I experimented with burlap stiffened with portland cement and gave it up and used the commercial stucco lath.

Then with a crew of volunteers we made a go of it and got the whole thing coated in one day.
The finished shell was no where near waterproof so my wife and I added two more layers after that with no wire, just a stucco of cement to try and seal the structure. We eventually used a concrete coating with latex to seal it up.
Well, you can see my post on the subject for full details.

I reckon with a big crew or a stucco pumping machine a feller could coat a pretty big wire structure in one go, but I generally work with little help and on a shoe string budget and don’t always have much help or coin to spend on a project.

That’s why I like concrete block for a root cellar; it’s easy for a feller to stack ‘em bit by bit.

Another problem is you are entirely on your own when it comes to ferro-cement building. No engineer can tell you if it will hold up or cave in on you when you back fill the earth over your root cellar.
Thanks to advice Tobus gave above I’m pretty certain my root cellar will last longer then I ever will.

Eh, I like ferro-cement and my play with it further down the road – I designed a nifty ferro-cement yurt using a simple cattle-panel framework, and I’d like to make a barrel vault out of it someday. But for a root cellar I decided to stick with a ( mostly ) conventional slab.
We poured our slab last Saturday.
Anyone know how long do I have to leave the props in and forms around it anyway??



I've been asking questions on a ferrocement forum. Apparently the higher strength comes from using multiple layers of lath (not chickenwire) with about 1/8th layer of concrete between.
Trowel the concrete into your 1st layer of lath, then put 1/4 inch layer on top and press the next layer of mesh into it, then repeat for as many layers as you need/want.

They're saying that this type of laminated ferro cement acts a bit more like plate steel instead of concrete and as a result the shape provides the strength moreso than the concrete itself.
Again, from what they've said on that board is that most engineering you can find on FC is either from marine engineering (used to make FC boats), or stuff from the 60's & 70's, particularly from South America.

Anyway, glad to hear you finished pouring the roof on your root cellar. I think the concrete will gain most of it's strength in 28 days, but I'm not sure how much sooner than that you can remove the forms and shoring.

#30 Tobus

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Posted 26 September 2011 - 10:20 AM

Anyone know how long do I have to leave the props in and forms around it anyway??

In the commercial construction industry, we usually strip the shoring after 3 days. However, this is with a high-early mix and plenty of test cylinders to ensure that it has reached adequate strength (ACI says you can strip an elevated slab when the concrete has reached around 75% of its design strength).

Normally, 28 days is the accepted duration for concrete to reach 100% strength. But the cure rate is not linear. So it doesn't take 75% of that time for it to reach 75% strength. A lot of guys that don't use professional concrete testing services will go ahead and strip after 7 days, which is about when normal Portland Cement will reach 75% strength. But unless you're in a hurry, I would recommend leaving the shoring in place as long as possible up to 28 days. This will minimize deflection from 'green' concrete issues, as well as ensure that any variations in temperature and moisture during the cure cycle have had a chance to work their way out.

And when you do strip the shoring, be very careful. Monitor everything to ensure that you're not seeing warning signs of imminent collapse. Every shore you remove will transfer loads to somewhere, and you may hear some noises or see visual indicators of your walls beginning to pick up the load. Just be careful, man! Stripping elevated formwork can be dangerous if you don't pay close attention to everything.

I've been asking questions on a ferrocement forum. Apparently the higher strength comes from using multiple layers of lath (not chickenwire) with about 1/8th layer of concrete between.
Trowel the concrete into your 1st layer of lath, then put 1/4 inch layer on top and press the next layer of mesh into it, then repeat for as many layers as you need/want.

They're saying that this type of laminated ferro cement acts a bit more like plate steel instead of concrete and as a result the shape provides the strength moreso than the concrete itself.
Again, from what they've said on that board is that most engineering you can find on FC is either from marine engineering (used to make FC boats), or stuff from the 60's & 70's, particularly from South America.

I don't know much about ferrocement practices, but I tend to agree that what you said is correct. It sounds like the lath is doing all the structural load-bearing work and the cement paste is pretty much only acting like 'glue' to hold it together. Kinda like a piece of built-in-place plywood, with the plies of wood gaining strength from the glue holding them together. In this case, from what you describe, the concrete never really gets utilized in compression. It's mostly shear strength, holding the lath layers together, that does the job.

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#31 LowKey

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Posted 28 September 2011 - 01:06 PM

I don't know much about ferrocement practices, but I tend to agree that what you said is correct. It sounds like the lath is doing all the structural load-bearing work and the cement paste is pretty much only acting like 'glue' to hold it together. Kinda like a piece of built-in-place plywood, with the plies of wood gaining strength from the glue holding them together. In this case, from what you describe, the concrete never really gets utilized in compression. It's mostly shear strength, holding the lath layers together, that does the job.

Wouldn't the concrete entrapped in the space (void?) be under compression?
I'm not an engineer and I honestly don't know, but in a curved piece of lathing bearing a load wouldn't those spaces deform under load if they were not filled with concrete?
If that is the case, wouldn't that mean the concrete is utilized in compression?
Sitting here typing this the thought popped into my head that in a curved piece of ferro-cement those (what the heck do you call the spaces in lathe :huh: ) cells when filled with concrete might be acting as very tiny bricks in an arch.

I imagine trying to calculate the stresses, ect on something like that would be a major PITA.

#32 Tobus

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Posted 28 September 2011 - 02:43 PM

Wouldn't the concrete entrapped in the space (void?) be under compression?
I'm not an engineer and I honestly don't know, but in a curved piece of lathing bearing a load wouldn't those spaces deform under load if they were not filled with concrete?

I suppose it really depends on the actual configuration and loading.

Anything that carries a load over a span to its supports is going to experience both shear and moment. Shear is pretty self-explanatory, but moment can be confusing. It's a bending stress, but it is actually a combination of internal forces including tension, compression, and shear across the member section (in different planes than the general shear forces of the whole).

So to answer your question in a roundabout way, if we are talking about a structural member like a floor slab or a roof, when it is loaded, it will experience all sorts of stresses. In bending moment, the top fibers of the member will be in compression while the bottom fibers will be in tension. There is a neutral axis where neither compression nor tension occurs, but there is a lot of shear there, and it's "in plane" shear. What this means for a ferrocement member is that the lath on top would be in compression and the lath on bottom would be in tension, but it would be the cement grout between them that is in shear. It is indeed also experiencing a component of compression from the actual load, as well as shear in other planes.

This is a difficult subject to describe without going into Mechanics of Materials terminology and Statics equations, but hopefully you get the gist. I'll use a simple example. Take two 2x4's to span across a hole in the floor. Lay the first one flat, and stack the other on top of it. You have double the depth (1.5" plus 1.5") for a total depth of 3". But you don't get the advantage of an actual 3" deep member unless you glue them together. Why? Because the shear plane at the neutral axis doesn't allow them to work together for a composite moment of inertia without the glue. The same would be happening with the grout between the lath in the ferrocement. So you are correct that it is in compression to keep the lath from simply deforming, but it also provides a shear plane to allow the lath to work together as a composite. And that's more important for strength than the compression forces.

And actually, calculating the stresses is a bit of a PITA but it can be done. It's a pretty precise set of calcs, though, and depends on all the section properties of the components, loads, etc.

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#33 EtdBob

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Posted 28 September 2011 - 04:09 PM

I am not in any hurry and will indeed leave the props in place for a full month.
In fact, the whole project is on the back burner untill we finish cutting our winters supply of firewood.
Thanks for the advice Tobus!

Steel lath is expensive. Moreso then rebar and concrete. I dunno if a thin ferro-cement shell built that way would be any cost savings over a traditional slab.
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#34 LowKey

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Posted 30 September 2011 - 01:13 AM

Thanks for the explanation Tobus, it does make sense to me.

EtdBob, for a short span your probably right. I don't know what lath costs these days.
It might become something to look at to span larger distances in an arch or folded plate*, either by itself or as a self supporting form for something like an arch or other curved surface.

Thanks to both of you for educating me!





* What nifty terms I run into when following a tangent B)

#35 EtdBob

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Posted 30 September 2011 - 05:19 PM

This is a difficult subject to describe without going into Mechanics of Materials terminology and Statics equations, but hopefully you get the gist. I'll use a simple example. Take two 2x4's to span across a hole in the floor. Lay the first one flat, and stack the other on top of it. You have double the depth (1.5" plus 1.5") for a total depth of 3". But you don't get the advantage of an actual 3" deep member unless you glue them together. Why? Because the shear plane at the neutral axis doesn't allow them to work together for a composite moment of inertia without the glue. The same would be happening with the grout between the lath in the ferrocement. So you are correct that it is in compression to keep the lath from simply deforming, but it also provides a shear plane to allow the lath to work together as a composite. And that's more important for strength than the compression forces


And therein lies my problem with ferro-cement built up in layers as Lowkey originally described -

Basic premise is that you prop up some extruded metal lathe, force concrete into it with a trowel, then add another layer of lathe, another of concrete, ect until you reach the required thickness.


So how do you avoid “cold joints” between all the layers? The adhesion between the layers is critical as Tobus described. Ferro-cement, built up in layers over time, does not have a great lock between the layers, does it? The rough surface would allow a new layer to “key in” to an extent, but that is no where near the strength it should have, especially for a root cellar roof.
So I’m back to square one with ferro-cement. Unless the whole thing is covered with cement all at once I don’t see how it can have much strength, and having to do it all at once limits it to small structures unless one has the equipment to pump the cement onto a large form all at once.

How the heck do they build large boat hulls out of the stuff?
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#36 Tobus

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Posted 30 September 2011 - 05:56 PM

For large applications, they usually use a spray application similar to gunite. No commercial operation is going to waste labor on trowels.

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#37 LowKey

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Posted 30 September 2011 - 08:56 PM

For large applications, they usually use a spray application similar to gunite. No commercial operation is going to waste labor on trowels.


From what I've read it's either mortar sprayers (followed by a float) or shotcrete/gunite as you said.
Where labor is REALLY inexpensive they swarm the project with lots of people, which is probably why FC is more common in developing nations.
Apparently they use it quite a bit in India. I ran across several apparently sizable engineering/construction firms that do a lot of FC, with a great deal of it done as precast pannels.

It's interesting stuff, but obviously it may not be the best application for everything. I am enjoying learning about it though. Looks like it works well for things like water tanks.

#38 John173

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Posted 01 October 2011 - 06:27 PM

I suppose it really depends on the actual configuration and loading.

Anything that carries a load over a span to its supports is going to experience both shear and moment. Shear is pretty self-explanatory, but moment can be confusing. It's a bending stress, but it is actually a combination of internal forces including tension, compression, and shear across the member section (in different planes than the general shear forces of the whole).

So to answer your question in a roundabout way, if we are talking about a structural member like a floor slab or a roof, when it is loaded, it will experience all sorts of stresses. In bending moment, the top fibers of the member will be in compression while the bottom fibers will be in tension. There is a neutral axis where neither compression nor tension occurs, but there is a lot of shear there, and it's "in plane" shear. What this means for a ferrocement member is that the lath on top would be in compression and the lath on bottom would be in tension, but it would be the cement grout between them that is in shear. It is indeed also experiencing a component of compression from the actual load, as well as shear in other planes.

This is a difficult subject to describe without going into Mechanics of Materials terminology and Statics equations, but hopefully you get the gist. I'll use a simple example. Take two 2x4's to span across a hole in the floor. Lay the first one flat, and stack the other on top of it. You have double the depth (1.5" plus 1.5") for a total depth of 3". But you don't get the advantage of an actual 3" deep member unless you glue them together. Why? Because the shear plane at the neutral axis doesn't allow them to work together for a composite moment of inertia without the glue. The same would be happening with the grout between the lath in the ferrocement. So you are correct that it is in compression to keep the lath from simply deforming, but it also provides a shear plane to allow the lath to work together as a composite. And that's more important for strength than the compression forces.

And actually, calculating the stresses is a bit of a PITA but it can be done. It's a pretty precise set of calcs, though, and depends on all the section properties of the components, loads, etc.



#39 John173

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Posted 01 October 2011 - 06:33 PM

I found this forum as I was searching for advice on building a root cellar roof our of concrete and wonder if you could provide some help with design. I am building a 2 room root cellar out of 8" block. Entry room inside dimension is 4' x 8' and main room inside dimension is 8' x 8'. I plan on a 6" concrete roof that will support worst case 3' feet of sand/gravel/rock and 300" of snow per year. I'm thinking of 5/8" rebar at 16" spacing. Will this be enough?
Thank you - John

#40 Tobus

Tobus

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Posted 03 October 2011 - 07:17 AM

300" of snow????!!!!

"You know who dies for their country, kid? Fuckin' rubes, that's who." - Nucky Thompson





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