Posted By FBBP on 03 Feb 2012 04:42 PM

Posted By Alton on 03 Feb 2012 09:58 AM
If the wire mesh standoffs have a G90 galvanized coating, then there should not be any concern about the wire mesh rusting.  If the wire mesh has less coating than a G90, then the finish coat over the shotcrete may be enough to keep oxygen from reaching the mesh.

Alton - I was concerned about the fact that the standoffs were site formed therefore assume open end cuts. OP stated that all columns were interior which makes it somewhat of a moot point. I would question the contention that they have a value equal to a 3/8" stirrup as they would not have full one inch concrete cover neither are they in position to perform that function but again this would have been over and above design so not important. Bob

jrobicheaux - The standoffs were site formed but they are 4ft long X 3in wide and the cuts are on the 3 in side.  The galvanization on the strand closest to the concrete surface is undisturbed.  The Fc of the concrete in the pour was ordered from the ready mix plant at 4000psi.  Pretty close the the 4200psi value that the ACI handbook refers to as "waterproof".  That plus the fact these columns are inside the garage/workshop and therefore not exposed directly to weather makes my corrosion worry bead go away.



The standoffs aren't structural and weren't the contributors to the As (Area of Steel) to which I referred.  I was suggesting that the Mesh Cage which surrounds the stirrups has continuous horizontal and vertical steel strands on 1 inch centers.   When you sum the area of steel in a double layer of mesh it works out to 0.105 in^2/ft.  A piece of #3 rebar has an area of 0.110 in^2.  Therefore a double layer of mesh 1 ft tall or if you prefer a single layer 2 ft tall, has very nearly the same area of steel as a single #3 rebar.  The Mesh Cage also has vertical strands so this calculus extends in that direction as well.

The mesh standoffs were installed to insure a minimum of 1 inch of concrete cover over the stirrups and 1.5 inches over the vertical #5 rebar.  If you look again at the original picture you'll see the achieved cover is closer to 1.5 for the stirrups and 2.0 for the #5s.

I've convinced myself that the Mesh Cage is a non-trivial contributor to both the bending moment reaction and hoop stress reaction of the column.  It certainly doesn't replace the rebar in the column and I'm not suggesting it should.  It just brings extra strength to the party in a sexy space frame sort of way.

Why is the ground bare? Are you not pouring a slab until after the walls?

You are correct,

This structure isn't supported by a slab it's on a poured 2.5 ft wide and 1 ft deep footing.  Post #19 on the blog

http://waterfrontbuildinginpanamaci...erted.html

shows what they looked like before being temporarily covered up by the construction process. 

Just about the last thing in the concrete phase will be to dig out the dirt and rebound from the shotcrete process, add a stone sub course, reinforcing wire and then pour a conventional slab whose edges will rest on the footers.  The slab will be isolated from the walls and columns by fiber insets as it's only purpose is as a garage/workshop floor, no structural contributions whatsoever.

Check your inbox, I sent you some info via private messaging.

Jim

I've added and interim update to the blog and a theoretical question that some of you "SRC Guru's" might find interesting.

http://waterfrontbuildinginpana...owner.html

Placing the concrete slab after the walls, floors and roof are shotcreted will allow for a clean slab.  Rebound (shotcrete that did not stick) from the shotcreting process really can make a mess on a nice slab that you might want to expose.  Another thing to consider, is that if the area has already been excavated where the slab will go, then most of the rebound can be left in the area after spreading it out.  Less work in spreading than having to shovel it to the exterior and then carrying it off.  Been there and done that.

Thanks Alton and Jim, that is super good information to know. I am planning to stain and polish my slab, so I want it to stay pretty!

Jim,

I wonder if columns lend themselves to post-tensioning.  Using multiple wire mesh cages, post-tensioned cables and a very limited amount of rebar might work as well as the typical reinforcement with rebar cages.  Post tensioning does require specialized equipment that might be more suitable for larger projects.

Since the wire mesh is galvanized and the rebar is covered well with concrete as in your case, I do not think there should be any problems with corrision in your salt air area.  With much less concrete coverage, protection from corrosion can be had by using G90 galvanized steel rebar or by using basalt Rock Rebar (non-metal).

Jim, please try to include a close up picture of Michael in your blog so I will be able to recognize him when he is not wearing his red hard hat.

For Beams perhaps not so sure about columns.

The Civil Engineering department at my university once made a concrete diving board using post tensioning.  As you can imagine it got poor reviews from the dive team but the demonstration was effective.  It showed you could make a relatively thin steel reinforced concrete "plank" that could be significantly deformed without spalling and failing. 

I think it has something to do with preloading the concrete so that the face that would normally be seeing tension when a conventional SRC structure is deformed instead just sees less compression.  This means of course that the face that would normally be seeing compression is seeing more compression.  Perhaps some wild Frank Lloyd Wright inspired cantilevered patio or roof would benefit from post tensioning.

For a column though, the loads are axial and the static loads before any moments show up to try to bend the column are considerable.  Now that I think about it wouldn't the static axial loads on a column be preloading the concrete without tensioning the vertical steel achieving essentially the same effect?

Jim

Posted By jrobicheaux on 07 Feb 2012 06:25 PM
For Beams perhaps not so sure about columns.

The Civil Engineering department at my university once made a concrete diving board using post tensioning.  As you can imagine it got poor reviews from the dive team but the demonstration was effective.  It showed you could make a relatively thin steel reinforced concrete "plank" that could be significantly deformed without spalling and failing. 

I think it has something to do with preloading the concrete so that the face that would normally be seeing tension when a conventional SRC structure is deformed instead just sees less compression.  This means of course that the face that would normally be seeing compression is seeing more compression.  Perhaps some wild Frank Lloyd Wright inspired cantilevered patio or roof would benefit from post tensioning.

For a column though, the loads are axial and the static loads before any moments show up to try to bend the column are considerable.  Now that I think about it wouldn't the static axial loads on a column be preloading the concrete without tensioning the vertical steel achieving essentially the same effect?

Jim

Not unless you tensioned it in all four directions. Think of a 4" column say ten feet tall. Put a spreader devise at mid point say 12". Run cables to all four points and over the spreader and it will be much stronger than the 4" column. Do only two sides and it will be much weaker than the column.

I'm absolutely certain I misunderstand what you are suggesting. 

A concrete column with no fixed (embedded in concrete) steel but instead a top and bottom cap connecting external tensioning cables also running top to bottom over a mid span external strut support four times the diameter of the column?

Jim

You did;-) I was referring to a steel column similair to a jib boom as an example. My point was that unlike a slab or diving board you would have to tension a concrete column in four directions to prevent the imposed load from bending it in the other direction. You would still have to have sufficient tie steel to prevent the post tension cables from breaking out. Don't think it would be very efficient but could probably be done.

I believe you're describing a Pre Tensioning technique, Post Tensioning requires the steel be isolated from the concrete.  This is normally done by running the steel which is four or five times stronger than ordinary rebar through lubricated plastic tubes or Sabots.  After the concrete is fully cured the steel is placed in tension with the load transferred to the concrete through the end anchorages. 

In Pre Tensioning, steel is placed in tension before the concrete is poured and bonds to it.  After the concrete cures the tension on the steel is released and the resulting compressive load is passed to the concrete through the steel/concrete bond.   Pre Tensioning has to be done before the pour.  Once steel is bonded to concrete any attempt to pre load the concrete by tensioning the steel becomes a kind of "self licking lollipop".  I believe Pre Tensioning can only be done in a manufacturing context.

Post Tensioning for columns is an interesting technique that requires ultra high strength steel and concrete and some pretty sophisticated equipment.  It appears to be a niche product particularly suited to highly corrosive environments like salt water. 

This is a great discussion but for me, until concrete gets North of $500 a yard, I think I'll stick with steel of the right strength and size, in the right places and concrete of the appropriate strength in the quantities needed.

Jim 

Posted By jrobicheaux on 08 Feb 2012 07:03 PM

I believe you're describing a Pre Tensioning technique, Post Tensioning requires the steel be isolated from the concrete.  This is normally done by running the steel which is four or five times stronger than ordinary rebar through lubricated plastic tubes or Sabots.  After the concrete is fully cured the steel is placed in tension with the load transferred to the concrete through the end anchorages. 

In Pre Tensioning, steel is placed in tension before the concrete is poured and bonds to it.  After the concrete cures the tension on the steel is released and the resulting compressive load is passed to the concrete through the steel/concrete bond.   Pre Tensioning has to be done before the pour.  Once steel is bonded to concrete any attempt to pre load the concrete by tensioning the steel becomes a kind of "self licking lollipop".  I believe Pre Tensioning can only be done in a manufacturing context.

Post Tensioning for columns is an interesting technique that requires ultra high strength steel and concrete and some pretty sophisticated equipment.  It appears to be a niche product particularly suited to highly corrosive environments like salt water. 

This is a great discussion but for me, until concrete gets North of $500 a yard, I think I'll stick with steel of the right strength and size, in the right places and concrete of the appropriate strength in the quantities needed.

Jim 

Jim - I agree that that economics don't make sense for a column. Post tension can be done internally or externally. The trick is to make sure the tension is being applied to the project. If it is internal most applications still require enough steel to make sure that the concrete beam cannot split. The last post tension I did was the most fun. We had to truncate to existing post tensioned beams to remove a portion of the building. The beams where about 250' long over about 10 supports. We removed about 50 feet. The Engineer I had designed an external tensioning system for the part we had to retain. A portion of the required tension was applied to the new system and an equal portion of the old cables were cut. Than more tension on the new cables and more cutting of the old. Back and forth. His #'s were so accurate that throughout there was less than 1/8 deflection in the beam. I know because I had glass curtain wall under a portion of the one beam and had no damage.

Posted By FBBP on 08 Feb 2012 07:44 PM

Posted By jrobicheaux on 08 Feb 2012 07:03 PM

I believe you're describing a Pre Tensioning technique, Post Tensioning requires the steel be isolated from the concrete.  This is normally done by running the steel which is four or five times stronger than ordinary rebar through lubricated plastic tubes or Sabots.  After the concrete is fully cured the steel is placed in tension with the load transferred to the concrete through the end anchorages. 

In Pre Tensioning, steel is placed in tension before the concrete is poured and bonds to it.  After the concrete cures the tension on the steel is released and the resulting compressive load is passed to the concrete through the steel/concrete bond.   Pre Tensioning has to be done before the pour.  Once steel is bonded to concrete any attempt to pre load the concrete by tensioning the steel becomes a kind of "self licking lollipop".  I believe Pre Tensioning can only be done in a manufacturing context.

Post Tensioning for columns is an interesting technique that requires ultra high strength steel and concrete and some pretty sophisticated equipment.  It appears to be a niche product particularly suited to highly corrosive environments like salt water. 

This is a great discussion but for me, until concrete gets North of $500 a yard, I think I'll stick with steel of the right strength and size, in the right places and concrete of the appropriate strength in the quantities needed.

Jim 

Jim - I agree that that economics don't make sense for a column. Post tension can be done internally or externally. The trick is to make sure the tension is being applied to the project. If it is internal most applications still require enough steel to make sure that the concrete beam cannot split. The last post tension I did was the most fun. We had to truncate to existing post tensioned beams to remove a portion of the building. The beams where about 250' long over about 10 supports. We removed about 50 feet. The Engineer I had designed an external tensioning system for the part we had to retain. A portion of the required tension was applied to the new system and an equal portion of the old cables were cut. Than more tension on the new cables and more cutting of the old. Back and forth. His #'s were so accurate that throughout there was less than 1/8 deflection in the beam. I know because I had glass curtain wall under a portion of the one beam and had no damage.

Wow! building these things seems hard enough. Sectioning an existing stressed beam while still under its operational loading and preserving its integrity and function is most impressive.  An undamaged glass curtain wall is certainly the most impartial and unforgiving measure of the skill required.

Jim

Jim,
Thanks so much for letting me stop by and see your build site! I am really excited about incorporating SCIP into my plans!
Catherine

Jim,

Thanks from me too.  I really enjoyed meeting you and the architect/builder.  I learned a lot about the SCIP system and how to design for it.

Contributors to this forum Alton and Catherine visited the site on Saturday 11 Feb.  It was rewarding to finally meet both of them in person.  I've posted an interim update to the blog at:

http://waterfrontbuildinginpanamacity.blogspot.com/

I'll be visiting the site tomorrow and Thursday so there will be an update.

Jim

I can understand if you don't want to discuss pricing but what are the costs associated with this SCIP home?

For the SCIP envelope alone; floors, walls, roof and beams, the finished cost work out to approximately $14/Ft ^2 of SCIP panel area.  Keep in mind this is influenced primarily by the cost of steel, secondarily by the cost of concrete and also by labor, and location particulars.

Not every project will have the same costs as mine.

Posted By jrobicheaux on 17 Feb 2012 10:47 AM
For the SCIP envelope alone; floors, walls, roof and beams, the finished cost work out to approximately $14/Ft ^2 of SCIP panel area.  Keep in mind this is influenced primarily by the cost of steel, secondarily by the cost of concrete and also by labor, and location particulars.

Not every project will have the same costs as mine.

Are you saying that the cost of the panels, labor, shotcrete, etc., are $14 per square foot of wall space?