Lbear
Veteran Member
Posts:2740
 |
| 17 May 2014 07:16 PM |
|
Just wanted to get some input on my rebar schedule for my 2nd floor InsulDeck. The engineer specs were: - 14" Total Floor Depth from foam to slab
- SLAB = 4" with #3 bars @ 12" o.c each way
- I-BEAM = 8" beam with one #5 bar on top and one #5 bar on bottom
- 3,000 psi mix or better
I was figuring they would have spec out two #4 bars at the bottom? I also thought they would do a 10" I-Beam and not an 8" What kind of floor deflection would I get? I wanted 1/480 or better... |
|
|
|
|
|
|
FBBP
Veteran Member
Posts:1215
|
| 17 May 2014 11:37 PM |
|
Impossible to say without redoing all the calc's. Thats what you paid the engineer for. If you want 1/480 make sure thats what he has calculated. Either trust your engineer or fire him. don't second guess. |
|
|
|
|
ksandjs
New Member
Posts:34
|
| 18 May 2014 09:18 AM |
|
what is your span? Our engineer pretty much followed the specs in the manual "Insuldeck - standard estimating reference guide" it can be downloaded from their site or quad deck. They do always add that an engineer required. |
|
|
|
|
ICFHybrid
Veteran Member
Posts:3039
|
| 18 May 2014 09:46 AM |
|
I was figuring they would have spec out two #4 bars at the bottom? That wouldn't be much for the depth of the floor. And, you must have fairly short spans because you can get into some bigger bar pretty quick when the span distance goes up. And, wondering why you have a 4" slab over (Instead of 2"-3")? Doesn't that just add unnecessary weight? I think the L/480 might be what the standard engineering tables for reinforced concrete flooring are based on. |
|
|
|
|
krom
New Member
Posts:58
|
| 18 May 2014 11:24 AM |
|
Posted By FBBP on 17 May 2014 11:37 PM
Impossible to say without redoing all the calc's. Thats what you paid the engineer for. If you want 1/480 make sure thats what he has calculated. Either trust your engineer or fire him. don't second guess.
X2 |
|
|
|
|
Lbear
Veteran Member
Posts:2740
|
| 18 May 2014 11:39 AM |
|
Posted By ksandjs on 18 May 2014 09:18 AM
what is your span? Our engineer pretty much followed the specs in the manual "Insuldeck - standard estimating reference guide" it can be downloaded from their site or quad deck. They do always add that an engineer required.
The longest span is 25' but we have some steel support columns that were added per the engineer. These steel columns will have their own footings and will be hidden within the 1st floor interior wood walls. |
|
|
|
|
Lbear
Veteran Member
Posts:2740
|
| 18 May 2014 11:52 AM |
|
Posted By ICFHybrid on 18 May 2014 09:46 AM
And, wondering why you have a 4" slab over (Instead of 2"-3")? Doesn't that just add unnecessary weight?
I think the L/480 might be what the standard engineering tables for reinforced concrete flooring are based on.
Two reasons, one is that we will have exposed 2nd story concrete floors and the engineer stated that a 2"-3" slab would definitely get significant cracks and he recommended nothing less than 4". The rebar schedule for the slab is #3 at 12" oc. It would be difficult to get proper concrete coverage on the bars once they are elevated if one only had 2" or 3" of concrete. Supposedly a 4" concrete slab when cured is a nominal 3.5" +/- The second reason is that I wanted the exposed thermal mass to be at 4" for my passive house design. |
|
|
|
|
Chris Johnson
Advanced Member
Posts:878
|
| 18 May 2014 03:46 PM |
|
Posted By Lbear on 18 May 2014 11:52 AM
Posted By ICFHybrid on 18 May 2014 09:46 AM
And, wondering why you have a 4" slab over (Instead of 2"-3")? Doesn't that just add unnecessary weight?
I think the L/480 might be what the standard engineering tables for reinforced concrete flooring are based on.
Supposedly a 4" concrete slab when cured is a nominal 3.5" +/-
a 4" concrete slab when cured...is 4"
I'm also thinking your dealing with an engineer unfamiliar with the products you are planing on using.
Don't hire a bridge engineer to design your house or else you are going to get a house built like a bridge.
You could be paying out unnecessary materials money that is not needed. |
|
| Chris Johnson - Pro ICF<br>North of 49 |
|
|
ksandjs
New Member
Posts:34
|
| 18 May 2014 05:25 PM |
|
I don't know how it prices out adding steel columns vs clear span. Our engineer clear spanned everything up to 30' using 12.5" forms, 2 #7 bottom, #6 top >4000 psi concrete 4" deck. After cured it is still 4" deck. It is interesting to look at their span charts as they give longer spans with 4" deck vs thinner decks. We considered putting in a support wall so we could use thinner decking less bar, but decided against it. Not sure what is the best price. |
|
|
|
|
Lbear
Veteran Member
Posts:2740
|
| 19 May 2014 12:37 AM |
|
Posted By ksandjs on 18 May 2014 05:25 PM
I don't know how it prices out adding steel columns vs clear span. Our engineer clear spanned everything up to 30' using 12.5" forms, 2 #7 bottom, #6 top >4000 psi concrete 4" deck. After cured it is still 4" deck. It is interesting to look at their span charts as they give longer spans with 4" deck vs thinner decks. We considered putting in a support wall so we could use thinner decking less bar, but decided against it. Not sure what is the best price.
I take that back, I was advised that the steel columns are for the roof loads and NOT for the InsulDeck. Therefore the span I have is 25' without supports. How deep were your beams? Were they 8" or 10"? Your span was only 5' more than mine but the rebar size your engineer called for is a lot larger and more bars. I have one #5 bottom and one #5 top while you have two #7's bottom and one #6 top. I can still change mine and I will call the engineer tomorrow to see what he has to say. |
|
|
|
|
Lbear
Veteran Member
Posts:2740
|
| 19 May 2014 12:46 AM |
|
ICFHybrid-
What was your rebar schedule and beam depth for your InsulDeck? Also, how far were the spans?
|
|
|
|
|
Lbear
Veteran Member
Posts:2740
|
| 19 May 2014 02:08 AM |
|
Posted By ksandjs on 18 May 2014 05:25 PM
It is interesting to look at their span charts as they give longer spans with 4" deck vs thinner decks.
That is correct. I noticed the same thing. A 4" slab on top will span higher distances than the 3" slabs and especially the 2" slabs According to InsulDeck on a 40psf load - 8" beam: 2" slab = can span 22' 4" slab = can span 25' So somehow that extra 2" of concrete slab gives it an extra 3' of spanning capabilities. So in essence the slab thickness plays a structural role in the system's spanning capabilities. |
|
|
|
|
ksandjs
New Member
Posts:34
|
| 19 May 2014 07:09 AM |
|
We used 12.5" forms that give 10.5" beam for both floor and roof, but no load bearing interior walls in basement or 1st floor. I am not an engineer, I just followed what they specified. I have had various comment about the rebar we are using (inspector, architect, other builders) in the footer, walls and floor/roof , however I'm not sure they are used to seeing an all concrete shell and understand the weight.
|
|
|
|
|
ICFHybrid
Veteran Member
Posts:3039
|
| 19 May 2014 08:56 AM |
|
What was your rebar schedule and beam depth for your InsulDeck? Also, how far were the spans? It is "7+3" which is 7" thick InsulDeck panels yielding a 5" high joist plus a three inch slab over. The steel was 2 sticks of #4 at the bottom on 14' multiple spans. |
|
|
|
|
Lbear
Veteran Member
Posts:2740
|
| 19 May 2014 05:35 PM |
|
I spoke to the engineer and was told that the one #5 bar on the bottom is more than adequate and that the InsulDeck span charts are based on a "pinned" and not a "fixed" condition. He also stated that "too much rebar" can cause a negative condition? I am not sure what that meant. Welcome to the world of engineer ego's I guess?? He was not really willing to "budge" on the schedule.
|
|
|
|
|
Alton
Veteran Member
Posts:2164
|
| 19 May 2014 05:42 PM |
|
About the only negative condition that I can think of is that the extra rebar would add weight to the assembly. |
|
Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
|
|
FBBP
Veteran Member
Posts:1215
|
| 19 May 2014 09:32 PM |
|
Bear - rebar for concrete is all about the cross section of steel buried in the concrete. If you take the area of a #5 bar, 5/8 inch diameter, as compared to two #4 bars, 1/2" times two, the actual area in cross section is not that much different.
To much rebar as in too many pieces can cause issues with the concrete not properly consolidating around them. However there is lots of room at the bottom of an insuldeck beam to properly enclose two pieces of steel.
Most rebar schedules are way over kill cause the engineer is to lazy to do the actual calc's. It is seldom an issue of an engineer calling for less then needed.
With regard to the depth of the deck, as you increase the depth, you increase the distance between the tension and the compression planes. The further apart the planes the longer the span assuming there is enough material in each plane to balance. |
|
|
|
|
Chris Johnson
Advanced Member
Posts:878
|
| 19 May 2014 09:52 PM |
|
If an engineer is not familiar with a product they generally 'overload' it slightly erring on the side of caution, this is why if your engineer is not familiar with Insuldeck or ICF construction then perhaps it is time to look for someone who is. I am not discounting any engineer here, but a guy who designs highways and bridges everyday or high-rise buildings is not well versed in residential houses.
There is such a thing as too much rebar, I have seen many times people refuse to pay an engineer to do something and just load up lintels with rebar...I have not seen a problem yet personally, but have heard of the catastrophic failures that can occur. I.E. typical concrete failures for various reason can be identified by a layman, 'Hey, there is a crack in that concrete' or 'I see that concrete beam sagging', with too much rebar it can look all fine and dandy and suddenly BOOM, it fails without warning and someone dies because there was no previous evidence of a problem. |
|
| Chris Johnson - Pro ICF<br>North of 49 |
|
|
Lbear
Veteran Member
Posts:2740
|
| 19 May 2014 10:44 PM |
|
Posted By FBBP on 19 May 2014 09:32 PM
Bear - rebar for concrete is all about the cross section of steel buried in the concrete. If you take the area of a #5 bar, 5/8 inch diameter, as compared to two #4 bars, 1/2" times two, the actual area in cross section is not that much different.
To much rebar as in too many pieces can cause issues with the concrete not properly consolidating around them. However there is lots of room at the bottom of an insuldeck beam to properly enclose two pieces of steel.
Most rebar schedules are way over kill cause the engineer is to lazy to do the actual calc's. It is seldom an issue of an engineer calling for less then needed.
Have you ever heard of rebar being called on the top portion of the InsulDeck beam? I did not see this shown in the InsulDeck manual. How would one even install a #5 bar on the top portion? You can't use a rebar chair so how do you suspend it in the air like that? I was also told by the engineer that the 4" slab on top of the beam portion plays a vital role in the beam/floor strength during the monolithic pour. Whatever "topping" you put over the beam becomes the total beam height. For example, if you have a 10" InsulDeck form with an 8" beam pocket and pour a 4" slab on top, you really end up with a 12" beam, not an 8" beam. Likewise if you only pour a 2" slab on top, you will end up with only a 10" beam. |
|
|
|
|
FBBP
Veteran Member
Posts:1215
|
| 19 May 2014 11:20 PM |
|
Bear - you will always need bar in the slab to tie it into the walls and to control shrink.
Does the engineer not call for ties or stirrups along the beams tying the top and bottom bars together?
••• Whatever "topping" you put over the beam becomes the total beam height.••• Exactly right. Bigger distance between tension and compression.
Picture the bottom bars as anchored to the walls. Now you load the middle of the floor. If there is not sufficient slab material to control the compressive strength of the load trying to pull the walls together so that the bottom bars can dip, there will be failure. The slab will buckle up and the beam will fail down. There must always be a balance between tension and compression.
Now if you have just the bottom bar with the slab poured over it 12 inches above it, there is nothing to stop the bottom bar from bursting out of the concrete beam bottom. If on the other hand, you place a bar near the top of the slab and wrap ties around the top bar down the side and around the bottom bar, now the bottom bar cannot burst free without pulling the top bar down as well. And yes this is laid out in most of the deck formulas.
Go to http://www.litedeck.com/pdf/Lite-Deck_Book.pdf Pages 9 and foreward show examples of "shear reinforcing"
|
|
|
|
|