Structural Member Embedded in Concrete?

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"...embed a light gauge steel stud... under tension to the point of its failure."

For what purpose? This is so far away from standard practice I cannot imagine the desired end. Will you elaborate, please?

Quite. <sigh>

Thanks for the response. I'm sorry the wording I chose was confusing. To be clear, I do not intend to set studs in concrete and then rip them apart. I was just trying to illustrate a connection good enough that the full strength of the stud could be assumed.

The stud is part of a retaining wall. The wall sits on a concrete footing. The stud is mechanically attached (with screws) to a member embedded in the footing. This attachment is made prior to pouring the footing and becomes completely encased by the footing concrete.

Presently, the calculation shows the wall's overturning moment being countered only by the mechanical fasteners attaching the stud to the footing member. There is no allowance made for the strength of the stud embedded in the concrete footing. I feel intuitively that the concrete contributes to the strength of the connection, but I am not able to quantify this effect. This is the premise of my original post.

So, in effect, a piling wall type of construction using light gage metal studs?

That's right Doorman, I think a piling wall best describes what is happening to the stud. Although the complete wall system may be better described as a cantilever wall, since there is a footing that extends back under the retained earth.

Thanks for the response.

I would think the cantilever wall Doorman shows is simpler and less expensive to construct than the composite structure you seem to be trying to put together.

As I see it the reinforcement consists of Sheetmetal fabrications. These are pretty expensive and the classic rebar would be cheaper to be manufactured. Furthermore the rebar would be bend around the corner and give much higher strength of the connection. (Which according the drawing is based on 4 screws loaded with sherring forces)

Most important for the performance of the retaining wall is the fact that the coverage of the reinforcement seems to be rather thin and the metal parts can start rusting. As a matter of fact steel rebar should be covered with at least 3cm of concrete to provide a protection against water penetration. In case of the retainingwall I recommend to add another 1.5 cm to this. The given design with -if I understand correctly- sheet metal based reinfordcements has the disadvantage that if rusting starts on the outside the water can work along the metal plate and can penetrate deep into the concret part -worse if the wall is errected in countries with harsh winters and frost. The rusting process of a metal part will create high pressure and lead to cracks in the surrounding concrete. These will open the water access along the fabrication to continue the rusting in side the retaining wall.

If standard rebar is used the outer rebars might be affected but the additional irons deeper inside the concrete are still well protected.

Another issue not covered is the formula of the concrete which will be used. There are special formulas for UnderWater concrete which are waterproof by design

Another important point is the chemistry of the concrete / iron junction- rebar is normally rusty (which is not the case in sheet metal fabrications). But the oxygen in the rust is an important point for the curing and stability of the concrete - iron junction. The result is sort of a "waterproofing" of the junction - to complicated to explain here.

The drawing does not show a) drainage pipes behind the retaining wall, b) a gravel / sandbed for the drainage pipes and the waterproofing of the backside of the retaining wall (no sharp edge on the outside, protective foil against the soil etc.).

Hi uli,

The sheet metal is cheap, we roll it out to exactly the length we need with all the cutouts in place. Allowing for labor, it is much less expensive than bending and tieing rebar. You state the rebar would bend around the corner and give much higher strength. How do you quantify that? Our formed steel has similar yield strength and cross sectional area, wouldn't the the tensile strength be similar? Your point about the screws is well taken, hence my original question regarding embedment.

I also appreciate your comment regarding corrosion. The components are coated to G90, although the concrete coverage is typically 2cm or less. We don't use "waterproof" concrete. You make an interesting comment regarding the rust and its effect on the joint.

The drawing is lacking some details. I did not intend to get into discussions aside from the steel members embedded in the footing. The rectangular shapes to the left and right of the footings are the drains. (Form-A-Drain, made by Certainteed). Of course, where prudent we use the drains and employ drain rock, filter fabric, waterproofing, etc.

OK now I see the picture and understand your intention - I personally would not buy such a structure

1) Rebar or sheet metal needs to be covered with concrete - at least 3 cm better 2 inch. Reasons: concrete if not 100% free of air bubbles - even when the crew used a vibrator - small fissures are always present. And especially the horizontal menbers have a high risk of getting rusty - and rusting iron leads to cracks penetrating deeper and deeper into the concrete. (Recently I travelled over US highways in Michigan and many bridges showed exposed rebar exact due to this fact. And understand these bridges are only exposed to (acid) rain! and able to dry off. In your case the chances of standing water (with acid pH value due to the refill soil or the eventual use of salt against ice) behind the wall are high.

Even when UV concrete is used and you drill a hole into it the first 1 to 2 cm of concrete will have a lower withstanding against the drill and the material is darker (water content) deeper you will find lighter material and higher resistance against the drilling. (less water..)

Especially the vertical sheetmetal is a real concern. a) you do not need so much metal here as it is not required due to structural requirements. And the high iron content is only reducing the strength of the concrete in this area. Think about the fact that during pooring the air bubbles can be trapped in the narrow grid reducing the stngth of the concrete - iron junction

b) if not covered well with concrete (or is not well galvanized) rusting will crumble a large area of the structure. And consider not only rust but also saltinity, acidity of the refill soil and frost affecting the structure. These can be rather nasty.

2) Compared to what is possible in the pre cast industry regarding use of CNC machine pre-bend rebar (irons and gridshaped reenforcement) as well as reuseable casting forms made of metal fabrications and screwed together you solution seems to be too costly.

Industry standard design of the precast form (e.g. "hanging" insertion of the rebar irons by using PVC tubing) will give a high productivity with low risk of failure. You could use a vibration table to improve the performance of the concrete. The possibility to add bitumen spraying and prefixing of "PVC nopps foil" to protect the wall will perfectly against water pentration. And you can do it with easy access and not in the rubble of the production site.

3) Another advantage of this apporach would be to have interlocking parts (footing area of your design) which would really stop the possibility that soil / water is seeping through from the refill side to the front - (consider the strong rainfalls due to global warming!!)

All will have a nice flat concrete apperance which can be painted from time to time or you can even have a casting front ffitting with a silicone rubber plate showing a "woodlike" structure" to please the eye when painted brown...

Transportation of the prefab part part is no added cost as your form as well as the concrete has also to reach the construction site.

Furthermore I used "new_builder" in my nick name not because as I have some 30 years exposure to even commercial building despite the fact that I am an electronics engineer. But it happend that I had to do turn-key projects from scratch.

Recently I build our own -retirement- house and the builder and I checked every cast exactly for the 4cm distance between rebar and "screwed together" metal casting form. Even some small precast "distance" plates where added to make really sure the rebar irons will stay in the correct position when pooring the concrete - and we did not have the length of rebar as in bridges (even here in Germany we have the a.m. problem!)

So now having contributed that - sorry Ican not shut up! - I will stop now and enjoy retirement.

Steel studs are engineered components designed for vertical loads. This is, as I said before, a very unusual intent for them.

Here is one of many property tables available for structural steel studs and their components. While I am still puzzled about the project, these table should help with your question.

The phrase "Light Gage Studs' infers components are 20 gage or lighter. I am assuming you are actually considering much heavier metal. Will you use galvanized? Corrosion will be a concern either way. The permanence and strength of concrete are wonderful properties, but the use must be carefully considered if these properties are to be utilized.

I am not too concerned about your drawing not showing gravel base, drainage, etc. We are not discussing those issues. This is obviously a dimensionless illustration for the discussion at hand..

Thanks Doorman, I appreciate you understanding that a sketch posted to this forum is not intended to show complete details. I am already familiar with the properties of the steel studs, my question is simply in embedding the stud in concrete to create a fully developed connection.

Yes, we are using 20 ga (33 mil) members, similar to 550S150-33. It is G90 for corrosion protection, and after pouring is fully covered by about 3/4" concrete, plus whatever finish cladding is desired.

So far this system has been used almost entirely for residential landscaping. There are just a few instances where we've gone higher than 4ft and been required to show calculations and get a permit. The principal purpose of the stud is for attaching the wall forms, the intention being that the stud and all forms will remain in place permanently. It is a big time saver to not have any form disassembly. With the stud in place the idea has been proposed that it could act as the vertical reinforcement for the retaining wall. It has roughly the same stressed area as #4 rebar, it has roughly 20 times the surface area as rebar for adhering to the concrete, it has similar yield strength (50ksi), etc. I'm simply trying to put together an intelligent argument either supporting or debunking this idea (see #5).

Thanks for the discussion.

"...the intention being that the stud and all forms will remain in place permanently."

Ah HA! So, the exposed face of the form (non-retained side) is the finished surface. Clever. I like it.

So, your question here, If I understand and you don't mind me restating it:
You are fabricating something similar to the sketch today, using deformed bar in a conventional manner as reinforcement. You are considering the possibility of counting on the steel studs as a partial or complete substitute for the rebar. There will be stud bridging for horizontal reinf.

Or am I way off in left field?

That is right Doorman. The only clarifications I would make are that the wall face form is actually expanded metal, which the concrete seeps through slightly during the pour. This concrete is then trowled to leave the form completely encased and a smooth finish on the wall. Also, we do still use conventional rebar for the horizontal reinforcement in the wall and footing.

Here is a photo of one of our mockups we use to explain the system to customers. (Horizontal rebar is not shown)

Good. The pic is useful. It is worth at least 810 words.

20 Ga G90 design thickness is .0396" (1MM), but some is the coating. Steel thickness is about .0359". This thickness varies within acceptable tolerances. Assume studs 8" web, 1-1/2" faces, 1/2" returns ≈ section area .431". #6 bar has a section area of .442". So the steel stud section is ≈ #6 deformed bar.

More assumptions:
Rebar would be spaced closer than 16" OC
Rebar is #6 minimum

Some of our PE members can advise further, but it appears the reinforcement scheme is undersized at least 50%.

As everyone here has said that this is a very unusual use of light gauge metal (steel?) wall studs, so I won't elaborate too much more about the subject. But, I do have some questions:

Q #1: What is the overall wall stem height?

Q #2: What type of backfill material are you using to fill in behind the wall?

Q #3: Are you a Licensed & Registered Professional Engineer, preferably a Civil or Structural Engineer?

Q #4: Are you designing this within your knowledge, skills, education, experience, and comfort zone?

I've done structural engineering for 34 years now, and in no way would I even consider using LGMS for a retaining wall, basically from a future corrosion standpoint, strength standpoint, and lastly from a pure economic standpoint. There are much better ways to design and construct a retaining wall that's also more durable, maintenance free, and long lasting. Structural concrete and steel deform rebar is the first choice. Second choice would involve Segmental Masonry Wall with or without a specific plastic geogrid or fabric. Third choice would involve driven steel or aluminum sheet piling, and depending on the retained height, retained soil, ground water levels, and superimposed dead & live loads, the wall may have to be anchored.......thorough design analysis and a solid geotechnical report would determine the best alternatives.

Seriously, from a Professional Engineering viewpoint you should seriously reconsider this far fetched retaining wall design and any subsequent construction of it.

Please have a great day!

===Signed CaptMoosie, BSCE/MSCE/PhD/PE

Civil, Structural & Environmental Engineer

Thanks for your response Moosie, I appreciate your questions, and have a few of my own.

Q #1: What is the overall wall stem height?

It varies, why are you concerned with the wall height instead of the retained earth height?

Q #2: What type of backfill material are you using to fill in behind the wall?

It's just native dirt, aside from gravel around the footing to facilitate drainage.

Q #3: Are you a Licensed & Registered Professional Engineer, preferably a Civil or Structural Engineer?

Here I must concede you have me at a disadvantage since I do not know the difference between Licensed and Registered. I am registered, but sometimes I say I'm licensed just to mix things up a bit. I have never described myself as registered AND licensed. Up to now I've considered them the same thing but you make it sound like they are not. What is the difference? Maybe your state is different than mine. Maybe it's time for lyn to post some more results from another of his "helpful" Google searches. Whatever, I'm not a SE so we leave it to someone else to stamp these submittals.

Q #4: Are you designing this within your knowledge, skills, education, experience, and comfort zone?

I feel pretty good about it for our typical installation, but obviously I'm looking to learn more.

We've put a lot of time into developing the simplest, most efficient process we can. For this application, I don't envision any of your "much better ways" suggestions being better. Do you have any others? We manufacture and install this "far fetched" type of wall just about every day, literally thousands of lineal feet of it are in place.

Thanks for your input.

If you want a stud set in/fixed to concrete to fail before the parent concrete fails then you need to minimise the tensile loading on the concrete itself. Best way to do that is to transfer the tensile load on the stud as deep as possible with an interference bend at the end. Can you cast J-bolts into the footing? J-bolts cast in situ are the normal way to provide studs that won't rip out of the concrete when subjected to service loading. eg steel frame building piers, steel bridges, tower feet, big plant, etc. The best way to obtain similar properties after casting is to drill the parent concrete and fix studs into the holes with an adhesive. Really deep holes. These are commonly referred to as chem-sets. There are many chemical setting anchor variants available that will suit your particular loading requirements and the site situation. Of course any anchor is only as good as the parent material. Is the footing material from a trusted source? Are you sure there actually is the expected amount of steel in it?

Thanks Wal,
The members will certainly be wet-set into the footing so no need to discuss epoxy. The footing form and rebar is all part of the monolithic wall system so its makeup is easy to confirm. I'm also trying to avoid J-bolts in order to maintain efficiency in our manufacturing process.

The procedure for calculating embed strength of structural members is pretty straighforward in ACI318-77 for slabs. But I don't know how to account for the added concrete material of the wall sitting directly on the footing. Do you have experience with this?

Studs as in vertical beams not captive pins. The photo which I've just seen makes it all clear now. That is a nice system you have there. That's a a cantilevered wall. Turn it 90 degrees and analyse the loading as you would a cantilevered floor with integral beams. Just looking at it I think the studs would fail, if overloaded, long before the embedded connection does. Empirically I would embed the studs to the full depth of the concrete in the footing. The concrete on top together with the hori rebars threaded through it would enhance the connection. Disregard this enhancement for your calculations. The enhancement would add to your factor of safety. I wouldn't depend on the enhancement to reduce your other structural obligations. A previous posting asked what type of backfill you are using. You said that you use native soil with stone only at the base for drainage. Granular backfill should be used to reduce the static loading and prevent the backfill from getting heavy when water logged. Weep holes or an engineered drainage solution at the back and bottom of the wall are not optional. I'm not sue if I have answered your questions. I like your system. Alot.

A design method for embedded steel menbers in concrete is given in the PCI Design Handbook.

The background to this method is in this link:

http://www.scribd.com/doc/48734778/Precast-concrete-connections-with-embedded-steel-mebers

Hi,

I love your innovation. One thing to keep in mind is that in reality, reinforcing bar bonds with the concrete through mechanical bonding of the raised sections on the bar. If you just have a shiny bar surface that is flat it doesn't bond to the concrete and won't then become a composite material and transfer stresses - ie you won't develop a lot of stress in the steel. I remember Professor Anderson from University of Technology of Sydney teaching me this in a class circa 1994. He was reminiscing when he was a Superintendent of Bridges and challenged a contractor or design of a Bridge which was using undeformed bar. He ended up doing his own tests and confirmed this mechanical bonding for himself. Worth keeping in mind why rebar is deformed.

The relevance for your system is that you are using steel studs that have no deformation on the surface to achieve this mechanical bonding to the concrete. Thus I would not expect the steel to develop a lot of stress from the concrete. IE under a lot of stress it will not really act as a composite material with the concrete as the concrete will strain across the surface of the gal steel and not develop stress. Therefore the concept of working out development length in the steel stud is a bit irrelevant. Sure it will develop some stress but not much.

Still - I like seeing new ideas and people who make up new things. If you've sold hundreds of these things then I guess the economics works out.

ex Civil Engineer (Australia)