Rammed concrete suspended wall - Strut and tie analysis?

[nathan7]

Hi All

I'm designing a rammed concrete house. I've got a 10m long wall suspended in the air. It is supported at the two ends by the boundary walls and in the middle by a return wall which span full height, therefore the wall is vertically supported at the two edges and in the middle. It's restrain at the top by the roof.

My suggestion was to embed a steel beam at the bottom of the wall but both the builder and the architect are concerned with crackings.

I guess that Strut and tie is the way to go but i've done some reseach and the prediction of the compression strut isn't straight forward.

has anyone have any experience with a similar design.

any help is appreciated.

cheers

 

[jayrod12]

There would be more visible cracking without the steel beam than with it.

Design your steel beam for strict deflection requirements and there shouldn't be any considerable cracking.

Even using strut and tie, the tension force at the bottom of the wall will open up cracks on the bottom.

 

[BAretired]

What is rammed concrete?

BA

 

[canwesteng]

How are you anchoring the steel beam? Seems like developing the tension in the beam is the bigger concern over identifying the struts. Generally, the simpler the truss and the closer you are to achieving 45 degree angles in your strut and tie model, less cracking will occur before the strut and tie mechanism develops. It's a lower bound theorem unless I'm mistaken, so any combination of valid struts and ties is fine as far as meeting strength requirements.

 

[BAretired]

I've heard of rammed earth but never rammed concrete. It is not found in a quick web search, either on this site or the internet. So what is rammed concrete? Can it be considered equivalent to poured concrete?

BA

 

[KootK]

Some additional thoughts:

1) You may want the steel beam as formwork support even if you don't want to use it explicitly as the tension tie. Depends on your situation.

2) Coming up with a strut and tie model should be pretty straight forward. We can help if you post a sketch of loads and proportions. For something like this, it usually just comes down to checking nodal stresses and anchorage at the support points.

3) An alternative to a strut and tie model is to use Park & Paulay's lever arm method.

4) One of the limitations of the strut and tie method is that it doesn't have much to say about serviceability limit states. Here, you could consider your tie element as a reinforced concrete tension tie with the dimensions of the concrete being concentric to the tie reinforcing. You would calculate estimated crack widths based on the tension in the tie at service loads and take that as an upper bound estimate of cracking. Alternately, you could work out an effective depth based on your strut and tie model and then enforce code mandated crack control reinforcing limits based on that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.