Creep in reinforced concrete 2

[saatsen]

I am reviewing a load-bearing wall system in which shot-crete is being used. Am I being overly critical of I comment that the design engineer has not accounted for creep deformation and shrinkage in the wall? I believe this is key to the strength of the wall. Your thoughts pleas!

 

[TehMightyEngineer]

Concrete typically doesn't creep in compression, unless you have some permanent out-of-plane loads I wouldn't worry about it. Shrinkage however could be an issue but I would imagine this is negligible for shot-crete.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[rapt]

If concrete does not creep in compression, when do you think it does creep?

 

[hokie66]

Concrete certainly does creep in compression, typically about 1.5 times the elastic shortening. But the OP's question is too vague to answer. Perhaps he can describe the structural concerns he has.

 

[saatsen]

The building is multustorey. My concern is that unlike regular in situ concrete cast with forms around till concrete hardens shotcrete does not have that benefit and therefore the effect of creep is heightened here.

 

[hokie66]

I think that particular concern is misplaced. In fact, properly designed and placed shotcrete concrete should be of better quality than concrete placed in forms, mostly because of the compactive effort involved in the spraying operation. Possibly you are confusing creep with plastic slumping or something.

 

[TehMightyEngineer]

I should rephrase my comment, by my understanding concrete creep typically isn't a concern in uniform, uniaxial compression. Flexure is typically the only time I've worried about long-term deflection (creep) of reinforced concrete.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[hokie66]

TME,
Another area where creep is important is in high rise building column shortening.

 

[rapt]

TME

As Hokie has mention, creep is very important in high rise columns and cores. It you have an asymmetric building with a concrete core are one edge with fairly low axial concrete stress under vertical loading, and columns at the far face with relatively high stresses under vertical loading, there will be a large axial shortening difference accentuated by a factor of 2 - 2.5 by creep which can result in significant rotation at the top of the building. I know of one case in the 1980's where the lift shaft was so far off straight that the lifts could not, run! Designers often use vertical prestress in the outside face of the core to compensate for this!

 

[TehMightyEngineer]

I stand corrected then. Like I said, my understanding was that concrete creep in compression was normally small enough to be ignored for most cases; though obviously in a high-rise everything gets compounded so that makes sense. I probably was misled as I don't recall the ACI 318 spec or any of my concrete textbooks even mentioning concrete creep in compression but, then again, I don't normally do high-rise concrete design so I may have overlooked it.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[BUGGAR]

Is he shotcreting a high rise? How tall is the wall?

 

[saatsen]

I'm grateful for all your responses. It's a 6-storey with 3.3m high walls.