Lightweight Concrete for Composite Construction 2

[abusementpark]

Are there any things to be concerned about when using lightweight concrete for composite steel construction?

It doesn't seem like there is much difference in the flexural capacities. Seems like it would be a good idea to use lightweight concrete to lighten the load on columns and foundations.

Thoughts???

 

[PEinVA]

As far as I've seen, another benefit is increased fire rating at thinner topping slabs.

I've also heard lightweight could have more cracking than normal weight, but I'm not sure if that is true or just something I "made" up.
(Check creep and shrinkage deflections!)

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[kslee1000]

Check ACI-318 Ch.11 for reduction on "Vc", and Ch.8 for calculating "E".

 

[bldgdesign]

I hear that it is harder to finish.

 

[jike]

I would consider using lightweight for fire rating, but that being said, I would estimate the cost to see which of the following options is cheaper:

Example (for 2 hour floor):
1) 3 1/4" Lightweight over deck
2) 4 1/2" Normal weight over deck
3) 2 1/2" Normal weight over deck plus fireproofing

I would only consider using lightweight for superstructure if it made a big difference in foundation type like changing from piling to spread footings.

 

[jike]

Lightweight is more difficult to finish. The aggregate keeps popping up to the surface. Finishers do not like it.

 

[lkjh345]

We have used lightweight concrete in composite construction a couple of times. Primarily, as mentioned above, to get increased fire rating without increasing the floor thickness.

We have had a few contractors price out Lt weight vs normal weight floors with similiar fire ratings. For example, a 3" deck with 2 1/2" light weight topping ( 1 Hr unprotected deck rating) versus a 3" deck with 3 1/2" normal weight concrete ( also a 1 hour rating). All the contractors agree that the normal weight floor comes out cheaper in my part of the country. The material comes out at about a wash (the light weight is more expensive per yd^3, but there is less of it), but the finisher needs to have more men at the site when they place the concrete becasue it is harder to finish.

These comparisions took into account the difference in steel framing costs, as we provided the contractors typical bay framings to price. The savings in steel could not overcome the extra cost in finishing.

Our conclusion is to use the thicker, normal weight floor unless other considerations dictate otherwise.

 

[Lion06]

I know that creep is not typically considered for deflections of composite members. That being said, S&J does talk about the effects of creep for composite members.
It references an ACI-ASCE Joint Committee that recommends using Ec/2 instead of Ec for calc'ing long term deflections. It also mentions that AASHTO-6.10.1.1b calls for Ec/3.

This part isn't related specifically to lightweight concrete, but the commentary in AISC does recommend using 0.75Iequ. for calc'ing deflection.

 

[csd72]

Structural EIT,

RE: "creep is not typically considered for deflections of composite members"

It is where I come from!

In fact I would say that creep/shrinkage is even more important in composite members as the top shrinks and the bottom doesnt at all giving additional curvature to the beam (and thus additional deflection).

 

[Lion06]

AISC has no provision for including creep in composite steel flexural member. What steel code do you use?

 

[kslee1000]

"It references an ACI-ASCE Joint Committee that recommends using Ec/2 instead of Ec for calc'ing long term deflections. It also mentions that AASHTO-6.10.1.1b calls for Ec/3."

What is the main factor for long term deflection?

 

[PEinVA]

SEIT,
While AISC has nothing in the code itself, it is mentioned in the commentary and an article by Roberto Leon is referenced. It is about serviceability and how sometimes creep and shrinkage can have an impact on your design, as well as camber and concrete placement.




RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[Lion06]

I know that it should be considered, I mention that myself and a couple references in one of my earlier posts. I was just noting that there is nothing in the actual AISC spec.