huangyhg

lightweight concrete for composite construction
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???
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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

check aci-318 ch.11 for reduction on "vc", and ch.8 for calculating "e".
i hear that it is harder to finish.
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.
lightweight is more difficult to finish. the aggregate keeps popping up to the surface. finishers do not like it.
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.

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
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
aisc has no provision for including creep in composite steel flexural
"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?

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