32 story building framed with lightweight concrete

[hippo11]

I thought this was kind of unusual...we are working on a renovation of an SOM bldg built in 1970 that is 32 stories tall, regular RC...but it turns out the RC is all lightweight...

I guess in 1970 maybe lightweight concrete wasn't as expensive, and it was worth it in material savings...

I had never seen this before...has anyone else?

 

[LittleWheels]

I had to maintain a couple of 1970's highway bridges built with lightweight aggregate. The aggregate was weaker than the paste, lots of reinforcement corrosion and consequential spalling, it wasn't pretty...

 

[hokie66]

It was quite common in the 1970's to consider lightweight concrete for building floors, not the columns. Savings in columns and footings would sometimes offset the additional cost of the lightweight concrete, which as I remember was about 10-15%, but that would have varied with location.

 

[concretemasonry]

There is a wide range of properties for lightweight concrete. Then density is not a measure of the real properties.

There are many different types of lightweight aggregates with different properties.

If you structure was built using a high quality rotary kiln aggregate it will perform well similar to some of the lightweight bridge decks. Other lightweight aggregates at that time may really have been by-products that are heavier and required a higher percentage and a lower percentage of normal, natural aggregates.

Unfortunately the lightweight aggregate industry at that time had a wide range of properties and levels of quality since structural lightweight concrete was a small part of their business.

Since you are working on the project it may be well to find out what type of aggregate was used if you are concerned about strength or durability.

 

[jike]

As hokie66 said, the floors were probably lightweight but the columns and footing normal weight. I have designed many buildings with lightweight floors.

The lightweight floors can be thinner and still achieve the required fire-rating. There also can be some additional benefit in the foundations depending upon the height and weight of the building.

It is a cost thing. As local costs change, decisions regarding the most economical structure will change. Our area doesn't use much lightweight anymore because of the higher cost of the thinner lightweight floors compared to the lower cost of the thicker normal weight floors.

 

[lkjh345]

When analyzing lightweight concrete structures from this era, don't forget the provisions regarding shear currently found in ACI 318 Section 11.2 ( if your in the US or other appropriate codes in your area ).

Shear in lightweight concrete was not particularly well understood or accounted for in the 70's. These provisions were added because of failures in lightweight concrete structures built in the 70's.

 

[hippo11]

Thanks everyone.

In the dwgs (from 1968) it specifies "Fsp=5.5"...

What does this mean? Does this mean

my fct=fc'/5.5,

in this case fct=4000 psi/5.5=727 psi?

 

[hokie66]

You need to refer to the ACI Code in use at the time the drawings were done, I think ACI 318-63. Fsp was defined as the ratio of splitting tensile strength to the square root of compressive strength. Fsp was a factor in the provisions used to determine the limiting shear stress for lightweight aggregate concretes. The factor was used for both working stress and ultimate strength design. The value of Fsp was limited to 4.0 without testing to determine the splitting tensile strength for the particular aggregate used, so testing would have been required to get a value of 5.5.