Flexicore Load Tables (Non Prestress vs Prestress) 1

[STR04]

I'm reviewing a Flexicore roof dated 1959. I found a load table from 1954 and compared this to a 1971 table. See attached links. The steel ratios are much higher for the 1954 compared to 1971 which leads me to believe they earlier are not prestressed and the latter is. What is everyone else's experience with Flexicore systems in the 1950's. Thanks in advance.

-STR04

 

[dik]

I'm not aware of any flexicore that wasn't pre-tensioned...

Dik

 

[BAretired]

I could not find the link to the 1954 load table.

BA

 

[Ingenuity]

The attached file has 1954, 1957, 1958, 1960, 1964, 1971 and 1978 FlexiCore load charts and data.

For the attached 1954 table (copy below) the tabulated bottom steel data notes 3/8", 7/16", 1/2", 9/16" prestressing configurations, HOWEVER, the stated AREA is the gross area of a SOLID ROD section, not a 7-wire strand of stated diameter.

Intersttingly, the FlexiCore pretension rods were stressed with a torque wrench: Photo from 1940's:

 

[Ingenuity]

And here is a old ACI paper entitled "Flexicore Floor and Roof Slabs" by G. Price, dated December 1948. It has some design and manufacture information that may assist you in your project.

 

[Ingenuity]

STR04:

I did a bit more research on the older (original?) Flexicore Co core slab planks. From the 1948 paper I referenced/attached in the above post, the steel used in the late 1940's (maybe 1950's too?) the yield stress of the steel was only 50 ksi - and therefore, (as was the case of the early pioneers of prestressed concrete) any/all prestress applied to a concrete element with such a low-tensile strength would be ZERO when you account for the most basic of concrete shrinkage strains and creep (for example: 20 ksi/29,000 ksi = 0.0069 in/in [690 microstrain] in rods at initial stressing if tensioned to 20 ksi), resulting is 100% prestress loss ==> hence, unless you know otherwise, I would assume the 1950's Flexicore slabs are indeed effectively NOT prestressed.

Also, the technique of pretensioning that was used in the late 40's by Flexicore using a torque wrench was questionable.

Also, it appears the rods are smooth, with no deformations. US Patent 2299111A [Link] seems to also infer smooth rods. The ends of the 'rods' are 'upset' with an area of 4x the rod area, however, your bond characteristics along the rods length should be based upon 'smooth'.