Flexicore 1971 - Hollow Core Slabs

[MiguelPenaWSE]

Afternoon everyone,
I am currently analyzing an existing structure (filing room with 8" thick precast slabs).
I was fortunate enough to find load tables for Flexicore Slabs dated 1971 (8"x24" section). These hollowcore slabs more-or-less match what we found on site.

Flexicore based their design on ACI 318-71 building code requirements.
I was wondering if someone had a copy of this document.

I am certain that back in the 70's, design was based on ASD requirements.
Flexicore's load table states that the Mu (kip-ft) is based on 1.4M(Dead) + 1.7M(Live). This one slab I am analyzing, according to Flexicore has a bending moment capacity of 50.75 kip-ft.

I am wondering if the 50.75 kip-ft Mu is the maximum bending capacity (no factor of safety) or if this 50.75 kip-ft has a "phi" reduced factor resistance in it.
If I had access to ACI 318-7 - I would be happy to read thru it and hopefully arrive to a conclusion.

Nowadays, we increase our loads by using load combinations (1.25DL + 1.5LL) and decrease the resistance by some "phi" factor. This can be thought as having two sets of Safety Factors.

Will my slabs fail if it happened to be loaded at 51 kip-ft instead of the tabulated 50.75 kip-ft.

Thank you!

 

[BadgerPE]

I was under the assumption that concrete design from this era would have been Ultimate Strength Design....a quick Google search turned this up. See the bottom of Page 1 for a paragraph that starts with "A.C.I. 318-71...."

Seminar on ACI 318-71

 

[Ingenuity]

Ultimate Strength Design first appeared in ACI 318 in the 1963 code, I believe.

Here is a part extract of ACI 318-71, Section 9, with some load factors and φ factors:

ACI sells a USB thumb drive of all ACI 318 documents from 1908 to 2005 - at a cost of less than one copy of the current code! Link

 

[BAretired]

I don't have a copy of ACI 318-71 but I do have a copy of ACI 318-63 which included both Working Stress Design (WSD) and Ultimate Strength Design (USD). The symbol M[sub]u[/sub] was included in USD but not WSD and was defined in Chapter 16 as ultimate resisting moment, so I would interpret the Flexicore value of M[sub]u[/sub] to be the ultimate moment capacity.

The attached PDF is taken from ACI 318-63.







BA

 

[msquared48]

I have the full '71 code and at least a portion of the '63 code in the back of one of my concrete textbooks. Marked up as H&(), but intact.

BA is correct in the timing of he new USD design, and old WSD design method in the '63 code.

Mike McCann, PE, SE (WA)

 

[dhengr]

MiguelPenaWSE:
You might do well to get (look at) copies of a couple Conc. Design textbooks from that era. M[sub]u[/sub] was the ultimate strength design moment, based on Ultimate Strength Design from that era, such as it was at that time. The 1.4D + 1.7L was right out of the ACI Code, as you can see above from Ingenuity’s attachment. There were also capacity reduction factors sprinkled throughout the design formulas. We didn’t factor loads up or use a phi (φ) value for Working Strength Design. But, most everyone was using Ult. Strength Design by the late 60's or early 70's, certainly the precasters were. The ACI-63 code had both WSD and USD and the ACI-71 (the next ed.) only had USD. Flexicore would have used Ultimate Strength Design for their planks as indicated by their 1.4D & 1.7L notation, and the moment they show as M[sub]u[/sub] is the ult. design moment, not a imminent failure moment, so I wouldn’t expect failure at 51 ft.-kips.

 

[saadpervez1]

If you have all the section details of the hollowcore slabs you can analyse it according to today's standards, there are softwares available for that.

 

[BAretired]

If the area and strength of strands is known, M[sub]u[/sub] can be calculated by hand.

BA

 

[Ingenuity]

If the area and strength of strands is known, Mu can be calculated by hand.

I agree, but those pesky precasters like to 'play' with their strand configurations (even mixing up diameters like 3/8", 7/16", 1/2" in the same plank), use 250 ksi vs 270 psi MUTS, change their stressing/jacking force etc.

For example, FLEXI-CORE have 7 different strand configurations for the same plank dimensions (above):

In extreme cases, we have undertaken invasive probing to determine strand diameter, # strands/spacing, clear cover, and even removed 5' long strand samples and had them tensile tested in a lab to determine grade. We take a 'stab' at the P[sub]j[/sub] and then calc the capacities by hand.