ACI 318 - History of Eq. 10-3 & Steel Stress fs in 10.6.4 1

[iv63]

We are in the process of providing an analysis of an existing, elevated reinforced concrete slab to handle additional load from rather heavy document storage file cabinets – versus typical office space loading. We’ve run into some notes on the existing building’s design drawings that we need some help in clarifying for the analysis.

Here is some background on the building and the slab:

Design/Built Time Frame: 1963
Floor Framing: Steel framing with A36 steel.
Concrete Strength: 3,000 psi @ 28 days for elevated slabs
Rebar: “All reinforcing steel shall be intermediate grade, new billet deformed bars A.S.T.M. A305, fs = 20,000 psi
Slab Thickness: 6 inches
Slab design: #4 @ 12” top and bottom / #3 temperature steel @ 18” max
Design Dead Load: 95 psf
Design Live Load: 65 psf
Span: 8’-2” c.c.

Questions:

The utilization of fs versus fy: We know that ASTM A-305 was superseded by ASTM A-615, but the use of fs is new to us. We found a reference in ACI 318-05, 10.6.4, that states fs = 2/3 fy. This would mean that fs = 20,000 equates to fy = 30,000 psi (Grade 30 steel) – which doesn’t make sense for the time frame – most reinforcement should be either Grade 40 or 60. After some local research, we found an engineer who believes the ratio was 1/2 in that time frame – which would then equate fs to fy as 40,000 psi (Grade 40). However, there is still another issue: if you use fy = 40,000, then the rebar design does not meet the minimum code requirements given by equation 10-3.
The building is in fine structural shape regarding the existing slab. It would be consistent that the building was designed and built in accordance with the code at that time. Does anyone know when Equation 10-3 came into practice? Or the other minimum rebar equation: 200*bw*d/fy? Or have any other explanation for this design?

Any help would be appreciated.

Regards,

IV

 

[jike]

Slab minimum steel is controlled by section 7.12 (referenced by 10.5.4 (ACI 318-02)

 

[hokie66]

I agree with jike, although some folks in a recent thread didn't read it that way. That provision has been essentially the same since ACI 318-63, in which it was in Section 911.

fs was used in the Working Stress Design method, and was the allowable stress. Intermediate grade bars of A15 or A160 had a yield stress of 40 ksi, and an allowable stress of 20 ksi. Structural grade had fy of 33 ksi, with fs of 18 ksi. 60 ksi steel had an allowable stress of 24 ksi. 40 ksi was still the most prevalent grade at the time, but was soon overtaken by 60 ksi.

 

[ishvaaag]

In Spain the geometrical reinforcement requirements are general and are to be respected anywhere, whilst the mechanical requirements use to be understood is proper to be enforced where reinforcement get its maximum value, be it positive or negative, and where in the absence of such minimum reinforcement a fragile failure would result. Explictly it says

"In all those cases in those that the failure of a section takes place for simple or compound flexure"

so we can interpret that in spite of the general acceptance of the prior criteria by some inspection agents, the requirement could be interpreted entirely general, especially when reinforcement follows very strictly the demand, and so the question also remains somewhat open in the spanish practice.