1931 Vintage Rebar & Concrete 1

[KarlT]

I am working on an analysis of an existing University Building that was constructed in Winnipeg, Manitoba, Canada in 1931. The client would like to throw an 8000 # mechanical unit on the existing concrete joist roof.

I do have a drawing of the original building. The roof construction note is as follows... 14" + 2-1/2", As = 0.84 square inches per square foot, min. joist width = 5". The span is approx. 36' and there is a 4" bridging joist at mid-span.

I would assume from this that the joists are a total of 16.5" deep (14" joist plus 2.5" slab), with a minimum width of 5" at the bottom of the joist. I am confused as to the square inches per square foot designation?

What grade/strength of steel and concrete would I assume? Didn't they use smooth bars back then? How would that affect the development length calculations?

I am going doen to look at the joists this afternoon. I may end up having them scanned to confirm As?

 

[JAE]

KarlT -
I've never seen As referred to in those terms - usually there is a beam/joist schedule where individual straight and bent bars are indicated along with any stirrups.

The straight bars were the bottom bars extending from support to support. The bent bars (or truss bars or crank bars) were bars that extended along the top of the beam/joist at the left support, turned down at some angle at about the 1/4 span point, ran horizontal along the bottom of the beam/joist to the other 1/4 span point, and then turned up at the same angle to proceed over the right support in the top of the beam/joist.

So bottom As = sum of bent bar plus straight bar.

Top As = sum of bent bar plus bent bar from adjacent span.

Usually the bent bars do not extend the proper distance as now required by ACI or your own code so there are problem areas near these 1/4 points.

Steel grade - according to CRSI - for 1931, you had the following bars - (ASTM A15)

Grade 33 (Structural) - Fy = 33ksi and Fu = 55 ksi
Grade 40 (Intermediate) - Fy = 40 ksi and Fu = 70 ksi
Grade 50 (Hard) - Fy = 50 ksi and Fu = 80 ksi

 

[KarlT]

Thanks JAE, I appreciate your input and advice - you have a lot of experience and I have learned quite a bit from you this last year!

What do you figure is a reasonable estimate of concrete strengths at that time? I suppose I could also test to confirm, but is 20MPa (2900 psi) reasonable to assume?

 

[JAE]

In the US - you can almost always assume structural concrete is at least 3000 psi unless its been damaged by salts. For flexural checks the f'c doesn't really affect the results much. For shear - its critical and many times governs the results.

You can easily take core samples and test them to get a better handle. Many times our clients need our response and are reluctant to pay extra for the testing lab to core and test. We many times report on the results, based on the assumed f'c (like 3000 psi) and then suggest a higher load capacity could be determined with tests (if shear controlled).