Historic rebar detailing 1B 1S

[LicensedToPEe]

Working on a project from 1925, concrete frame building constructed by the War Department Quartermaster Corps Construction Services at Ft Benning, GA. The 1925 concrete details use the truss (bent) bars (round in section) in conjunction with straight bars (square in section).

Q1: For a designation like this: 2 1ØS 1 1☐B, how would one read it? Does that mean (2) 1" DIA STRAIGHT bars (1) 1" SQR BENT bar? The bent bar being 1 would only end up having a single bar in the top reinforcement over the supports. See beam mark RB10 (top left corner in attached PDF).

Q2: For a designation: 2 3/4"Ø 1S 1B, how would one read it? Does that mean (2) 3/4" DIA bars, 1 STRAIGHT, 1 BENT? Does that mean there is only 1 top bar (the 1B that is being bent) on one side of the beam at the supports? (top middle of page in attached PDF).

Q3: Roughly in the middle 3/5-ths of the beam, there are no top bars (the way bent bars work). The PDF indicates 3/8"ø stirrups. Would they place say #3 or #4 (round or square) top support bars in order to hooks the stirrups or would they be wrapped around slab bars (in & out of page).

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1678446694/tips/1925_conc_framing_details_ft_benning_GA_j0eypd.pdf[/url]

 

[kipfoot]

Q1 & Q2: I read it the way you do.
Q3: I would not assume that there are additional horizontal top bars, but rather that the stirrups would/could hook onto horizontal bars in the slab.

 

[Lomarandil]

agreed with kipfoot

 

[LicensedToPEe]

@kipfoot: Are we then accepting that the would be only be a single top bar? Hasn't there been a consensus that there should be at least 2 bottom bars and 2 top bars (at supports) to keep the stirrups anchored as well as some idea of nominal continuity reinforcement? Based on the sections (see originally linked PDF), the top bar would not be centered but rather offset to the side. It just seems a bit odd as we "naturally" tend to strive for symmetry.

Looking at say RB4 section (see originally linked PDF), the profile of the bent bar is such that there is hardly any top steel at the supports (i.e., it extends like 3" before it bends down). It seems this was only some anchoring requirement. What would be the basis/logic to take one of the 2 bottom bars and bent it up, then hook into column?

 

[kipfoot]

Are we then accepting that the would be only be a single top bar?

I'm willing to accept that, yes. I also see it in, for example, the 1957 CRSI manual.

Hasn't there been a consensus that there should be at least 2 bottom bars and 2 top bars?

also yes. ACI 318 9.7.7.1 The concept of integrity steel was incorporated as part of ACI 318-89

What would be the basis/logic to take one of the 2 bottom bars and bent it up, then hook into column?

maybe to account for some negative bending that wasn't accounted for in the beam design, controlling top cracks at the support.

 

[JAE]

Most old plans I've worked with you do typically have one bent bar creating one top bar each end.
But usually, there's an adjacent span, with a similar beam with IT'S bent bar providing a single top bar each end....thus you end up having two top bars for negative moment over the support.

However, note that back in 1925, etc., the extension lengths of rebar rarely was consistent with ACI's required top bar cutoff extension lengths....usually a bit too short.