1927 Concrete Building

[rholder98]

I am analyzing a concrete building to determine the capacity of the 8th floor. Surprisingly, I have drawings. The joists and beams are scheduled, so I have the reinforcing. The schedules list, for example, '1 7/8" dia. mark 703'. The bar bending diagram tells me what mark 703 looks like. However, each joist has one or two bars marked "S+". S+ is not in the diagram, so I think it is just additional straight bars. I just can't tell if it goes in the top or the bottom. Can anyone shed some light?

 

[rholder98]

Okay, I think "S+" is really "St." I still can't figure out where it goes.

 

[JAE]

St means that these bars are "straight" bars. In the era you describe, engineers would most often specify combinations of straight and bent bars alternating within a slab or beam.

I would thin that your 703 bar is what we would call a "crank" bar or a "truss" bar....where its left portion is up high (a top bar) - it transitions down diagonally to a bottom bar in the middle, then transitions up to a top bar on the right.

So at the bottom of your slab/beam, you have a straight bar plus a bent bar for your +As. On the left, you'd have the bent bar from your particular span, plus the bent bars from the adjacent left span for your left side -As. Similar for your right side -As.

 

[rholder98]

Thanks for the response. I agree with the "straight" designation. After consulting my Kidder-Parker, I was able to convince myself that straight bars would by default be placed in the bottom, despite the fact that it may leave stretches of the beam unreinforced in the top. However, one of the beams has 3 straight bars, 2 trussed bars, and a "W" shaped stirrup (4 verticals). The geometry of the stirrup makes it look like the three straights would go in the top.

Anyhow, without knowing the original use of the building, I have to assume office live load at best. That's what I'll tell the client, unless I find overwhelming evidence otherwise.

 

[JAE]

All the "straights" that I've ever seen are bottom bars unless specifically noted otherwise. There was a tendancy to be concerned much more with positive bending and then consider negative bending only as a secondary issue....sort of like using the bent bars simply for some crack control on top.

In any case, most of these older beams will generally be controlled by their shear capacity. Low f'c values and minimal stirrups. When you try to back-calculate an allowable live load, the shear controls most always over the positive and negative moment capacities.

 

[archeng59]

interesting, my partner and I just discussed this very thing about an old building he is reviewing. He had a set of drawings showing both straight and bent bars on the framing plans, but he could not determine if the straight bars were in the bottom or top. The way the bars were shown on the plans, it appeared that the straight bars were bottom bars because they were shown below the bent bar similar to this:

___ ___
\________/
----------------

the sizes and spacing was provided on the plan, but nothing indicating location in the slab as either top or bottom straight bars. a schedule provided the lengths of the bent bar. I do not recall if the straight bars were designated "St".

my thoughts were that the bottom bars were straight bars, not top bars as my partner thought they might be. he checked the slab two ways, with the bars as top and bottom bars. the slab checked out with the straight bars as bottom bars but not with them as top bars.

 

[samdamon]

Rholder, you might be aware of this but I'll mention it. When looking at the shear capacity of your old existing concrete beams, keep in mind that the concrete code did not require steel shear stirrups when Vu/(phi Vc) > .5 until about late in the 1960's (Not exactly sure of the date, someone please correct me if I'm wrong). Early concrete designers relied much more on the shear strength of the plain concrete. Something to consider in your review. Regards.

 

[SlideRuleEra]

rholder98 - To get an idea of floor design and live load requirements for that time period, you may want to look at the "Floor & Floor Loads" section of "Carnegie Pocket Companion - 1923" on the home page of my website (link below). That section begins on page 260 with typical live load tables on page 265.

My father always referred to the zig-zag rebar described above as "galloping steel" - Contractors hated it since it was very difficult to keep in proper position before concrete placement.

http://www.SlideRuleEra.net