older concrete beam, bent bars

[nongamer12]

i'm working on a portion of an existing concrete building from 1962. i have access to the original drawings. a concrete beam reinforcing schedule has columns for: top bars, bottom bars, bent bars, stirrup marks, and ties.

we're adding some load to a specific concrete beam. the beam is listed to have 2-#10 top bars, 4-#10 bottom bars, 3-#10 bent bars, "3104" stirrup mark, and ties are #2 @ 12" o.c.

my question is: at the mid-span of the beam, do i have 4-#10 bottom reinforcing bars, or 7-#10 bottom reinforcing bars (from adding the 3 bent bars to the 4 bottom bars)?

any other information i can provide? thanks for reading and your anticipated replies!

 

[dik]

You should have 7#10 bars, if these have been placed according to practice at the time.

 

[ron9876]

I agree but you should have a bar bending diagram on the original drawings. Be sure you consider the strength of the reinforcement for that era.

 

[nongamer12]

there is a bend diagram, showing the bend location near the supports. i'm assuming 40 ksi steel. i just was wondering about whether those "bent bars" were in addition to the bottom bars, or if the "bent bars" were the number of bottom bars to be bent near the supports.

 

[JAE]

Yes - agree with dik - the bent bars were usually intended to occur at the bottom in the midspan region and turn up at each end and provide negative, top reinforcement at the support regions.

 

[BAretired]

I agree with Dik and JAE about (3+4) = 7-#10 in the bottom between bend points. Over the support, you might have (3+3+2) = 8-#10 bars or (3+3+2+2) = 10-#10 bars depending on whether the 2-#10 top bars were simply lapped at midspan or were lapped across the support to about the quarter span from each side. The beam bending diagram should make that clear.

The unusual part of the specification, in my view is the #2@12 ties. I don't remember ever using ties and stirrups in the same beam. Some people called stirrups ties, but they were never as small as #2. Are the stirrup spacings given on the drawings?

BA

 

[nongamer12]

thanks for all the replies helping me to verify the 7-#10 in the bottom at mid-span.

the #2 @ 12" ties is just stated in the schedule; there is no beam elevation to show these pieces.

 

[msquared48]

If there is any doube take a rebar scanner to the bottom of the beam and verify the number, but I also agree with 7.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

It is possible the bars are in two layers. If so, the bent bars would likely be in the upper layer. Would a scanner detect the bars in that situation?

To analyze the beam, you must also know the amount of negative steel over the columns and the spacing of the stirrups.

BA

 

[msquared48]

Depending on the width of the beam, it could be two layers. You'd have to see what the minimum width of the beam would be for 4 and 7 bars for another indicator of what was done.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Agree that there should be 7 bars bottom and 8 bars top, in accordance with normal practice, but the details on the drawings should confirm that. I think the stirrups are #2@12", and the "3104" mark shows the configuration of the stirrup. 1/4" smooth bars were sometimes used for stirrups.

 

[BAretired]

hokie,

I suspect that "3104" represents a #3 bar bent to a configuration represented in a chart somewhere on the drawings as "104", at least that is the way I have seen it on shop drawings in the past. I believe the stirrups are #3 bars, but I do not know why #2@12 ties have been added.

#2 are plain (undeformed) bars. I doubt that they would be much use as stirrups in a beam which required so much reinforcement as 7-#10.

BA

 

[hokie66]

You are probably right. Maybe the #2 "ties" were used between the zones where shear reinforcement was actually required by the code?

 

[paddingtongreen]

Didn't we use the sloped section of the bent bars as shear reinforcing? it crossed the shear-tension cracks. I'm hesitant because I didn't work much on concrete in my early years.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

paddingtongreen,

Bent bars were considered very efficient because they saved the development length for the top bars and the bottom bars and, as you noted, acted as shear reinforcement too. Material savings were offset by higher labor costs in placing the bent bars.

BA