Concrete Beam Advice 1

For a 20ft span I would 100% be supplying stirrups, even if they're just nominal little 6mm wires (I would use 10mm though)
They have many uses

1) Provide a construction cage to support the beam bars during the pour to ensure your required lever arms are achieved
2) Provide a sleep-at-night factor (20ft is a decent span!)
3) Protect against potential overload
4) Protect against builder fuckups
5) No doubt help to control cracking - this seems to be a high-end home if you've got a 20ft concrete beam, so why risk it?
I would see that this would do the job of the verticals you're considering adding

P.S - those loads mean nothing to me as I can't relate to 'murican loads so this, for me, is a non-numerical decision - at the end of the day, a 20ft clear span beam with no shear reinforcement looks like a mistake to me

If you're worried about cracking you could always add longitudinal bars halfway down the section too
Technically that is what we are required to do under our codes for deeper sections - it will help restrain cracks forming from the mid-depth

XR250 said:

Should I specify stirrups regardless of the shear?

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I do. #4 stirrups will fit decently in a 10" wall too.

XR250 said:

I am going to have (4)#4 at the top of the beam also to anchor it into flanking wall. Is this a good idea?

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Definitely. It's pretty common practice to provide simple support beams with top steel on the order of 0.25X - 0.33X the bottom steel.

XR250 said:

Anything else I should be thinking about?

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Make sure that your bottom steel is properly anchored at the supports. This is usually only tricky if one end of the beam bears on a perpendicular wal.

Thanks GAC and KootK - I appreciate it.

I would reduce number of the top steel bars. Your spacing will be tight, I am getting ~ 1 5/8" clear spacing. I prefer a minimum 2" clear spacing for concrete consolidation but do your code checks and standard concrete vibrators head sizes are 1 1/2" or 2"

GC-Hopi said:

I would reduce number of the top steel bars

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I already changed it to two bars based on KootK's input. I have the bars stacked two high anyway so there should have been enough room.

Thanks

I'd be hesitant to provide any more than 2 bars in the top other than for supporting the rebar cage. If you are designing as simply supported, in theory you wouldn't need any top bars. If you are providing top bars, I'd definitely think twice about connecting them to anchoring them into another wall. They WILL transfer moment. Also, take a look at your beam dimensions, it may qualify as a deep beam and require skin reinf.

Agree with the comment that they will transfer moment
Is that likely to be a bad thing? I'm imagining them fixing into a solid wall that will be able to handle the induced moment quite easily
An advantage that I see with doing so is to give stability to the end of the beam for any out-of-plane loads e.g. wind, seismic, impact, or just to provide lateral buckling restraint

MotorCity said:

Also, take a look at your beam dimensions, it may qualify as a deep beam and require skin reinf.

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I was wondering about that and am not familiar with the design of deep beams. It is 36" deep x 10" thick. Does that qualify? What would I need to do differently?

I just double checked our code, skin reinforcement doesn't kick in until the beam depth exceeds 1.0m (40")
If you could, my inclination would be to make the beam shallower but wider
Gives more space within the stirrup cage to assemble a proper-looking beam and vibrate concrete

Obviously less efficient that way so you'll need to check numbers, geometric constraints etc

In my opinion, the primary reason that one wants top steel here is to preserve the shear capacity of the beam when it inevitably cracks in negative moment near the supports. In this situation, your shear depth basically drops to zero. From a detailing perspective, I'm of the opinion that there's no such thing as a simple span member in monolithically cast concrete.

XR250 said:

It is 36" deep x 10" thick. Does that qualify?

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36" is the limit so you don't need to do anything here. If it were deeper, you'd need to calc some horizontal bars along the side faces of the beam. #5@12" or something like that. In these situations, I usually just have adjacent wall horizontals run through.

Thanks Koot.

Ties for sure to form up the downturn.

A common sense view says that, for a big beam like this 36" beam proposed, a crack could start at the top or the bottom under many load cases
Thermal, shrinkage, faceloads, impact, even just lifting stress or shoddy pouring
If there is no steel at the top then nothing restrains that crack
I would always put a bar or two at the top just to provide SOMETHING

One of the reasons that I like stirrups for this particular situation is that I feel there is some potential for differential settlement between beam supports. That might exacerbate shear and lead to unpleasantness in the absence of reinforcing.

While you are adding stirrups why not add some top bars and a layer of side bars mid depth. This would take the rod buster no time at all to place and the extra $$ in steel is negligible in my opinion.

If you really wanna save your client a few pennies you could run tighter stirrups near the supports and space them quite far apart in the middle 2/3 of the beam...or wherever you decide.

According to classic concrete fundamentals, the stirrups should arrest any diagonal shear cracks before they propagate and need to rely on the top bars.

@motorcity I'm not sure if you're responding to me there?
In my head, you can't really have stirrups without top steel, or else the stirrup cannot be anchored or supported during the pour

MotorCity said:

According to classic concrete fundamentals, the stirrups should arrest any diagonal shear cracks before they propagate and need to rely on the top bars.

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I don't believe that to be correct MC. Rather, I feel that a shear crack has to propagate across the entire cross section and induce a degree of real shear slip before the stirrups become meaningfully engaged.

What is fundamental, in my opinion, is the shear truss concept upon which we base out stirrup reinforcement. In this context, attempting to use the full shear depth of the member when top steel is not provided is akin to trying to use the blue webs of the truss shown below in the absence of the chord segments shown in green.

To an extent, this problem diminishes once the beam ends hinge and the "truss" transmogrifies into the simply supported version that is consistent with a "bottom steel only solution" and the top chord becomes a compression element across the entire span. My concern with that, however, is whether or not a catastrophic shear failure may be induced before that outcome is brought about.

There is one example out in the wild that leads me to believe that this can be a practical concern rather than merely a theoretical one. In the literature dealing with hollow core planks, there is concern for the failure shown in the second sketch below when plank ends wind up being significantly "end clamped" at their supports.