Equal amounts of reinforcement in doubly reinforced concrete grade beam

[Woody1515]

Hello,

I come across many examples of concrete grade beams on helical piles. Most have 2-15M bars top and bottom for flexural strength. When determining the flexural strength of the grade beam, can you simply conservatively ignore the compressive reinforcement when determining the flexural strength? Since the compressive and tension reinforcement is the same, I find this somewhat confusing. These beams are also often quite very under reinforced, which can make the neutral axis above the ”upper reinforcement”.

 

[hokie66]

Yes, ignore the "compression" reinforcement in cases like that.

 

[Woody1515]

That’s what I thought. I’m the case where you would include the compressive reinforcement in your calculations, the strain in the compressive reinforcement is equal to the strain in the concrete at the same level (strain comparability). This is most often a compressive strain. Stupid question, but does that mean the compressive reinforcement is in compression? If so, what is the force in the compressive reinforcement?

 

[EngineerEIT]

Reinforcement above the neutral axis (for positive bending) will be in compression. For strains less than the yield strain, the force will be equal to the strain in the bar * modulus of elasticity (F=E*ε, hooke's law).

 

[jayrod12]

I find for many grade beams, the design neutral axis depth is so close to the centroid of the upper bars that you get no substantial contribution. Top steel really only contributes when you have a large amount of bottom reinforcing where you are essentially approaching a balanced strain condition. And it helps with long term deflection regardless.

 

[KootK]

Since the compressive and tension reinforcement is the same, I find this somewhat confusing.

I suspect that much of your confusion may be mitigated by knowing what an experienced designer's intent would normally be with the doubly reinforced sections:

1) These are generally roughly designed and roughly constructed elements. There's usually just no great benefit to having discontinuous top steel as you might expect in above grade structure.

2) In addition to the compression steel being flexurally ineffective, it may also be flexurally invalid depending on your stirrup spacing. Most folks feel that compression steel cannot be relied upon unless less it's tied back to the body of the section at sufficiently small intervals. Most designers will not be doing that in these situations.

3) One of the main reasons for using grade beams is to smooth out any potential differential deflection between piles. Or at least smooth out the effect that such deflection would have upon the structure above. If you imagine what your moment diagram would look like under this load case, you'll see a demand for reinforcing on both sides of the beam for pretty much the entire length. Obviously, it's not possible to know in advance if a particular pile will be a high point or a low point.

4) Grade beams are often exposed to temperature change and can be highly restrained axially by the lateral restraint provided at each pile. This means that there's a high probability of developing axial tension stresses and cracking in the grade beams. Viewed in that light, having some rebar on both faces makes sense.

5) Because of #4, it's entirely possible that grade beams will have full depth tension cracking present when they are called upon to resist transverse loads from the stuff stacked upon them. When that happens, it is nice to be able to utilize your compression steel for flexural stiffness prior to closing of those tensile cracks in bending.

Because we suck at mentoring junior engineers these days, many designers assume that the only loads on their grade beams are the transverse ones induced by gravity acting upon the stuff stack upon them. Not so.

 

[Woody1515]

Thanks for the input everyone, I have a much better understanding now. Good point KootK regarding the lateral restraint of the grade beam. Is this something you would actually run the numbers for?

 

[KootK]

Is this something you would actually run the numbers for?

Nah. Once in a while I revisit the math that went into specifying the distance between control joints but that's about it.