In plain concrete what is the effective depth value "d"

[gasma1975]

Hi,

In a plain concrete beam, what is the value of the effective depth ? Do we take the full depth of the beam or if the ACI code is limiting us ? I don't see the article. Thanks for helping

 

[TehMightyEngineer]

I don't think there is anything limiting your d from ACI.

The effective depth is defined as "the distance from the extreme compression fiber to the centroid of the longitudinal tensile reinforcement." Therefore, it would NOT be the full depth because the centroid of your tensile reinforcement will be the centroid of the the tensile half of the section.

Thus, your d = 0.75*depth of section

EIT with BS in Civil/Structural engineering.

 

[Splitrings]

It is not d=0.75h. That would mean that concrete has the same strength in tension as it does in compression. The modulus of rupture is what governs the strength of unreinforced bending members. It is covered in 318.

 

[jdgengineer]

I don't do a lot of design for plain concrete, but why would you need to know an "effective d"? I would think that this term wouldn't apply to plain concrete. Chapter 22 of ACI 318 has provisions of how to calculate Moment capacity, shear, and axial forces of plain concrete -- none of them use the term d.

 

[TehMightyEngineer]

But wouldn't the strain diagram be uniform until cracking (failure) of the unreinforced beam?

Either way you're right, I forgot about chapter 22 which is all about plain concrete. In there it states that you use the full depth, h, unless you're cast against soil in which case you use h - 2 inches (for the uneven excavation).

EIT with BS in Civil/Structural engineering.

 

[Splitrings]

You could solve for an effective d by assuming a modulus of rupture value at the extreme tension face. You know that T times its moment arm must equal C times its moment arm. A trial and error solution until you get a balance condition (sum of the moments equals zero). I am guessing d is going to be pretty close to 2/3 of h.

 

[BAretired]

The effective depth of plain concrete has no meaning as there is no reinforcement. In CSA A23-94, the terms b and h are used to represent width and depth of section.

The assumption is that you assume linear stress-strain relationship in tension and compression. The code specifies maximum stress values in tension and compression.

For footings poured against soil, h used in calculations should be taken as 50mm (2") less than specified thickness.

BA

 

[chicopee]

The effective depth of plain concrete is not the total depth of the slab but the depth unaffacted by the material of the concrete bed. If stones are used in this bed, the concrete will penetrate some of the void between the stones and that is ineffective concrete.

 

[rowingengineer]

Why would you design a beam as a plain concrete member?

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[shobroco]

I didn't even read this thread until today because I thought the question was pointless. Now having read it, I still think so. Why would you design a beam as a plain concrete member? Does d even have a meaning in plain concrete? If you really need or want to design a plain concrete member, assume h=L/2 & there is no bending to deal with or reinforce it.

 

[cooperDBM]

As BA says, effective depth only has meaning for reinforced beams.

If you want to see how effective plain concrete beams are take a look at the stone beams on top of the Parthenon. To get a span to depth ratio greater than about 2 someone invented arches.

 

[BAretired]

Plain concrete is normally limited to stocky pedestals, short walls, pad footings, deep foundations and slabs on grade. It cannot be used where ductility is required. It is not a section of the code which I have used very often, but it comes in handy when checking existing unreinforced foundations or walls.

When designing new concrete members, I would not recommend using the Plain Concrete provisions of the code.

BA

 

[Splitrings]

The OP was asking what is the "d" value of an unreinforced concrete member. Every member subjected to bending has a "d" value. It doesn't matter if it is a concrete, wood or a steel member or your left arm. The "d" value is the distance between the resultant tensile force and the resultant compressive force. This distance times either the compressive force or the tensile force is the moment capacity. Whether you are compfortable with unreinforced concrete or not, these members do exist.

 

[chrislaope]

I also think this question is pointless (maybe a bug). However pointless or not, I fully understand why gasma1975 raised this question in OP. I was actually totally confused at OP's question in a very special moment that was many years back. And still I wish I can get a nominal correct answer. I see most people in this forum has a (structural) title, I am just puzzled that did you guys never have such a moment that you were asked such question as raised in this post?

 

[gasma1975]

The reason why I've asked that question, is that I see many strip footing without any bending reinforcement. And in all of my books it is designed as a beam with reinforcement. So I tried to figure out how to design a beam without reinforcement, to see at what point the rebar is necessary. This is why I asked how to determin the term "d" in plain concrete.

Gasma1975

 

[CANPRO]

Splitrings...I believe that 'd' is the from the resultant tensile force to the extreme compressive fibre...

 

[DST148]

@Splitrings - The OP asks about effective depth in a plain concrete. Your definition of "d" in your last post is different than the one given in ACI 318.

 

[Splitrings]

Caneit....The d value is as you said. This value has no meaning or use other than an easy way for us measure or grasp. Ultimately the value of (d-a/2) or the moment arm is used to calculate the strength of the section. The point I was trying to make was that every member, regardless of its material type, when subjected to bending forces has a moment arm or distance between the two resulatant tensile and compressive forces.

 

[Splitrings]

I tried to put quotes around d to indicate I wasn't referring to it actual definition but that ultimately we are trying to determine the moment arm. I should have made that more clear.

 

[nutte]

Every member subjected to bending has a "d" value.

Not when using "d" as defined by ACI 318. "d" is the distance from the extreme compression fiber to the centroid of the tensile reinforcing. Unreinforced concrete has no tensile reinforcing, so "d" does not apply.

As has been noted, if you want to check the capacity of an unreinforced section, use Chapter 22. The bending capacity is based on the elastic section modulus, which is based on the overall depth, "h."

... I see many strip footing without any bending reinforcement. And in all of my books it is designed as a beam with reinforcement.

You have multiple book sources showing this? Do you care to post one for us to look at? This seems highly unusual.