Moments after rebar placement 5

[howarts]

If a beam or column has certain moments and after you put rebars.. what would happen to the moments? For instance. A beam has certain moment at midpan, when you put the rebar to address the tension (bottom bars).. what would happen to the moments? I assume the beam would no longer deflect for the moments? But there should still be moment.. how much would be the retained moments or actual bending of the beam if there is sufficient tension bars?

 

[hokie66]

Moment with reinforcement = moment without reinforcement...unless the beam fractures without the reinforcement, in which case there would be zero moment.

 

[howarts]

Moment with reinforcement = moment without reinforcement...unless the beam fractures without the reinforcement, in which case there would be zero moment.

With reinforcement, there is less deflection/deformation.. but can you cite an example where moments/bending is not proportional to deformation?

Can you give a daily example of bending/moment that is similar in two objects but one of them has more deformation (in beam/column case no rebars)?

 

[hokie66]

Are you just trying to be silly, or argumentative, or both?

 

[howarts]

A more senior structural engineer told me when you put rebars in the beam, you straighten the beam and the moment/bending get less.. so I'm trying to find basis in his argument. But for beam with tension bars, the deflection is less. Isn't the moment same as deflection? I just want another example of an object where the bending or moment is not proportional to deflection.

 

[IDS]

Is this related to your Etabs question?

If you are talking about analysis, the deflection is controlled by the flexural stiffness of the section, which is defined at the start of the analysis, and is usually based on the uncracked concrete section, ignoring reinforcement. Adding the reinforcement doesn't increase the uncracked stiffness very much, and the stiffness after cracking is much less than the uncracked stiffness. For normal design purposes the design is based on an elastic analysis, using the uncracked section, with some moment re-distribution allowed, as defined in the applicable code.

If you are talking about adding reinforcement to an existing concrete structure, we need more details of what is actually being done.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[howarts]

No. It's not about adding reinforcement to existing concrete structure.

Let's take the example of a simple beam supported by two columns on the left and right. He stated that by putting more bars in the negative moments at support.. he can straighten the beam and control the moments at midspan and using less tension bars at midspan.

This won't work if moments before and after rebar placement of beam is the same.

 

[molibden]

This is moment redistribution. I know it only for continuous beam. Beam section cracks above support so the moment redistributes to midspan. So you get bigger moment at midspan and smaller moment at support.
For a rigid frame I am not sure if we have it in the code.

 

[klaus.]

@howarts
I think you are mixing different things up
By changing the stiffness of a section you can change the moment distribution in a system unless the system is statistical determined like a beam on 2 supports
Now in concrete structure you can modify the section stiffness by the rebar

==> So yes, by modifying rebar you can influence the bending moments unless the system is statically determined




best regards
Klaus

 

[KootK]

I think that you would benefit from googling the concept of "static moment". The static moment remains unchanged regardless of the designer's choice to distribute that moment between mid-span and support moments. As IDS mentioned, rebar has little impact on the distribution of elastic analysis moments. What rebar does do is allow the designer some degree of flexibility in deciding upon a plastic distribution of design moments to be used to satisfy overall equilibrium.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[howarts]

I think that you would benefit from googling the concept of "static moment". The static moment remains unchanged regardless of the designer's choice to distribute that moment between mid-span and support moments. As IDS mentioned, rebar has little impact on the distribution of elastic analysis moments. What rebar does do is allow the designer some degree of flexibility in deciding upon a plastic distribution of design moments to be used to satisfy overall equilibrium.

rebar has little impact on the distribution of elastic analysis moments? But without rebar, the moments of beam won't even reach elastic moments (service level moments). As review "Plain concrete beams are inefficientas flexural members because the tensile strength in bending (modulus of rupture) is a small fraction of the compression strength. As a consequence, such beams fail on the tension side at low loads long before the strength of the concrete on the compression side has been fully utilized. For this reason, steel reinforcing bars are placed on the tension side as close to the extreme tension fiber as is compatible with proper fire and corrosion protection of the steel."

So what rebar does is to increase the moments before the concrete beam fails.
But for steel i-beam, there is no rebar.
A reinforced concrete beam won't be a beam at all with any elastic moment capacity without any rebars.

 

[klaus.]

Well ...concrete without rebar is pretty useless ....


best regards
Klaus

 

[KootK]

So what rebar does is to increase the moments before the concrete beam fails.

Sounds like you've got it all figured out then.

Recognize that the distribution of moment capacity is a distinctly different thing from the distribution of moment demand. Your original question pertained to the distribution of moment demand. Rebar disposition primarily affects the distribution of moment capacity.

As a hyperbolic example, consider a gravity loaded, simple span beam with lots of rebar up top but none in the bottom. It has tons of moment capacity! But none of that capacity is distributed in a way that aligns with the distribution of moment demand so it's worthless.

So did you investigate tge static moment concept as I recommended? Do you understand it? Did it not address your fundamental question in any way?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

Kootk - do you have a specific document on the "static moment" concept you could link to? I think just browsing the first links that come up in a Google search would be extremely confusing for someone not experienced in structural analysis and design.

Actually it was extremely confusing for someone who is experienced in structural analysis and design as well.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[howarts]

I found the structural plan made by my friend.

I asked him why there were massive bars at midspan and only few at support. He said he wanted to strengthen the beam at midspan so the beam would be more straight at the support and it needs few bars. I told him that may be wrong and told him to redesigned it based on normal procedure. He is actually a beginner like me but he is one year ahead in field experience in structural engineering. He designed moment frames yet he didn't even know the meaning of plastic hinges and capacity design. That's why I'm questioning his reasoning that straightening the midspan can straighten the support.. do others also do this? is it valid moment redistribution?

(Kook.. i can't find any "static moment" thing at google related to structural design.. so hope you can address IDS question too above, thanks)

 

[BAretired]

I think you were correct in suggesting that your friend re-design it using normal procedure. Using his method, a continuous beam would develop significant cracking near the supports because the elastic moment would be much greater than the moment capacity of the beam at supports. His beam may not fail but it would not meet normal practice.

BA

 

[mtu1972]

I'm sorry but those details really suck. 4 top bars but 2 are almost halfway down. 9 "bottom" bars and only 3 are at the bottom. It's time to review your textbook from Reinforced Concrete Design I; or go and buy one if neither of you have one.

gjc

 

[rapt]

I think you need to read a good text book on the fundamentals or reinforced concrete design and learn how it all works. It is beyond responders on this website to provide the amount of training/education in reinforced concrete design that you need.

 

[howarts]

I know many basic. My friend has now designed 5 buildings and supervising them simultaneously at present. I told him why the bars at midspan are not at bottom and mid height and he replied "the area covered by actual moment reaches into the mid height of the beam section at midspan so it's ok the rebars reaches up at mid height". I told him the bottom bars can yield first before the upper bars because of the distance to the neutral axis. But he doesn't seem to understand the concepts of neutral axis, strain and stress. So it's frustrating. He said he finished civil engineering and took up masteral in structuring engineer. Maybe he forgot his lessons. But i'll make him review it and let him get a teacher to refresh his memory or his buildings would pretty dangerous. Thank you to all your suggestions especially about the difference between moment demand and moment capacity. He seemed to mix the two. But I'm still googling about static moments so hope Kootk can shed more light on it as IDS also requested.

 

[hokie66]

If you can tell us where these buildings are located that your friend has designed, it would be helpful, so we can be sure to avoid them.