Rigid or semi-rigid con. in RC structures 1

[Baltali]

What is the effect of shear reinforcement or bending reinforcement in beam-column joints for degree of fixity?

How can we decide whether we use semi-rigid or rigid in modelling?

Could you refer me some books or articles about this topic?

 

[JAE]

I would use it always as rigid in reinforced concrete frames.

THe moment of inertia of various members should be adjusted, though, for cracked or semi-cracked conditions (i.e. the Ie in ACI 318).

 

[Baltali]

By the way, some people say that anchorage length and bent part of the anchored rebar affects the fixity.

What do you think about it?

 

[slickdeals]

I think now we are getting into full blown finite element analysis with solid elements, rebar, bonding etc. etc. I don't think anyone accounts for that in real design as budgets won't support it.

In order to model rigidity of every joint, you would have to assign springs at each one of these. There has been a lot of research on panel joint deformation for steel structures using various models such as the Krawinker and Helmut models.

As JAE mentioned, I have used rigid connections in concrete frames. Note that most programs don't calculate stiffness based on reinforcing. It just uses gross section properties.

 

[rowingengineer]

I half agree with the above, while joints are not normally modelled in a similar way to steel in regards to semi-rigid connections. Concrete must be modelled to account cracking of the members, aka concrete is a semi-rigid material. There are reinforcing arrangements that ensure the joints reach the design moments and shears.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[csd72]

Baltali,

These are all very fundamental questions, I suggest that you do some reading on this to familiarise yourself with concrete design and detailing. There are some free publications on the internet if you google it.

This is not the place to learn about concrete design except for specific questions.

Just remember that the computer program relies on the accuracy of your input - garbage in garbage out- despite what they claim the most important design still happens in your head.

 

[Baltali]

@csd72

thank for your response but,

I've already wrote in my post that "Could you refer me some books or articles about this topic?"

That means you should not advice me to read something.
You should advice me to read some spesific book or article.

Please google it and I strongly believe that you can not find any related article about my question. Because I do not post here without searching.

Besides this, if you have any idea about this very simple topic, i am gladly welcome to hear your comments.

If you even read previous posts, you can see that there is not already a single answer to my question.

Regards,

 

[rowingengineer]

Baltali,
The problem is your question uses term normally reserved for steel structures, where you have a rigid type structure for the members but semi-rigid connections.

Concrete on the other hand is not a rigid structure (it can be rigid at low loadings or posttensioned to a point), concrete is semi rigid. This means the semi-rigid modelling undertaken in steel structure is not used for concrete structures. There are however reinforcing models that help to ensure that your joint reflects analysis.

Try searching for opening of knee joints, and efficiency, I have attached a discussion in concrete international to help you start check out the second last page.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[csd72]

http://www.concretecentre.com/onlin...ry/publication_details.aspx?PublicationId=613

http://www.concrete.org.uk/

http://www.concretecentre.com/pdf/CCIP_Concrete_Building_Scheme.pdf

http://www.cement.org/buildings/res...e=&searchterm=&searchtype=&cat2ID=2&pagenum=2

 

[Baltali]

@rowingengineer

thank you for your attachment. that would be quite useful.

@csd72

thank you but I could not find any related topic in your references.


Alright guys. Lets make my question more spesific.

In a slab-shear wall connection, is there any differences in moment transfer (full moment transfer or semi) if I supply a 200cm top rebar anchorage in to the wall or I supply a 100cm top rebar anchorage?

 

[rowingengineer]

Yes, there would be a difference; normally you can't develop full moment transfer from the shear wall to the slab. How you would get a correct stiffness for the modelling I don’t know, too many issues for my liking, I normally make a fairly conservative assumption.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[rapt]

Baltali,

Now you have changed it to a reinforcement detailing question.

We would need a plan view and section of the connection detail you are suggesting showing slab reinforcement and wall reinforcement to comment fully.

 

[Baltali]

@rapt

no it is related with the first question in my thread.

"What is the effect of shear reinforcement or bending reinforcement in beam-column joints for degree of fixity?"

 

[Baltali]

Please see the dwg document in the attachment.

There are two figures that shows only the top reinforcement in a slab-column joint. (both bottom reinforcement of the slab and the bending,shear reinforcement of the column are still there but are not shown.)

Is there any difference in the moment transfer through slab to the column between those figures?

Should I model both of these connections FIXED in my model?
Or the figure 2 should have a semi rigid connection?

 

[slickdeals]

Baltali,
How do you plan on modeling a semi rigid connection? What software are you using? What kind of a building is this? What are your other assumptions?

It seems like you are spending too much time analyzing something that you may not be able to properly model instead of spending the time to properly detail.

 

[Baltali]

Asssume the joint detail is going to be formed as in the figure 2 in the real construction. Besides this, the model has been prepared with fixed connection.

Now, if a moment release occurs on that joint in reality, then there will be an increase in the span moment. In other words the moment graph will shift downwards. But i modelled the structure with a fix joint and i put enough reinforcement at the midspan and it is not safe enough anymore.

 

[hokie66]

Simple...put more reinforcement at midspan. Too much thinking about modeling, not enough about how the structure works. No matter what you do in the way of detailing, you will lose effectiveness at a corner, and the amount you lose depends on a lot of things which you may not be able to control. The reference given by rowingengineer above is a good starter.

 

[irizarry]

Baltalli, unless your doing nonlinear, plastic analysis, you should assume the connection to be rigid (full moment transmission). If you need to take into consideration cracked or semi rigid sections, normally codes use decreased Inertia factors (like 0.35 for beams and 0.7 for columns), but this is only where extreme events are likely to occur and the structure is likely to surpass the elastic, linear limits.

 

[rowingengineer]

Agree with hokie, more reo required in the mid span. i don't agree with Irizarry on this topic.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[rapt]

Baltali,

Either of those 2 details could give a full connection. Depends on Bar diameter, hook/cog bend details, wall thickness and wether or not the bar can develop full capacity at the centre of the wall.

But there are other variables. How is it being constructed? Are the walls slipformed with shear pockets, or are they poured to the underside of the slab at each floor before the slab is poured? If slipformed, I would not assume full connection

I agree with Hokie and RowingEngineer that it is conservative to assume that you cannot achieve a full connection and assume some of the negative moment capacity is lost and the moments redistribute to the bottom of the slab.

But this is a basic engineering question for you to answer. Can you develop the full bar capacity at the location where it is needed? It depends on the design code you are using and thus the bend details for the hook/cog. ACI318 development hook/cog riules for bars develop the full capacity. As3600 rules develop 50% unless you go to a lot more detail. BS8110 you need to calculate the development depending on the situation. Your design code gives you ways of determining this. You do not need to ask us.

As we do not know the answers to most of the questions I have raised, we cannot answer you. But it is simple engineering, you should not have to ask! Or if you need to ask, ask your supervisor.