Composite slab

[agps]

Dear coleagues!

I´m calculating a composite slab with a Zinc plate below.
How can I get the moments on the slab between the "beff"?
Should I consider it as one or should I calculate the metalic beam and the concrete slab (between beff) as separated elements?

I´m using EuroCode 4 for this.

I hope I could be explicit enough.

Thank you.

agps

 

[sdz]

What sort of zinc plate? Do you mean sheet metal structural formwork? e.g.

http://www.bluescopesteel.com.au/go/product/lysaght-bondek

 

[agps]

Hi!

Thank you for acknowledge.

Yes, exactly.

Regards,


agps

 

[sdz]

I have no knowledge of Eurocode 4.
I would expect that the manufacturer would have design information. The metal formwork initially has to support the weight of the wet concrete but after the concrete sets it should behave as a composite section.
At worst design for the metal formwork to be "lost" formwork, i.e. it contributes zero long term strength, as might be the case if it rusts out.

 

[agps]

In that case (lost formwork ´cause rusting), how to analyze the reinforcement between the beams???

Because what I understand from you is that initially it´s a composite section but in a long term will become a normal concrete reinforced slab.

How to check the reinforcement in the middle of the slab? (If you don´t mind tell me how you do it with your code, it doesn´t matter which one is it. It can´t be far from EC4)

I don´t have any information from the manufacturer

 

[rapt]

agps,

From your first email, I get the impression that you are trying to analyse for this slab in a FEM analysis. Is this correct.

1 The normal assumption with these types of slabs is for 1 way action as the stiffness in the slab direction across the decking sheets is much less than in the main spanning direction. One way action cannot be analysed in FEM! It is possible to analyse the whole slab in FEM if the reduced stiffness is allowed for in the cross direction but then it has to be reinforced in the same way.

2 If the concrete and the decking are acting together compositely, then they are connected. You cannot treat them as separate. Normally an elastic analysis of the concrete slab is done, as for RC design.

I would talk to the supplier about design methods (they might know something or they might not) or find a good book on the subject (do not know of one personally).

 

[agps]

rapt,

Thank you for your answer.

Actually I´m doing hand calculation for this case but I´ll use your advise to cross results from hand calculation to FEA.

I didn´t explain myself well but my question is very simple:

When you try to find out the 'beff' (effective span) in composite slabs for positive and negative moments (this beff depends on the span and supporting conditions), normally there´s "left-over" area of the slab between the beams (concrete+steel) verification width (beff) because the distance between 2 beams is usually smaller than the total span of the slab.

Usually I only consider the reinforcement on the support section where you find stress (negative moments).

So, is this area on the slab between the 2 beams and outside the 'beff' distance from each beam, automatically verified, using the same reinforcement from the one used over the supports or they have to be verified in a different way?

Should I consider a normal concrete slab between 2 composite beams for the analysis?

Thank you,

agps

 

[hokie66]

agps,

It sounds to me as if you are confusing the effective width of the concrete flange of the composite beam with the span of the one way slab. The one way slab span is the centre to centre spacing of the steel beams, or in some codes it may be defined as the clear distance between the steel beam flanges. The defined effective width of the concrete slab has no effect on the slab span.

 

[agps]

hokie66,

Thank you for your answer.

Can you please explain in a different way. Although I understood your explication I couldn't understand completly your conclusion.

PS - I refered to beff as effective "span". I ment "widht". Tk you

 

[hokie66]

agps,

As I understand your system, you have one way slabs spanning onto steel beams framing in the opposite direction. Whether the steel beams are composite or not does not affect the span of the slab.

If you are trying to figure how much reinforcing to specify in the slabs parallel to the beams, use the temperature and shrinkage requirements of your code. The slab does not "span" in that direction.

 

[rapt]

agps,

I thought my advice was NOT to use FEM!

I do not think any of us really understand what you are asking!

 

[agps]

rapt and hokie66,

My apolögies if I wasn´t clear enough explaining my doubt and thank you for your time.

My doubt was clear by the answers of both of you.

Sincerely

agps

PS - Can you advise me any documentation about the subject?

 

[sdz]

agps,

Your first stop should be to ask a more senior engineer where you work. Most of this stuff is pretty basic. Most senior engineers are quite willing to help out.

 

[agps]

Hello sdz,

I understand that they might be basic for your knowledge, but please understand that not all of us work in the same areas and time to time we have to work with new concepts.

Unfortunetly I don´t have anyone in my work with knowledge in composite sections to discuss this or other issues besides what is writen in EC4.

Therefore that´s the reason why I´m using this forum.

Thank you for your time as well.

Regards

 

[sdz]

contact me at aarceng@yahoo.com and I'll see what help I can give you.

 

[kslee1000]

agps:

Regarding your question on reinforcing on the strips beside the effective width, look up for "Lateral distribution of reinforcement for one way slab" on ACI and/or AASHTO code. That will answer your question.