one way slab behavior 2

[nada2013]

Hi, I just want to ask about the behavior of a slab supported in two sides only and has (L/S) less than 2? Can I treat it as a one way slab? I mean if I use ANSYS program shall I get a curvature in the transverse side of it?

 

[JedClampett]

It's hard to figure out what you're asking without a sketch. What is L and S? But if you have a slab supported on two sides, it should be treated as a slab supported on two sides and one way analysis used.
As far as your last question, I'm not sure what program ANSYS is, but you should be doing one way analysis by hand.

 

[nada2013]

Thank you Jedclampett, I mean by L the length (span) of slab and S the width (short direction). I read in many concrete design books that the slab treated as one way slab if it is supported in two sides regardless of its longer span to shorter span ratio, but I am not quite sure if this principal applicable according to “ plate and shells theory” which used in the ANSYS program.

 

[BigInch]

One way slab is more applicable to established concrete design principles for slabs with reinforcement directed between supports, rather than what method might be employed by any analysis program, be it ANSYS, Mathcad, Excel, or your pencil for that matter. Doing it "your way" would require that concrete actually has some usable tensile strength when used without reinforcement in the unsupported direction.

You must always ensure that your chosen model is appropriate for the material behavior, the response you hope to mimic, and the boundary and loading conditions. Never attempt the reverse.

Independent events are seldomly independent.

 

[nada2013]

Thank you too BigInch , but if anyone use Timoshenko method to analysis a slab which has longer span to shorter span ratio less than 2 and supported on two sides only and then use FE(Finite Element) model in any program such as ANSYS, difference results will appeared due to the two way spanning action. Now I am a researcher and want to study the behavior of one way slab, my models slabs has (L/s) less than 2, can I analyzed them as one way slab or not? This is exactly my question, and I know than one way slab according to Timoshenko neglect the two way spanning near supports?

 

[BigInch]

I'll answer a question with a question. What's the ultimate tensile bending stress of unreinforced concrete? Now calculate the ultimate bending moment of that section of concrete using that ultimate stress. What is that in relation to the allowed bending moment of reinforced concrete section in the other direction?

Independent events are seldomly independent.

 

[nada2013]

Sorry but I am talking about Reinforced concrete slab not unreinforced, when I used finite element analysis the slab concave in both direction, this phenomena called two way action, but Timoshenko said that when slab supported in two sides only it will behave as one way slab regardless to (L/S) ratio “concave in one direction only”, now I must find the moment by treated this slab as one way or two way?

 

[BigInch]

NO NO NO You confuse concrete design principles with the L/S span resistance capacity of homogenous materials.

Whether a concrete slab is "one or two way" depends on the direction(s) that the reinforcement runs in. It is a one way slab when the slab has tension reinforcement spanning between two supports. A slab is "two way" when tension steel runs in two directions. The supports can be 2, on opposite sides, or 4, one support at each edge, however it is the tension steel running both ways that makes a concrete slab 2 way. A concrete slab with tension steel in one direction only, can be supported on all sides, but still only be one-way, because it has no capacity to resist bending moment perpendicular to the direction of reinforcement.

If the material is homogenous in each direction, YES, then whether it behaves as one way, or two, way depends on L/S and location of supports, but that is not true for concrete with tension steel spanning only between 2 opposing supports. Concrete is not homogenous in both directions when the reinforcement is only in one direction.

Independent events are seldomly independent.

 

[nada2013]

Ok, I understand this point, now if the slab strengthen with reinforced mesh this steel mesh different in each direction “ as you said non homogeneous” and has (L/S) less than 2 But supported in two sides only, is it one way slab or two way slab?

 

[BigInch]

I myself would ignore the bars not spanning between the two supported sides and consider the slab as one-way. Why complicate your analysis? To save 0.1 mm2/m in the perpendicular direction, I don't think so. I wouldn't put perpendicular steel in there in the first place, unless only to provide temperature and shrinkage steel in that direction, so I would ignore any two-way bending moment action there might be. I would design for bending resistance only one way, between supports. I can see you want to complicate your life to save the 0.1 mm2 and it's not worth any more of my time continuing to go on about it, so please use ANSYS, but don't take all of next week to do it.

Independent events are seldomly independent.

 

[nada2013]

Hahahah. Thank you so much for your expression, I just want to be sure about the validity of one way analysis and I see now it is suitable.

 

[BigInch]

Very much so. But if you choose to use one-way action with 4 edge supports, make sure you put the rebar in the shortest direction, as that is the direction that will be "stiffer" against bending and thereby will be sapping up the majority of the bending stress.

Hint: for simple load case on flat square or circular plates, and you want to design using 2-way action, look for Roark's formulas. Much faster than ANSYS.

Independent events are seldomly independent.

 

[hokie66]

I think I disagree with what BigInch is saying, which is a rare occurrence.

If a slab is supported on two sides only, it is a one way slab, no matter the reinforcement. The loading can change this somewhat, as there will be purely one way action for uniform loading, but for point loads, two way action is required to distribute the load over a width.

For a slab supported on four sides, two way action always exists, also regardless of the reinforcement, but is sometimes ignored in modelling.

Reinforced concrete does behave as a homogeneous material...until it cracks.

 

[BigInch]

I don't think we disagree. Point load distribution over a width, I've always considered to occur within thewidth described by the punching planes determined by the slab thickness, and ignored the rest of the width, even though it might indeed be effective. I do tend to go for the practical quick solution, and conceed that there will always some difference between practical and theoretical.

Independent events are seldomly independent.