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[b]ABAQUS CAE - tensile behavior input for RC Slab[/b]

helmi30 (Structural) (OP)
18 Jun 10 5:33
Dear Abaqus users,

Iam modeling and analysing a simply supported RC Slab with Concrete Damaged Plasticity on Abaqus CAE. Concrete slab is modelled with C3D20 solid elements and steel bars are modelled on Truss element T3D2 which are embedded into concrete. My results show very high force vs deflection graph, i.e. the loading force is high (around 800-100kN) when the behavior of slab begins to yield. These values are far out when compared to my experimental test results which is around 80kN. I have chosen a tensile stress of concrete to a minimum value of 0.05 to 0.1% of fcu (41.24N/mm2), i.e. around 2.06 to 4.12 N/mm2, for Abaqus input tensile behavior. My query is that how to lower down the force vs graph curve so that it would match the experimental tests results? Thanks in advance for any kind helps.  
mrgoldthorpe (Mechanical)
18 Jun 10 12:57
I assume you mean the maximum force predicted is in the range 80 kN to 1000 kN? If that's the case, a factor of 10 on the test results is surprisingly high (If that's the case, you have posted that mistake in three different places in eng-tips without noticing - so are you scrutinising your model sufficiently?)

I suggest you first of all do a 'back of the envelope' calculation to check what maximum load might be expected.

I also strongly suggest that you carefully check that you are modelling the test correctly in terms of things like material properties, truss element attributes, model boundary conditions, position/distribution of load, proper account taken of symmetry conditions (if any), coupling between the concrete C3D20 and T3D2 re-inforcing elements.
 
helmi30 (Structural) (OP)
19 Jun 10 5:17
Thank you mrgoldthorpe for your reply and comments. Actually, Abaqus gives yield load of around 800 - 1000kN higher than experimental test results with 80kN yield load. To the best of my knowledge, I have thoroughly and meticuluosly checked my models, boundary conditions, elements type. I have done lots of trial and errors with different parameters, but Abaqus has shown a very stiff and high yield load as compared to experimental tests. Please advice me further, thanks
ASoliman (Geotechnical)
15 Jul 10 8:03
Here is a thought; If you have imbedded the entire reinforcement truss part into the concrete slab part, then it will be all constrained during the load application as if it is entireley mobilized to resist the loads.  In reality not the entire reinforcement rod is anchored into the concrete since there is a cracked zone where the bending moment is max.  So I suggest you split the reinforcement bars and only embed the end segments.  For example, if you have a slab span of 12m, split the reinforcement bar into three segments each is 4m long and only embed the end segments and leave out the mid segment.  This way it will simulate the actual behavior better.
Elabbasi (Mechanical)
15 Jul 10 15:43
What are dimensions of the concrete block, and the number and diameter of the steel bars?
helmi30 (Structural) (OP)
21 Jul 10 13:57
ASoliman - Do you mean leaving out the middle segment of steel bars as if there is totally no steel rods at middle segment or to choose different element type rather than truss elements?

Concrete slab dimension is 1600*1600*150mm thk. 6 numbers of 12mm diameter bars in each direction. Uniformly distributed loading at the top entire area of slab. How to set Abaqus so that the analysis will stop at failure? So far my curve of force vs deflection goes on and on, in plastic region without stopping.
Elabbasi (Mechanical)
22 Jul 10 11:33
You need to check your model carefully. If the tensile strength of concrete is 10% (not 0.1%) of its compressive strength, and based on the dimensions you provided the concrete by itself without any reinforcement can carry a much higher load than 80kN before yielding.

Nagi Elabbasi
Veryst Engineering
ASoliman (Geotechnical)
27 Jul 10 7:49
Actaully what I was suggesting is use the Partition Edge option in the part module to partition the wire reinforcements each into three segments.  Then in the constraints module, use the Embedded Region option to embed the reinforcement into the concrete slab, but when you select the embedded region only select the two ends of the segmented wire reinforcements and leave out the middle segments.  This way it will model the actual bonding of reinforcement in concrete more realistically and the middle segment will not constrain the concrete and will be free to deform.
helmi30 (Structural) (OP)
5 Aug 10 17:07
Since my model consists of a quarter of the original slab due to the geometrical symmetry, I have partitioned the steel bars into three segments. However, I have embedded only one of the end segments of the steel bar. After running the analysis, I noticed that Abaqus gave negative result noted 'Too many attempts made for this increment'. For information, my slab consists of an orthogonal steel reinforcements, i.e. it has 2 way steel rods. Iam still wondering this problem.

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