List of questions:This page contains frequently asked questions related to composite behavior.
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Why am I getting unexpected response for composite section? (Impact of Support Conditions)
Extended Question: I have models 1 and 2 (please see the description below), but I am getting unexpected deflections for model 1. The deflection form model 1 is smaller than expected.
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The axial force in the girder is the net effect of stresses acting on the girder. The applied moment is resisted by the entire composite section, with tension stresses below the neutral axis of the cross-section. Since most (if not all) of the girder is located below the neutral axis of the composite section, this results in axial force (tension) in the girder.
How can I model partially composite sections?
Extended Question: I have a 85% composite design with concrete over steel girder. If in the SAP2000 model I put 0.85 for the membrane stiffness modifiers in the shell element, will it simulate the 85% composite action?
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Alternatively, if the composite section is modeled by a single frame element, the stiffness modifiers would need to be derived for the entire section.
Why I am getting jumps in my frame moment diagram? Why does the moment diagram follow saw-tooth pattern?
Extended Question: I modeled composite section using frame members for the girders and shells for the deck. However the frame moment diagram does not appear to be correct, since there are sudden jumps, resulting in a moment diagram which looks like a saw-tooth pattern.
Answer: Please note that the deck and the girders are connected to common joints to model a composite action. The jumps in the moment diagram at these connection points are caused by forces that are being applied by the deck to the girders at these connections. You can reduce these jumps if you refine the discretization for the shell or frame elements to make these connection points distributed closely to each other.
Is there any difference in modeling composite sections in SAP2000 and ETABS?
- The modeling and analysis is similar between the two programs, however ETABS can perform a design for the composite section. This design capability is not included in SAP2000.
How can I obtain design forces for T-beam composite section modeled by frame and shells?
Extended question: I have a concrete floor modeled using finite element for slab and frame members for girders, beams and columns. The beam are girder members are offset to their physical locations. I found that the direct reading of member forces and moments cannot be directly used for the design of the composite T-beam section. How do I get the correct member forces for design?
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- You could create a sequence of section cuts to obtain the design forces - see related FAQ on sequence of section cuts for additional details. This may require significant effort if done manually, but it would be a reasonable approach if it is automated using Application Programming Interface (API). The discretization should be refined as needed in order to adequately define the section cuts.
- You could also replace the rectangular beams in your structure with T-Beam section and modify the adjacent shell elements (using property modifiers), such that they do not contribute the same stiffness and weight as the T-beams in the relevant directions. Then the T-Beam frame forces would directly correspond to the design forces for the composite section.
How can I model noncomposite action for girders modeled by frame elements and slab modeled by shell elements?
If the slab and the girder are connected to common joints then the composite action would be considered (please note that joint offsets and frame insertion points can be used to offset the slab from the girder). The composite action would be also considered the slab and the girder do not share any common joints, but full body constraints are used connect joints on the girder with corresponding joints on the slab.
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