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Why
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is my composite section generating unexpected response
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measures?
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.two comparable models, described below, but the deflection of Model 1 is less than expected. Is there an explanation?
Model 1:
- Steel girder modeled by Composite section with steel-girder frames and concrete-deck by shell elements shells.
- U1, U2, and U3 restraints assigned at both ends to either end of the girdergirders.
Model 2:
- Just one A single composite-section frame element with composite section created in the section designerSection Designer.
Answer: The unexpected results for the model with shells over steel girders may be caused by the boundary condition, since you are providing longitudinal restraint at both ends of the girder. For this particular model, the longitudinal restraint is not located at the centroid of the cross-section and it results in a longitudinal force acting on an arm about the neutral axis of the composite section. This introduces additional moment which impacts the response. While this is kinematically correct behavior, it does not correspond to the other model (with section designer section) in which the composite beam is essentially supported at its centroid.
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?
Answer: The total stiffness of the composite section (let's consider concrete deck on a steel girder) comes from the following three sourcesof Model 1 may be attributed to its boundary conditions. The longitudinal restraints at either end of the girder are not located at the cross-section centroid, causing longitudinal force to act along a moment arm extending from the neutral axis. This behavior induces an additional moment which influences flexural response and vertical displacement. While kinematically correct, this behavior is different from that of Model 2 because of these boundary conditions.
The resultant axial force is the net effect of longitudinal stresses acting along the composite section. When longitudinal restraints are higher in the cross section, the neutral axis moves upward as well. To maintain equilibrium with the tensile forces located below the neutral axis (now greater because a larger portion of cross section is below the neutral axis), compressive forces are introduced at the restraints. These axial forces may be released with the assignment of a roller support at either end. This would allow axial force to transfer into longitudinal displacement.
How are partially-composite sections modeled?
Extended Question: For a concrete-deck over steel-girder section that is 85% composite, will assigning a shell-element membrane-stiffness modifier of 0.85 simulate 85% composite action?
Answer: Given this design, composite-section stiffness is a function of three sources which include the following:
- Stiffness of the girder about its own center of gravity.
- Stiffness of the deck about its own center of gravity.
- Additional contribution of the deck and the girder girder and deck stiffness contributions about the center of gravity of the entire section.
Changing the deck membrane modifier to 0.85 would directly affect the 1st source of the stiffness, and indirectly affect the 3rd source of the stiffness. However, the 85% composite action would allow some slip between deck and the girder and , therefore only the 3rd source of the stiffness should be affected.
Therefore, in In a detailed model with , where the deck and the girder explicitly modeled (which seems to be your case), your could reduce the 3rd source of the stiffness by connecting the girder and the deck with flexible links instead of rigid or fixed links. You would need to derive the stiffness of these links based on the discretization of your model and the stiffness may be reduced using flexible links (rather than rigid or fixed) to connect the girder to the deck. The stiffness derivation for these flexible links would be based on model discretization and the prescribed 85% composite action.
AlternativelyFurther, if the composite section is modeled by a single frame element is used to model the composite section, the stiffness modifiers would need to be derived for the entire section.
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What causes jumps in
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the frame moment diagram
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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.
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of a composite section?
Answer: The deck and girders of a composite section are connected through common joints. During composite action, forces are transferred from the deck to the girders at through 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 otherjoints. These forces, concentrated at the connection points, cause jumps in the moment diagram. Discontinuities may be reduced by refining the discretization of frame and shell elements such that girder and deck connection points have closer spacing.
Is there any difference
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between the modeling of composite sections in SAP2000 and ETABS?
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Composite-section modeling and analysis is similar between
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SAP2000 and ETABS. One additional capability of ETABS is that the software can design the elements of a composite section.
How are design forces obtained for a composite section modeled using frame and shell elements?
Extended question: I have modeled a composite reinforced-concrete T-beam floor modeled system using finite element elements for the slab and frame members elements for the 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 read for the design of the composite T-beam sectionprocess. How do I get the correct are member forces obtained for design?
Answer: It is important to remember that you are using finite element model with shells to model the deck and frames to model the supporting girders. If you need to obtain design forces for the design of T-Beam which corresponds to the girder and a tributary slab width, you would need to combine the corresponding forces from the frame and shell elements to obtain reasonable design forces for this composite T-Beam section.
Although there is no direct way to this in the current version (V14.0.0 as of 5/2009) of the program, you could try one of the following approaches:
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Design of a T-beam floor system is dependent upon the forces on both the girder and the tributary slab width. Design forces are derived as a combination of those within the frame and shell elements which compose the composite system.
Design forces may be obtained using either of the following methods:
- Users may create a sequence of section cuts to obtain the design forces - see , as described on the Section cut FAQ page, item "Can the program display force diagrams based on a 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 to obtain design forces. Done manually, this process may require significant effort. A more practical approach may be to automate the process using the Application Programming Interface (API). The Please note that discretization should be refined as needed in order necessary to adequately define the section cuts.
- You could also replace the rectangular beams in your structure Users may also obtain design forces by replacing rectangular beams with T-Beam section and modify the sections, then using property modifiers to modify 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 . Frame forces in the T-Beam frame forces Beams would then directly correspond to the composite-section design forces for the composite section.
How
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is non-composite action modeled for frame-element girders and shell-element slabs?
Answer: When common joints connect a girder and a slab, composite action occurs. Users may offset the slab from the girder using 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 . Composite action may also result when girders and slabs do not share any common joints, but instead are connected through full-body constraints are used , which connect joints on the girder with corresponding joints on the slab.To model noncomposite action (with girder joints to certain corresponding slab joints.
Non-composite action results when the girder and the slab both contributing stiffness, but with slip individually contribute to stiffness properties. Slip is allowed along the slab-girder interface). To model non-composite action, the girder and the slab should not share the same common joints, but corresponding joints they should be connected such that they corresponding joints share the same vertical deflection in gravity direction. For example, for a horizontal girder with a noncomposite slab, you could use connected to a non-composite slab may be modeled using the equal Z constraint to model this condition.
To model a A condition when where the slab does not contribute any stiffness, you could use property modifiers to reduce the to stiffness may be modeled using property modifiers where flexural and axial stiffness of the slab are reduced in the direction parallel with the to girders. I would recommend to use very Very small values for the property modifiers, say on the order of 1e-3 , are recommended to avoid the numerical problems that which may occur when zero modifiers are used. Property modifiers for result from zero-modifier application. To model the stiffness contribution from forms, property modifiers may also be applied in the direction perpendicular to the girders could be also modified to account for the stiffness of the forms.