FAQ

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.

Model 1:

• Steel girder modeled by frames and concrete deck by shell elements.
• U1, U2 and U3 restraints assigned at both ends of the girder.

Model 2:

• Just one frame element with composite section created in the section 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?

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Modeling

How do I model an object with different tensile and compressive stiffness relationships?

Extended Question: I am modeling a composite brace in which a standard steel shape is enclosed within concrete. I would like only the steel to carry tension, while both the concrete and the steel carry compression. How do I achieve this behavior during a linear-dynamic analysis?

Answer: Rather than using a frame object, you can simulate this behavior by drawing a two-joint multilinear elastic link. Within the link definition, tensile and compressive force-displacement relationships may be drawn differently, and may follow any monotonic pattern desired.

See Also:

• CSI Analysis Reference Manual (Chapters 14 and 15)

• SAP2000 - 18 Gap Elements - CSI Watch & Learn video

How are partially composite sections modeled?

Extended Question: For a concrete-deck over steel-girder section that is 85% composite, will the assignment of a shell-object 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:

1. Stiffness of the girder about its own center of gravity.
2. Stiffness of the deck about its own center of gravity.
3. 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 that explicitly models 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 a single frame object is used to model 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?

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:

• 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.

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Is there any difference between the modeling of composite sections in SAP2000 and ETABS?

Answer: Composite-section modeling and analysis is similar between SAP2000 and ETABS. One additional capability of ETABS is that the software can design the objects within a composite section.

Noncomposite action

How is noncomposite action modeled for frame-object girders and shell-object slabs?

Answer: When common joints connect a girder and a slab, composite action occurs. The slab may be offset from the girder using joint offsets and frame insertion points, as described further in the Composite section tutorial. Composite action may also result when girders and slabs do not share common joints, but instead are connected through full-body constraints which connect girder joints to certain corresponding slab joints.

Noncomposite action results when the girder and slab both individually contribute to stiffness properties. Slip is allowed along the slab-girder interface. To model noncomposite action, the girder and slab should not share common joints, but they should be connected such that corresponding joints share the same vertical deflection. For example, a horizontal girder connected to a noncomposite slab may be modeled using the equal Z constraint.

A condition where the slab does not contribute to stiffness may be modeled using property modifiers where flexural and axial stiffness are reduced in the direction parallel to girders. Very small values on the order of 1e-3 are recommended to avoid the numerical problems which may result from zero-modifier application. To model the stiffness contribution from forms, property modifiers may also be applied in the direction perpendicular to girders.

Composite response

Why is unexpected response generated for a composite section?

Extended Question: I have two comparable models, described below, but the deflection of Model 1 is less than expected. Is there an explanation?

• Model 1:
  
  • Composite section with steel-girder frames and concrete-deck shells.
  • U1, U2, and U3 restraints are assigned to either end of the girders.

• Model 2:
  
  • A single composite-section frame object created in the Section Designer.

Answer: The unexpected results of 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.

What causes jumps in the moment diagram of a composite frame?

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 through these joints. 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 objects such that girder and deck connection points have closer spacing.

Design forces

How are design forces obtained for a composite section modeled using frame and shell objects?

Extended question: I modeled a composite reinforced-concrete T-beam floor system using finite elements for the slab and frame objects for the girders. I found that member forces cannot be directly read for the design process. How are member forces obtained for design?

Answer: Design of a T-beam floor system is dependent upon the forces within both the girder and the tributary slab width. Design forces are derived as a combination of those within the frame and shell objects which compose the composite system.

Design forces in may be obtained using either of the following methods:

• Create a sequence of section cuts, as described on the Section cut FAQ page, 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). Please note that discretization should be refined as necessary to adequately define the section cuts.

• Obtain design forces by replacing rectangular beams with T-Beam sections, then use property modifiers to modify adjacent shell objects 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.

To model noncomposite action (with the girder and the slab both contributing stiffness, but with slip allowed along the slab-girder interface), the girder and the slab should not share the same joints, but corresponding joints should be connected such that they share same deflection in gravity direction. For example, for a horizontal girder with a noncomposite slab, you could use equal Z constraint to model this condition.

To model a condition when the slab does not contribute any stiffness, you could use property modifiers to reduce the flexural and axial stiffness of the slab in the direction parallel with the girders. I would recommend to use very small values for the property modifiers, say 1e-3, to avoid numerical problems that may occur when zero modifiers are used. Property modifiers for the direction perpendicular to the girders could be also modified to account for the stiffness of the forms.

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• (not applicable to SAFE program as weight of slab/T-beam flange overlap is not double counted in SAFE). Frame forces in the T-beams would then directly correspond with composite-section design forces.

Composite design

How are composite and noncomposite sections designed in the same model?

Extended Question: I am using auto selection to design a steel structure with both composite and noncomposite sections. Composite design returns the correct results, but the noncomposite design for the gravity system does not generate results except for columns. Noncomposite member check is shown as composite design. How is this resolved?

Answer: The design procedure may be changed by selecting the appropriate members, then switching from Composite Beam Design to Steel Frame Design through Design > Overwrite Beam Design.