Modelling Options for Composite Cross-Sections

Technical Article

In RFEM, there are different options to model composite cross‑sections. In the following example, three different modelling options for a composite cross‑section, consisting of a rolled steel section HEA 300 and a rectangular cross‑section made of concrete w/l = 100/30 cm will be displayed and explained.

Structure

The composite beam is supported as single‑span beam at a length of 15 m. In reality, the composite is created by using headed stud shear connectors which are welded every 1.25 m. The self‑weight is assumed as loading (Figure 01).

Figure 01 - Structure

Option1: Composite Using Common Nodes by Member Eccentricity

Both cross-section elements are modelled each as (2 ⋅) 12 ⋅ 1.25 m long member elements (member type beam). Since both cross‑sections are initially located on the same line, the function ‘Allow Double Members’ under ‘Edit’ has to be activated so that they can be considered separately but are still supported at the same end node. The corresponding member eccentricity has to be assigned to all 2 ⋅ 12 member elements so that the top edge of the steel section is equal to the bottom edge of the concrete cross‑section (Figure 02).

Figure 02 - Option 1: Composite Using Common Nodes by Member Eccentricity

In this way, the twelve individual composite elements are connected with each other at their respective member start and member end.

Option 2: Composite Using Rigid Elements

The same modelling takes place here for the elements like for Option 1, with the difference that no eccentricities are assigned to the cross‑sections but both member cross‑sections are on two separate lines. To create twelve composite elements of the total of 24 member elements, the individual cross‑sections are connected with each other by rigid members. At both supports the rigid member is divided into two members each so that the nodal supports are located in the composite joint as in Option 1. The support is carried out with rigid members (Figure 03).

Figure 03 - Option 2: Composite Using Rigid Elements

The rigid members connect both complete cross‑sections in the same way as an eccentricity. This option offers the possibility to modify stiffnesses of the rigid members similarly to the existing headed stud shear connectors at their ends or to even replace them by other member types. In addition, there is the possibility to see the internal forces of the couplings (rigid members). Therefore, it is necessary to activate the option ‘Results on Couplings’ in the Display navigator under ‘Results’ → ‘Deformation’ → ‘Members’ (Figure 04).

Figure 04 - Activating "Results on Couplings"

Option 3: Composite by Member Type 'Rib'

This option is based on a completely different modelling. The concrete cross‑section is modelled as a surface, the steel cross‑section as a rib. The eccentricity of the rib can be defined within the ‘Edit Rib’ dialog box on the +z-side of the surface. To obtain the same support situation as in Option 1 and Option 2, the surface (and thus automatically also the rib) can be connected on both sides to the support with the rigid member, from its central axis to its +z‑side. Furthermore, to obtain internal forces for the complete cross‑section, the option ‘On Members Addition of Surface Components’ has to be activated in the Display navigator → ‘Results’ → ‘Members’ → ‘Ribs - Effective Contribution on Surface/Member’). Find more information about the member type rib in the corresponding FAQs.

Summary

Whereas the first two options are a kind of framework model and thus bending moments are jumping favourably due to the couplings, the third option represents an ideal composite cross-section. Due to the internal forces integrated over the concrete surface, the internal forces have a different size than in the first two options and have to be evaluated differently (Figures 05 and 06).

Figure 05 - Option 1 and Option 2: Display of Bending Moments My in Steel Beam

Figure 06 - Option 3: Display of Bending Moments My in Composite Cross‑Section

However, a comparison of the deflection reveals that all three options can be used for modelling (Figure 07).

Figure 07 - Deflection of all Three Options

Moreover, concerning the shear effect between concrete cross‑section and steel cross‑section, it can be stated that the results are comparable. The longitudinal shear force VL is displayed when activating the internal force VL under ‘Result Diagram’ (right‑click the rib) (Figure 08).

Figure 08 - Display of Longitudinal Shear Force VL

This compares to the shear force of the rigid members of Option 2 (Figure 09).

Figure 09 - Shear Force of First Rigid Member from Support

Internal forces
and deflections
in comparison
 
Option 1
 
Option 2
 
Option 3
My [kNm] 32.64    32.64    240.50   
u [mm] 20.10    20.10    20.40   
VL [kN] 132.30    132.30    128.30   

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