Composite design in SCIA Engineer 16 - EN 1994
SCIA Engineer 16 features a comprehensive solution for the modelling, analysis and design of composite beam floor systems. New functionality and extensions have been added to the module esacbd.01.01, with a focus on analysis possibilities, fire-safety design, and design in serviceability limit states.
Two principle demands emerge during the design of floors with composite beams:
- the accurate numerical modellisation and analysis in a 3D FEM environment,
- the code-based design of individual structural members (ULS and SLS checks).
SCIA Engineer addresses these demands with a module that makes it possible to apply engineering theory to everyday structural design.
Accurate FEM-based calculation
The Composite Analysis Model (CAM) in SCIA Engineer is used to accurately model the entire structure and account of its varying properties during the stages of construction, service, and exploitation. Deformations and load effects obtained from the various stages of construction are superimposed, taking into account (1) the presence or lack of shear connection between beams and slab and (2) creep in the concrete slabs.
The EN 1994-1-1 standard stipulates which parts of a composite slab can be considered to contribute to the strength and stiffness of the composite beam. In SCIA Engineer 15, the effective width of the composite beam was already calculated automatically. This included the automatic detection of:
- span length;
- distances to neighbouring elements in the 3D model (i.e., other beams, openings);
- distances to slab edges;
- boundary conditions in neighbouring spans.
An orthotropic slab with ribs
The CAM calculates the exact orthotropic properties of corrugated steel decks and composite slabs (corrugated steel deck & concrete topping). These orthotropic properties, as well as the augmented contribution of the steel beams (depending on the degree of composite action), are used in the FEM calculations.
The CAM can represent the steel ribs in a composite slab in two different ways. The first method (via a "standard composite action") eliminates normal forces that may arise from eccentricity between the slab and beam in the FEM model by assuming the slab and beams are located at the same level. This idealisation is suitable for the majority of composite floors. The second method (an "advanced composite action") uses the actual cross-section properties and alignment of beams. As a result, normal forces will be generated in both beam and deck as a result of the actual eccentricity of the 1D member (also taking into account the degree of composite action). The latter approach is useful if horizontal loads could lead to additional bending in the composite beams. The EUROCODE, though, does not prescribe a method for taking the normal forces into account in the design verifications for composite beams.
Internally, the calculation distinguishes between three FEM submodels (with different stiffness of the composite slabs) – one for the construction stage and two for the final (composite) stage. The stage-based model evaluates rheological effects (i.e., creep) by distinguishing between long and short-term load cases in the final (service) phase.
There are no limitations on the structural system, span type, or the arrangement of beams within a slab. In addition, composite beams may be simply supported, continuous, or cantilevered; they can have any orientation within the slab without any limitation in the functionality. Unusual geometries can be modelled and analysed accurately with limited additional user input.
Finally, a shear stud library, as well as steel decking libraries with common profiles from European and North American manufacturers are also provided.
The design of composite beams is performed according to the EN 1994-1-1 standard and includes the following features:
- Ultimate Limit States (ULS) design checks for both the construction and final (composite) stages. The following is included:
The contribution of concrete slabs to the resistance of steel beams in the final (composite) stage is taken into account within the effective slab width of a beam;
- Shear lag is taken into account at multiple locations along the beam length (in effective-width calculations for the composite stage as per EN 1994-1-1, 220.127.116.11);
Section classification of the steel beam is done in the construction and composite stages (as per EN 1993-1-1, 5.5.2); minimal reinforcement in the slab is evaluated as per EN 1994-1-1, 5.5.1 (5);
- The capacity of shear connectors and effect of number and strength of connectors on the overall beam resistance is evaluated as per EN 1994-1-1, 6.6.4;
- The positive and negative flexural capacity of the composite beams is evaluated as described in EN 1994-1-1, Chapter 6;
- Verification for longitudinal shear is performed as per EN 1994-1-1, 18.104.22.168;
- Verification for crushing of concrete flange is performed as per EN 1992-1-1, 6.2.2 (6);
- The shear capacity and shear buckling of composite beams is evaluated as per EN 1993-1-1, 6.2.8 (based on the steel section alone);
- Serviceability Limit State (SLS) design checks for the construction and final stages:
- Deflection controls are based on the FEM deformation (from variable and total loads) and take into account pre-cambering of beams.
- Floor vibration checks according to simplified theory based on natural frequency.
- Limitation of crack widths according to EN 1994-1-1, 7.4.
The simplified approach for limitation of crack widths in composite slabs was included in SCIA Engineer v15.3, as described in EN 1994-1-1, §7.4. 2 and §7.4. 3.
- The method ensures that cracks are limited to an acceptable width.
- It is required that a minimum reinforcement according to EN 1994-1-1, 7.4.2 be provided;
- The calculated minimal reinforcement is compared to the one provided by the user and a warning is sent if both the bar diameter and bar spacing do not comply with the recommendations of EN 1994-1-1, §7.4. 3.
- In version 16.0, it became possible to evaluate the contribution of variable loads to the final deflections obtained in the FEM calculations. This means that all required deflection checks can now be performed.
- A simplified vibration check was also added in version 15.3. A natural frequency is calculated for each beam based on its deflection under permanent loads. This natural frequency is then compared to a minimal natural frequency requirement (a hard-coded value of 4 Hz is used as a limit).
These additional verifications complement the set of SLS checks for composite floors.
Main advantages of the composite design module
- The multi-model approach of the CAM allows for parallel checks for construction and composite stages to be performed without modifications to the model.
- Partial shear connection between the composite slab and the steel beams is taken into account both during the FEM analysis and in the design checks.
- Creep in concrete slabs is taken into account during the FEM analysis (using a modular ratio for the concrete Young's modulus).
- Both longitudinal and transverse alignment of the steel decking are supported.
- Section classification takes into account the actual position of the neutral axis in the composite cross-section.
- Calculated resistances in ULS are based on a plastic distribution of stresses in the section.
- SLS checks take into account the stages of construction and pre-cambering of beams.
- Additional checks control the minimal degree of shear connection, minimal area of longitudinal reinforcement, etc.
- National annex support.
- Detailing provisions, e.g., on the spacing and diameter of shear connectors, are taken into account.
- All unity checks values may be (1) plotted along the beams in the 3D window and (2) listed in tables.
- Brief & summary calculation outputs in table form are provided.
- The detailed calculation output is provided with rendered formulas, intermediate calculation steps, realistic dynamic drawings of the composite system, of the derivation of plastic moment resistance and location of plastic neutral axis.
The composite functionality benefits from a number of already existing functionalities in SCIA Engineer:
Limit state design based on partial safety factors in accordance with EN 1990;
- Actions in accordance with EN 1991; combination of actions in accordance with EN 1990;
- Action effects calculated in an elastic global analysis (in the Composite Analysis Model environment).
- Negative flexural strength can only be evaluated for beams with full composite action (as specified in EN 1994-1-1).
- Only symmetric beams are supported.
- Only beams of section classes 1 and 2 are supported.