Composite design in SCIA Engineer 16 - AISC 360-10
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 composite module (esacbd.01.05) with a focus on analysis capabilities and optimization, also focusing on serviceability limit states.
Two principle demands emerge during the design of floors with composite beams:
- the accurate numerical modelling 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 and account for the varying properties of the structure throughout the construction and service of the building. Deformations and load effects obtained from the various stages of construction are superimposed, while taking into account (1) the presence or lack of shear connection between the beams and the slab and (2) creep in the concrete slab.
The AISC 360-10 stipulates which parts of a composite slab can be considered to contribute to the strength and stiffness of a 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.
- distance to neighbouring elements in the 3D model (i.e., other beams, openings),
- distance to slab edges.
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 the 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. However, the AISC code 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.
Standard shear stud as well as steel decking profiles from US manufacturers (including Vulcraft, Verco, and Canam) are included making it easy to assign these to a slab.
The design of composite beams is performed according to the AISC 360-10 standard and includes the following features:
- Ultimate limit states (ULS) design checks for both the construction and composite (final) stages. The following is included:
- Ability to design according to both Load and Resistance Factor Design (LRFD) and Allowable Strength Design (ASD);
Composite action (the contribution of concrete slabs to the resistance and stiffness of the steel beams) in the final stage is included through automatically calculated effective widths;
- Positive & negative flexural strength is evaluated per AISC 360-10, Chapter I3.2;
- Shear strength (including shear buckling) is evaluated per AISC 360-10, Chapters I4 & G2;
- The strength of headed shear studs is evaluated per AISC 360-10 Chapters I3.2 & I8;
Shear studs can be input manually and verified or can be automatically calculated;
Number of required studs can be designed using uniform or segmented layout (in the case of point loads on the beam) also taking into account detailing provisions for the spacing of studs.
- Serviceability limit state (SLS) design checks for both the construction and final stages, including the following:
- Control of deflections based on actual FEM deformation (from variable and total loads) including the effects of beam camber;
Camber can be automatically calculated as well as defined as an absolute or relative value;
- Other detailing provisions (related to concrete slab, steel deck and shear studs) are taken into account as per AISC 360-10 Chapter I.
- An automatic optimization routine is available; it allows the user to design the composite beams according to the following criteria:
- ULS unity check values in the construction stage;
- ULS unity check values in the final stage;
- SLS unity check values in the construction stage;
- SLS unity check values in the final stage;
- Detailing conditions for the placement of studs.
The optimisation routine chooses a suitable steel profile, determines the optimal number of studs and required camber for the beams. Additionally, multiple options for the optimization routine are available including strategies which lead to the lightest beam, fewest studs or balanced condition. This allows the user to have full control over the internal optimization leading to the most economical solution.
Design/Optimisation for composite beams
An intelligent optimisation routine has been implemented to enable the efficient design of composite floors. AutoDesign now takes into account the following four limit states:
- ULS unity check values in the construction stage,
- ULS unity check values in the final stage,
- SLS unity check values in the construction stage,
- SLS unity check values in the final stage,
as well as the detailing conditions for the placement of studs, in order to determine an optimal beam section and stud layout.
To begin the AutoDesign, the degree of composite action must be set, as this value is used in the FE calculation and thus influences the results of the FEM analysis. Based on this required degree of shear connection, and options related to the optimisation strategy (see further), the optimisation routine will modify:
- the beam cross-section,
- the number of studs,
- the camber of the beams,
in order to obtain a solution that complies with strength and serviceability limit states and detailing conditions.
Options in the Composite Setup allow the user to determine what is the preferred outcome of the optimisation routine. The user may select:
- to minimize the beam size -- this means that the AutoDesign will look for a solution that utilizes a degree of shear connection that is as high as possible (100% if detailing conditions allow);
- to minimize the number of studs -- this means that the AutoDesign will search for a larger steel beam and provide as few studs as possible while still complying with the detailing provisions and the requirement of 25% minimal composite action.
- a balanced approach -- this means that the AutoDesign will find a balance between the beam size and number of studs (using a maximum degree of shear connection of 55 %);
- to manually input a desired degree of shear connection, based on which the routine will find possible solutions.
The user may also choose to limit or prevent cambering the beams or restrict the beam height, for example, when the floor depth is a limiting factor.
- Required construction and final stage load cases and combinations must be defined.
- After the FEM calculation, the composite design can be run in the Composite service -- it will take into account ULS and SLS combinations for the different stages and propose an outcome of the optimisation (a new W-shape, number of studs and camber) that can be applied to beams in the model.
- After the model is updated with the new sections and new degrees of composite action, it can be recalculated to obtain FEM results that are up-to-date with the current model state.
It is also possible to fine-tune the updating of beam properties according to the proposals obtained from the optimisation routine.
- As a general rule, beams that share the same cross-section will be updated together to the cross-section that is required for the critical member in that group of beams. Additionally, it is also possible to perform the optimisation for a selection of members. Then the user can choose whether to update beams only inside the current selection, or also change the cross-section of beams in the rest of the model that have the same cross-section as the selected beams.
- Splitting and unifying cross-sections is also supported (as a general functionality of SCIA Engineer).
- The user may also reject the proposed model update, change the options related to design and rerun to obtain different output.
The design outcome is best summarised in the 3D window of SCIA Engineer.
- Labels on the beams display the designation of the W-shape, the number if studs (in uniform or segmented layout) and the required camber of the beam.
- The label is shown in the middle of the beam, oriented along the beam length, and its size can be adjusted to allow for enhanced readability depending on the floor size and layout.
Main advantages of the composite design module
- The multi-model approach of the Composite Analysis Model allows for parallel checks for construction and composite stages to be performed without modifications to the model.
- Partial connection between the composite slab and steel beams will be taken into account both during the FEM analysis and in the design checks.
- Creep in the 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.
- 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 camber of beams.
- Detailing provisions such as the spacing and diameter of shear connectors are taken into account.
- All unity checks values may be plotted along the beams in the 3D window and listed in tables.
- A brief calculation report is provided in table form.
The detailed calculation output contains rendered formulas, intermediate calculation steps, realistic dynamic drawings of the composite system, the derivation of plastic moment resistance and location of plastic neutral axis.