AISC composite design extensions in SCIA Engineer 16.1
Version 16.0 of SCIA Engineer saw the release of an integrated composite design module (esacbd.01.05) according to the Specification for Structural Steel Buildings AISC 360-10. The unique about the released functionality was the integration of code-based member verifications into advanced design workflows, such as slab based finite element analysis including nonlinearities. The consequence was that the user could now use the same platform for the creation of sophisticated models in an advanced 3D analysis environment while, in that same platform, run design checks and code-compliant optimisation on all or selected members from the 3D model.
The functionality that translates advanced finite element analysis (FEA) into output that can be streamlined for code-specific checks is known as the Composite Analysis Model (CAM). The CAM ensures that:
- the stages of construction (and exploitation) are reflected in the analysis;
- the influence of creep is taken into account for long-term loads;
- the composite deck is represented in the 3D model by realistic stiffness in all directions;
- the composite deck and supporting beams are (partially) connected, resulting in additional stiffness against bending in the beams;
- sheeting contributes to the stiffness of the structure in both construction and final composite stages, also keeping sheeting orientation in mind;
- primary and secondary beams are recognised and treated differently during, e.g., the determination and layout of studs;
- the composite deck and beams are modelled in such a way that no axial forces exist due to eccentricities (because the code only describes the verification of bending moment and shear force);
- beam effective widths are automatically calculated such that the internal forces in the beams contain the integrated stresses in the slab within the effective width.
When composite decks are present in your model, the software knows that a staged analysis should be performed. Different load cases are calculated with different stiffness, based on when these loads are first applied on the structure (in construction or final stage) and (2) what the duration of these loads is (short or long term). From a modelling perspective, the composite slab consists of a shell with partially connected rib elements. Additional modelling tricks are put in place to take into account the specific detailing and behaviour of composite structures.
SCIA Engineer's esacbd.01.05 also provides a sophisticated multi-parameter optimisation routine (Composite AutoDesign), which automatically updates cross-section sizes, number/spacing of studs along the beams, and required camber in order to provide an economic design that satisfies in one go all strength, serviceability and detailing requirements.
In an effort to continually expand the software’s functionality, the latest version of SCIA Engineer brings further extensions to the composite workflow. The following sections outline the extensions to the module for composite design (esacbd.01.05) in version 16.1.
Addressing web openings
An essential new addition to our composite module is the support for design of web openings, as described in AISC Design Guide #2: Steel and Composite beams with web openings.
This new extension allows users to do the following:
Define all aspects of the web opening shape (circular or rectangular), size, location and whether or not stiffener plates should be included.
- Calculate the moment and shear resistance at the web opening such that these can be compared to the existing internal forces in the beam at the opening location. Separate calculation routines are included for the determination of beam resistance in construction and in final stage.
- Include all relevant output of the web opening design in the calculation report including details of the opening and all strength level checks for bending and shear resistance.
Include the strength level web opening checks into the composite beam autodesign optimization. This means that the optimal composite beam is selected based on all strength, service and detailing requirements.
Provision for the weight of fresh concrete
Fresh concrete weighs more due to its high water content; it is estimated that the density of concrete is around 4% higher in the fresh state than in the dry state. In addition to that, the dead weight of fresh concrete should be considered as a variable load (with corresponding load factors in LRFD) due to the delivery methods and the fact that liquid concrete can heap on top of an individual sheet of deck or beam.
In version 16.1 of SCIA Engineer, the following functionality is added in order to properly account for the weight of fresh concrete:
- The material properties of concrete have been extended with an additional (editable) property "Density in fresh state." By editing this value, the user may specify how much the nominal concrete density should be increased during the construction stage;
- The increased self-weight of fresh concrete is taken into account in the construction stage. The weight of all other load-bearing elements and materials remains unchanged in both construction and final stage.
- The necessary load cases pertaining to the self-weight of the structure are created automatically and assigned to the correct stage (construction or final).
- The weight of fresh concrete is, by default, considered as a variable load on the structure due to the possibility of heaping during pouring; however, the user may edit this and consider the weight of fresh concrete as a permanent load;
- Precautions are taken to ensure that the user cannot, by mistake, include both concrete weights (fresh and final) in the same result class (and thus account for the concrete twice).
Neglecting composite action in negative moment regions
AISC 360-10 allows the designer to choose one of two approaches for the calculation of negative moment resistance of composite beams. The first approach is to consider the contribution of a tensile component in the slab reinforcement when the beam is loaded with negative bending moments. In the second approach, only the steel cross-section is considered in the determination of negative moment capacity.
SCIA Engineer v16.0 only supported the more advanced approach: namely, we always took into account the reinforcement contribution to the moment resistance and we assumed that composite action is the same in positive and negative moment regions.
Version 16.1 allows the user to choose whether or not slab reinforcement should be included in the calculation of negative moment capacity, thus allowing for the recognition of the more conservative approach.
Various small improvements
Along with the significant developments mentioned, other small improvements have also been included in version 16.1. These are as follows:
- Composite Setup improvements:
- values that are not currently used (due to the dependencies with other settings) are hidden;
- defaults have been established specifically for the IBC design environment.
- AutoDesign improvements:
reduction in the required number of iterations (which requires a recalculation of the structure)
- detailing check for minimal flange thickness has been added to the optimisation routine;
- secondary beams use only a uniform stud layout, regardless of the presence of point loads.
- Calculation report improvements:
- the moment diagram plot has been improved and made more readable;
- detailing checks are included for the geometry of web openings and beam flange thickness.
SCIA Engineer's module for composite design is a powerful tool for analysis and optimisation of composite floors. Its main advantages are the seamless integration into a 3D modelling and design environment and the automatic handling of stages in the background without additional user input.
For reference, the following are important milestones of the recent composite developments within SCIA Engineer:
- Stage analysis automation: the creation of the CAM (version 14.1);
- Libraries: sheeting and stud connectors (version 14.1);
- Added design checks for construction and final stage, detailing conditions (version 15.0);
- Removal of parasitic moments due to slab action with addition of one-way deck option (version 15.3);
- Multi-parameter composite AutoDesign (version 16.0);
- Result labels on the 3D structure (version 16.0);
- Usability improvements (version 16.0);
- Web opening checks and their inclusion in AutoDesign (version 16.1);
- Reducing number of iterations during AutoDesign (version 16.1).