EuroCode 4 composite floor design

 

18.0

SCIA Engineer v18.0 offers significant new functionality in composite beam design according to EN 1994-1-1: from AutoDesign of both cross-section and number of studs, camber design, taking into account web openings, all the way to the support for nonlinear and stability analysis for structures with composite floors, the new version has a lot to offer to composite floor designers.

These design and analysis enhancements complement previously released features like:

  • a stages analysis model that simulates the structural response in construction and final stages and includes creep deformation and partial shear connection between the steel beams and composite slab;
  • rigid, semi-rigid and flexible floor diaphragms;
  • tributary area logic for the distribution of gravity loads;
  • automatic handling of fresh concrete weight during the construction stage;
  • fire resistance checks for composite beams in both construction and final stage.

All these capabilities make composite design in SCIA Engineer a powerful tool for building designers.

Highlights
A versatile design tool for composite floors complies with design requirements of EuroCode 4 and SCI publications and is scalable towards advanced numerical simulations.
A sophisticated design routine cycles thought all checks (ULS, SLS, detailing conditions) in both construction and final stages, and proposes suitable cross-section, stud layout, and camber to fulfil all these design requirements.

Reporting in various forms provides the user the right output in all steps of the design process: from brief overviews, scalable summaries, error/warning/note indicators and design labels in the 3D scene, to a detailed report with all used formulas and references, the composite documentation is a key feature of SCIA Engineer.

Support for isolated web openings in the composite beam is one of the final steps towards an all-included composite coverage in SCIA Engineer.

A building system firmly established in UK design practice

Composite design functionality in SCIA Engineer pertains to floors consisting of symmetrical steel I-beams (e.g., HE, UB, IPE, or sheet-welded) that support a concrete slab. Steel sheeting is welded on top of the steel beams, and a layer of concrete is poured onto the sheeting. A reinforcement mesh is added to the concrete slab to counteract cracking and failure modes such as longitudinal shear and the crushing of concrete.

The steel beams act as ribs in the composite floor. Shear connection is achieved by welding headed stud connectors to the top flange of the beams; these studs partially prevent slip between the slab and steel beams (or fully, depending on the design).

This building system is much preferred in UK and US design practice since it offers many advantages: 

  • Formwork is entirely replaced by the use of steel sheeting.
  • Propping can be avoided in most design situations, leading to a largely reduced cost of the floor and a speed up of execution.
  • A simplified response of the structural system eliminates the need for profound reinforcement design for the slab.

The design methods associated with this building system are proven by both practical experience and extensive experimental campaigns executed in both academic context and by industry.

SCIA's 3D composite floor solution

What SCIA Engineer offers for composite floors is an efficient 3-dimensional design tool that complies to EN 1994-1-1 rules and is scalable towards advanced numerical simulations (2nd order analysis, eigenvalue stability and modal analysis, etc.). The analysis and design framework accommodates (besides the isostatic beam case) cantilever and continuous beams.

In addition, the module is simple enough, yet flexible: irregular geometries, special loading scenarios and preferences in design approach are easily taken into account. The solutions offered for the floors are economical, yet safe, as these fulfil all design code requirements in both construction and final stages, ultimate and serviceability limit states and detailing conditions.

Accounting for web openings

The EC4 composite module in SCIA Engineer v18.0 supports design for beams with isolated web openings. The user can input an opening on each composite beam and take its influence into account in the ULS checks in construction and final stages as well as in the beam AutoDesign. Web openings can be rectangular or circular, with any size and at any location along the beam.

The design is performed based on the SCI P355 “Design of composite beams with large web openings” publication and is benchmarked to the examples provided there. The rest of the checks on composite beams are also benchmarked to the examples in SCI P359 “Composite design of steel framed buildings.”

It is planned to extend the support for web openings in the following version of SCIA Engineer: the plan is to soon support multiple and closely-spaced openings and reinforced openings.

Stud and cross-section AutoDesign

The composite module is equipt with a sophisticated design routine that cycles thought all checks (ULS, SLS, detailing conditions) in both construction and final stages, and proposes suitable cross-section, stud layout, and camber to fulfil all these design requirements.

In case the user agrees with the proposed changes, SCIA Engineer adapts the cross-sections of composite beams in the model; in addition, the composite action between each beam and its relevant slab is adapted to reflect the designed stud layout. This is necessary in order to achieve correspondence between the design checks and the analysis model. In this way, the user is certain that the design assumptions are reflected in entities that are derived from the FEM model, like deflections, vibration frequencies, member stiffness, etc.

The optimisation (AutoDesign) can be guided in a few ways:

  • The user can select an "optimisation strategy:" one can choose to either minimise the steel beam size (thus using more studs and higher composite action), he can choose to minimise the number of studs (resulting in larger steel beams) or he can choose a balanced approach in between these two options.
  • The user can limit the beam height, which is useful if the floor height needs to be restrained.
  • The routine offers splitting of cross-sections: new sections can be created for the beams selected by the user with appropriate sizes based to utilisation.
  • The routine also offers to unify cross-sections in case separate groups of members happen to converge to the same I-profile from the Profile Library.
  • In stud design, the routine automatically switches to (and from) 2 number of studs per row, if needed to achieve better shear connection and if allowed by the user.

A current limitation of the AutoDesign routine is that cross-section optimisation is done only for hot-rolled profiles from the Profile Library. Sheet-welded sections are only checked, and only stud and camber design are offered for these.

Stud layouting

For each beam, two stud layouts are always provided: a segmented stud layout and a uniform one. The segmented stud layout represents the minimal number of studs needed along the beam; it is relevant for composite girders and beams that carry significant vertical point loads. The uniform layout uses uniform stud spacing along the beam, thus it requires a higher number of studs, but it simplifies execution and prevents mistakes on the construction site.

Both layouts fulfil detailing conditions and comply with the assumptions in the 3D FEM model. The user can choose which stud layout will be executed on site and does not need to take further action inside the model depending on which layout is chosen. The segmented and uniform stud layouts for secondary beams without point forces are identical.

The designed stud layout is drawn in the check output and provides a clear overview of spacing and the sizes of segments on the beam.

Result labels

The design can easily be reflected on the floor plan by using result labels. A label is plotted on each composite beam and it contains:

  • the cross-section name (e.g., UB356/127/33)
  • the required number of studs (e.g., [28] for uniform layout and [8,4,8] for segmented layout)
  • the required camber (e.g., c=15 mm)

Labels let the user summarise the design on a plan view; each beam gets a label (e.g., UB356/127/33 [28] c=15 mm) which contains the most relevant information for third parties (draughtsmen, contractors, collaborators). The same information can also be exported to Revit via the Revit link.

In addition, the number of studs per beam are a separate result variable of the composite check. This means that the number of studs needed can be plotted on the beams, listed in the Brief output and in Table Results. Via Table Results, the total number of studs for the whole structure is easily obtained by a copy to a spreadsheet. This is useful when evaluating the cost of construction for the project.

In this version, the number of studs per beam corresponds to the segmented stud layout, therefore to the minimal number of studs that are needed. This can be improved in the following version to allow the user to specify which stud layout is relevant for him/her.

Camber design

SCIA Engineer allows the user to counteract SLS issues in composite floors with camber. Camber on composite beams can be designed or inputted by the user. In the case of design, the user can set a maximal value in mm (overall or different per beam).

Beam camber is considered not just in the middle of the span: in the case when large point loads are present away from the centre of the span, deflection checks may become critical at a location different from midspan. Still, a fraction of the camber is available there and the checks take that into account.

Unique design reporting

Reporting composite checks is truly a unique feature in SCIA Engineer. Various levels of output are provided:

  • All formulas and design decisions are reported in the Detailed report; references are added there to all formulas from the EuroCode and SCI publications that have been used; dynamic images describe the input and the derivation of plastic neutral axis and bending moment capacity.
  • The Standard report contains a summary of input data and a list of the current utilisation levels for all checks per stage and limit state. It lets one easily pinpoint any existing issues with the currently used cross-sections, slab and sheeting geometry, reinforcement, studs or camber. Colour coding values there makes it really easy to distinguish the failing unity checks.
  • The Brief output summarises the outcome for all composite members in a table (one row per composite member). The table contains the final utilisation ratio, the calculated values of effective width along the length, the design labels and the minimal required number of studs.
  • The uniform and segmented stud layouts are drawn in both the Detailed and Standard report.
  • An overview table of all detailing checks is provided in the Standard report.
  • Errors, warnings and notes are reported in both the 3D scene and in all reports. EWN reporting in the 3D scene lets the user discover issues by hovering directly on the composite beams: indicators are pointing to the beams where issues are found, e.g., in properties or settings, in assumptions of the checks that are not fulfilled, in issues encountered during optimisation. Information is provided about how the user should address these issues.

Analysis extensions

Second-order and stability analysis of structures with composite floors is available since SCIA Engineer v18.0. The analysis is done in a single stage; therefore, a relevant composite stage needs to be selected. Automatic selection is done based on the load cases present in the nonlinear/stability combination:

  • in the case where all load cases are in construction stage, the construction stage stiffness is used for the analysis.
  • in the case where at least one load case is in final stage - long-term, and no load case is in final stage -short term, the final stage - long term stiffness is used for the analysis.
  • in the case where at least one load case is in final stage - short-term, the final stage-short term stiffness is used for the analysis.

The user can overwrite which stage is used for each nonlinear combination.

Another great feature that significantly speeds up composite design is the "locking" of results. Available since version 17.1, Result Locking lets the user keep the FEM results after making modifications to the model that would normally require for the results to be purged. Keeping results longer lets the user continue working and optimising his structure with the old values of internal forces and deflections.

Notifications that results are outdated are provided in the 3D scene, Report Preview, Engineering Report, Table Results; so there is no danger of using invalid results for the final reporting. A recalculation of the model will eventually be required, also because SLS utilisation is only going to be fulfilled after the stiffness of the new sections has been used in FEM analysis.

 

22/05/2018