Test article Fire resistance - automatic temperature calculation
Fire design is based on temperature calculation using the incremental method of EN 1993-1-2 - 4.2.5. Engineers do not need to make temperature calculations on their own anymore or rely on additional manual solutions, such as spreadsheets.
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- Bolted connections
- Welded connections
- Whatever
Bolts and welds are the most difficult elements in the design of steel connections. Excel spreadsheets very often simplify their calculation. Modeling them in general FEM programs is complicated because these programs do not offer the predefined sets of elements. That is why the CBFEM method was developed and implemented into IDEA StatiCa.
Bolt model according to CBFEM
IDEA StatiCa has a unique method in its solver, the Component-based Finite Element Method (CBFEM). The bolt model used in CBFEM is described and verified to several steel design codes. The load resistance and deformation capacity are also compared to the main experimental research programs.
In the Component-Based Finite Element Method (CBFEM), bolt with its behavior in tension, shear, and bearing is the component described by the dependent nonlinear springs. The bolt in tension is described by spring with its axial initial stiffness, design resistance, initialization of yielding, and deformation capacity. For the initialization of yielding and deformation capacity, it is assumed that plastic deformation occurs in the threaded part of the bolt shank only.
In our Theoretical background, you can find more information on how the CBFEM method describes and verifies bolts. If you want to know a bit more about CBFEM in general, the full General theoretical background is definitely the best place to start from.
Bolts according to design codes
Let's take a look at how CBFEM approaches bolts from the point of view of individual design codes. So far, IDEA StatiCa supports eight design codes where design and/or detailing of bolts and preloaded bolts are being solved.
Check of bolts and preloaded bolts according to Eurocode
The initial stiffness and design resistance of bolts in shear are in CBFEM modeled according to Cl. 3.6 and 6.3.2 in EN 1993-1-8. The spring representing bearing and tension has a bi-linear force-deformation behavior with an initial stiffness and design resistance according to Cl. 3.6 and 6.3.2 in EN 1993-1-8.
Detailing
Checks of bolts is performed if the option is selected in Code setup. Dimensions from bolt center to plate edges and between bolts are checked. Edge distance e = 1.2 and spacing between bolts p = 2.2 are recommended in Table 3.3 in EN 1993-1-8. Users can modify both values in the Code setup.
Check of bolts and preloaded bolts according to AISC
The forces in bolts are determined by finite element analysis. The tensile forces include prying forces. The bolt resistances are checked according to AISC 360 - Chapter J3.
Detailing
The minimum spacing between bolts and distance to the bolt center to an edge of a connected part is checked. The minimum spacing 2.66 times (editable in Code setup) the nominal bolt diameter between centers of bolts is checked according to AISC 360-16 – J.3.3. The minimum distance to the bolt center to an edge of a connected part is checked according to AISC 360-16 – J.3.4; the values are in Table J3.4 and J3.4M.
Check of bolts and preloaded bolts according to other standards
- Check of bolts and preloaded bolts according to CISC (Canada)
- Check of bolts and preloaded bolts according to Chinese standard (GB)
- Check of bolts according to Hong Kong Code (HKG)
- Check of preloaded bolts according to IS 800 (India)
- Check of bolts and preloaded bolts according to SP (Russia)
- Check of bolts and preloaded bolts according to AS (Australia)
Bolt detailing
How to set the distances
Edge distances used for bolt bearing resistance must be relevant for general plate geometries, plates with openings, cutouts, etc.
The algorithm reads the real direction of the resulting shear force vector in a given bolt and then calculates the distances needed for the bearing check.
The end (e1) and edge (e2) distances are determined by dividing the plate contour into three segments. The end segment is indicated by a 60° range in the direction of the force vector. The edge segments are defined by two 65° ranges perpendicular to the force vector. The shortest distance from a bolt to a relevant segment is then taken as an end, or an edge distance.
There exist several options for how to treat welds in numerical models. The large deformations make the mechanical analysis more complex, and it is possible to use different mesh descriptions, different kinetic and kinematic variables, and constitutive models. The different types of geometric 2D and 3D models and thereby finite elements with their applicability for different accuracy levels are generally used. The most often used material model is the common rate-independent plasticity model based on the von Mises yield criterion. Two approaches that are used for welds are described. Residual stress and deformation caused by welding are not assumed in the design model.
The load is transmitted through force-deformation constraints based on the Lagrangian formulation to the opposite plate. The connection is called multi-point constraint (MPC) and relates the finite element nodes of one plate edge to another. The finite element nodes are not connected directly. The advantage of this approach is the ability to connect meshes with different densities. The constraint allows modeling the midline surface of the connected plates with the offset, which respects the real weld configuration and throat thickness. The load distribution in the weld is derived from the MPC, so the stresses are calculated in the throat section. This is important for the stress distribution in the plate under the weld and for modeling of T-stubs.
Plastic stress redistribution in welds
The model with only multi-point constraints does not respect the stiffness of the weld, and the stress distribution is conservative. Stress peaks that appear at the end of plate edges, in corners, and rounding, govern the resistance along the whole length of the weld. To eliminate the effect, a special elastoplastic element is added between the plates. The element respects the weld throat thickness, position, and orientation. The equivalent weld solid is inserted with the corresponding weld dimensions. The nonlinear material analysis is applied, and elastoplastic behavior in equivalent weld solid is determined. The plasticity state is controlled by stresses in the weld throat section. The stress peaks are redistributed along the longer part of the weld length.
The elastoplastic model of welds gives real values of stress, and there is no need to average or interpolate the stress. Calculated values at the most stressed weld element are used directly for checks of the weld component. This way, there is no need to reduce the resistance of multi-oriented welds, welds to unstiffened flanges, or long welds.
Constraint between weld element and mesh nodes
General welds, while using plastic redistribution, can be set as continuous, partial, and intermittent. Continuous welds are over the whole length of the edge, partial allows users to set offsets from both sides of the edge, and intermittent welds can be additionally set with a set length and a gap.
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