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Effective width is a standard property of plate ribs in SCIA Engineer. It is used for several purposes in a usual analysis model. There are two sets of values defining the effective width of a plate rib:
It is important to note, that none of the above affects the stiffness of the system. Modifying the effective width does not modify the properties of the cross-sections in the analysis model.
There is however a difference when using the Composite Analysis Model, which uses the effective width in a third way:
Important: unlike in all other cases, in a plate rib with standard composite action, the effective width affects the stiffness of the beam
The effective width of a plate rib may be defined manually or automatically. Available properties can be found in " Rib in the slab". This document focuses on the automatic calculation of effective width.
The effective width of the plate ribs can be defined manually by the user or it can be calculated automatically by SCIA Engineer.
In the case of automatic calculation, the effective width is determined according to the following geometric rules:
beff,left, beff,right are the left and right hand side effective width of the rib
Le is the equivalent span length of the considered beam, assumed equal to the distance between zero-moment points; in SCIA Engineer; it depends on the boundary conditions at each end of the span.
SCIA Engineer uses the general method (from EN1994) which defines Le as a ratio of L. Only for AISC (IBC) code, the simpler rule Le=L is used in all cases.
a is a configurable constant; it is taken = 8.0 by default, which corresponds to the specifications of the EN1994 and ASCI (IBC) codes for the effective width in the analysis model.
badj,left, badj,right are the maximum values of effective width on the left and right hand side due to adjacent entities. The effective width on a side may not exceed:
Only entities that fulfil all the following conditions are taken into account. Those are plate ribs, outer edges or opening edges that are:
The distance to the adjacent entity is calculated at mid-length of each member. The effective width is calculated as a uniform value per member. That assumption corresponds to the mentioned design codes.
A 1D member may not cover more than one span for the automatic calculation of effective width. If it does, the calculation of effective width will fail and the rib will switch back to manual input. Continuous beams should therefore be modelled as multiple 1D members, rigidly connected to each other.
It is however possible, in the case of nearly parallel beams, to take into account the variable spacing of the beams by splitting each span into several members.
The current implementation supports only the method described here. For special, unsupported use cases, it is recommended to use manual input of the effective width.
The effective width can be defined manually or calculated automatically (see " Rib in the slab").
When the property shape of rib is set to T-symmetric, slab left, slab right or asymmetric, effective values are to be defined manually.
When shape of rib is set to automatic, the effective width is calculated automatically from the geometry of the structure. The calculation of the effective width is performed for the entire structure when running the analysis. It can also be triggered by clicking the action button calculate effective width at the bottom of the property sheet of the considered beam.
The elements that influence the value of the effective width are
The span length does not necessarily correspond to the length of the considered 1D member. A span may be composed of several beams.
A span is defined according to the information that can be found in the
In the example above, the span is composed of 3 beams. The span is simply supported. Two possible boundary conditions at 1/3 and 2/3 of the span are disabled, because there is no support nor supporting column in those points.
Important: effective width is calculated as a uniform value for each span. Distinct spans usually have different effective width values. As SCIA Engineer supports only uniform effective width on a beam, it is not allowed for a beam member to exceed one span. Therefore, any inner support or supporting member along a beam will be ignored during the automatic calculation of effective width. Only supports and supporting members located at an end of a beam will be taken into account.
Always split members at supports, in order to obtain correct calculation of the effective width. The examples here are not OK: there is either an inner support on the beam, or a column connected to the beam via an inner node. One can see, that the beam is made of one member covering two spans. Those boundary conditions will be ignored by the automatic calculation of effective width.
The beam should be split in two members, as shown in the following examples. The boundary conditions are then applied to beam end nodes and will be taken into account correctly.
Additionally, please note, that boundary conditions for the calculation of effective width may be modified manually via the
The general settings for the calculation of effective width can be found in the solver setup. They are located there, because they influence the results of the analysis and may also, in some cases (Composite Analysis Model), influence the stiffness of the analysis model.
| Parallelism tolerance | maximum angle between a beam and its considered adjacent entity, for them to be considered parallel |
| Span length ratio L/beff,max (1 side) | ratio of the span length used for the calculation of the effective width; the reference span length will be divided by that value to obtain the effective width on each side of the beam |
| Span length correction | span length adjustment coefficients for the calculation of the equivalent span length Le (see theory above) |
| Simply supported beam | coefficient for a simply supported beam; at each end of the span, there is no continuity to another beam and there is a support or a supporting member (e.g. a column) |
| Inner span | coefficient for an inner span; at each end of the span, there is continuity with another beam and there is a support or a supporting member (e.g. a column) |
| End span | coefficient for an end span; both ends are supported and there is continuity with another beam at one end of the span |
| Cantilever | coefficient for a cantilever span; one end of the span is supported, the other one is not |
Those settings are located in the solver setup, because they influence the analysis model
In the current implementation, the effective width, is calculated separately on each side (left and right) of the rib as the smallest of the three following values:
SCIA Engineer stores the calculated effective width values for both the internal forces and check effective width. However, the checks might use or not use those stored values when determining the resistance of the cross-sections. Please refer to the detailed description of the considered checks for more information about this topic.
For more details "Composite Analysis Model - Theoretical background".
The effective width for internal forces can be graphically displayed on the analysis model, in two different ways.
