Limitazioni del software
In tutti i programmi di verifica FEA generali, incluso RFEM, ogni asta è rappresentata da un singolo elemento lineare situato al baricentro della sezione. Questo è indicato come vista di rendering 'wireframe'. The member includes internal information such as the material properties as well as the geometric properties including width and height to be used in both the analysis and design calculations. However, when intersecting members are modeled, the program has no immediate information such as how the intersecting members are oriented relative to each other or in the case of members bearing on one another, the bearing area is unknown. The program is only aware of these line elements connected at a single nodal point.
Quando aste di piccole dimensioni sono collegate ad aste-trave più grandi mediante appoggio sulla faccia superiore o quando le aste sono vincolate ad una delle estremità, o lungo la lunghezza, da un tipo di vincolo esterno, la tensione dell'asta perpendicolare alla fibratura dovrebbe essere inclusa nelle verifiche. The NDS Sec. 3.10.2 [1] and CSA O86 Clauses 6.5.7 and 7.5.9 [2] design checks for bearing perpendicular to the grain both require the net bearing area to be known. Because of this requirement, design software may forgo important factors such as the bearing area factor, Cb, [1] or the length-of-bearing factor, KB, [2], or the design check is ignored altogether.
With the new RFEM 6 feature "design supports", it is now possible to define such information and carry out the full design checks for compression stress perpendicular to the grain.
Flusso di lavoro NDS 2018
When creating a new design support definition, the Type should be set to "timber". This will activate the relevant input data according to the NDS standard. The Direct Support checkbox indicates this definition type will be used for the compression check perpendicular to the grain. The Support Length is user-defined and should specify the total length of the bearing area. The Support Width is automatically set as the width of the relevant member, but can be overwritten. Support from Edge indicates whether the bearing support is located at the upper +z axis of the member, the lower -z axis, or both. The Inner Support checkbox indicates whether the Cb factor should be applied to the member’s compression design value perpendicular to the grain, Fc⟂, based on Sec. 3.10.4 [1]. If this option is checked off, an end support will not consider Cb. Il valore di progetto a compressione dell'asta definito nella NDS varia a seconda del limite di deformazione selezionato.
Una volta che le informazioni di input del vincolo esterno di progetto sono complete, i vincoli esterni di progetto possono essere applicati ai nodi pertinenti e alle aste sulla struttura. Multiple design supports may be needed, as different bearing scenarios may occur at different locations. RFEM consente all'utente di impostare un Nome definito dall'utente per più definizioni di vincolo esterno di progetto nella parte superiore della finestra di dialogo per applicazioni più rapide e convenienti.
The section proof will be carried out in the Timber Design add-on considering Sec. 3.10.2 [1]. With a glued-laminated member, for example, the adjusted compression design value perpendicular to the grain, Fc⟂’, is defined in Table 5.3.1 [1] with the following equation.
| Fc⟂ | Valore di progetto della compressione di riferimento perpendicolare alla fibratura |
| CM | Coefficiente di umidità di esercizio |
| Ct | Coefficiente di temperatura |
| Cb | Coefficiente dell'area di appoggio |
| KF | Coefficiente di conversione del formato (solo LRFD) |
| φ | Coefficiente di resistenza (solo LRFD) |
The Cb factor is further defined in Sec. 3.10.4 [1]. However, Cb is only applicable if the Support Length defined under the Design Support definition is less than 6 inches and farther than 3 inches to the end of a member. Additionally, Cb is only applicable for inner supports, which also must be indicated under the definition type as indicated above. If these criteria are met, the Timber Design add-on will automatically calculate and apply Cb based on the following equation.
| lb | La lunghezza del cuscinetto misura parallelamente alla fibratura |
The section proof design check ratio is determined by the ratio of the demand compression force perpendicular to the grain to the adjusted compression design value perpendicular to the grain. All NDS variables, formulas, and code references are indicated directly in the Timber Design add-on design check details providing clear and traceable results.
CSA O86:19 Flusso di lavoro
When designing according to CSA O86:19, the same workflow above for the NDS can be followed for the design support definition type. There are a few key differences with the CSA standard, however. A new option, Check Critical Bearing, will determine if the compression load is applied within a distance from the center of the support equal to the depth, d, of the member indicated in Fig. 6.2 [2]. If so, the factored compressive resistance perpendicular to the grain is reduced by a factor of 2/3 as shown in Clause 6.5.6.3.1 [2] for sawn lumber and Clause 7.5.9.3.1 [2] for glued-laminated timber. Dovrebbe essere utilizzata anche l'area di appoggio media.
The Limit of High Bending Stresses for Factor Kb is also set in the design support definition type. The length of the bearing factor, Kb, can be applied to the compressive resistance if the bearing area does not occur in positions of high bending stresses indicated in Clause 6.5.6.5(b) [2]. The user can set the ratio of demand moment to moment capacity to be considered a high bending area.
With a glued-laminated member, for example, the factored compressive resistance perpendicular to the grain, Qr, is defined in Clause 7.5.9.2 [2] with the following equation.
| φ | = 0,8 |
| Fcp | = fcp (KD KScp KT ), dove fcp è la resistenza specificata a compressione perpendicolare alla fibratura, |
| Ab | Area di appoggio |
| KB | Coefficiente della lunghezza del cuscinetto |
| KZcp | Coefficente dimensionale per portanza |
The section proof design check ratio is determined by the ratio of the demand factored bearing force to the factored compressive resistance perpendicular to the grain. Tutte le variabili, le formule e i riferimenti di codice CSA O86 sono indicati direttamente nei dettagli dell'add-on Verifica legno.
Sommario
Bearing stresses perpendicular to the grain are an integral part of member design according to both NDS 2018 and CSA O86:19. The bearing area is typically an unknown variable for this design check when using general analysis and design software, as all members are represented by an internal line element connected at nodal points only. However, the new "design support" feature in RFEM 6 now allows users to specify the length and width of the bearing area, paving the way for compression design checks perpendicular to the grain that previously were not possible.
