In the Members | Base tab of the 'Ultimate configuration' or 'Strength configuration' dialog box, you can make basic settings for the design of members and member sets.
The 'Design parameters' are divided into several categories, which differ depending on the design standard. The following sections describe the parameters of these standards:
EN 1995
General
The 'Perform stability analysis' check box controls whether, in addition to the cross-section checks, stability analyses such as against flexural buckling or lateral-torsional buckling are performed. You should deactivate the check box if you do not want to perform a stability analysis or if you have already taken the stability effects into account in the determination of internal forces (for example, by a second-order analysis with imperfections and/or stiffness reduction).
If the stability analysis is activated, effective lengths must be defined and assigned to the respective members or member sets. If this is not the case, no stability analysis is possible. After the calculation, a corresponding message is displayed in the Errors & Warnings result table. Tension members are excluded from this, as they can only absorb tensile forces and do not require stability analyses.
Utilization limits for ignoring internal forces and stresses
The design standards, which are mostly designed for manual calculations, contain design check formulas or interaction conditions that are often only designed for certain combinations of internal forces. However, within the framework of the structural analysis of the 3D model, very small values for internal forces are usually obtained. Although they are insignificant for the ultimate limit state from an engineering point of view, strict adherence to the design standard prevents certain design checks or enforces the use of less favorable interaction formulas. The utilization limits therefore offer a simple and transparent way of neglecting stresses from very small internal forces in the designs and avoiding the problems mentioned above.
The limit value η describes the ratio of the existing stress to the design value of the strength, which is determined in a simplified way via the cross-section properties. The limit ratios of the design resistances are derived from the standard specifications and are preset with very small values to enable a simplified design. Therefore, you should only use high limit values for testing purposes.
If an internal force is below the limit value condition, the design is performed without a warning that the internal force is neglected. However, in the design check details, you can use the note 'Negligible' for the design internal forces to check which internal forces were neglected due to the limit values and were not taken into account in the design. If all internal forces are negligible at a location, only the check of the negligible internal forces is given as the design.
Curved and saddle members
Using the check boxes, you can control whether the check of the stresses perpendicular to the grain direction should also be performed for non-uniform cross-section distributions. This tension perpendicular to grain check often proves to be a weak point for curved members.
'Saddle members' represent members with a section distribution of the type 'saddle'. For these members as well, you can specify whether the tension perpendicular to grain check should be performed.
Cut-to-grain angle limitation
For members with a linear or tapered section distribution, the wood fibers are cut through in the beveled edge. According to [1] 6.4.2, the influence of the cut-to-grain angle α on the stresses at the cut edge must be taken into account. According to the German national annex, this angle must not be too large and may be a maximum of 24°. You can adjust the default limit value if necessary to enable further design.
System strength according to 6.6
If the continuous load distribution system is able to transfer the loads from one member to adjacent members for similar structural components with equal distances (for example, ceiling joists all equal next to each other), the strength of the structural components may be increased by the factor ksys according to [1] 6.6. In this case, select the check box.
Increase of the shear crack factor
This category is available for a design according to the German national annex. It concerns the influence of cracks on the check of the shear resistance of members subjected to bending. If you activate the option, the factor kcr for the shear design of softwood cross-sections at a distance of more than 1.5 m from the member ends is increased by 30%. It is assumed that the cracks there only have minor dimensions.
NDS
General
The 'Perform stability analysis' check box controls whether, in addition to the cross-section checks, stability analyses such as against flexural buckling or lateral-torsional buckling are performed. You should deactivate the check box if you do not want to perform a stability analysis or if you have already taken the stability effects into account in the determination of internal forces.
For further information, see the section General in the description for standard EN 1995.
Utilization limits for ignoring internal forces and stresses
The design check equations of the design standards are often only designed for certain combinations of internal forces. As a rule, however, the structural analysis of the 3D model also results in very small values for "secondary" internal forces, which are insignificant for the ultimate limit state from an engineering point of view, but prevent certain design checks when strictly adhering to the standard. The utilization limits make it possible to neglect certain internal forces in the design and thus avoid the problems mentioned above. The limit value η describes the ratio of the existing stress to the design value of the strength.
For further information, see the section Utilization limits in the description for standard EN 1995.
Torsion calculation
Using the check boxes, you can control how torsion is taken into account in the designs:
- Check torsion limit only: Further design is permissible if the shear stress caused by torsion does not exceed the design value of the strength according to [2] Chapter 3.4. If it is above the limit value, no design is possible.
- According to Timber construction manual: The torsional stresses are designed according to the specifications of [3] Chapter 4.6. In the section Utilization limits, you can specify the limit ratio η above which torsional stresses are generally relevant for the design.
- Ignore: The shear stresses due to torsion are checked in the design but have no influence on the design. If the design value of the strength is exceeded, the program only outputs an informative note.
Positive or negative bending about y-axis
According to [2] Table 5A and Table 5C, specific design values apply to structural components made of glued-laminated timber subjected to bending about the y/x-axis for positive bending (bottom side of the beam under tensile stress) and negative bending (top side of the beam under tensile stress). Specify whether the bottom side of the members to be designed is in the positive direction of the local z/y-axis or in the opposite direction.
Curved members
Curved glued-laminated timber beams are exposed to radial stresses that must be taken into account according to [2] Equation 5.4-1. The 'radial stress check' is therefore activated by default. For more information, see the technical article Design of curved glued-laminated timber beams according to ANSI/AWC NDS.
User-defined size factors
The size factor CF is regulated in [2] Section 4.3.6 for dimensional lumber and in Section 6.3.7 for round timber poles. If you select a check box, you can set the factor manually in each case.
User-defined flat use factors
The flat use factor Cfu is regulated in [2] Section 4.3.7 for dimensional lumber and in Section 5.3.7 for glued-laminated timber. You can adjust this factor in each case.
User-defined stability factor for built-up columns
If a factor Kf other than that specified in [2] Section 15.3.2.4 is to be used for determining the stability factor for built-up columns, you can set it here.
User-defined volume factor
You can manually specify the volume factor Cv for glued-laminated timber if it is not to be determined according to [2] Section 5.3.6.
CSA O86
General
The 'Perform stability analysis' check box controls whether, in addition to the cross-section checks, stability analyses such as against flexural buckling or lateral-torsional buckling are performed. You should deactivate the check box if you do not want to perform a stability analysis or if you have already taken the stability effects into account in the determination of internal forces.
For further information, see the section General in the description for standard EN 1995.
Utilization limits for ignoring internal forces and stresses
The design check equations of the design standards are often only designed for certain combinations of internal forces. As a rule, however, the structural analysis of the 3D model also results in very small values for "secondary" internal forces, which are insignificant for the ultimate limit state from an engineering point of view, but prevent certain design checks when strictly adhering to the standard. The utilization limits make it possible to neglect certain internal forces in the design and thus avoid the problems mentioned above. The limit value η describes the ratio of the existing internal force to the design value of the cross-section resistance.
For further information, see the section Utilization limits in the description for standard EN 1995.
Positive or negative bending about y-axis
According to [2] Table 5A and Table 5C, specific design values apply to structural components made of glued-laminated timber subjected to bending about the y/x-axis for positive bending (bottom side of the beam under tensile stress) and negative bending (top side of the beam under tensile stress). Specify whether the bottom side of the members to be designed is in the positive direction of the local z/y-axis or in the opposite direction.
Curved members
Curved glued-laminated timber beams are exposed to radial stresses that must be taken into account according to [4] Section 7.5.6.7. The 'radial stress check' is therefore activated by default.
Select the 'end type' of the beam in the list:
- Curved end
- Tangential end
The end type determines the minimum radius of curvature to be applied according to [4] Table A.7, based on the laminate thickness for glued-laminated timber specified in the standard parameters tab.
Maximum taper angle
Set the limit value of the angle α for tapered glued-laminated timber components. If the angle is greater than this limit value, no design of the beam is performed.
After the calculation, you can check the taper angle and the limit value in the design check details.
Reduction of compressive strength for connection of built-up compression members
According to [4] Section 6.5.5.4, the compressive strength of built-up components is limited to a certain percentage of the member compressive strength. Depending on the fastener, the strengths must be reduced as follows:
- Nailed connections: 60%
- Bolted joints: 75%
- Split ring connections: 80%
You can adjust the factored compressive strengths if necessary.
