Combined analysis and design choices for steel structures
For many steel structures a separate gravity design is not necessary and you can simply proceed directly to a Static design.
In some circumstances however, you may find it preferable to adopt a two-stage workflow in which gravity members are sized prior to the full design.
The benefits of this are:
- For larger models it can significantly speed up the analysis and design time
- If lateral resisting systems are not modeled, then mechanisms can result and the analysis may not find a solution. Since these lateral resisting systems are perhaps unknown early in the project (or insufficient systems have been provided), running a gravity design has the added benefit of automatically fixing column nodes (not attached to a diaphragm) horizontally, which stabilizes the model.
The gravity design stage enables you to design all members (and hence size those members with the Autodesign property checked), for only the active gravity combinations (this will include the Construction Stage combination). As an example, beams within a rigid floor diaphragm are unaffected by lateral load, and hence they can be sized using only the gravity combinations.
Gravity design is initiated by clicking Design Steel (Gravity).
After the gravity design has been completed, by default all steel members are reset to check design mode.
You may then decide to reset certain members to auto design e.g. columns and beams in 'moment frames', columns and braces in vertical or horizontal braced bays. In such cases, when the full static design is performed member 'pre-sizing' will take place and for members resisting lateral loads the new section size will be used if it is larger than that which resulted from the gravity design.
Full static design is initiated by clicking Design Steel (Static) or Design All (Static) on the Design tab.
All beams, columns and braces that do not have the Gravity only property checked, are designed or checked for all active combinations; Gravity only design members are designed or checked for the active gravity combinations only.
As part of the full design process a 3D Analysis is performed, for which you must select (via Design Options) the analysis type. The choice of analysis type will depend on the code being designed to.
Designing individual members for gravity only
By default, when a full static design is run, all members are designed for both gravity and lateral combinations.
You can however tell Tekla Structural Designer to only consider gravity combinations for the design of specific members. This is achieved by checking Gravity only in an individual member's properties, i.e.
- Gravity only checked - designed for gravity combinations only
- Gravity only unchecked - designed for all combinations types: gravity, lateral and seismic
Setting columns that do not help resist lateral loads to be designed for gravity loads only, will reduce the overall design time.
Engineering judgement will be required when identifying members as being 'gravity only'.
- if an inclined braced member connects to a beam, axial force in the brace (from both gravity and lateral loads) puts the beam into bending and therefore the beam should be designed for both gravity and lateral loads.
- potentially, beams in a sloping roof would also need to be designed for both gravity and lateral load