You can use 2D and 3D sketches to define the basic framework of a
weldment structure, then create structural members containing groups of sketch
segments.
You can also add items such as gussets and end caps using tools on the
Weldments toolbar.
For more information about
weldments
(external link to MySolidWorks.com), see
SOLIDWORKS Tutorials:
Weldments by clicking
.
When you create the first structural member in a part, a weldment feature 
is created and added to the
FeatureManager design tree.
The software also creates two default
configurations in the ConfigurationManager: a parent configuration Default<As
Machined>
and a derived configuration Default<As
Welded>.
On a per document basis, you can suppress the automatic creation
of the
<As
Welded>
configuration. Before adding weldments to a new document, click
Options
(Standard toolbar). On the
Weldments page of
Document
Properties, clear
Create derived
configuration.
You can also configure multiple weldment profiles of structural members
as library features. You can add different configurations of a structural member and
save them as one profile in a library feature. For example, instead of having 50
separate library feature files for square tubing sizes, you can have one library feature
file with 50 configurations that you can control by a design table.
When creating or editing structural members, you can select the
configured library features in the Structural
Member PropertyManager.