The Full Family of Structural Sections
A steel building is assembled from several families of hot-rolled sections, each doing a different job: H beams for columns and primary beams, I beams for long-span flexural members, U channels for purlins and edge members, and angles for bracing and connections. Together they form an efficient and economical frame, and the grades ASTM A283, Q235 and Q345 cover the strength range of most projects.
Section Families and Their Roles
H shape beams: wide parallel flanges; columns and primary beams in portal and multi-storey frames
I beams: taller narrow profiles; beams carrying bending in long spans
U channels: open U-shaped sections; purlins, girts, edge beams and secondary members
Angles: L-shaped sections; bracing, brackets, lintels and connection cleats
Material Grades and Properties
ASTM A283: a carbon steel plate and structural grade with minimum yield 205 MPa for Grade C, used for general building and low-stress members
Q235 (GB/T 700): minimum yield 235 MPa, tensile 370-500 MPa; the economical general structural grade
Q345 (GB/T 1591): minimum yield 345 MPa, tensile 470-630 MPa; the higher-strength grade for primary members
Selecting the Right Section
The choice of section follows the load path: columns need stability in two directions, so wide-flange H sections are preferred; beams need depth for stiffness, so I sections and deep H sections work well; secondary members favour lightweight channels and angles. The grade is chosen from the governing design code, balancing strength against material cost.
Dimensional Range and Tolerances
Hot-rolled sections are produced to GB/T 11263 for H beams, GB/T 706 for angles and GB/T 707 for channels, with tolerances and theoretical weights listed for every size. Standard lengths are 6 m and 12 m, and all sections can be cut, drilled, welded or galvanized to the fabrication requirements.
Frequently Asked Questions
Q: What is the difference between an H beam and an I beam?
An H beam has wide parallel flanges, while an I beam is taller and narrower with tapered flanges; H sections are preferred for columns, I sections for bending members.
Q: What is ASTM A283 used for?
A283 is a carbon steel grade for general structural and plate applications where the lower yield strength of about 205 MPa is acceptable.
Q: Which grade should a building frame use?
Q235 suits light and secondary frames, while Q345 is recommended for primary columns and beams to reduce section size and weight.
Q: What are channels used for in steel buildings?
Channels are used as purlins, girts, edge beams, bracing and secondary members where their open U-shape suits light loads and easy connections.
Q: Are all sections available in the same lengths?
Yes, H beams, I beams, channels and angles are normally supplied in 6 m and 12 m lengths, with cut-to-length service available.