1. The Load Cases on an H-Beam
An H-beam in a structure carries some combination of bending, shear, axial compression and torsion. A floor beam is dominated by bending and shear, a column by axial compression and buckling, and a rafter by combined bending and compression. Each load case has its own check, and the governing case determines the section that is selected.
2. Bending and Shear Capacity
The bending capacity of a beam is the product of the section modulus and the yield strength of the material, reduced by the safety factor of the design code. The shear capacity is governed by the web area and the shear yield strength. For an H-beam the flange carries most of the bending moment, while the web carries most of the shear, which is why the H shape is efficient for combined bending and shear.
3. Deflection and Serviceability
Even when the strength is sufficient, a beam must not deflect so much that it damages the finishes, misaligns the equipment or feels unsafe. The deflection under service loads is limited by the design code, typically span over 200 to span over 400 depending on the element and the use. Because deflection depends on the moment of inertia rather than the section modulus, a deep section is often chosen to satisfy the serviceability limit.
4. Lateral Torsional Buckling
A beam that is not laterally restrained can fail by lateral torsional buckling before it reaches its full bending capacity: the compression flange moves sideways and the section twists. The resistance depends on the length between restraints, the section geometry and the moment distribution. Restraining the compression flange at intervals, or choosing a wider flange, raises the buckling resistance significantly.
5. Column Stability and Buckling
A column under axial compression fails by buckling about its weaker axis unless it is braced. The buckling resistance depends on the slenderness, which is the effective length divided by the radius of gyration, and the design curves of the code. Wide-flange H-sections have similar radii of gyration about both axes, which is why they are efficient columns, and bracing at mid-height reduces the effective length and increases the capacity.
Frequently Asked Questions
Q: What governs the bending capacity of an H-beam?
The section modulus of the beam and the yield strength of the grade, reduced by the safety factor of the design code.
Q: What is lateral torsional buckling?
It is a failure mode of unrestrained beams where the compression flange moves sideways and the section twists before the full bending capacity is reached.
Q: How is deflection limited?
By the serviceability limits of the code, typically span over 200 to span over 400, checked under the service loads.
Q: Why are H-sections good columns?
The wide flanges give similar radii of gyration about both axes, which raises the buckling resistance about the weaker axis.
Q: How can lateral buckling be prevented?
By restraining the compression flange at intervals, using a wider flange section, or reducing the unrestrained length.
Q: What is the slenderness of a column?
It is the effective length divided by the radius of gyration, and it determines the buckling resistance through the design curves.