Mounting Hollow Section Frames: A Complete Engineering Guide

Based on UTMK Technical Department Materials

Square and rectangular hollow sections (SHS and RHS) are among the most efficient materials for steel frames. They are widely used in the construction of hangars, warehouses, canopies, commercial pavilions, and various engineering structures.

The popularity of this material is easily explained: high rigidity with relatively low mass, convenient mounting, and the ability to implement complex designs with minimal steel consumption. However, to achieve a reliable and economical result, it is necessary to correctly design the connection joints, calculate the stiffness of the elements, and rationally cut the material.

This guide covers the practical principles of working with hollow sections from UTMK: calculations, recommendations for trusses and columns, as well as ways to minimize metal waste during fabrication.

Key Properties of Hollow Sections

A hollow section is a closed metal profile of square or rectangular cross-section made of structural steel. It is manufactured using hot-rolling or cold-forming methods (according to EN 10210 or EN 10219 standards).

The closed cross-section provides a significantly higher moment of inertia than open profiles of the same mass. For a practitioner, this means: less steel — more rigidity.

Main advantages of hollow sections:

• high flexural stiffness (resistance to bending)
• significant torsional resistance
• uniform load distribution
• excellent weldability
• ease of mounting for frame structures

The most common sizes used in construction:

For load-bearing frame elements, the wall thickness should be at least 3 mm. When choosing a profile, rely on load calculations rather than just a "safety margin."

You can purchase hollow sections for your project in the UTMK catalog — a wide range of sizes is available in stock.

Key Frame Connection Joints

A joint is the connection point of several structural elements. Joints often determine the strength of the entire structure, so their design deserves special attention.

1. Butt Joint

Used for extending elements. There are three execution options:

• perimeter welding
• internal sleeve (insert)
• cover plates

The strength of the joint is calculated using the formula:

σ = N / A

where: σ is stress; N is the load force; A is the cross-sectional area.

To reinforce butt joints, internal sleeves with a length of 2–3 times the profile height are often used.

2. T-Joint

The most common in trusses: one element is attached to the side wall of another with a fillet weld. To improve welding, a shaped cut — known as a "fish mouth" — is often made. This increases the contact area and enhances weld strength.

3. Corner Joint

Used in frames and chassis, performed at angles of 45°, 30°, or 60°. For high loads, always add gusset plates or stiffening ribs.

Calculation of Element Stiffness

Structural stiffness is the ability to resist deformation under load. Simply put: no amount of strength will save a structure if it deflects excessively.

Beam Deflection Formula

To check if a beam will deflect under a uniform load, engineers use the following formula:

f = (5qL⁴) / (384EI)

where: f is deflection (mm); q is the uniform load (kN/m); L is the element length (m); E is the modulus of elasticity of steel (~210,000 MPa); I is the moment of inertia (cm⁴).

The allowable deflection for roof beams is usually L/200. If your result exceeds this limit, a larger section or a smaller beam pitch is required.

Moment of Inertia of a Hollow Section

The larger the moment of inertia, the stiffer the profile. For a rectangular closed section:

Ix = (bh³ − b₁h₁³) / 12

where: b, h are external dimensions; b₁, h₁ are internal dimensions (external dimension minus twice the wall thickness).

Column Stability: Euler’s Formula

Columns work under compression. The main danger with incorrect calculation is loss of stability (buckling). The critical load is determined by Euler’s formula:

Pcr = π²EI / (KL)²

where: Pcr is the critical load; K is the effective length factor; L is the actual length of the column.

The factor K depends on how both ends of the column are secured:

Choosing the wrong K factor is one of the most common errors in field calculations. For example, a cantilever fence post (K=2.0) requires a profile twice as powerful as an identical post pinned at both ends (K=1.0).

To prevent buckling:

• reduce the effective length of the element
• increase the profile cross-section
• use braces and struts

Designing Trusses with Hollow Sections

Trusses allow for covering large spans with minimal steel weight. Main elements: upper chord, lower chord, diagonals (braces), and verticals (struts).

Most Effective Truss Schemes

• Triangular — the most rigid construction. Provides uniform load distribution and high stability.
• Parallel Chord — used in canopies, warehouses, and industrial buildings.
• Arched — used in hangars, sports facilities, and exhibition pavilions. Arches handle snow loads exceptionally well.

Column Selection: Reference Table

Profiles for columns in stock: 60×60×3 / 80×80×3 / 100×100×4 / 120×120×5 — check availability and price in the UTMK catalog.

This data is for reference. For critical structures, always perform a full engineering calculation in accordance with Eurocode 3 (EN 1993).

Steel Cost Optimization

The designer's main task is to achieve maximum strength with minimum mass. Here are principles that yield real results.

Geometry Over Thickness

A triangle is a geometrically rigid shape. Therefore, a correctly designed truss made of thin metal proves to be stronger and cheaper than a massive beam. Adding braces is much more effective than increasing the profile's wall thickness.

Design for 6-Meter Lengths

Steel products are supplied in standard lengths of 6 and 12 m. With poor planning, waste can reach 15–20% of the order.

Example of a correct approach:

• 6 m → 3 elements of 2 m each → 0% waste
• 6 m → 3 m + 3 m → 0% waste
• 6 m → 2.5 m + 2.5 m + 1 m → the remainder is used for braces and brackets

Utilize Offcuts

Scraps from standard lengths are ideal for braces, reinforcements, and mounting brackets. Include this in the specification beforehand.

Minimize Number of Welds

Fewer welds mean fewer deformations, less time, and cheaper production. Where you can combine two elements into one — do so.

Ground Pre-assembly

Assemble trusses horizontally on the ground and lift them ready-made. From practice: this technique reduces mounting time by 15–20%.

Practical Mounting Recommendations

1. Geometry Control

Before welding, be sure to:

• check diagonals — the difference should not exceed 2 mm per meter
• secure elements with clamps or tack welds
• use mounting templates for repeating joints

2. Welding Sequence

1. mounting with tack welds
2. geometry verification
3. final welding

This order minimizes metal deformation from thermal effects.

3. Corrosion Protection

For the longevity of structures, corrosion protection is a requirement, not an option. Main methods:

• priming + painting — for indoor and enclosed structures
• hot-dip galvanizing — for outdoor and industrial conditions
• powder coating — for decorative and light structures

Hot-dip galvanizing according to the EN ISO 1461 standard provides a service life of 20 to 65 years, depending on the coating thickness and environmental aggressiveness. This is one of the most effective protection methods for structures operated outdoors.

Checklist Before Ordering Steel

Before finalizing your specification, check:

• moment of inertia is calculated for all load-bearing elements
• the K factor is determined for each column
• element lengths are multiples of 6 or 12 m (minimum waste)
• cutting offcuts are planned for braces and brackets
• connection joints are designed for welding
• corrosion protection method is selected
• a control deflection calculation is performed for floor/roof spans

Adhering to these principles allows for building structures with minimal steel mass, predictable characteristics, and a long service life.

Select your profile and check availability in the UTMK hollow section catalog.