Designing Composite Beams with Web Openings: A Practical Guide

This article presents guidance and recommendations on the design of composite steel beams with web openings to Eurocode 4

The provision of beams with web openings has become a common method of incorporating services within the structural depth of a floor. Whether it is an isolated opening in a rolled section or a set of regular openings in a fabricated member, the introduction of an opening in the web alters the stress distribution in the member and influences the structural behaviour. Beams with multiple openings behave as Vierendeel girders. In composite beams, the bending resistance of the top T (above the opening) may be increased by allowing for composite action. Although specialist software is available to design beams with web openings, guidance regarding sizing, spacing and best practice is useful for engineers performing scheme design.

This article presents guidance and recommendations on the design of composite steel beams with web openings to Eurocode 4

Types of Web Openings

Web openings in beams can vary in spacing and shape to meet specific requirements or be uniformly spaced along the beam’s length. Depending on the design choice, beams with web openings can be fabricated using one of three methods:

1. Hot-rolled I sections with individually cut openings.
2. Hot-rolled I sections cut along the web profile and re-welded to form a deeper beam with regular openings.
3. Fabricated sections made from three plates with openings cut in the web plate either before or after forming the I section.

The shape of the web opening, often circular or rectangular for convenience, depends on the designer’s preference and the opening’s purpose (Figure 1). While elongated openings are possible, they may require reinforcement. Traditionally, beams with regular openings were castellated with hexagonal openings. Advances in manufacturing technology have popularized cellular beams with circular openings, which can be asymmetric or tapered in depth if fabricated from plates. Larger bottom sections are advantageous for maximizing composite resistance.

Cellular beams with regular openings are typically used as secondary beams, where design is usually controlled by serviceability requirements. These beams range from 900–1200mm in depth, depending on the floor zone.

Fabricated beams with isolated openings are often utilized for long-span primary beams, where high shear forces are critical. Large openings should ideally be positioned in low-shear regions. If elongated openings are needed, local reinforcement with horizontal stiffeners can enhance the bending resistance of the T-sections above and below the opening.

Design Considerations

Web openings significantly reduce shear resistance due to the substantial loss of the web but only slightly affect bending resistance since flanges primarily carry bending loads. The structural resistance of a beam with web openings depends on its global resistance in flexure, lateral torsional buckling, and shear, or the local resistance of the web posts (between openings) and the T-sections, whichever is more critical. In many practical scenarios, beams are laterally restrained, so local effects around the openings govern the design.

Designers need to consider the following failure modes for a composite beam (Figure 2):

1. Global bending, which depends on composite action at the opening position.
2. Vertical shear, due to the reduced web area.
3. Local Vierendeel bending, caused by transferring shear force across the opening, consistent with the bending moment’s rate of change along the beam.
4. Web-post horizontal shear, which may dominate for closely spaced openings, depending on the web-post’s minimum cross-sectional area.
5. Web-post bending, which may dominate between closely spaced rectangular openings.
6. Web-post buckling, which may dominate for slender web-posts.
7. Shear buckling, which may dominate for slender webs.

Vierendeel Bending

Shear transfer across openings occurs through Vierendeel bending in the top and bottom T-sections, with the maximum moment at the four corners of the opening. The flexural resistance of the T-sections under Vierendeel bending is critical, especially for long openings. Therefore, there’s usually a limit on the length-to-depth ratio of the opening. In a composite beam, local composite action between the top T and the slab can increase resistance to Vierendeel bending, potentially allowing for larger openings compared to non-composite steel beams.

Practical Guidance for Positioning Openings in Beam Webs

When designing beams with large web openings, no further consideration of the openings’ effect on section properties is needed if certain rules are followed. Geometric limits for designing composite beams with web openings are outlined in “Design of Composite Beams with Large Web Openings.” During the initial design stage, the bending moment resistance of a composite beam with web openings can be approximated using specific equations.

For unstiffened openings:

M_{o,Rd}=M_{Rd}(1-0.35h_o/h)

For horizontally stiffened openings:

M_{o,Rd}=M_{Rd}(1-0.2h_o/h)

Where: M_o,Rd is the bending resistance of the composite beam at the opening; M_Rd is the bending resistance of the unperforated composite beam; h_o is the depth of the opening; h is the depth of the steel beam

Reinforced Openings

If local resistance to Vierendeel bending or local buckling in the web T needs enhancement, beams with openings can be reinforced with horizontal stiffeners. Recommended detailing limits for openings and stiffeners in high-shear zones of beams subject to uniformly distributed loads or multiple point loads are provided as Figure 3. Simple design rules for reinforced rectangular openings include:

• The distance between the edges of the openings should be at least 1.5 times the length of the longer opening.
• The stiffener reinforcing the opening should not form an outstand exceeding 10 times its thickness.
• The minimum projection of the stiffeners past the opening should be at least 20 times the stiffener thickness.
• The horizontal centerline of the opening should be located within d/10 of the beam’s longitudinal centerline, where d is the web depth between fillets.
• Shear stress on the net shear area at the opening should not exceed fy/(√3γM0).
• Stresses at the edge of the opening at the top and bottom T-sections under local axial forces, Vierendeel bending, and shear forces should meet specific criteria.
• The distance from any part of the opening to the nearest point load should be at least the depth of the beam.
• Stiffeners should preferably be attached by fillet welds on both sides of the web, with the weld throat thickness being at least half the stiffener thickness.
• The minimum offset distance from the edge of the opening should allow for at least a 5mm leg-length fillet weld.
• The anchorage length of the stiffener beyond each end of the opening should generally be at least 25% of the opening length, with welds checked for force transfer over this length.

Stiffener Types

Horizontal stiffeners (Figure 4) are most commonly used to reinforce web openings. Vertical stiffeners may be necessary at point loads and secondary beam connections. Vertical stiffeners should generally be full depth unless the unstiffened web part can prevent local buckling. Ring stiffeners welded around the inside of circular openings can help prevent web-post buckling for class 3 webs.

End Posts

The solid material between the end of the beam and the first opening is called the “end post.” The recommended minimum width of the end post in a beam with circular openings is at least half the diameter of the first circular opening (Figure 5). For beams with rectangular openings, the end post should be at least as long as the first rectangular opening. The longitudinal shear and buckling resistance of the end post often depend on the type of connection used. Using a full-depth end plate can improve end post stability by smoothly transferring shear forces into the web and partially restraining the web against buckling.

Serviceability

At the scheme design stage, engineers must estimate the additional shear and bending deflections caused by each opening. A relevant SCI Advisory Desk note suggests allowing for a 3% additional imposed load deflection for each opening, provided the composite section at these openings also meets the resistance criteria for shear, overall bending, and local (or Vierendeel) bending. The additional deflection due to openings in a cellular beam is typically 12–15% of that in an unperforated beam of the same depth. “Design of Composite Beams with Large Web Openings” provides guidance on calculating perforated section properties and additional deflections due to shear and bending for a single opening and approximating these effects for beams with multiple openings.

Software

Specialist manufacturers of cellular beams and fabricated beams provide software for engineers to design members with web openings. This software can be used for a wide range of applications, assisting in the effective design of these members.

Summary

For projects requiring column-free spaces and the integration of structural and service zones, long-span solutions using beams with web openings are popular. Large openings in the beam web introduce additional considerations regarding reduced resistances and practical detailing limits. Guidance and support from beam manufacturers enable effective design, contributing to integrated long-span, lightweight steel solutions.

Also See: Designing a Composite Steel Beam to Eurocode 4 | Worked Example

Sources & Citations

• Johansson B. and Unosson E. (2006) “NCCI: Design rules for web openings in beams” [Online] Available at: www.steelbiz.org/Handlers/ResourceLookInsideHandler.ashx?ResourceID=1001384 (Accessed: August 2024).
• Lawson R. M. and Hicks S. J. (2011) “Design of Composite Beams With Large Web Openings” Ascot, UK: SCI.
• Steel Construction Institute (2014) “Part 11: Beams with web openings” Composite and Steel Construction Compendium.