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Concrete Slab Thickness in Composite Deck Systems vs. NIAZIT OWSJ Roof System

Introduction

Concrete slab thickness is one of the most influential parameters in the structural behavior of steel floor systems. In conventional composite and steel deck floors, slab thickness typically ranges from 70 to 80 mm, while in Open Web Steel Joist (OWSJ) systems—particularly the NIAZIT OWSJ Roof System—the slab thickness can be reduced to approximately 50 mm.

This difference is not a simplification, but the direct result of a different structural design philosophy, based on SJI standards and the National Building Regulations.

Structural Role of the Slab in Composite and Steel Deck Floors

In composite and steel deck systems:

• The concrete slab is a primary load-bearing element

• Concrete actively participates in bending resistance

• Slab thickness directly affects flexural capacity, deflection, and vibration control

Due to large spacing between secondary beams, the slab behaves as a one-way slab with a relatively large effective span, making slab thicknesses below 70 mm generally impractical and non-compliant with serviceability requirements.

Design Philosophy of Open Web Steel Joists (OWSJ)

According to Steel Joist Institute (SJI) specifications:

• OWSJs are designed as fully independent load-bearing members

• Dead and live loads are carried by the steel joists themselves

• Deflection and vibration are controlled at the joist design stage

Therefore, the concrete slab is not considered a primary structural member in flexure.

Role of the Slab in the NIAZIT OWSJ Roof System

In the NIAZIT OWSJ Roof System, the concrete slab functions as:

• Load distribution layer

• Structural diaphragm

• Continuous walking and finishing surface

• Protective cover for steel components

As a result, slab thickness can be safely reduced to 50 mm, provided that execution and detailing requirements are satisfied.

Joist Spacing and Its Relationship with Slab Thickness

In composite floors, increasing beam spacing directly increases slab bending moments, requiring thicker slabs.

In contrast, in OWSJ systems:

• Joist spacing does not transfer bending responsibility to the slab

• Load-bearing remains within the steel joists

• Slab effective span remains limited

Role of Bridging and Diagonal Bracing (SJI Requirements)

SJI requires the use of horizontal and diagonal bridging to:

• Prevent lateral-torsional buckling

• Control construction-stage instability

• Ensure load sharing between joists

When diagonal cross-bridging is designed according to SJI λ/r limits:

• Joist lateral stability increases

• Relative displacement between joists decreases

• Joist spacing can be increased within SJI limits without increasing slab thickness

Relationship Between Bridging and Slab Thickness

With proper bridging:

• The slab is not subjected to secondary bending effects

• Cracking and excessive deflection are avoided

• A 50 mm slab remains fully adequate

Without adequate bridging:

• Slab is unintentionally forced into structural action

• Increased cracking and serviceability issues occur

• Either slab thickness must increase or the system becomes non-compliant

Conclusion

The reduced slab thickness in the NIAZIT OWSJ Roof System is the result of:

• SJI-compliant joist design

• Proper diagonal bridging

• Structural independence of steel joists

• Elimination of concrete from primary flexural resistance

This approach represents a modern steel-oriented design philosophy, optimizing material efficiency while maintaining full compliance with international standards.

🔹 Final Comparison Table

The reduced slab thickness in the NIAZIT OWSJ Roof System is a direct result of SJI-compliant joist design and proper diagonal bridging, not a compromise in structural safety.